bailey ness and upcycling

Plastics are now indispensable in our daily lives. However, the pollution from plastics is also increasingly becoming a serious environmental issue. Recent years have seen more sustainable approaches and technologies, commonly known as upcycling, to transform plastics into value-added materials and chemical feedstocks. In this review, the latest research on upcycling is presented, with a greater focus on the use of renewable energy as well as the more selective methods to repurpose synthetic polymers. First, thermal upcycling approaches are briefly introduced, including the redeployment of plastics for construction uses, 3D printing precursors, and lightweight materials. Then, some of the latest novel strategies to deconstruct condensation polymers to monomers for repolymerization or introduce vulnerable linkers to make the plastics more degradable are discussed. Subsequently, the review will explore the breakthroughs in plastics upcycling by heterogeneous and homogeneous photocatalysis, as well as electrocatalysis, which transform plastics into more versatile fine chemicals and materials while simultaneously mitigating global climate change. In addition, some of the biotechnological advances in the discovery and engineering of microbes that can decompose plastics are also presented. Finally, the current challenges and outlook for future plastics upcycling are discussed to stimulate global cooperation in this field.

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Review

Upcycling to Sustainably Reuse Plastics

Xin Zhao,* Bhanupriya Boruah, Kek Foo Chin, Miloš Đokić, Jayant M. Modak,

and Han Sen Soo*

Dr. X. Zhao, B. Boruah, Dr. K. F. Chin, Dr. M. Đokić, Prof. H. S. Soo

Division of Chemistry and Biological Chemistry

School of Physical and Mathematical Sciences

Nanyang Technological University

21 Nanyang Link, Singapore 637371, Singapore

E-mail: xinzhao@ntu.edu.sg; hansen@ntu.edu.sg

B. Boruah, Prof. J. M. Modak

Department of Chemical Engineering

Indian Institute of Science

CV Raman Avenue, Bangalore, Karnataka 560012, India

Prof. H. S. Soo

Artiﬁcial Photosynthesis (Solar Fuels) Laboratory

Nanyang Technological University

50 Nanyang Avenue, Singapore 639798, Singapore

The ORCID identiﬁcation number(s) for the author(s) of this article

can be found under https://doi.org/10.1002/adma.202100843.

DOI: 10.1002/adma.202100843

% for textiles.[] The annual production

is still growing and is expected to con-

tinue, meaning that it is imperative for

us to plan for a future that will have even

more plastics.

Yet, the ubiquity of plastics has been

recognized to be a double-edged sword.

It was estimated that  Mt of virgin

plastics were produced between  to

, but  Mt of them had become

plastic waste with % accumulated in

landﬁlls or the natural environment such

as the waterways and the ocean.[a] In

 alone, around  Mt or % of the

annual production had become waste,

with only % recycled, % combusted,

and the remainder exposed to the environ-

ment (Table1).[] Over  Mt of plastics

are believed to be added to the aquatic

ecosystems every year since ,[] with

samples being discovered in some of

the remotest marine ecosystems like the

Mariana trench, including some dating as

far back as .[] The increase in ocean

plastics has been especially dramatic since

the s.[a] Besides being potentially toxic by compromising

the immune system in dierent organisms, plastics have also

been found to disrupt global biogeochemical cycles such as

nitriﬁcation and denitriﬁcation.[]

Furthermore, the enormous volumes of plastics being

produced, converted, and disposed have been estimated to

contribute almost  Mt of CO -equivalents (COe) in green-

house gas (GHG) emissions in , with the majority (%

or  Mt) generated at the production stage, % or  Mt

during conversion, and the remaining % or  Mt from the

end of life disposal (Table2).[] Alarmingly, the current trajec-

tory in growth of plastics consumption suggests that around

 Mt of CO e in GHG emissions (% of the global carbon

budget) is expected by .[] In , owing to the COVID-

pandemic, even more plastic wastes are being generated due

to increases in the use of food and other packaging materials,

as well as the proliferation of single use disposal face masks.

This has resulted in a surge in criminal recycling scams where

plastic wastes from Europe and North America are being ille-

gally exported to Asian countries to artiﬁcially inﬂate the recy-

cling rates in the countries of origin.[] Patently, more sustain-

able approaches are needed to manage the entire life cycle of

plastics from production to end of life.

To manage the inexorable growth of plastics production

and waste, one strategy can be to adopt a zero waste hierarchy

that was proposed by the European Commission's Waste

Plastics are now indispensable in daily lives. However, the pollution from plas-

tics is also increasingly becoming a serious environmental issue. Recent years

have seen more sustainable approaches and technologies, commonly known

as upcycling, to transform plastics into value-added materials and chemical

feedstocks. In this review, the latest research on upcycling is presented,

with a greater focus on the use of renewable energy as well as the more

selective methods to repurpose synthetic polymers. First, thermal upcycling

approaches are brieﬂy introduced, including the redeployment of plastics for

construction uses, 3D printing precursors, and lightweight materials. Then,

some of the latest novel strategies to deconstruct condensation polymers to

monomers for repolymerization or introduce vulnerable linkers to make the

plastics more degradable are discussed. Subsequently, the review will explore

the breakthroughs in plastics upcycling by heterogeneous and homogeneous

photocatalysis, as well as electrocatalysis, which transform plastics into more

versatile ﬁne chemicals and materials while simultaneously mitigating global

climate change. In addition, some of the biotechnological advances in the

discovery and engineering of microbes that can decompose plastics are also

presented. Finally, the current challenges and outlook for future plastics upcy-

cling are discussed to stimulate global cooperation in this ﬁeld.

1. Introduction

Plastics and other synthetic polymeric materials have become

integral and indispensable to our daily lives. Plastics are now

essential for consumer electronics, construction and engi-

neering, packaging, healthcare, and even space exploration. In

, around  megatons (Mt) of plastics was produced, of

which –% were used for packaging purposes, followed by

–% for the building and construction sector, and around

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Framework Directive (Figure1a).[] In this hierarchy, the

existing, predominant illicit and indiscriminate disposal of

plastics in the environment and landﬁlling are clearly the least

desirable since they contribute the most pollution and GHG

emissions with absolutely no economic beneﬁts. Top of the

hierarchy should be to redesign the plastics life cycle so that

they can be reused, recycled, repaired, or composted to create

a circular economy with minimal need to produce more virgin

plastics.[] These aspirations are ideal and will require time as

well as global coordination between governments, industry, and

consumers, which will be discussed further in the Conclusions

and Outlook. And ultimately, a zero-waste hierarchy does not

really address the fate of the plastics that have already been

discarded and are polluting the environment. In the interim,

the next best options will be to reduce consumption, reuse

more, recycle, and upcycle. In this review, we will refer to recy-

cling mainly as the process of mechanically processing waste

plastics and converting them into objects with similar or lower

value and usually the same chemical composition. Upcycling

is a complementary set of processes, which we deﬁne to be

the creative chemical transformation of the plastics into

value-added materials and products, and is essentially using

waste plastics as chemical feedstocks.

Some have argued that mechanical recycling should be

preferred since it has a lower environmental footprint than

upcycling.[b,] While this may be ostensibly reasonable based

on energy considerations, this approach is fraught with many

problems. Among the seven most commonly encountered plas-

tics as classiﬁed by the Plastics Industry Association (Table3),[]

only two, PET and PE representing –% and around %

respectively of the annual plastics production (Table ) can be

easily recovered and mechanically recycled.[,,] In principle,

although PP and PVC are also thermoplastics that can be sub-

jected to molding (Figure d), extrusion, and heat pressing,

their contribution to mechanical recycling is only less than

% of the plastics production.[b,] There are several qualitative

problems that hinder more widespread exploitation of mecha-

nically recycled plastics:[,]

i) Inability of the recycled plastic to replicate the mechanical

properties (e.g., strength) of the virgin plastics.

ii) Residual color in the recycled plastics owing to minute

amounts of impurities.

iii) Poor reproducibility in quality of recycled plastics owing to

the varied feedstocks.

iv) Consumer- and market-driven requirements and regula-

tions.

v) Residual odor in recycled plastics.

vi) Visual defects in the recycled plastics.

vii) Loss of functionalities in the recycled plastics.

viii) Health and safety concerns from employees in the produc-

tion process arising from odors of the recycled plastics.

A common misconception about why plastics recycling

remains abysmal in most of the biggest economies in the world

is that recycling is not economically viable. Among the coun-

tries that consumed the most plastics in the world in , the

highest amount recycled (Table4) was in South Korea (%)

Adv. Mater. 2021, 2100843

Table 1. Distribution of plastics produced and turned into post-con-

sumer waste in 2015.[2]

Type of Plastic % Total Production

[407 Mt]a)

% Plastic Wasteb)

[302 Mt]

Polyoleﬁns 57 53

Polyvinyl chloride 12 5

Polyethylene terephthalate 10 11

Polystyrene 8 6

Others 13 25

a)Mt = megatons. b)Recycled: 14%, Landﬁll: 40%, Leakage to environment: 32%,

Combustion: 14%.

Table 2. Global greenhouse gas emissions over the entire life cycle organized according to the most common types of plastics and their life cycle

stages in 2015[6].

Type of Plastic Life Cycle Stage Greenhouse Gas Emissions

[megatons]

Type of Plastic Life Cycle Stage Greenhouse Gas Emissions

[megatons]

Polyamide and acrylic Production 214 Polystyrene (PS) Production 88

Conversion 159 Conversion 31

Polypropylene (PP) Production 135 Polyvinyl chloride (PVC) Production 79

Conversion 93 Conversion 23

Polyurethane (PUR) Production 132 Additives Production 55

Conversion 32 Conversion 26

Low-density/linear low density poly-

ethylene (L/LLDPE)

Production 126 Others, e.g., polycarbonates (PC) Production 45

Conversion 70 Conversion 17

Polyethylene terephthalate (PET) Production 110 High density polyethylene (HDPE) Production 101

Conversion 27 Conversion 58

All Total production 1085 All Incineration 96

All Total conversion 535 All Recycling 49

All End of life 161 All Landﬁll 16

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followed by Germany (%) and Spain (%).[, a] Japan leads

the way in plastic waste recycling per capita with an impres-

sive % rate, while Singapore has one of the highest per capita

production of plastics waste (≈ kg in ) and yet a plas-

tics recycling rate that hovers around –% only (Figurec).[]

Remarkably, multiple consultancy groups have recently made

business cases for the economic beneﬁts of increased levels

of recycling. McKinsey suggested that proﬁts as much as

US$ billion may be achievable for the plastics and petrochem-

ical industries if % of the plastics produced are recycled, while

the Boston Consulting Group showed that the proﬁt margins

for a recycling plant in Singapore can be as high as % with

a % rate of return.[a,] A circular economy is projected to

be valued at US$. trillion by .[] Nonetheless, the global

average recycling rate remains around %, while the amounts

of plastic waste disposed and incinerated are expected to

steadily increase beyond  (Figure b).[a,] And to directly

quote a recent study, "Recycling delays, rather than avoids, ﬁnal

disposal".[a] Clearly, more sustainable alternative approaches

and technologies are needed to incentivize concerted action to

overcome the challenges of the plastics pollution.

Beyond mechanical recycling, upcycling is an appealing and

re-emerging method to add value to plastics recycling, despite

the higher anticipated energy costs.[b,,,,,b,c,] Chemically

transforming the plastics oers more versatility in using the recy-

cled plastics for novel applications and overcomes the qualitative

problems described above in mechanical recycling.[,b,c,a,]

Moreover, upcycling can create opportunities for existing plastic

waste in landﬁlls and the marine ecosystem to be converted

into chemical feedstocks, which are additional beneﬁts over a

circular economy. Some of the low-hanging fruits in this arena

include the solvolysis or transesteriﬁcation of condensation

polymers such as PET, PC, and PUR.[,,c,a,b] These upcycling

methods have historically involved established thermal pro-

cesses familiar to the petrochemical industry and some have

even been commercialized, albeit brieﬂy. For example, Eastman

Adv. Mater. 2021, 2100843

Figure 1. a) Milliken's version of a zero waste hierarchy proposed by the European Commission's Waste Framework Directive to guide plastics life cycle

management. b) Historical data and projections of the cumulative plastic waste produced and disposed from 1950–2050. Adapted with permission.[1a]

American Chemical Society.

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Kodak had operated a plant to recover dimethyl terephthalate

from PET owing to the high price of oil during the s energy

crisis.[] However, polyoleﬁns cannot be readily chemically con-

verted back into their monomers and are typically pyrolyzed

into longer chain hydrocarbons.[b,,,c,b] Some of these

thermal upcycling strategies can be integrated into the prevailing

petrochemical infrastructure and supply chain, and are

expected to be cost competitive when oil prices are moderate to

high at a base case scenario of US$ per barrel.[a,a] However,

there has been an oil glut over the past decade starting with the

global ﬁnancial crisis in , culminating in a previously only

theoretical possibility of negative oil prices (albeit only brieﬂy)

Adv. Mater. 2021, 2100843

Table 3. Structures and typical uses of the most common consumer plastics as classiﬁed by the Plastics Industry Association (PIA)[12].

PIA Code Polymer Structure Common Uses

1 PET Mineral water and

carbonated drink

bottles

2 HDPE Soap, detergent,

and bleach con-

tainers, trash bags

3 PVC Plumbing pipes,

cables, fencing

4 LDPE Grocery bags, cling

wrap

5 PP Reusable food

containers, bottle

caps

6 PS Styrofoam, utensils,

packaging peanuts

7 Others (e.g., polylactic

acid PLA, PC, PUR)

Utensils, plates,

cups

Table 4. Consumption and recycling rates of the countries using the most plastics in 2017[9a].

Country Estimated Plastics

Consumption

[kilotons]

% Recycled % Incinerated Country Estimated Plastics

Consumption

[kilotons]

% Recycled % Incinerated

China 88132 10 40 Italy 5671 27 33

US 30834 10 15 Indonesia 5019 2 5

India 14958 – 2 Thailand 4683 20 25

Japan 8571 21 59 France 4527 21 42

Brazil 8015 2 5 Iran 4330 9 –

Germany 7856 38 61 UK 3431 29 31

Turkey 6958 10 0 Vietnam 3332 8 20

Russia 6946 4 0 Saudi Arabia 3189 10 0

South Korea 6926 59 25 Spain 3110 34 16

Mexico 5884 10 14 Canada 3052 25 3

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during the midst of the COVID- pandemic in . Thus,

merely upcycling plastic wastes into fuels will probably not be

able to supplant the convenience and low prices of crude oil in

the foreseeable future.

1.1. Survey of Previously Published Review Articles

Given the urgency and global scale of the problem, several

reviews have been published recently about plastics recycling.

Trenor and co-workers described some of the technical gaps that

need to be overcome by all the stakeholders in the plastics com-

munity, policy makers, and the public to achieve circularity.[]

Beke and co-workers examined dierent types of recycling,

such as primary, secondary, tertiary, quaternary, and biological

recycling, and also provided a number of case studies on pro-

jects implemented in Europe.[b] They also evaluated the patent

literature and the markets for the recycled materials. Among the

reviews, the most frequently encountered topics include chem-

ical recycling, biodegradation, and mechanical recycling.

