Sustainable Polymers Using Renewable Resources For Today and

Tomorrow

Yunqing Zhu; Charles Romain; Charlotte K. Williams*

Department of Chemistry, Imperial College London, London, UK, SW7 2AZ.

c.k.williams@imperial.ac.uk

Preface

The article highlights recent developments using renewable resources, such as carbon

dioxide or biomass, to make polymers. In particular, the use of monomers such as carbon

dioxide, terpenes, vegetable oils and carbohydrates to make elastomers, plastics, hydrogels,

flexible electronics, resins, engineering polymers and composites are described. Examples

of efficient catalysis to produce monomers, selective polymerizations and recycling or

upcycling of waste materials are provided. The opportunities to use bio-based polymers in

areas beyond packaging, including in high-value areas are outlined. Where relevant, life

cycle assessments are used to quantify the environmental benefits for sustainable polymers.

Introduction

Modern life relies on polymers, from the materials in our clothing, houses, cars, and

airplanes through to sophisticated applications in medicine, diagnostics and electronics, and

the vast majority of these polymers are derived from petrochemicals. Many polymers also

contribute significantly to a better quality of life and environment, for example as materials

enabling water purification or as improved fuel-economy polymer composites in aerospace.

Even though only ~6% of worldwide oil production is used to produce plastics, there are

environmental concerns associated with both the raw materials used to make them1 and

end-of-life options.2 Whilst there is no panacea to these complex environmental problems,

one option is to develop more ‘sustainable’ polymers. Research activities have primarily

focused on both replacement of fossil raw materials with renewable alternatives, with the

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products being known as bioderived polymers, and on end-of-life options, including materials

suitable for recycling or (bio)degradation. It is important to note that simply because a

material is bioderived does not guarantee its biodegradation, and that a few petro-polymers

are biodegradable. The potential for sustainable polymers is stimulated by recent policy,

legislation and international agreements, such as Paris COP21.3 Nonetheless, any

commercial application of bioderived polymers may benefit from improved environmental

performance (as well as supporting policy or legislation) but it will require favorable

economics and equivalent or better material properties, including features such as thermal

resistance, mechanical strength, processibility and compatibility. Taken together, these are

tough criteria and may explain in part why there are currently rather few commercially

successful sustainable polymers. To quantify this, in 2014, for an annual polymer production

of over 300 Mt, only 1.7 Mt of bioderived polymers were produced, with the three major

products, by volume, being polyethylene terephthalate (PET), polyethylene (PE) and

polylactide (PLA).4 There are generally two approaches to preparing sustainable polymers:

1) to improve the environmental impacts of current monomer and polymer production, for

example by using biomass to make known monomers/polymers, such as the PET or PE

mentioned above or 2) to prepare new ‘sustainable’ material structures derived from

renewable raw materials, such as PLA and other materials highlighted here.

Here, some of the opportunities for sustainable polymers are highlighted using four different

renewable raw materials: carbon dioxide, terpenes, vegetable oils and carbohydrates (Figure

1). These feedstocks enable production of polymers and materials spanning a whole range

of properties and applications. The use of modified natural polymers, such as cotton, silk,

thermoplastic starch, cellulose derivatives and natural peptides, is not discussed and the

potential for polymers from lignin is only briefly mentioned. The examples are selected if they

meet some of the overarching challenges:

1) The transformations of both the renewable resources and the polymer productions must

be highly efficient so as to reduce costs. Using mixtures of raw materials, lower purity

monomers or ‘up-cycling’ of agricultural/industrial waste materials is important.

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2) Materials must show complementary or improved properties compared to those available

today. Opportunities in high-value markets, such as thermoplastic elastomers, rigid plastics

or polyols may present more favorable economics compared to packaging.

3) Life cycle analyses (LCA) should be used to quantify and compare materials against

existing petrochemical benchmarks — it is usually used to assess environmental impacts

and outputs associated with polymer production. Whilst the need for such comparisons may

seem obvious, there are complexities associated with selecting appropriate benchmarks,

boundaries and data.5 Currently many materials at an early stage of development are not

routinely examined by LCA in academic publications. Nonetheless, where such studies exist

the key findings are included, particularly if they are relevant to the design and development

of future materials.

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Figure 1: Illustrates some of the polymers which can be produced, using biomass and

carbon dioxide as replacements for petro-plastics.

1) Up-cycling Carbon Dioxide into Polymers

Using waste greenhouse gases like CO2 to prepare useful and valuable polymers has long

been an academic interest and is now on the cusp of commercialization. It represents a rare

example of a chemical process which consumes carbon dioxide as a reagent. It allows 30-

50% of the mass of the polymer to derive from CO2, with the remainder being petro-derived,

and it delivers both economic and environmental benefits (Figure 2).6-12

Polymers are produced by an alternating copolymerization of epoxides, commonly propylene

oxide, and carbon dioxide. The process is critically dependent on the catalyst applied and

global efforts have focused on improving and understanding the catalysis.6,13-19 Generally,

homogeneous catalysts deliver a much higher carbon dioxide uptake into the polymer,

resulting in balanced epoxide/CO2 enchainment and producing aliphatic polycarbonates. In

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contrast, heterogeneous catalysts require significantly higher pressures and result in lower

carbon dioxide incorporation, thereby producing poly(ether carbonates), where the ether

linkages result from sequential epoxide enchainment. It is also feasible to selectively

combine CO2/epoxide copolymerization with other bio-based monomers so as to produce

block copolymers comprising ester, ether and carbonate blocks.20-23 Although such methods

are at an early stage, they highlight the need for more selective chemistry using monomer

mixtures and the potential to control properties by monomer sequence control: both are

signaled as areas for future developments.

The current commercialization of polymers from CO2 addresses two distinct polymer

molecular weight regimes and application areas:

1) Low molecular weight hydroxyl end-capped polycarbonates or poly(ether carbonates) are

applied as polyols in polyurethane manufacture.6,11 By developing polyols with low viscosities

and glass transition temperatures there may be opportunities to substitute common petro-

polyols used to make furniture foams, adhesives, apparel and resistant coatings.15

2) High molecular weight polycarbonates are already used as binders/sacrificial materials

and by improvements to the properties future opportunities may include applications as rigid

plastics and in blends with petro-polymers.12,24

An important benefit of CO2 usage, which is distinct to many bio-derived monomers, is that

polymer production is easily compatible with current petro-polymer manufacturing

infrastructure. In particular, polymerizations proceed using existing reactors, processing and

purification methodologies. Furthermore, there is no dependence on agriculture or complex

monomer pre-treatments and transformations.

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Figure 2: Illustrates schematically how to CO2 may be up-cycled into valuable polymers.

An obvious question from the point of view of sustainability is whether the process is really

compatible with recycling of waste CO2 emissions? It has recently been shown that

successful polymerization can be achieved using epoxides and carbon dioxide emissions

captured from a UK based coal-fired power station.11 The catalytic performance and product

quality using the captured gas were nearly equivalent to those achieved using ultra-pure

carbon dioxide gas. The entire process was surprisingly tolerant of contaminants, including

water, nitrogen, oxygen, as well as small-molecule amines and thiols which may be present

from the CO2 capture process.

LCA was used to compare the production of polyether carbonate polyols, from petro-derived

propylene oxide and carbon dioxide, with polyether polyols, produced using entirely petro-

derived propylene oxide. The study showed that even with only partial carbon dioxide

substitution, net GHG and fossil resource depletion reductions were ~ 11-20%.25 It is,

perhaps, important to emphasize that the environmental benefits