Chemical processes for recycling plastics are undoubtedly

the most common. For instance, Thiounn and Smith focused

on the academic research and actual industrial processes for

the chemical recycling of three major plastics: PET, PE, and

PP.[] Hong and Chen discussed the latest developments in

the chemical recycling of condensation polymers by depolym-

erization and subsequent repurposing.[b] Likewise, Coates and

Getzler also advocated the concept of degrading "waste" conden-

sation polymers into monomers before repolymerization.[] They

emphasized the value of this approach both from an energetic

analysis and the broad range of polymeric products that can be

accessible. Weckhuysen and co-workers presented their views

about the most-promising technologies for chemical recycling,

such as solvolysis, dissolution/precipitation processes, pyrolysis,

and thermal upcycling.[c] However, they also included a brief

survey about some emerging technologies, namely mechano-

chemistry, ambient temperature photo-reforming, design for

recycling, and biotechnological approaches.

Besides chemical recycling, the biodegradation and mechan-

ical recycling of plastics are also commonly investigated. In this

context, there have been a number of reviews about the bio-

degradation of PS and modiﬁed PS by Ho and co-workers,[]

the use of enzymes from bacteria, fungi, and other microbes

to digest synthetic polymers by Ma, Guo, and co-workers,[]

the biodegradation of PUR by fungal enzymes by Aguilar and

co-workers,[] and an exploration of the optimal conditions

for the environmental biodegradation of oil-based plastics by

Raddadi and Fava.[] For mechanical recycling, Vilaplana and

Karlsson discussed how quality materials can be produced from

used plastics by modelling the degradation processes, the life

cycle of the recycled plastics, and dierent strategies to upgrade

the polymers into new products.[]

In addition to recycling, the use of waste plastics as raw

materials and chemical feedstocks is also gaining popularity.

For example, Dunn and co-workers examined the technologies

of converting waste plastics into fuels, especially diesel, as an

approach to reduce the reliance on fossil fuels.[] Zhuo and

Levendis explored some of the latest reports on using waste

plastics for carbon nanotube synthesis.[] Another approach

toward sustainability is to synthesize materials with feed-

stocks that are renewable, as discussed by Schneiderman and

Hillmyer.[c] Waymouth and co-workers took on the sustain-

ability by comprehensively surveying many of the existing cata-

lytic technologies available to create sustainable polymers.[a]

Despite all these reviews, the combination of plastics upcycling

with renewable energy has not been thoroughly explored.

1.2. Scope of the Review

Here, we present the latest research on less traditional upcy-

cling methods, with a greater focus on non-thermal processes

as more sustainable, value-added, and selective methods to

valorize plastics waste into chemical feedstocks to supplement

crude oil. We will ﬁrst begin with a survey of less common

thermal upcycling approaches including the repurposing of

plastics for construction uses, D printing precursors, and light-

weight materials. We will then examine ways in which plastics

can be thermally converted into high quality carbon materials

such as graphene or carbon nanotubes (CNT). We will discuss

some of the latest novel strategies to solvolyze condensation

polymers or introduce vulnerable linkers to make plastics

more degradable, but we emphasize that more extensive

reviews on these topics can be found elsewhere.[b,,,,c,,–]

Subsequently, the review will explore plastics upcycling in the

context of artiﬁcial photosynthesis (AP) and the use of renew-

able energy to simultaneously address global climate change

and environmental problems (Figure  2 ).[] AP is broadly

deﬁned as the use of light energy to mediate chemical changes

while storing energy, to distinguish it from photocatalysis.[]

AP has predominantly emphasized the formation of solar fuels

in the form of simple molecules like H and CO, coupled with

the formation of an innocuous by-product like O from water

oxidation (Figurea, top left).[c] However, these AP systems

are unlikely to be cost-competitive with fossil-fuels without

additional value-add from the oxidative half-reaction, which

is where the valorization of plastics and biomass (Figure b)

can play an important role in creating this value to make AP

more sustainable and economically viable (yellow star in

Figurea, top right).[,] Photocatalysis by heterogeneous and

homo geneous catalysts, as well as electrocatalysis, will all be

discussed in the context of upcycling plastics to more versatile

ﬁne chemicals and materials, while simultaneously mitigating

global climate change.[] We will also review some of the bio-

technological breakthroughs in the discovery and engineering

of microbes that can decompose plastics, which give additional

impetus to redesign for biodegradability.[] Finally, we will con-

clude with an outlook and a call for more concerted eorts from

governments, industries, and consumers, as well as greater

interdisciplinary collaborations between chemists, materials

scientists, and engineers to develop innovative solutions for

upcycling plastics and tackling global climate change.

2. Thermal Upcycling Approaches

As described above, recycling plastics with heat is most estab-

lished with more comprehensive reviews available elsewhere.

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In this section, we examine two ways to thermally upcycle

plastics, namely, the repurposing of plastics for entirely new

applications distinct from the original function, and the depo-

lymerization of plastics by chemical processes.

2.1. Repurposing Plastics

Upon analyzing the qualitative problems behind why mechanical

recycling of plastics is not more widely adopted,[] many of the

reasons arise from unrealistic expectations about what the recy-

cled product should be. Currently, mechanically recycled plastics

are often processed to become direct replacements for similar

functions that they were originally manufactured for, which is

why virgin plastics often have to be blended in or additional

energy consuming processing is required. However, plastics

are not uniform, discrete molecules. It may not be possible

for recycled plastics to achieve the same level of mechanical

strength, clarity, color, smell, and other physical properties as

their original form. Rather than treating recycled plastics in the

allegorical lens of asking a ﬁsh to climb a tree, we should instead

rethink and redesign them for novel applications that maximize

their utility. We can learn from how the petrochemical industry

was able to monetize almost all the fractions of crude oil from

natural gas to bitumen, so that the "waste" or E-factor is very

small for the industry as a whole.

2.1.1. Construction Materials

The construction sector is the second largest consumer of

plastics, using up around –% of the annual plastics pro-

duction.[] PVC and PP are used for plumbing, pipes, and

insulation, while PC is used for interior ﬁnishing, among

numerous applications. These purposes rely on the lightweight,

moldability, corrosion resistance, and insulating properties

of plastics, but typically do not have load-bearing functions.

Lately, there has been renewed interest in the use of recycled

plastics for functions such as bricks, concrete, paving addi-

tives, or interior design purposes.[] PET, LDPE, PP, and waste

rubber have all been melted and mixed with other materials to

create bricks.[d,k] In particular, PET drink bottles were rinsed,

dried, shredded, and melted with river bed sand in dierent

ratios before being cast into bricks with molds (Figure  3 a).[d]

Compression strength (Figureb), water absorption, ﬁre resist-

ance, and heat insulation tests were all conducted on the recy-

cled plastic bricks, with normal clay bricks as the control.[d]

A : PET to sand ratio was found to have the highest com-

pressive strength compared to : or : ratios,[d] whereas a

waste PP-waste rubber-CaCO brick was found to have com-

pressive strength (. N mm− ) exceeding that of clay bricks

(. N mm− ).[k] Moreover, the recycled plastic bricks absorbed

less water (<%) relative to clay bricks (–%) and were also

better heat insulators, although the plastic-containing bricks

started to melt at °C, much lower than the clay variants.[d]

The authors concluded that the recycled bricks could help to

abate environmental pollution and were suitable for the con-

struction of sanitary landﬁlls or water conservation purposes,

but not where ﬁre resistance is critical.[d,k]

Besides bricks, the other application that has seen the most

research for recycled plastics in construction materials is in

concrete. Waste plastics have been mixed with blast furnace

slag, cement, sand, and water to create lightweight aggregate

concrete.[c,e,f,j] In a study comparing recycled PET with blast

furnace slag, ﬂy ash, and volcanic ash for preparing concrete,

the density, compressive and tensile strength, elasticity, and

microstructure were all examined  days after fabrication.[e]

The compressive strength and density of the concrete con-

taining %, %, and % waste plastics to replace natural

aggregates were all monotonically lower, although the structural

Adv. Mater. 2021, 2100843

Figure 2. a) Achieving cost-competitive artiﬁcial photosynthesis (AP) by the development of scalable chemical processes and the production of value-

added ﬁne chemical and pharmaceutical products. b) Proposed design of photoelectrochemical cells that can harvest sunlight to simultaneously

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eciency (deﬁned as the ratio of compressive strength to den-

sity) of the sample containing the most waste plastics was only

around % lower.[e] However, the transition zone between

the waste PET lightweight aggregates (WPLA) and the cement

was consistently wider than that for the natural aggregates, as

illustrated in the SEM images (Figure  4 a).[e,f] This was attri-

buted to the smoother morphology of the PET and the higher

hydrophobicity, which interfered with cement hydration at the

interface.[e,f ] Thus, this phase separation and the transition

zone may become the strength limiting factors for concrete

containing recycled PET. On the contrary, a SEM image of

the transition zone between cement and porous PUR foam

(Figure b) revealed eective inﬁltration of the cement into

the micron-sized pores.[c,f ] These studies suggest that waste

plastics cannot be considered as a monolithic class of mate-

rials since the range of physical and mechanical properties vary

widely; successful upcycling of plastics as construction mate-

rials will need further collaborative research between materials

scientists and both mechanical and civil engineers.

2.1.2. New Applications

Beyond recycling waste plastics as construction materials,

there have been exciting advances to develop entirely new

functions for the upcycled plastics. In a study that employed

"design from recycling" and "design for recycling" principles,

PET-contaminated PP were mixed with dierent additives and

mechanically ground together with a twin-screw extruder to

produce a medium-stiness, high-impact new material.[i] In

another interesting development, recycled PET (rPET) ﬁbers

were hydrolyzed with tetraethoxysilane (TEOS) to form a

rPET-silica aerogel (Figure5a).[] The rPET ﬁbers of dierent

thicknesses and lengths were mixed in dierent concentrations

with the TEOS to produce aerogels of densities ranging from

.–. g cm− .[] The aerogels showed thermal con-

ductivities (.–. W mK− ) comparable to PUR foams

(.–. W mK− ) and insulation boards (.–. W mK− ).

Notably, the rPET-silica aerogels are less brittle and softer with

compressive Young's moduli in the range of .–. kPa,

lower than the stier pure silica aerogels (.–MPa).[] The

SEM image of the aerogel revealed the formation of silica clus-

ters randomly distributed in the rPET ﬁber matrix (red ovals

Adv. Mater. 2021, 2100843

Figure 3. a) Photograph of plastic sand brick created by melting used

PET bottles with river bed sand and casting the mixture in a brick mold.

b) Photograph of compression strength test of the plastic sand bricks

IOP Publishing Ltd.

Figure 4. a) SEM images of the transition zone in concrete between cement paste and natural aggregate (left) or waste PET lightweight aggregates (WPLA,

right), 28 days after preparation. The more hydrophobic nature and spherical shape of the plastic aggregates in WLPA results in a wider phase separation,

which limits the strength of the concrete. b) SEM image of the concrete derived from mixing PUR foam lightweight aggregates (LWA) with cement paste,

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in Figureb), verifying that the silica particles and rPET cross-

linked eectively.[] Moreover, after treatment of the as-synthe-

sized aerogel with chlorotrimethylsilane, the aerogels acquired

superhydrophobic behavior with an average water contact angle

of .° (Figure c).[] The combination of good thermal

insulation properties, increased mechanical ﬂexibility, and

superhydrophobicity create opportunities for using these rPET-

silica aerogels in multiple insulation applications that were not

previously available to PET only in any form.[]

Another novel application was reported by Gaikwad et al.

who used electronic waste (e-waste) plastics, consisting mainly

of PC from disposed printers, as D printing ﬁlaments after

granulating the e-waste, drying at °C, and then subjected to

extrusion at around °C (Figure  6 a).[] As the control, virgin

acrylonitrile butadiene styrene (ABS) pellets were also extruded

into similar ﬁlaments under the same conditions. The D

printed products from e-waste were found to be more ductile

and elongated at lower stress, eventually fracturing with rough

surfaces only, which contrasts with the combination of rough

surfaces and river cracks that arise from brittle failure in the

ABS products (Figureb–d).[] Based on solid-state C NMR

spectroscopic analyses of the e-waste polymer before and after

multiple extrusion cycles, there were increased signal integra-

tions at  and . ppm, which were attributed to terminal

phenyl groups and quaternary carbon atoms respectively.[]

Based on these measurements, the authors proposed that

during the extrusion process at around °C, some of the PC

polymeric chains cleaved at the ipso-carbon of the bisphenol

A monomer and disproportionated, leading to the formation

of terminal phenyl groups (Figure e).[] Both the e-waste

PC and virgin ABS ﬁlaments could be printable at least three

cycles, but there was a % decrease in GHG emissions based

on a life cycle assessment when e-waste was used instead of

virgin ABS for producing the D printing ﬁlaments.[] This is

an archetypal example of how waste plastics can be upcycled

for totally new functions in the embryonic ﬁeld of D printing,

which has fewer status quo biases and pricing advantages for

using virgin plastics.

2.2. Functionalization

While mechanical recycling typically has a smaller carbon

footprint, the market for recycled plastics in new applications

is still relatively small. Consequently, thermal methods for

plastics upcycling have historically involved some form of depo-

lymerization or chain scission. Here, we examine four recent

approaches for thermally processing waste plastics, including

the transformation into carbon or doped carbon materials, con-

version into value-added small molecules, functionalization at

Adv. Mater. 2021, 2100843

Figure 5. a) Photograph of recycled PET-silica aerogel disc after rinsing with dichloromethane and treatment with chlorotrimethylsilane. b) SEM image

showing the dispersion of silica microparticles on the PET ﬁbers. c) Contact angle measurement depicting the superhydrophobic properties of one of

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the periphery of the polymer chain for new purposes, and the

insertion of vulnerable linkers to facilitate degradation.

2.2.1. Upcycling to High Quality Carbon Products

One of the most common ways to upcycle waste plastics by

thermal chemical transformation is to turn them into carbon-

based materials or doped carbon materials. This concept is

sensible since most plastics, especially the polyoleﬁns, are

essentially very long chains of hydrocarbons. Under suitable

conditions at high temperatures, dehydrogenation and

crosslinking can occur to turn the plastics into dierent carbon

materials. Waste plastics such as LDPE, HDPE, PET, and PS

have been pyrolyzed at – °C to give conducting solid

carbon microspheres.[] Recycled plastics have also often been

converted into porous carbon materials. Recycled PET was

heated under a N atmosphere between –°C to become

microporous carbon materials that were employed to remove

the GHG CF , which has a  times higher global warming

potential than CO .[] In another instance, waste PE was

blended with graphene oxide and pyrolyzed at  °C to form

a composite of graphene and mesoporous carbon that was suit-

able for high-voltage supercapacitors.[] Likewise, PUR foam

waste had also been pyrolyzed under Ar at  °C to become

N-doped (– wt%) carbon materials with speciﬁc surface areas

up to  m g− and was also explored for supercapacitor

purposes.[] These examples and others that describe the

carbonization of waste plastics into carbon nanomaterials have

already been reviewed elsewhere.[]

Lately, Lisak and co-workers have reported the more

controlled, catalytic pyrolysis of waste plastics over nickel

supported on CaCO to produce multi-walled carbon nano-

tubes (MWCNTs), which were then used for electrocatalytic

O reduction.[] In one of the studies, plastic packaging waste

containing % or % PET was ﬁrst pyrolyzed at  °C on

Fe O with ZSM- as support to give pyrolysis oil.[e] The non-

condensable gas components from this process were then

reacted with the nickel/CaCO catalyst to produce modest yields

(.–.%) of MWCNTs.[e] Notably, when the heterogeneous

electron transfer rates of the MWCNTs from the pyrolyzed rPET

were compared with glassy carbon (GC), Pt/C, and commer-

cially sourced MWCNTs (CCNTs), the rates were similar within

experimental error (Figure  7 a).[e] The MWCNTs were found to

catalyze the O reduction reaction with overpotentials down to

only − .V for the ones derived from % PET (Figureb,c),

which makes them suitable and aordable electrode materials

in fuel cells. TEM images of the MWCNTs prepared from the

% PET sample clearly showed the presence of tangled

nanotubes (Figure d).[e] The ability to turn waste plastics

selectively into CNTs or D materials like graphene will

undoubtedly add value since these high quality carbon materials

can be exploited for a number of optoelectronic applications.

In a similar concept, Lee and co-workers upcycled PE thin

ﬁlms into carbon nanosheets (CNSs) for use in organic photo-

voltaics.[] After spin-coating LLDPE solutions on quartz, the

Adv. Mater. 2021, 2100843

Figure 6. a) Overall process for the conversion of e-waste plastics into ﬁlaments suitable for 3D printing. b) Photographs comparing the 3D printed

products from virgin ABS and recycled e-waste PC after tensile tests, showing the similar performances. c) Stress–strain curves for the 3D printed

products made from virgin ABS compared to recycled e-waste PC. The recycled PC is more ductile at lower stress and elongation. d) SEM images

of the fractured surfaces for recycled PC (top) compared to virgin ABS (bottom). The recycled PC mainly shows rough surfaces from brittle failure,

whereas the virgin ABS sample additionally shows river cracks, consistent with ductile failure as well. e) Proposed mechanism for the polymer chain

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ﬁlms were then heated to °C ﬁrst before carbonizing under

N at °C (Figuree).[] The Raman spectra of the carbonized

ﬁlm clearly displayed the D, G, and D bands (, , and

 cm− ) that are diagnostic of graphene-like structures, which

contrasts with the aliphatic C-H bands for PE (– cm−)

and the CC and CO bands for the partially oxidized interme-

diate ﬁlm (Figuref ).[] XPS data and TEM images conﬁrmed

the graphitic nature of the CNSs ﬁlms.[] In four-point probe

measurements comparing the conductivity and sheet resist-

ance of the CNSs from PE in this study with CNS derived from

polymers with intrinsic microposority (PIM CNS), polyacry-

lonitrile (PAN CNS), graphene oxide (GO), and graphene, the

conductivity reached a maximum of  S cm− (Figure g),

which was superior to all other samples besides graphene

( S cm− ).[] The high optical transparency of the PE-derived

CNS also made it suitable as a transparent conductive ﬁlm

similar to indium tin oxide, so the authors demonstrated that

the CNS could be used in an organic photovoltaic with respect-

able power conversion eciencies of up to .%.[]

Although the yields of some of these carbon materials may

not be high and the applications are still fairly niche currently,

the products are of high value and the transformations are tol-

erant of non-uniformity in the feedstocks since the reactions

are conducted at such high temperatures that the impurities

will be eliminated. The technologies for upcycling waste plas-

tics to carbon nanomaterials and porous carbon materials are

clearly established, but the main limitation will be identiﬁca-

tion of downstream demand for the broad range of accessible

products now.

2.2.2. Upcycling to Valuable Small Molecules

The thermochemical conversion of waste plastics to smaller

molecules is by far the most commonly encountered method for

chemical transformation. By subjecting mixed and single stream

waste plastics to temperatures ranging from – ° C,

a broad spectrum of products ranging from pyrolysis oil to

Adv. Mater. 2021, 2100843

Figure 7. a) Comparison of the heterogeneous electron transfer rates ( k0 ) of glassy carbon (GC) with carbon nanotubes (CNTs) derived from 12% (PET-

12) and 28% (PET-28) PET in plastic waste packaging. b) Linear sweep voltammograms (LSVs) for the oxygen reduction reaction (ORR) comparing GC

with the CNTs from PET-12 and PET-28. c) Onset potential values for the ORR using the CNTs from upcycled plastics. d) TEM image of the CNT from

f) Raman data comparing the original PE ﬁlm with the fabricated CNS. g) Comparison of the electrical conductivity of CNS derived from dierent sources

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syngas and H have been produced.[c,c,e,] The products

are typically complex mixtures containing saturated hydrocar-

bons, oleﬁns, aromatic compounds, and gases like CH , CO, and

H , with little selectivity in the product distribution. [c,c,e,]

Nonetheless, pilot plants and even industrial scale plants have

been demonstrated to use waste plastics for energy recovery,

syngas production, or monomer recovery by companies such

as SABIC, Petronas, Dow, Shell, BASF, RECENSO GmbH,

and TEXACO.[c,b-d] Essentially, petrochemical companies

are able to consider waste plastics as if they are like crude oil

and subject the plastics to cracking and distillation using their

existing infrastructure.[c,b-d] However, few processes are

currently operating, likely due to a combination of low oil prices

and lack of government incentives or regulations, which means

that the pyrolytic processes are still not cost-competitive with

crude oil.[c,b-d] Moreover, the end-of-life GHG emissions are

actually only marginally lower than incineration with energy

recovery and is roughly equivalent to disposal in landﬁlls,[c] so

the pyrolysis of waste plastics may not be so environmentally

beneﬁcial either.

Lately, there has been renewed interest in focusing on single-

stream plastics like PE and converting it under milder, more

controlled conditions to achieve higher product selectivity and

hence more valuable products. Chow et al. employed Fenton

chemistry to partially oxidize PE to a mixture of carboxylic

acids under relatively mild conditions.[] Commercial pellets

of LDPE and HDPE were ﬁrst pre-treated by grinding into

– µ m-sized powders, dissolved in chloroform at  ° C,

and sulfonated (at every fourth ethylene unit on average) using

chlorosulfonic acid.[] After FeCl was introduced as the source

of FeIII , excess H O was added in the dark to initiate the Fenton

reaction.[] Over the course of less than  min, the sulfonated

PE degraded with up to % weight loss to give ≈.% CO

and the remaining carbon became a mixture of carboxylic acids

as distributed in Figure  8 a,b.[] The dissolved organic carbon

contents matched the weight loss of PE and were character-

ized and quantiﬁed through a combination of HPLC/ESI-MS

and GC-MS.[] Butanedioc acid, which can be used for manu-

facturing nylon was the biggest component (.%, Figurea),

while other mono- and di-carboxylic acids made up majority of

the remaining products.[] This process is promising in that

carboxylic acid ﬁne chemicals are generated, which raises the

quality and value of the upcycled PE, especially if the product

selectivity can be further improved.

In another novel development, actual PE was subjected to

hydrogenolysis over Pt nanoparticles supported on SrTiO

to give a narrow distribution of high quality liquid products

including lubricants and waxes (Figure c).[] The catalyst

consisted of SrTiO nanocuboids with average sizes of nm

on which  nm Pt nanoparticles were uniformly deposited

(Figure d).[] The team examined commercially sourced PE

with Mn ranging from –  Da and also an actual

plastic bag with Mn of around   Da.[] After thermal

hydrogenolysis of up to  h at  °C and  psi H , the

molecular weight dispersity of the PE decreased down to .

from . and a Mn of Da for the actual plastic bag sample.[]

Representative data from another PE sample is depicted

in Figure e.[] Impressively, the recovery yields after  h

typically reached >%, with Mn ranging from –Da and

dispersities of .–., even after multiple cycles.[] Through a

combination of  C magic-angle-spinning ssNMR spectroscopy

and DFT calculations, the authors attributed the high selectivity

to the favorable adsorption of high molecular weight PE on the

Pt nanoparticles instead of the SrTiO support, as well as

the abundance of Pt edge sites that helped to suppress over-

hydrogenolysis to lighter hydrocarbons.[] Furthermore,

the eective epitaxial support of Pt nanoparticles on SrTiO

minimized sintering and hence prolonged the catalyst lifetime.[]

Notably, the Pt/SrTiO catalysts were superior for hydrog-

enolysis of PE compared to commercially available Pt/Al O ,

which was proposed to be due to the uniform size and distri-

bution of Pt nanoparticles on SrTiO that enabled the long PE

chains to adsorb on multiple Pt nanoparticles simultaneously.

In these recent studies, the greater emphasis on product

selectivity and higher quality valuable ﬁne chemicals has

created better opportunities for upcycling waste plastics

thermochemically. The carboxylic acids can be utilized for pre-

paring new synthetic polymers, while the liquid hydrocarbons

can be valorized into detergents, cosmetics, coatings, and other

specialty chemicals. Additional work to change and improve the

selectivity for other applications or even more narrow product

streams will undoubtedly make these processes more versa-

tile and cost-competitive. Nonetheless, the need for elevated

temperatures in thermochemical upcycling will still impose

substantial energy costs and lead to GHG emissions, which

can ideally be overcome by the use of non-thermal processes

relying on renewable energy as described later in this review.

2.2.3. Transformation for Alternative Applications

In addition to depolymerization strategies, there have been

established methodologies to upcycle polymers by functionali-

zation reactions so that the products can be used in dierent

applications. One route is to derivatize only the periphery of the

polymer,[] which has been thoroughly reviewed by Leibfarth

and co-workers recently,[] while the other route is to depo-

lymerize and repolymerize condensation polymers. Here, we

will describe the general approaches and highlight only some

recent speciﬁc examples.

The post-synthetic introduction of functional groups on

plastics is very well established and some of the common

methods are illustrated in Figure  9 . [] PS and polysulfone

(PSU) containing aromatic rings are most readily upcycled by

the introduction of functions on the phenyl ring. Friedel-Crafts

alkylation and acylation in the presence of Lewis acids are

frequently employed to decorate the benzene ring. Radical reac-

tions with hypervalent iodine reagents and peroxides have also

been utilized for triﬂuoromethylation of the benzene rings or

the insertion of halides and N along the polymer backbone.

In addition, metalation and subsequent quenching with electro-

philes has also been frequently employed. More recently, Bae

and co-workers exploited iridium chemistry to install bispinacol

boronate esters on PS and PSU, which opened the door to

further cross-coupling chemistry by Suzuki reactions.[d,j,p,]

Besides plastics containing aromatic rings, saturated polyole-

ﬁns can also be modiﬁed, although the predominant methods

involve radical processes (Figure ).[] Manganese porphyrins

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and nickel complexes were used for the oxygenation of PE and

other polyoleﬁns, while azodicarboxylates have trapped radicals

on the polymer chain to form hydrazides. Hillmyer, Hartwig, and

co-workers also developed rhodium-catalyzed methodologies to

decorate PP with bispinacol boronate ester to facilitate subsequent

cross-coupling reactions.[b,] In addition, Coates, Goldman, and

co-workers have demonstrated that iridium catalysts can be used

for transfer hydrogenation of poly(-hexene) to produce alkenes

along the aliphatic side-chain.[l,m,] There is clearly a long his-

tory of methodologies for post-synthetic functionalization of poly-

mers as summarized in Figure,[] and these can undoubtedly

be valuable to upcycle waste plastics for new purposes.

The other common way to upcycle plastics is to deconstruct

condensation polymers (e.g., PET and PC) either fully or par-

tially, followed by repolymerization. A notable example is the

trans-esteriﬁcation of rPET with ethylene glycol (or other diols

like ,-butanediol) using titanium butoxide as the catalyst at

 ° C to form shorter PET oligomers (Figure  10 a).[] Subse-

quently, muconate or acrylate esters can be added for melt

blending at °C to produce the rPET-based unsaturated poly-

ester (UPE), following which the UPE was dissolved in a reactive

diluent (e.g., styrene or acrylate esters) with azobisisobutyroni-

trile as a radical initiator (Figurea).[] After the crosslinking,

the resin was coated on a ﬁberglass mat to give the upcycled

Adv. Mater. 2021, 2100843

Figure 8. a) Product distribution of carboxylic acids after Fenton degradation of sulfonated polyethylene (PE) using FeCl3 and H2 O2 . b) Fenton oxidative

suitable as lubricants and waxes, using Pt nanoparticles on SrTIO3 nanocubes as catalysts. d) Electron micrographs of 2nm (avg.) Pt nanoparticles

deposited on 65nm (avg.) SrTiO3 nanocubes used as the hydrogenolysis catalysts. e) Time progression of the weight distribution of products during

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ﬁberglass reinforced plastic (FRP).[] In terms of the mechanical

properties like the storage and loss moduli, the FRPs containing

rPET showed performances comparable to or even exceeding

those of a FRP derived from petroleum-based isophthalic acid,

,-propanediol, and maleic anhydride (Figure b).[] The

upcycled FRPs in this study exhibited high and favorable glass

transition temperatures between –° C.[] More critically, a

cradle-t ,,c,a,b o-gate life cycle assessment comparing petro-

leum-derived FRP with the upcycled rPET ones suggested that

there could be up to % savings (vs MJ kg− ) in energy

during the supply chain process for feedstocks, transportation,

and electricity (Figurec).[] This translated to GHG emissions

reductions of up to % for the petroleum base case (. kg

CO e kgFRP− ) relative to rPET bioderived muconic petro-acrylic

FRP (. kg CO e kgFRP− ).[] In addition, the rPET-derived

FRPs only require around  MJ US$− in energy, whereas

petroleum derived FRPs need  MJ US$− and conventional

recycling of PET needs  MJ US$−.[] Thus, a compelling

business and environmental case can be made for more sustain-

ably upcycling PET to FRPs via this process.

Another interesting application of rPET was reported by

Hedrick, Yang, and co-workers in the synthesis of antimicro-

bial polyionenes by aminolysis of the polyester linkages.[]

Drug-resistant bacteria have become increasingly common,

which has spurred the development of novel antimicrobial ther-

apies. Inspired by how nature uses cationic antimicrobial pep-

tides, cationic polymers have been prepared, which can attach

by electrostatic interactions to bacterial membranes and trigger

destabilization and lysis.[] PET was subjected to aminolysis at

 ° C in neat amines to give the corresponding terephthala-

mide monomers, after which benzylic electrophiles were added

and the SN  nucleophilic substitution reactions were conducted

at °C (Figure  11 a).[] The resultant cationic polyionenes were

then tested against K. pneumoniae and P. aeruginosa, where it

was found that .% of the bacteria cells were killed within

 min (Figure b).[] The TEM images of Mycobacterium

tuberculosis before and after treatment with a previously

reported polyionene B[] clearly displayed the disrupted bac-

terial membranes after  h (Figurec).[] Conceivably, rPET

can be employed, which will allow waste plastics to be upcycled

for more valuable therapeutic applications.

A dierent application for upcycling waste PC to polymer

electrolytes for batteries was described by Sardon and

co-workers.[] PC containing bisphenol A (BPA) has received

controversial attention and has been banned in many countries

because of the possible endocrine disrupting health eects of

Adv. Mater. 2021, 2100843

Figure 9. Various thermally activated approaches to transform commodity plastics and plastic waste into more value-added derivatives.[39]

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leached BPA. Nonetheless, BPA-PC is still found as a thermo-

plastic in vehicles, smartphones, and optical devices owing to

its transparency and high temperature resistance, which also

hinder their recyclability.[] Saito et al. used ,-propanediol,

,-butanediol, and ,-pentanediol for the trans-esteriﬁcation

of BPA-PC at °C in the presence of ,,-triazabicyclo[..]

dec--ene and methane sulfonic acid as the catalyst to yield

essentially quantitative amounts of BPA and the respective car-

bonates containing diols (Figurea).[] Dierent ratios of the

carbonate diols were then mixed with dimethyl carbonate and

heated using N,N-dimethylaminopyridine (DMAP) to produce

the aliphatic polycarbonate electrolytes.[]

To evaluate the performance of the polycarbonates as electro-

lytes for lithium ion batteries, the ionic conductivity was meas-

ured between room temperature and  °C for each sample

(Figure12b). [] Interestingly, the copolymers exhibited – orders

of magnitude higher conductivity than the photopolymers, and

the best performing samples exhibited higher ionic conductivities

than previously reported solid polymer electrolytes, boding well

for their applications in solid state batteries.[] Nonetheless,

after operating at °C with a current density of .mA cm−,

the resistance of one of the samples increased from  to

 Ohms, suggesting the formation of a solid electrolyte inter-

face layer (Figurec).[] These results indicate that waste PCs

can indeed be upcycled into polycarbonate electrolytes, although

further research will be needed to corroborate their long term

performances. Moreover, plans have to be made to repurpose the

BPA co-product as well before this approach can be sustainable.

Beyond these illustrative examples, the Chen group has an

established program showing that trans-esteriﬁcation by using

bioderived γ -butyrolactone as a solvent and co-monomer, as

well as other polymer solvolysis-condensation sequences, can

be used to upcycle and introduce new functions and lives to

condensation polymers.[] Their work and those of others have

been more comprehensively reviewed previously, focusing on

depolymerization and repurposing processes to chemically

recycle polymers, and will not be further discussed here.[b]

2.2.4. Introduction of Vulnerable Linkers

Finally, a dierent thermal approach was adopted recently to

enhance the degradability of plastics by design. Thermoset

plastics typically have high densities of crosslinks, which result

Adv. Mater. 2021, 2100843

Figure 10. a) Reaction pathway to upcycle PET by sequential transesteriﬁcation with bio-based ethylene glycol and then oleﬁnic diacids, followed by

crosslinking into ﬁberglass reinforced plastics (FRP). b) Comparison of the petroleum derived FRP with dierent forms of FRP obtained by incorpo-

rating bio-based monomers, showing the similar or superior storage and loss moduli of the upcycled plastics. c) Signiﬁcant reductions in supply chain

energy costs in the production of upcycled PET containing bio-based monomers relative to petroleum derived FRP. Reproduced with permission.[40]

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in desired properties like chemical and thermal resistance,

as well as tensile strength, but in turn reduces their degrada-

bility.[] Around  Mt of thermoset plastics are produced every

year, constituting around % of the polymers manufactured.[]

In a recent report, Johnson and co-workers developed a model

using the Miller-Macosko and Flory-Stockmayer theories[] to

predict that thermoset plastics may be more degradable if the

number of crosslinks and the number of cleavable bonds are

similar.[ ] To test this concept, they used ring opening metath-

esis polymerization (ROMP) with a Grubbs second-generation

catalyst to synthesize polydicyclopentadiene and introduced

–% of a cyclic silyl ether monomer (iPrSi or EtSi) as shown in

Figure13.[] The tensile tests indicated that samples with % or

% iPrSi exhibited almost identical Young's moduli and elon-

gations at break relative to undoped samples, although samples

with % or more of the silyl ether had lower moduli and were

less rigid.[] Although the % iPrSi doped sample had a slightly

lower glass transition temperature of °C compared to °C

for the undoped sample, the high-strain-rate projectile impact

tests displayed essentially identical behaviors.[] Most remark-

ably, the samples with .–% silyl ether monomers completely

dissolved when tetrabutylammonium ﬂuoride (TBAF) was used

to cleave the silyl ethers, whereas the undoped and .% doped

samples remained mostly intact.[] In fact, samples containing

the silyl ethers gradually turned into sub- nm particles after

 days of exposure to UV radiation in seawater.[] The weight

average molar masses of – kDa from gel-permeation chro-

matography (GPC) for the degraded samples and the diusion-

ordered spectroscopic data were both consistent with individual

uncrosslinked polymer strands.[] Moreover, the degraded prod-

ucts could be reprocessed by ROMP after introducing the silyl

ethers again to produce thermoset plastics with similar elastic

moduli and other mechanical properties.[] This represents an

attractive proof of concept of imparting vulnerable linkers to

improve the degradability of plastics at the design stage, while

still preserving virtually identical mechanical properties and

hence functions as the original material.

3. Non-Thermal Upcycling

3.1. Photocatalytic Transformations

3.1.1. Heterogeneous Photocatalysis

As discussed in the previous section, thermochemical pro-

cesses such as pyrolysis, cracking, and solvolysis are all estab-

lished protocols for upcycling waste plastics, some of which

Adv. Mater. 2021, 2100843

Figure 11. a) Aminolysis of PET into α,ω-diamines (compounds 1), followed by polyaddition polymerization with electrophiles to generate polyionenes

(compounds 2 and 4). b) Kinetics showing the antibacterial ecacies of the polyionenes against two dierent kinds of bacteria. c) TEM images showing

how a previously published polyionene (50B) damaged the membranes of Mycobacterium tuberculosis before (top) and after 24 h (bottom) of treatment.

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were demonstrated in pilot plants. Notably though, few of them

are operational right now, indicating that they may not yet be

suciently cost-eective. Instead of heat, which is usually

obtained from fossil fuel combustion now, renewable energy

such as light can also initiate photochemical processes such as

the degradation of plastics. In fact, there is almost an equiva-

lent amount of sunlight that reaches the surface of Earth as the

energy consumed by all of humanity within one year.[] Despite

this, solar energy contributes only around .% of the global

electricity needs.[] There are of course challenges in using

natural sunlight for industrial processes such as its diuse

nature, the diurnal and seasonal variations, the intermittency,

and the broad spectral distribution. However, if light could be

deployed to upcycle plastics, it could, in principle, be a

low-carbon or carbon-neutral way to simultaneously address

the environmental externalities and GHG emissions, and also

add economic beneﬁts to nominal waste materials.

In the context of photocatalysis and AP, Fujishima and

Honda demonstrated almost ﬁve decades ago that UV radiation

could be absorbed by TiO to split water with Pt co-catalysts.[]

Since then, TiO and related UV-absorbing semiconductor

materials (e.g., ZnO) have dominated the attention of

researchers with a multitude of applications, including some

from our team for the photocatalytic degradation of organic

pollutants.[] By considering plastics waste as macromolecular

organic pollutants, there have been numerous reports on

using TiO nanomaterials,[] doped TiO ,[] TiO with grafted

co-catalysts or polymers,[] and ZnO[b,] nanomaterials for

Adv. Mater. 2021, 2100843

Figure 12. a) Organocatalyzed transesteriﬁcation of BPA-derived polycarbonate with short-chain diols (top) followed by polycondensation with dime-

thyl carbonate (DMC) to give linear, aliphatic polycarbonate electrolytes (bottom). b) Arrhenius plots of the dierent polycarbonates, demonstrating

their ionic conductivity as electrolytes together with 30 wt% lithium bis(triﬂuoromethanesulfonylimide). c) Electrochemical impedance spectroscopy

measurement on one of the polycarbonates in a lithium symmetric cell conducted at 70°C and applying a current density of 0.05mA cm−2 , showing

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the photocatalytic oxidative decomposition of dierent types of

plastics. Nevertheless, these studies showed only partial con-

version of the plastics (mostly PE and PS), which may produce

micro- or nano-plastics and does not fully overcome the pollu-

tion problem.

An advance was reported by Zhang and co-workers who

screened several types of TiO P nanoparticle ﬁlms and

discovered that the ones made with Triton X- as a non-ionic

surfactant were able to completely mineralize (.%) nm

PS within  h.[] They ﬁrst prepared TiO P powders by

grinding and then mixed the TiO with Triton X- (TXT),

water (WT), or ethanol (ET) to produce the precursor solutions,

which were then separately dropcast on pre-cleaned FTO plates,

dried, and calcined at  ° C.[] The powder XRD and Raman

spectroscopic data of the three TiO ﬁlms were very similar,

but the Nyquist plots suggested that the TiO ﬁlm prepared

using Triton X- had the lowest impedance, suggesting that

it showed the most eective photogeneration, charge separa-

tion, and transport of electron–hole pairs.[] When the ﬁlms

with nm to  µm PS spheres were illuminated by  and

nm UV, FE-SEM images of the samples with TXT showed

complete disintegration in  h (Figure  14a), whereas the WT

and ET samples showed slower decomposition (Figureb).[]

Gas sampling of the headspace from the reaction showed the

steady growth of CO as the main product and CO as the other

major component (Figurec).[] The mechanism of the photo-

catalysis was probed by in situ diuse reﬂectance infrared

Fourier transform spectroscopy (DRIFTS) and indicated the

oxygenation of PS by the growth of OH (– cm− )

and CO (– cm− ) stretches (Figured).[] In com-

parison with a previous study where TiO P grafted with

FeII phthalocyanine showed only partial decomposition of PS

after  days of irradiation (Figuree),[] the performance for

the TiO ﬁlms prepared with TXT deﬁnitely show a marked

improvement.[]

While the complete mineralization of waste plastics by light

can resolve the environmental pollution of plastics, it will con-

tribute to a similar level of GHG emissions as incineration of

plastics for energy recovery, but with fewer beneﬁts. Thus, plastics

upcycling should emphasize the generation of recoverable

and valuable products. A step in this direction was reported

by Sun, Xie, and co-workers who employed Nb O monolayer

nanosheets to completely degrade three types of plastics under

AM .G simulated sunlight to produce small amounts of acetic

acid.[] This paper is unusual in that PE, PP, and PVC were

shown to fully oxidize within , , and  h respectively to

give mostly CO and less than .% acetic acid (Figure  15 a).[]

The authors monitored the reaction progress of PE photo-

degradation with in situ FTIR spectroscopy and detected the

formation of oxygenated species assigned to CO and COOH

stretches between – cm− (Figure b).[] They also

conﬁrmed the formation of acetic acid with isotope labeling

experiments in D O using a high resolution mass spectrometer,

where they observed a distribution of isotopomers ranging

from CH COO– to CD COO– (Figure c).[] These experi-

ments and others conducted with  O and H O conﬁrmed

that the oxygen of the acetic acid originated from both O and

water, while the hydrogens were from water.[] Based on some

additional in situ electron paramagnetic resonance spectro-

scopy, UV/vis spectroscopy, and DFT calculations, the authors

proposed that the acetic acid formed in a two-step process

(Figured).[] The photogenerated holes ﬁrst oxidized water to

produce •OH radicals, which completely degraded PE to CO .[]

Most of the photoelectrons were directed toward reducing O

to water, while a small fraction reduced CO to •COOH, which

coupled to oxalic acid (HOOC-COOH).[] Further reduction of

the oxalic acid to acetic acid would then take place, although

their calculations suggested that poor adsorption of HOOC-CO•

and poor desorption of acetic acid limited the eective photore-

duction performance.[] Despite the low acetic acid yields, this

paper marks an important demonstration that light can be used

to upcycle dierent types of plastics selectively to C feedstocks,

pending further improvements on the product yields.

Another aspect that necessitates progress is the harvesting of

a broader swath of the solar spectrum through the use of other

visible light absorbing semiconductors.[b,] In spite of the

fact that Sun's and Xie's team used AM .G radiation, Nb O

nanosheets is still a wide bandgap (. eV) material[] that

can only absorb UV radiation, which constitutes less than %

of the solar spectrum. There was a study where boron-doped

Adv. Mater. 2021, 2100843

Figure 13. Introduction of small quantities of cleavable silyl ester linkers into thermoset polycyclopentadiene to facilitate degradation and upcycling,

with minimal impact on the mechanical properties.[47]

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cryptomelane, a manganese oxide mineral K(MnIV MnII)O ,

was found to exhibit superior photocatalytic activity to undoped

cryptomelane for the oxidation of PE over  h (Figuree).[]

The authors proposed that the Lewis acidic boron dopant

captured some of photogenerated electrons that helped to

prevent recombination of electron–hole pairs.[] No recoverable

products were isolated in this study, however.[]

In an alternative approach, Reisner and co-workers pio-

neered the photoreforming of some types of plastics to produce

predominantly H as a solar fuel, with several small molecules

such as acetate, formate, and glycolate as the carbon-based

products using visible light responsive photocatalysts, namely

CdS quantum dots (QDs) and carbon nitride.[] CdS has a bulk

bandgap of .eV for visible light absorption and suitable band

positions (conduction band =  −.V and valence band =  + .V

versus normal hydrogen electrode) that are favorable for the

photoreforming reaction.[c] They conducted the reactions

in alkaline aqueous solutions, during which thin Cd oxide/

hydroxide shells (CdOx ) would form on the surface of the CdS

QDs that appeared to prevent photocorrosion.[c] A variety of

polymers were tested including polyvinyl pyrrolidone (PVP),

polyethylene glycol (PEG), PE, PVC, polymethyl methacrylate

(PMMA), PS, PC, PLA, PET, and PUR, among which PLA and

PET exhibited the highest reactivities (Figure  16 a).[c] They

observed that the pre-treatment (alkaline hydrolysis at  °C for

 h) improved the reactivities for both PET and PLA since it

led to the release of monomers or oligomers into the solution,

which were then photoreformed more rapidly.[c] In addition

to H generation, an added beneﬁt in their process is the for-

mation of organic products such as formate (c), glycolate (g),

ethanol (h, m), acetate (i), and lactate (l) (Figureb) derived

from the plastics, which can potentially be recovered, instead

of just CO .[c] The authors also explored the applicability of

this methodology to real-world PET plastics, which account

for % of the global plastics production and is usually more

challenging to recycle due to the presence of additional ﬁllers,

cross-linkers, and antioxidants. In their experiment, part of a

PET water bottle was subjected to photoreforming and contin-

uous H evolution was recorded up to  days (Figurec).[c]

Reisner's team continued investigating several less toxic

and cheaper alternative photocatalysts to CdS. Unfortunately,

Cd-free nanocrystals such as ZnSe QDs did not produce H under

their employed experimental conditions.[c] However, they

subsequently reported that a carbon nitride/nickel phosphide

(CNx |Ni  P) composite photocatalyst also showed promising

photoreforming activity to convert plastics into H and a variety

of carboxylates.[b] Carbon nitride is a visible light responsive

photocatalyst with a bandgap of .eV, while Ni P was used as

an ecient cocatalyst to boost the H evolution eciency.[b]

TEM studies on the composites show that the Ni P was in the

form of nanoparticles (.± .nm in diameter) randomly dis-

tributed on the CNx nanosheets (Figured).[b] XPS measure-

ments conﬁrmed that the Ni P was likely bonded to CNx based

on the binding energy shifts attributed to CN and Ni-P in the

high-resolution C and Ni regions (Figure e,f), which sug-

gested an electron density shift from CNx to Ni P.[b] This inter-

action was proposed to improve electron extraction and thereby

enhance the photoreforming eciency.[b] The CNx | Ni P photo-

catalyst generated H from several types of plastics, including

polyester microﬁbers, pieces of a PET bottle, and even PET

that had been contaminated by soybean oil (Figure g).[b]

The authors suggested that the photoreforming activity from

Adv. Mater. 2021, 2100843

Figure 14. a) SEM images of PS spheres irradiated with 365nm UV radiation without catalyst (top) and in the presence of Triton-X TiO2 (TXT, bottom),

illustrating the complete disintegration of PS. b) Comparison of the PS degradation eciencies of TiO2 prepared in water (WT) and ethanol (ET) with

TXT. c) Mineralization products from PS photodegradation by TXT. d) In situ DRIFTS study of PS during photodegradation by TXT, indicating the

degradation of PS by TiO2 and TiO2 grafted with FeII phthalocyanine (FePc-TiO2) under actual sunlight. Reproduced with permission.[57] Copyright

2008, Elsevier B.V.

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the polyester microﬁber was likely the highest because it could

disintegrate most easily and expose more surface area for reac-

tions during the course of the irradiation. Nevertheless, the

H yields from CNx | Ni P were noticeably lower than those of

CdS/CdOx (Figure c,g). They concluded that CNx | Ni  P

required an electron transfer process from the light absorber to

the cocatalyst, which likely limited its photocatalytic eciency

in comparison to CdS/CdOx .[b]

Undoubtedly, Reisner's work on photoreforming plastics

with visible light absorbing photocatalysts are signiﬁcant mile-

stones, but many challenges regarding real-world applications

still remain. Some of the major obstacles are the incomplete

plastics conversion and the necessity for the highly caustic

alkaline pre-treatment. Furthermore, the alkaline conditions

can initiate a variety of side reactions, such as aldol conden-

sations, leading to poor product selectivity.[b] Hence, it is

imperative that future studies in this ﬁeld focus on developing

more ecient photocatalysts that can breakdown real-world

plastics completely, while generating valuable organic products

selectively. In addition, complex or hazardous pre-treatment

processes should ideally be minimized.

3.1.2. Homogeneous Photocatalysis

Although heterogeneous photocatalysts are usually more robust

during operating conditions, there are limitations to the cata-

lytic eciency. Many plastics will not be soluble in aqueous or

Adv. Mater. 2021, 2100843

Figure 15. a) Degradation of dierent plastics by Nb2 O5 under UV radiation into CO2 and acetic acid over 90 h. b) In situ FT-IR of PE photodegrada-

tion showing the formation of oxygenated products. c) Mass spectrum of the acetic acid products from PE photodegradation conducted in the pres-

ence of D2O. d) Band edge positions and proposed two-step mechanism for CC cleavage and subsequent bond formation from PE. Adapted with

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organic media so the catalytic processes can only take place at

solid-solid interfaces, which will likely be extremely low. On

the other hand, homogeneous photocatalysts can dissolve and

adsorb on the plastics, meaning that every catalyst molecule

can, in principle, be reactive. Another alternative that employs

homogeneous species is to employ a photoassisted Fenton reac-

tion to produce reactive oxygen species that will mineralize the

waste plastics.

Yu, Lam, and co-workers discovered that broadband UV

radiation from a  W Hg(Xe) lamp promoted the oxidative

decomposition of PS by Fenton chemistry, whereas heating

up to °C in the absence of light did not result in observable

degradation.[] They obtained PS from Styrofoam lunchboxes,

which had to be partially sulfonated with concentrated sulfuric

acid at °C ﬁrst.[] Subsequently, FeCl and H O were added

to the sulfonated PS and the reaction mixture was illumi-

nated with broadband UV to initiate the photoassisted Fenton

process.[] Over the course of  min, the sulfonated PS

decomposed as veriﬁed by SEM studies (Figure  17a), producing

dissolved organic species, while the pristine PS remained unre-

active.[] The authors conducted adsorption experiments for

 min to show that the sulfonate groups were necessary to coor-

dinate FeIII to bring the reactive oxygen species in close prox-

imity to the PS with minimal deactivation due to diusion.[]

Furthermore, the absence of any degradation for the thermal

reaction in the absence of UV led the authors to suggest

that high valent FeIV (O) or FeV(O) intermediates may be the

actual active species, analogous to biological cytochrome P

enzymes.[] Their proposed mechanism for the complete min-

eralization is depicted in Figure b, where coordinated FeIII

reacts with H O in the presence of UV to form •OH, •OOH,

and other oxidants that cleave the CC bonds of the PS back-

bone and also the aromatic rings.[]

Inspired by these results and previous studies on high

valent Fe(O) chemistry including our own,[] we sought to

photochemically generate FeV (O) complexes with water as the

oxygen atom source for more selective oxygenation of sub-

strates such as organic pollutants and plastics to recoverable

products.[] This is because as mentioned in the previous sec-

tion, the mineralization of PS photochemically is not an ideal

solution since it mainly results in GHG emissions with no

value-add. As early as , Mita etal. had explored the use of

UV-photosensitization with benzophenone (BP) to oxidatively

degrade PS without mineralization by a radical chain process at

 °C.[] BP is a well-known triplet photosensitizer, which can

also behave as a hydrogen atom abstracting agent in the excited

triplet state. On irradiation with nm UV using a high pres-

sure Hg lamp,  BP* would form and abstract a hydrogen atom

from the methine position of PS to form two benzylic radicals

(Figurec).[] The radical on PS can then undergo β -scission

to release an α-alkylstyrene product and form another benzylic

radical on PS (Figurec).[] The radical on PS can also abstract

a hydrogen atom from another PS methine position to propagate

the radical chain process.[] Alternatively, chain termination

Adv. Mater. 2021, 2100843

Figure 16. a) Photoreforming dierent plastics to produce H2 using CdS/CdOx quantum dots under ambient conditions with simulated sunlight.

b) 1 H NMR showing the formation of degradation products from PET after 24 h of photoreforming. c) Performance of untreated and alkaline-treated

of carbon nitride/nickel phosphide (CNx |Ni2 P) photocatalyst for plastic photoreforming. XPS spectra for the C 1s e) and Ni 2p f) edges for CN x |Ni2 P.

g) Comparison of the long term photoreforming performance of dierent plastics using CNx |Ni2 P as the photocatalyst under ambient conditions with

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or crosslinking can occur to stop the PS degradation.[] By

monitoring the kinetics of the polymer molecular weights by

GPC, the authors found that the ratio of crosslinking to main

chain scission was .–., with more β-scission at higher

benzophenone concentrations.[] Although the conversions

and yields were not reported since this study was about under-

standing the kinetics and mechanism of the process, it still

demonstrated a possible strategy to unravel polyoleﬁns, espe-

cially PS, into small molecular ﬁne chemicals.

In the same vein, our team was interested in β-scission as

a strategy to systematically unravel plastics photochemically by

cascade CC bond cleavage. Since the revival of photoredox

catalysis for organic synthesis by MacMillan, Fukuzumi,

Stephenson, Yoon, and others,[] there has been a steady

renaissance in the use of radical chemistry for synthetic appli-

cations. The controlled cleavage of CC bonds is acknowledged

to be challenging owing to its non-polar nature and rela-

tively high strength, although signiﬁcant developments in the

radical activation and functionalization of CC bonds, with

an emphasis on both synthetic outcomes and reaction mecha-

nisms, have been reviewed recently.[] Our team was especially

interested in employing photoredox catalysis to eect selective

and mild CC bond cleavage reactions under ambient or close

to ambient conditions.

To this end, our team has prepared VV complexes supported

by hydrazone-imidate ligands that absorb visible light through

ligand-to-metal charge-transfer (LMCT) processes, which could

serve as photocatalysts to selectively cleave the CC bonds adja-

cent to alcohol groups.[] This reactivity was ﬁrst demonstrated

in several representative lignin model compounds, from which

valuable aryl aldehydes and formates were obtained in moderate

to high yields under ambient conditions (Figure  18 a).[] In

addition, DFT calculations indicated that the incorporation

of electron-withdrawing groups at selected positions on the

aryl rings of the ligand is crucial to promoting the LMCT

process, thus increasing the product yields and overall

reaction rates.[] Thus, we subsequently expanded the library

of our VV complexes,[] and identiﬁed V as the fastest and the

most robust photocatalyst in the series by performing detailed

kinetic studies (Figureb). With the optimal photocatalyst in

hand, we then further expanded our substrate scope to include

more general, non-benzylic substrates such as small aliphatic

alcohols and even macromolecules like hydroxyl-terminated

synthetic polymers.[] Over  unactivated alcohols underwent

Adv. Mater. 2021, 2100843

Figure 17. a) Photoassisted generation of reactive oxygen species by Fenton chemistry for the degradation of sulfonated polystyrene. The SEM image

shows the degradation of microspheres over 60 min, while the chart shows the changes in dissolved organic contents (DOC) for the sulfonated poly-

styrene over time. b) Proposed mechanism for the photoassisted Fenton degradation of sulfonated polystyrene by reactive oxygen species and radical

degradation of polystyrene with UV radiation.[64]

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selective CC bond cleavage under exceptionally mild condi-

tions, while producing chemical feedstocks containing carbonyl,

alcohol, and formate functional groups as the products

(Figurea).[] Remarkably, hydroxyl-terminated polymers such

Adv. Mater. 2021, 2100843

Figure 18. a) Functional group tolerance and substrate scope of homogeneous vanadium photocatalyzed C  C bond cleavage. Adapted with permis-

Chemical Society. c) Cascade CC bond cleavage in biodegradable and non-biodegradable polymers. The sites of CC bond cleavage are highlighted

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as PEG , its co-block polymer with polycaprolactone (PCL-

PEG-PCL), and even plastics containing the non-biodegradable

PE backbone were completely consumed and converted into

formic acid and formate esters (Figurec).[]

We had further conducted isotope-labelling studies and

other experiments to gain insights into the CC bond cleavage

reaction mechanism. The  H- and C-labelled substrates were

subjected to the photocatalytic conditions and the  H NMR

spectroscopic data after the photoreactions indicated that the

isotopes were not exchanged or scrambled during the C

C bond

activation process, thus precluding the traditional benzylic

hydrogen abstraction and oxidation pathways.[] In addition,

according to our DFT calculations, the mechanism of this

unique LMCT-mediated C C bond cleavage reaction in

aliphatic alcohols starts with a single electron transfer (SET)

from the redox noninnocent ligand to the VV center after visible

light irradiation (Figure  19 a).[] In the photoexcited state, the

C C bond cleavage occurs with a low activation barrier of only

.kcal mol− , which will be spontaneous at room temperature,

and a carbonyl group forms from the alcohol proximal to the

cleaved CC bond (Figurea).[] The remaining alkyl radical

fragment will then react with O from air to aord another

oxygenated compound, and the transient VIV species is regener-

ated back to the active VV complex in the presence of O. Based

on the insights from these small molecules, we believe that the

same mechanism is operational for synthetic polymers.[] The

peroxide intermediate in Figurea will undergo a subsequent

O O bond cleavage to give an alcohol as well.[] Building on

this proposed mechanism, for synthetic polymers, we believe

that the initial step is the coordination of an alcohol group to

the VV center (Figureb).[] After the loss of the ﬁrst carbon

as formaldehyde (which goes on to become formic acid), the

initially formed alcohol intermediates will behave as substrates

again for subsequent CC bond cleavage, which can lead to

the unprecedented, photodriven cascade CC bond cleavage

of hydroxyl-terminated synthetic polymers that we observed

(Figureb). [] In principle, our methodology can be extended

to degrade and repurpose any polyoleﬁn if a hydroxyl group is

available on the polymer chain to initiate the process. Our work

has thus set a benchmark in the homogeneous photocatalytic

valorization of non-biodegradable plastics.

Following our team's ﬁrst report on the LMCT-mediated

photochemistry in ,[] there has been a steady increase

in the number of reports on exploiting LMCT for photoredox

or photosensitizer applications. For instance, Milsmann and

co-workers have developed a photoluminescent ZrIV complex with

,-bis(pyrrolyl)pyridine ligands,[] in which the visible light

absorption bands originate from LMCT, as supported by experi-

mental and computational studies. The authors used their com-

plexes as photocatalysts for the dehalogenation of alkyl halides,

reduction of electron-deﬁcient oleﬁns, and reductive coupling

of benzyl bromide via photoredox catalysis, which suggested

that the ZrIV complexes could be more earth-abundant and

aordable substitutes for the more expensive [Ru(bpy) ]+ .[]

Similarly, Wärnmark's group has prepared an FeIII complex,

[Fe(phtmeimb) ]+ (phtmeimb = phenyl[tris(-methylimidazol-

-ylidene)] borate− ), which exhibited intense photoluminescence

with a then record-high  ns lifetime for Fe photo sensitizers

and a % quantum yield via a spin-allowed transition

from a doublet LMCT ( LMCT) state.[] They also reported

potential photoredox activity from the complex's  LMCT state

toward both electron donors and acceptors through bimolecular

quenching studies with methyl viologen and diphenylamine.[]

Additionally, Zuo's group has also demonstrated the application

of a LMCT-homolysis pathway by inexpensive Ce salts as photo-

catalysts to generate alkoxy radicals from alcohols, and then

utilized these alkoxy radicals for δ-selective C−H bond func-

tionalization reactions.[] Likewise, with the aim of replacing

expensive, precious-metal containing, Ru- and Ir-based photo-

catalysts, Schelter recently reviewed the use of cheap, earth-

abundant, and readily available lanthanides as photocatalysts.[]

These instances portend the promise of homogeneous photoca-

talysis driven by LMCT in future plastics degradation processes.

Although the upcycling of plastics to small molecules like

formic acid by depolymerization can be more versatile since

the products are chemical feedstocks, it will likely be energy

intensive, even if renewable energy can be used. Another ener-

getically less costly approach would be to repurpose plastics by

post-synthetic modiﬁcations by using light, instead of heat as

described in previous sections. Leibfarth and co-workers has

instituted a program on upcycling plastics containing aromatic

groups (e.g., PS) by photocatalytic ﬂuoroalkylation reactions.

Building on the seminal work by Stephenson and co-workers

in using a complex of triﬂuoroacetic anhydride and pyridine

N-oxide as the precursor to generate CF radicals photochemi-

cally,[] they applied this same tactic to triﬂuoromethylate PS

and observed that they could steadily increase the number of

CF groups with higher number of equivalents of the reagents

(Figure  20 a). [] The protocol is essentially the same as the

original with Ru(bpy) Cl as the photosensitizer, although the

exclusion of air is no longer necessary.[] Since ﬂuorinated

organic materials are known to become more hydrophobic, they

introduced increasing lengths of ﬂuorinated alkyl groups on PS

by the same photochemical method and prepared a new per-

ﬂuoroheptylated PS with a water contact angle of ° exceeding

the contact angle of ° in pristine PS (Figureb).[] Further-

more, the photocatalytic reaction could be extended beyond per-

ﬂuoroalkylation to the installation of CClF and CBrF groups

(Figurec), and could also work on post-industrial PS, post-

consumer PS, and other plastics like PC, PET, and Tritan.[]

It was recognized that the use of Ru(bpy) Cl as the photo-

sensitizer in the halogenoalkylation reactions was undesir-

able due to the expense of Ru, which is not cost-eective for

large scale upcycling of plastics, and also the fact that trace

heavy metal impurities are not acceptable for microelectronic

and biomedical applications.[] Consequently, Leibfarth's team

screened a set of organic photocatalysts (Figured) and used

F NMR spectroscopy and GPC to monitor the degree of alkyla-

tion.[] The GPC data indicated that the phenoxazines and

phenothiazines appeared to result in chain coupling reactions

that led to low levels of CF installation, while the phenazines

produced the optimal results.[] They were able to accomplish

up to  mol% addition of CF groups to PS and also extended

the substrate scope to perﬂuoropropylation and the addition of

CClF .[]

Other than the use of exogenous photocatalysts to prepare

derivatives by post-synthetic modiﬁcations, photoactive poly-

mers can also be directly transformed. Studer and co-worker

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exploited the Norrish type I chemistry of carbonyl compounds

and prepared photoactive PS with -hydroxy--methyl--phenyl-

propan--one on the aromatic ring (Figure  21 a).[] After illumi-

nation with  nm to photoexcite the ketone and trigger the

Norrish type I CC bond cleavage (Figurea), the radical was

trapped with several dierent types of nitroxides including ones

with polyethylene glycol or biotin for biomedical applications.[]

The products were characterized with a combination of  H

NMR spectroscopy (Figureb), FT-IR spectroscopy, and GPC,

which veriﬁed the installation of nitroxide groups on the PS.[]

Figure 19. a) Proposed mechanism of C  C bond cleavage after LMCT by molecular vanadium photocatalysts, as suggested by DFT calculations.

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While the upcycling of plastics by photocatalytic post-syn-

thetic modiﬁcations should consume less energy and result in

lower GHG emissions, the reports have thus far been mainly

restricted to polymers containing aromatic groups, especially

PS.[–] In addition, the market for the functionalized poly-

mers is currently unknown. Although, in theory, ﬂuorinated

PS should be more hydrophobic and will undoubtedly have

dierent mechanical properties, there are still no major uses

reported for them. Likewise, while nitroxides have been used

to modify zeolites,[] there is no clear large-scale demand for

these polymers. Thus, in order to achieve more sustainable

photocatalytic or photosynthetic upcycling of plastics, a combi-

nation of controlled depolymerization to known ﬁne chemical

feedstocks and astute choices of post-synthetic modiﬁcations of

Figure 20. a) Photocatalytic triﬂuoromethylation of polystyrene with visible light. The successful introduction of CF3 groups was veriﬁed by GPC and 19 F

NMR spectroscopy. b) Contact angle measurements highlighting the increased hydrophobicity of a series of perﬂuoroalkylated poystyrenes, suitable for

coating applications. c) Substrate scope demonstrating the generality of this halogenoalkylation approach. Parts (a) to (c) of this ﬁgure are reproduced

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certain plastics to desired products will likely be more practical

and eective in attracting industry interest.

3.2. Electrocatalytic Transformations

Electrocatalysis can be a clean and sustainable technology when

the electricity is derived from renewable resources such as solar,

wind, or geothermal energy. The energy from these renewables

can be conveniently utilized to convert nominal "waste" or

by-products of respiration and human activities like water, CO ,

N , biomass, and discarded plastics to produce more usable

chemicals such as H , CO, NH, formic acid, and methanol.[]

There have been studies on applying electrocatalysis to trans-

form most of the by-products described above into chemical

feedstocks.[] However, remarkably, there has been very little

research on employing electrocatalysis to upcycle plastics such

as PET into value-added products.

Recently, Myren et al. reported the electrochemical depoly-

merization of PET plastics in a two-compartment reactor sepa-

rated by a glass frit, commonly called a H-cell.[] The cathode

compartment consisted of a glassy carbon plate as a working

electrode and Ag/AgCl (  NaCl) as a reference electrode.

Carbon cloth was used as a counter electrode in the anode

compartment. Electrolysis was carried out in a : methanol/

water system with .  NaCl as a supporting electrolyte and

PET was suspended in the cathode compartment. As shown

in Figure  22 a, at a constant potential of −. V, the necessary

basic conditions were generated in situ, which facilitated the

hydrolysis of PET to terephthalate in .% yield. In addition,

the formation of CO was observed in the headspace of the

anode compartment, which was attributed to the migration

of the terephthalate product across the glass frit of the H-cell

and subsequent Kolbe decarboxylation. The conditions and

yields of terephthalate and CO for PET depolymerization are

summarized in Figureb, together with a control experiment

conducted in water only as the solvent, which showed negli-

gible PET degradation.

One of the main advantages associated with this technique is

the room temperature operating condition, which makes it less

energy-intensive than conventional heating or even microwave

techniques. Another beneﬁt is that the in situ electrochemical

generation of basic conditions in a protic medium eliminates

the need for the storage of highly caustic or corrosive solu-

tions, which can make the system more cost-eective. However,

this research is still in its infancy since other parameters can

be investigated to enhance the product yields or increase the

conversion of PET. Some of the variables include tuning the

applied potential to change or increase selectivity of the desired

products, mild heating to raise the conversions, testing of other

solvents, screening dierent electrocatalysts to enhance the

current densities or lower the overpotentials, using dierent

semi-permeable membranes instead of just a glass frit to

circumvent product crossover, or adopting a totally dierent

electrochemical cell design.

Another potential strategy to raise the conversion eciency of

an electrocatalytic depolymerization system is to exploit Fenton

chemistry and use what is commonly known as the electro-

Fenton process. The Fenton reaction traditionally involves the

generation of hydroxyl radicals (•OH) and other reactive oxygen

species through a mixture of H O and soluble FeII salts as

shown in Equation ().[] These radicals are strong oxidizing

agents, which are capable of oxidizing almost any organic

pollutant to CO and water, but they typically cause uncon-

trolled chain reactions.[] On the other hand, electrochemistry

oers a controlled way to produce H O in situ in a protic

medium via a two-electron reduction of O by Equation ()

or a two-electron oxidation of water by Equation().[] For an

electro-Fenton process, low concentrations of −  Fe II can be

added to the electrochemical cell with the generated H O to

initiate the Fenton reaction. The FeIII ions can be reduced at the

cathode by Equation () back to FeII ions, which in turn

Figure 21. a) Photoactivated Norrish type I pathway to upcycle functionalized polystyrenes with nitroxyl radicals. Reproduced with permission.[68]

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propagate and enhance the formation of •OH by the Fenton

reaction, eventually accelerating the rate of organic pollutant

decomposition in the reaction media.[] The electro-Fenton

reaction has been successfully applied for the oxidative deg-

radation of a wide variety of organic pollutants, such as poly-

chlorinated phenols,[] antibiotics,[] pesticides,[] nonsteroidal

anti-inﬂammatory drugs,[] and synthetic dyes.[] Therefore,

we believe that the electro-Fenton reaction can potentially

be employed as a more energy-ecient method to degrade a

broader variety of waste plastics, including not only PET but

also polyoleﬁns and other condensation polymers, while being

paired with another value-added electrochemical reaction at the

counter electrode.

Fe Fe OH OH

II III •

+→

−

()

++ →

+−

()

OH O2

22 2

→+ +

()

III II

+→

− ()

3.3. Natural and Biological Conversion

As we continue to develop more sustainable strategies

to upcycle waste plastics, especially the existing ones, we

should remember that the priority of the zero waste hier-

archy (Figurea) is still to rethink and redesign materials and

their life cycles to target a circular economy.[–] If plastics are

designed to be more chemically degradable or biodegradable, it

will doubtlessly beneﬁt the upcycling eorts as well. However,

it is critical to recognize that there will always need to be some

compromises between the degradability, desired mechanical

properties, and price of the material among several factors.

Furthermore, degradability under laboratory conditions or even

in a compost may not be equivalent to degradability in a dif-

ferent environment such as seawater or in landﬁlls.[] Addi-

tional nuances to keep in mind are that materials derived from

biomass are not necessarily biodegradable and that depends on

the chemical structure, the presence of additives, and even the

crystallinity.[] In the next section, we will review recent eorts

at understanding the factors inﬂuencing the degradability of

plastics and discuss the most exciting developments in bioengi-

neering microbes that can consume plastics.

3.3.1. Factors Aecting Degradation

Earlier in Section .., we discussed how susceptible, cleav-

able monomers could be inserted into thermoset plastics to

make them more recyclable with minimal impairment on the

mechanical properties and performance of the materials.[]

The chemical structure is just one of the multiple variables

to be considered though. In a study on the biodegradable

PLA, higher temperatures and relative humidities, as well as

exposure to  nm UV radiation all accelerated the rate of

decomposition, highlighting the importance of environmental

factors in inﬂuencing the kinetics of plastics decomposition.[]

In another discussion about PS, the introduction of additives

such as decabromodiphenyl oxide (ﬂame retardant), mineral oil

(lubricant), alicyclic bromine (antioxidant), and bis(,,,-tetra-

methyl--piperidyl)sebacate (UV stabilizer) helped to improve

the shelf life, but also made the PS more resistant to biodegra-

dation.[] Even the role of physical agitation cannot be neglected

as described in research on using nanoparticle tracking analysis

and FE-SEM to monitor the particle sizes and morphologies of

PS spheres subject to rotation mixing, shaking, ﬂowing and no

agitation.[] The nm diameter PS particles aggregated over

 days at  °C without agitation, but shaking prevented the

aggregation without signiﬁcant shape changes, whereas both

Figure 22. a) Production of base from protic solvents at the cathode for the electrochemical depolymerization of PET, starting from neutral conditions.

b) Conditions and yields of terephthalate and CO2 in H-cell for PET depolymerization.

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rotation mixing and ﬂowing caused the partial disintegration of

some PS particles into nanoplastics <nm in size.[]

Other than external environmental factors, the polymer

microstructures and physical properties also play vital roles

in determining degradability. Wei, Zimmermann, and

co-workers examined how the crystallinity of dierent PET sam-

ples aected the rates of hydrolysis by a thermophilic polyester

hydrolase Thermobiﬁda fusca cutinase TfCut extracted from

Bacillus subtilis after incubation between –° C.[] They exam-

ined the weight loss of postconsumer PET packages from Agri-

pack, Carton Pack, and amorphous PET ﬁlms of µm thick-

ness from Goodfellow Ltd after incubation with TfCut at  °C

over  h and observed full decomposition of the amorphous

sample within  days, while the weight loss plateaued at –%

for the remaining two samples (Figure  23 a).[] By conducting

dierential scanning calorimetry (DSC) experiments on all

Figure 23. a) Comparison of the degradation of dierent PET samples after 168 h incubation with the enzyme TfCut2 derived from B. subtilis. b) Cor-

relation of the PET degradation with the degree of polymerization in the samples. c) Proposed mechanism for the facile enzymatic degradation of PET

with mobile amorphous fractions (MAF) over the slower degradation of the rigid amorphous fractions (RAF). Adapted with permission.[93] Copyright

2019, Wiley-VCH. d) Machine learning correlation of the crystallinity (represented by enthalpy of melting) and the hydrophobicity (represented by the

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three samples, they discovered that the amorphous PET ﬁlm

consisted mainly of a mobile amorphous fraction (MAF) with

a lower glass transition temperature and higher cold crystalliza-

tion temperature, while the Agripack and Carton Pack samples

had additional rigid amorphous fractions (RAFs) that had higher

glass transition temperatures which were more ordered.[] The

signal integrals from  H NMR spectra of the samples were also

employed to estimate the degree of poly merization by taking

the ratio of the chain end hydroxy signals to the terephthalate

units.[] The authors found that the degree of polymerization for

the amorphous sample (grey diamonds, Figureb) decreased

linearly with weight loss, whereas the other two samples (Agri-

pack, black triangles; Carton Pack, black circles) showed reduc-

tions in degree of polymerization up to a % weight loss before

leveling o. They proposed that the enzymes initiated the deg-

radation by ﬁrst randomly inserting scissions at susceptible

sites before releasing the monomers at chain ends systemati-

cally (Figurec).[] The Agripack and Carton Pack samples had

more RAFs and crystalline components, so there were fewer

vulnerable sites initially for the enzyme to target, leading to

slower monomer release and weight loss (Figurec).[]

In another groundbreaking study, Mathers and co-workers

compiled a database of over  plastics, physical properties,

and experimental degradation data from literature and utilized

machine learning to analyze it.[] After a pre-screening, they

identiﬁed seven physical properties that could be correlated

to the biotic and abiotic degradation rates, namely density,

molecular weight, glass transition temperature, crystallinity,

enthalpy of melting, sp carbons, and a measure of hydropho-

bicity called LogP (SA)− .[] Among these, they correlated the

rates of abiotic and biotic degradation with the hydrophobicity,

enthalpy of melting, and crystallinity (Figure d).[] Biotic

degradation rates were noticeably higher for more hydrophobic

and less crystalline samples.[] Overall, they employed data

analytic tools to identify vital physical properties of plastics that

inﬂuenced the rates of plastics degradation and proposed a

predictive model based on them.[]

3.3.2. Biodegradation

Given that majority of the plastics that have ever been syn-

thesized are now in landﬁlls or in the marine ecosystem,[a]

there has been ongoing interest in the identiﬁcation of path-

ways for natural degradation in the presence of air, water, and

sunlight, such as biodegradation. There had been a report as

early as  on the characterization of the metabolites from

isotactic polypropylene biodegradation by microbial communi-

ties in various soil samples.[] Most of the studies were on the

discovery, characterization, and occasional isolation of enzymes

from bacteria, fungi, soil microbes, marine microbes, or worms

in communities where the plastics are typically found.[–,–]

In most of the cases, only partial decomposition of the waste

plastics was observed.[-,,b,c,e-g,i-l,n] The studies can

be broadly classiﬁed into systems that biodegrade polyoleﬁns

and the others that target condensation polymers by hydro-

lases and are summarized in Table5.[–,–] More detailed

discussions about the biology, metabolite characterization, and

degradation eciencies of these biodegradation case studies

are reviewed elsewhere. These include the biodegradation of

PS and modiﬁed PS,[] enzymatic degradation of plant biomass

and synthetic polymers,[] PUR biodegradation by microbial

enzymes,[] and biodegradation of oil-based plastics in the

environment.[] Here, we focus on the latest breakthroughs

in the biodegradation of polyoleﬁns and PET, including some

especially exciting work on bioengineering enzymes isolated

from bacteria to improve their catalytic activity and thermal

robustness.[m,o]

Over % of the plastics produced and more than % of the

plastics waste generated annually are polyoleﬁns (e.g., PE, PP,

PVC, PS, PMMA) so understandably, there has been intensive

interest on ﬁnding solutions to biodegrade them (Table).[] By

nature of the lack of reactive functional groups in polyoleﬁns,

they are invariably mineralized to CO , but only partially thus

far. After the discovery by Jiang and co-workers that the meal-

worm larvae of

Tenebrio molitor could digest PS in their guts into CO and

lipids,[] Criddle and co-workers extended the study to include

the digestion of PE as well as the identiﬁcation of two spe-

cies of bacteria, Citrobacter sp. and Kosako nia sp., most likely

responsible for the biodegradation.[b] They fed the mealworms

PE only, PS only, combinations of both PE and PS, and also

combinations of plastics and bran and found that the survival

rates exceeded .% after  days.[b] More interestingly,

when they examined the % plastic mass loss, the ones fed a

combination of plastics and bran resulted in higher mass loss,

although the mass loss for the mealworms fed a combination

of PE and PS exceeded % for PE and was >% for PS too

(Figure  24 a,b). [b] They also examined the excrement (frass)

from the mealworms by GPC and found that the mealworms

fed PE and bran showed reductions in M w of .% and M n

of .%, ascertaining that PE was indeed partly digested.[b]

Furthermore, the  H NMR spectra of the residual polymer in the

frass for mealworms fed dierent diets showed that only those

consuming PE showed alkene C–H peaks around . ppm

(Figure c).[b] Together with FT-IR data that indicated the

presence of C–O (– cm− ) and O–H (– and

– cm− ) stretches in the partly digested residual PE

(Figured), the authors acquired conclusive evidence for the

biodegradation of PE in addition to PS.[b] They extracted the

gut walls of the mealworms, sequenced the DNA and RNA with

kits from Illumina, analyzed the microbial communities, and

observed that the aerobic bacteria Citrobacter sp. and Kosako nia

sp. were most abundant and associated with the mealworms

that consumed plastics.[b]

In related reports by Li, Kim, and co-workers, it was found

that Pseudomonas aeruginosa strain DSM   isolated from

the Zophobas atratus superworm larvae[] were able to digest PS

even more rapidly than mealworms.[f ] The larvae of dark and

yellow mealworms as well as superworms had all been revealed

to biodegrade PS in the past decade.[,f,–] Kim et al. cul-

tivated  superworms and  mealworms on PS for  days

and compared the amounts of PS consumed during this period

(Figuree).[f ] The consumption rate by the superworms was

over six times that of mealworms, although the consumption

rate when normalized to the weight of the worms was still .

times higher for the mealworms relative to superworms.[f ]

Likewise, they also analyzed the gut microbiome of the

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superworms and out of four strains, they observed that the

P. aeruginosa bacterial strain constituted around % of all the

sequenced bacteria with a new strain DSM   identiﬁed

for the ﬁrst time.[f ] They then isolated the P. aeruginosa strain

DSM   and cultured them on PS for  days, after which

they subjected the residual PS through a battery of experiments

including XPS, ATR-FTIR spectroscopy, NMR spectroscopy, and

contact angle measurements.[f ] They consistently observed an

increase in the hydrophilicity of the PS surface that had been

exposed to biodegradation, concurring with the surface oxida-

tion of the PS.[f ] This was further conﬁrmed by the presence

of new methylene protons at . ppm from α-protons adja-

cent to a carbonyl group and two new stretches at  (C O)

and  (OH) cm− (Figure f ), thus providing deﬁnitive

evidence that the PS was indeed being digested by this new

bacterial strain.[f ]

Although polyoleﬁns constitute most of the plastics produced

and discarded currently, they are more challenging to biodegrade

Table 5. Latest biodegradation technologies for dierent plastic types.

Linkage Type Polymer Backbone Linkage M [Da] Degrading microorganisms/processes

Polyoleﬁn Polyethylene C C bond 5 × 104 –3 × 105 Bacteria, fungi, soil microbes[20]

Polypropylene C C bond 2 × 103 –6 × 104 Bacteria, soil microbes[20]

Polystyrene C C bond 1 × 105 –4 × 105 Bacteria[20]

Polyvinyl chloride C C bond 1 × 105 –2 × 105 Bacteria, fungi, marine microbes[20]

Linear low density polyethylene

and high density polyethylene

C C bond 1 × 105 –2 × 105 Bacillus sp. BCBT21[96c]

Polystyrene C C bond – Bacteria[17]

Polyethylene and polystyrene C C bond 7 × 104 –2 × 105 Bacteria Citrobacter sp. and

Kosakonia sp. from mealworms

Tenebrio molitor[96b]

Polystyrene C C bond 1 × 105 -4 × 105 Pseudomonas sp. DSM 50 071 from

superworms Zophobas atratus [96f]

Polyethylene C C bond 2 × 104 –1 × 105 Bacteria Enterobacter asburiae YT1 and

Bacillus sp. YP1 from waxworms

Plodia interpunctella[96n]

Low density polyethylene C C bond – Bacteria Pseudomonas sp. and fungus

Aspergillus niger[96j]

Low density polyethylene C C bond – Bacteria Pseudomonas, Bacillus,

Brevibacillus, Cellulosimicrobium,

Lysinibacillus sp. and fungus

Aspergillus sp.[96i]

Polyethylene C C bond – Marine fungus Zalerion maritimum [96k]

Vulcanized and nonvulcanized

polyisoprene

C C and CC bonds 1 × 104 –1 × 105 Enzymes from Ceriporiopsis

subVermispora FP-90031, Coriolus

fungi, and other commercial sources[96l]

Polyisoprenes and vulcanized

natural rubber

C C and CC bonds 3 × 105 –2 × 106 Lipoxygenase, horse radish peroxidase,

and Fenton reactions[96d]

Hydrolyzable condensation

polymer

Polyurethane Urethane 5 × 104 –3 × 105 Bacteria, fungi, soil microbes[19]

Polyethylene terephthalate Ester 3 × 104 –8 × 104 Ideonella sakaiensis 201-F6[96o]

Polyethylene terephthalate Ester 2 × 104 –4 × 104 Hydrolase TfCut2 from bacteria

Thermobiﬁda fusca[93]

Polyethylene terephthalate Ester – PET hydrolase from wild type and

site-mutated Ideonella sakaiensis [96h]

Polyethylene terephthalate Ester – PET hydrolase from wild type and

site-mutated Ideonella sakaiensis

201-F6[96e]

Polyethylene terephthalate Ester – PET hydrolase from wild type and

site-mutated Ideonella sakaiensis

201-F6[96m]

Polyethylene terephthalate Ester – PET hydrolase from wild type and

site-mutated Ideonella sakaiensis

201-F6[96a]

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and mechanically recycle, partly because of the inertness of their

polymer backbones and also the fact that many of them are found

in composites with multiple components. Consequently, mainly PE

and PET are recycled mechanically in meaningful quantities.[a,]

Besides, the biodegradation of polyoleﬁns usually resulted in

mineralization and the release of GHGs, whereas the biological

hydrolysis of PET can conceivably regenerate the monomers eth-

ylene glycol and terephthalic acid again, which creates opportuni-

ties to upcycle them back to pristine PET or other ﬁne chemicals.

In this context, Oda, Miyamoto, and co-workers made a

pioneering discovery of a new bacterium in , Ideonella

sakaiensis -F, which could consume PET as its major carbon

and energy source.[o] They further narrowed down the activity

to two proteins, one that they termed PETase at least . times

more active than previously reported enzymes for PET hydrol-

ysis to mono(-hydroxyethyl) terephthalic acid (MHET), and a

second they named MHETase, which was responsible for fur-

ther hydrolyzing MHET to ethylene glycol and terephthalic acid

(TPA).[o] Building on these results, Woodcock, McGeehan,

Beckham, and co-workers structurally characterized the PETase

and bioengineered double mutants that outperformed the wild-

type.[a] The PETase digested PET predominantly to MHET,

Figure 24. a) Degradation of PE and PS plastics by Tenebrio molitor mealworms fed either plastics only or a combination of plastics and bran over

32 days. b) Final % mass loss of the plastics at the end of 32 days. c) 1H NMR spectra of the excrement (frass) obtained from the mealworms, showing

the presence of alkene C-H chemical shifts. d) FT-IR spectra of the metabolized plastics in the excrement, indicating the presence of new C-O, CO, and

Zophobas atratus superworm and mealworm, with the photograph showing the resulting PS. f) FT-IR spectra conﬁrming the presence of new C = O and

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with only trace quantities of TPA and bis(-hydroxyethyl) tere-

phthalic acid (BHET) as the products (Figure  25 a).[a] Patently,

the PETase is indispensable, which is why they obtained a

high resolution (. Å) X-ray crystal structure of the wild

type enzyme to gain deeper insights on the substrate speci-

ﬁcity and compare its active site with existing enzymes known

to hydrolyze PET.[a] Based on the sequence homology of the

PETase with the Thermobiﬁda fusca cutinase, they determined

that the PETase had a more polarized surface charge.[a] They

then engineered the wild type PETase to give a double mutant

(SF/WH) by switching serine- to phenylalanine-

and tryptophan- to histidine- to try to make the PETase

more similar to cutinase enzymes.[a] Remarkably, the SF/

WH mutant was more active than the wild type during  h

of incubation, with over . times higher crystallinity change

(.vs .%, solid green, Figureb), higher amounts of MHET

released (blue diagonals), albeit lower TPA formed (black

hatchings).[a] Analysis of the crystal structure of the double

mutant and computational modeling of the induced ﬁt docking

revealed a narrower binding cleft for PET, with enhanced

aromatic interactions by phenylalanine- with terephthalate

over the original serine- amino acid residue (Figurec).[a]

The authors also found that PETase could depolymerize

polyethylene-,-furandicarboxylate (PEF), another aromatic-

containing polyester, but not aliphatic ones.[a]

Nonetheless, it is vital to recognize that bioengineering is still

an art and systematic improvements are not always guaranteed.

Earlier studies on the protein crystallography of the PETase

isolated from Ideonella sakaiensis had shown that almost all

single mutations from the wild type resulted in inferior enzy-

matic activities.[e,h] However, co-crystallizations of the PETase

with several substrate (MHET) and product analogs (e.g.,

p-nitrophenol, p-nitrophenylacetate) oered additional insights

into the catalytic mechanism of PET hydrolysis.[e] Several

single mutations were made to the PETase at locations around

where the substrates such as MHET would bind and the relative

activities of the mutant PETases compared to the wild type for

producing MHET and TPA were monitored.[e] As illustrated

in Figured, all the mutants showed poorer activities than the

wild type for the hydrolysis of PET to both MHET and TPA.[e]

With the structural insights from the substrate binding and the

relative enzymatic activities, they proposed the mechanism in

Figuree where the apo-enzyme has a shallow pocket on the

protein surface for substrate binding and the tryptophan-

has many possible conformations.[e] Once the PET binds, a

carboxylate group along the polymer chain will be oriented by

the catalytic triad of methionine-, tyrosine-, and serine-

 so that the terephthalate benzene ring is T-stacked with

tryptophan-.[e] Subsequent nucleophilic attack by water is

similar to other cutinases, after which the hydrolyzed benzoate

can π -stack with tryptophan-. [e]

Very recently, additional progress was made to enhance

the sustainability of PET upcycling by PETase biodegradation.

Marty, Duquesne, Andre, and co-workers explored a number

of enzymes including the PETase isolated from Ideonella

sakaiensis, Thermobiﬁda fusca hydrolases, and a leaf-branch

compost cutinase (LCC).[m] They found that LCC was at least

 times more active than all the other enzymes tested.[m]

By employing enzyme contact-surface analysis and molecular

docking to model how typical substrates can bind to the active

site of LCC, they identiﬁed  amino acid residues in the ﬁrst

contact shell of the active site for mutagenesis and created

 variants.[m] After extensive testing, they found that a quad-

ruple mutant FI/DC/SC/YG (ICCG) showed the

optimal compromise in enzymatic activity, cost, and thermal

stability, using  milligrams of enzyme per gram of PET to

hydrolyze > % of the plastic within  h to produce TPA at

a rate of . grams per liter per hour.[ m] This translated to a

cost of only % of the ton-price of virgin PET to recycle  ton

of PET.[m] After purifying the TPA to .% by conventional

processes, they went a step further to demonstrate that it

could be used to prepare virgin PET with Mn of   Da

with mechanical and optical properties similar or better than

petroleum-based PET.[m] This latest study suggests that the

upcycling of PET to its monomers by biodegradation can be

sustainable and cost-competitive with PET from crude oil in the

near future.

Besides converting the PET monomers back into PET, they

can also be biologically valorized into other ﬁne and speciality

chemicals. For instance, bioengineered Escherichia coli were

used for the conversion of TPA into ,-dihydroxybenzoic

acid ﬁrst, and then elaborated into vanillic acid, muconic

acid, gallic acid, pyrogallol, and catechol in moderate to high

yields (up to .%).[g] The ethylene glycol was fermented by

another bacteria, Gluconobacter oxydans KCCM  , into gly-

colic acid, a cosmetic ingredient.[g] These recent studies have

validated the technological and economic viability of upcycling

some types of plastics such as PET into its monomers or other

ﬁne chemicals by biological processes. Besides creating novel

technopreneurship opportunities to add value to future plastics

waste, these bioengineered enzymes or bacteria may even

be suitable for release into landﬁlls or marine ecosystems to

biodegrade all the existing post-consumer and post-industrial

plastics already indiscriminately disposed of, provided that

more rigorous studies are conducted on the environmental

impact. Nevertheless, further research will still be necessary to

evaluate if a similar approach is feasible to upcycle polyoleﬁns

and what recoverable products may be possible, which is espe-

cially crucial given that polyoleﬁns are still manufactured in far

larger quantities.

4. Conclusions and Outlook

There has been a steady drumbeat of attention on the deterio-

rating problem of global plastics pollution over the past decade.

Several global organizations including the United Nations

Environment Program (UNEP), the Organization for Economic

Co-operation and Development (OECD), and the Chemical

Sciences and Society Summit (CS) bringing together chem-

ical societies in Europe and Asia have released white papers

and policy documents to guide plastics management and dis-

posal in the past few years.[] Two papers have also modeled

the projected growth of plastics pollution up to  and 

based on "business as usual" versus dierent levels of interven-

tion.[] In all the scenarios, the situation is dire with marine

plastics pollution anticipated to average  Mt in  and

 Mt in  for business as usual compared to  Mt in

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Adv. Mater. 2021, 2100843

Figure 25. a) Application of a PET digesting enzyme from the bacteria Ideonella sakaiensis 201-F6 to hydrolyze PET into terephthalic acid and esters.

b) Improved PET digestion by the double mutation of PETase (Trp159 to His159, Ser238 to Phe238), which results in higher crystallinity changes (green

solid) and MHET (blue diagonal lines) production. c) Enhanced π-stacking and hydrophobic interactions by Phe238 with terephthalate compared to

levels of MHET and TPA with dierent single mutations compared to the wild type (WT). e) Proposed mechanism of enzymatic PET hydrolysis. Adapted

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 and – Mt in .[] Even including all the ambi-

tious commitments by governments right now, the leakage of

plastics into aquatic ecosystems is predicted to reach  Mt

in .[] The conclusion was that pre-consumer and post-

consumer interventions individually or increased waste

management alone with current technologies will all not be

sucient to bring the plastics pollution quantities back to 

levels.[] Instead, extraordinary, revolutionary, and coordinated

actions will be necessary, but even then, will only alleviate but

not solve the problems.[]

Besides disposal in landﬁlls and leakage into the aquatic eco-

systems, incineration for energy recovery represents the next

biggest end of life plastic management method,[a,] especially

in Europe where around  plants combust around % of

the collected plastic waste.[] However, we need to acknowledge

that although incineration decreases plastic waste volumes and

generates small amounts of electricity, it inevitably produces

toxic by-products, exacerbates social justice issues since incin-

eration plants are sited near the poor, and ultimately just trans-

forms the global plastics pollution problem into global climate

change.[,] It is increasingly recognized that mechanical recy-

cling is by no means a solution, with only about % collected

for recycling, % reused as downcycled post-consumer plastics,

and only about % really recycled into their original purposes

worldwide.[–,] Instead, Kammen and Bazilian[] advocate

a mindset change that resonates with our team[b,c] and also

others[b,–,,c,b,,,] about how we should be mining

plastics (and other wastes) into resources using less energy-

intensive new technologies with fewer GHG emissions. Plastics

upcycling into chemical feedstocks is not just an utopian ideal

but has been commercialized by Dow Chemical Company and

Fuenix Eco Group[ ] and can actually be a sustainable and

proﬁtable business model if implemented correctly.[a,a]

Furthermore, the development of upcycling technolo-

gies is only part of the downstream plastics life cycle man-

agement. Several outstanding problems that also have to be

resolved before we can transform plastics into resources are the

following:[a,c,a]

i) Waste separation processes need to be improved. In many

countries, recycling is still not widely adopted and plastics are

mixed in with other municipal waste streams and discarded

in landﬁlls.

ii) At the next stage, the separation of plastics waste into single

material streams for upcycling is also a formidable challenge.

Other than the fact that there are so many dierent kinds of

plastics and the labeling of materials is not mandatory, many

plastics are also mixed composites. Even if consumers are

well informed, they may not be able to voluntarily separate

the plastics.

iii) Even with predominantly single material streams like PET

or PE, contamination by food, additives, coloring, and other

substances will complicate the processing and add additional

costs to the entire upcycling life cycle.

Going forward, we recommend the following actions:

i) There needs to be more coordination and collaboration

between government, industry, and academia to prioritize

and fund the development of plastics separation and upcy-

cling technologies. It is manifestly certain that isolated initia-

tives and even the existing policy promises are inadequate for

coping with the proliferation of plastics waste in the environ-

ment.[,,]

ii) Academia and industry need to cooperate to create upcy-

cling technologies that are cost-competitive and sustainable.

It is no longer sucient to just innovate without a business

model in mind.

iii) There should be more international and local government

eorts to promote recycling/upcycling and penalize disposal

with enforcement. Currently, it is too easy for high-income

countries to export plastic waste to lower-income countries

on the pretext of recycling with no traces of the actual fate of

the plastics.

iv) Policies should be introduced to oer more convenient and

eective recycling choices for consumers. In many countries,

recycling bins are intended for mixed recyclables composed

of plastics, paper, glass, and metals, which has resulted in

a vicious cycle of low recycling rates since waste sorting

adds costs. It is not unreasonable to expect a relatively high

compliance if dedicated recycling bins for only plastics are

available.

v) Governments, non-proﬁt organizations, and academia

should work together to inform and encourage consumers to

choose upcycled and/or biodegradable materials.

vi) More interdisciplinary research, even beyond chemistry,

chemical engineering, and materials science[] is needed to

create innovative products derived from plastics upcycling.

Civil engineering and architectural insights are need-

ed to turn plastics into novel construction applications;

biomedical engineers and medical doctors have to ensure

that upcycled plastic products are safe and eective for thera-

peutic purposes; electrical engineers and computer scientists

should be involved in designing new materials that go into

consumer electronics, inter alia.

Global plastics pollution is patently a challenging problem

that will require unrelenting eorts, but we believe that upcy-

cling to sustainably reuse plastics should play a pivotal role in

relieving the problem.

Acknowledgements

H.S.S. acknowledges that this project is supported by A*STAR under the

AME IRG grants A2083c0050, A1783c0003, A1783c0002, and A1783c0007.

H.S.S. is also grateful for the Singapore Ministry of Education Academic

Research Fund Tier 1 grants RG 111/18 and RT 05/19.

Conﬂict of Interest

The authors declare no conﬂict of interest.

Keywords

artiﬁcial photosynthesis, photocatalytic plastics degradation,

photoreforming, plastics biodegradation, plastics upcycling, sustainable

chemistry, thermal upcycling

Received: February 1, 2021

Revised: March 23, 2021

Published online:

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Adv. Mater. 2021, 2100843

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www.advmat.dewww.advancedsciencenews.com

Xin Zhao is a research fellow at Nanyang Technological University (NTU), Singapore. He

received his B.Sc. from China Agricultural University in 2009 and Ph.D. from Nanjing University

in 2014. He joined the School of Materials Science and Engineering at NTU as a research fellow

in 2014 working with Prof. Zhong Chen, and then moved to the Division of Chemistry and

Biological Chemistry to join Prof. Han Sen Soo's team. His current interests include photoelec-

trochemistry for solar energy conversion and the use of earth-abundant elements to valorize

biomass and degrade environmental pollutants by (photo)electrochemical technology.

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2100843 (38 of 38)

www.advmat.dewww.advancedsciencenews.com

Han Sen Soo is an associate professor at Nanyang Technological University (NTU), Singapore.

He graduated from MIT with bachelor's and master's degrees and completed his Ph.D.

work at UC Berkeley. Subsequently, he joined the Lawrence Berkeley National Laboratory as

a postdoctoral fellow, working on materials and nanotechnology. He started his career in the

Division of Chemistry and Biological Chemistry at NTU in 2012. The overarching theme of his

research program is to ﬁnd ways to use solar and other forms of renewable energy to create

fuels and chemical feedstocks from "waste" materials, such as photodriven plastics upcycling.

Adv. Mater. 2021, 2100843

Nonbiodegradable plastics with inert and saturated C–C backbones comprise the majority of global annual plastics produced, including the ubiquitous polyolefins (polyethylene and polypropylene) and polystyrene. Unlike polymers with cleavable bonds, such as polyesters, polyurethanes, and polycarbonates, these inert plastics are the most challenging to upcycle and cannot be easily broken down by chemical or enzymatic means. Thus, they mostly end up in landfills or are incinerated to produce copious amounts of greenhouse emissions. In recent years, increased research effort has been focused toward the upcycling of low-value plastic waste to give them a new lease of life. However, the unreactive C–C polymer backbones of these plastics have posed formidable challenges for attempts at post-synthetic chemical functionalization and conversion into commodity chemicals. In this Perspective, we discuss these inert plastics as large untapped resources for the production of new functional polymeric materials and valuable industrially relevant feedstock, such as dicarboxylic acids and aromatic compounds. The exciting pioneering work featured herein will hopefully inspire a change in the way we view these waste plastics from a chemical dead-end to versatile raw materials, forming the basis of a more sustainable materials economy.

Plastic pollution is a planetary threat, affecting nearly every marine and freshwater ecosystem globally. In response, multilevel mitigation strategies are being adopted but with a lack of quantitative assessment of how such strategies reduce plastic emissions. We assessed the impact of three broad management strategies, plastic waste reduction, waste management, and environmental recovery, at different levels of effort to estimate plastic emissions to 2030 for 173 countries. We estimate that 19 to 23 million metric tons, or 11%, of plastic waste generated globally in 2016 entered aquatic ecosystems. Considering the ambitious commitments currently set by governments, annual emissions may reach up to 53 million metric tons per year by 2030. To reduce emissions to a level well below this prediction, extraordinary efforts to transform the global plastics economy are needed.

Plastic pollution is a pervasive and growing problem. To estimate the effectiveness of interventions to reduce plastic pollution, we modeled stocks and flows of municipal solid waste and four sources of microplastics through the global plastic system for five scenarios between 2016 and 2040. Implementing all feasible interventions reduced plastic pollution by 40% from 2016 rates and 78% relative to 'business as usual' in 2040. Even with immediate and concerted action, 710 million metric tons of plastic waste cumulatively entered aquatic and terrestrial ecosystems. To avoid a massive build-up of plastic in the environment, coordinated global action is urgently needed to reduce plastic consumption, increase rates of reuse, waste collection and recycling, expand safe disposal systems and accelerate innovation in the plastic value chain.

Peyton Shieh
Wenxu Zhang
Keith E. L. Husted
Jeremiah A Johnson

Thermosets—polymeric materials that adopt a permanent shape upon curing—have a key role in the modern plastics and rubber industries, comprising about 20 per cent of polymeric materials manufactured today, with a worldwide annual production of about 65 million tons1,2. The high density of crosslinks that gives thermosets their useful properties (for example, chemical and thermal resistance and tensile strength) comes at the expense of degradability and recyclability. Here, using the industrial thermoset polydicyclopentadiene as a model system, we show that when a small number of cleavable bonds are selectively installed within the strands of thermosets using a comonomer additive in otherwise traditional curing workflows, the resulting materials can display the same mechanical properties as the native material, but they can undergo triggered, mild degradation to yield soluble, recyclable products of controlled size and functionality. By contrast, installation of cleavable crosslinks, even at much higher loadings, does not produce degradable materials. These findings reveal that optimization of the cleavable bond location can be used as a design principle to achieve controlled thermoset degradation. Moreover, we introduce a class of recyclable thermosets poised for rapid deployment.

Microplastics (MPs), which are small plastic debris of ≤5 mm size, are polluting the oceans with negative consequences for their biota. In this work, visible-light photocatalysis of high-density polyethylene (HDPE) and low-density polyethylene (LDPE) MPs in aqueous medium using a mesoporous N–TiO2 coating is proposed as an alternative for fighting MP pollution. Spherical primary HDPE MPs were extracted from commercially available facial scrubs, while film-shaped secondary LDPE MPs were obtained from a plastic bag. For each plastic, two different sizes were tested. Degradation was measured by mass-loss and carbonyl-index (CI) calculation. The results obtained reveal that the photocatalytic degradation of HDPE and LDPE MPs using an N–TiO2 coating was affected by the size and shape of the MPs. Smaller MPs led to higher degradation, while film-shaped MPs led to lower degradation that was related to a poorly illuminated and oxygenated reaction medium. These results set the basis for further investigation on the on the design of more effective photocatalytic-reaction systems for decreasing MP inputs to the environment.

Recently, the environmental impacts of microplastics have received extensive attention due to their accumulation in the environment. However, developing efficient technology for the control and purification of microplastics is still a big challenge. Herein, we investigated the photocatalytic degradation of typical microplastics such as polystyrene microspheres (PS) and polyethylene (PE) over TiO2 nanoparticle films under UV light irradiation. TiO2 nanoparticle film made with Triton X-100 showed complete mineralization (98.40 %) of 400 nm PS in 12 h, while degradation for varying sizes of PS was also studied. PE degradation experiment presented a high photodegradation rate after 36 h. CO2 was found as the main end product. The degradation mechanism and intermediates were studied by in-situ DRIFTS and HPPI-TOFMS, showing the generation of hydroxyl, carbonyl, and carbon-hydrogen groups during the photodegradation of PS. This study provides a green and cost-efficient strategy for the control of microplastics contamination in the environment.

Julien Warnan
Erwin Reisner

From the understanding of biological processes and metalloenzymes to the development of inorganic catalysts, electro‐ and photocatalytic systems for fuel generation have evolved considerably during the last decades. Recently, organic and hybrid organic systems have emerged to challenge the classical inorganic structures through their enormous chemical diversity and modularity that led earlier to their success in organic (opto)electronics. This Minireview describes recent advances in the design of synthetic organic architectures and promising strategies toward (solar) fuel synthesis, highlighting progress on materials from organic ligands and chromophores to conjugated polymers and covalent organic frameworks.

Tessa H. T. Myren
Taylor A. Stinson
Zachary Mast
Oana Luca

This work describes new methods for the chemical recycling of end-use poly(ethylene terephthalate) (PET) in batch, microwave and electrochemical reactors. The reactions are based on basic hydrolysis of the ester moieties in the polymer framework and occur under mild reaction conditions with low-cost reagents. We report end-use PET depolymerization in refluxing methanol with added NaOH with 75% yield of terephthalic acid in batch after 12 h, while yields up to 65% can be observed after only 40 min under microwave irradiation at 85 • C. Using basic conditions produced in the electrochemical reduction of protic solvents, electrolytic experiments have been shown to produce 17% terephthalic acid after 1 h of electrolysis at −2.2 V vs. Ag/AgCl in 50% water/methanol mixtures with NaCl as a supporting electrolyte. The latter method avoids the use of caustic solutions containing high-concentration NaOH at the outset, thus proving the concept for a novel, environmentally benign method for the electrochemical recycling of end-use PET based on low-cost solvents (water and methanol) and reagents (NaCl and electricity).

Stijn Billiet
Scott Trenor

Plastic packaging has gained an increasing amount of attention in all aspects of society. Over the past several decades, plastics became the material of choice due to their excellent properties, performance, and economics, but the end of life of plastics is not well managed. This has led to plastic waste in our environment, especially the oceans, rivers, and estuaries, driving legislative, industrial, and voluntary initiatives to make the necessary pivot to circularity. While the plastics recycling industry has made many advances in its relatively short life, there are still many technical and societal hurdles to be overcome. The goal of this work is not to provide a complete review of recycling as it pertains to circularity, but rather to highlight the technical gaps that need to be collaboratively addressed by the entire plastics community to achieve circularity. Each stage along the path, from design of packaging and materials of construction to sortation, recycling, and reprocessing are ripe for innovation. The most relevant issues are introduced to provide a starting point for research across all fields of polymer science to aid in reducing the environmental impact of plastic packaging waste.

Keita Saito
Coralie Jehanno
Leire Meabe
Haritz Sardon

The constant increase of plastic waste released into the environment is a global problem which is of increasing concern to the general population. Although there are many different approaches to the recycling of plastics, chemical recycling is currently seen as one of the most promising technologies in that it allows plastic waste to fit into a sustainable, circular economy. Herein we investigate the chemical recycling of Bisphenol A polycarbonate (BPA-PC) using diols of different chain lengths to yield Bisphenol A and innovative carbonate-containing diols. Subsequently, the latter are polymerised into a series of unique value-added aliphatic polycarbonates (APC). The new polymers obtained by this method have shown promising values of ionic conductivity that make them attractive candidates to be implemented as sustainable polymer electrolytes for solid-state batteries. This procedure opens the way for recycling methods to produce unique, innovative materials using plastic waste as an alternative sustainable feedstock.

Thermo-chemical processes for converting plastic wastes into useful materials are considered promising technologies to mitigate the environmental pollution caused by plastic wastes. In this study, polyethylene terephthalate (PET) plastic wastes were used to develop cost-effective and value-added porous carbons; the developed porous carbons were subsequently tested for capturing CF4, a greenhouse gas with a high global-warming potential. The activation temperature was varied from 600 °C to 1000 °C and the mass ratio of KOH/carbon ranged from 1 to 3 in the preparation process and their effects on the textural properties and CF4-capture performance of the PET plastic waste-derived porous carbons were investigated. The CF4-adsorption uptake was dictated by the specific surface area and pore volume of narrow micropores less than 0.9 nm in diameter. PET-K(2)700, which was developed by KOH activation at 700 °C and KOH/carbon mass ratio of 2, showed the highest CF4-adsorption uptake of 2.43 mmol g⁻¹ at 25 °C and 1 atm. Also, the CF4-adsorption data were fitted well with the Langmuir isotherm model and pseudo second-order kinetic model. The PET plastic waste-derived porous carbons exhibited a high CF4 uptake, good CF4/N2 selectivity at relatively low CF4 pressures, easy regeneration, rapid adsorption/desorption kinetics, and excellent recyclability, which are promising for practical CF4-capture applications.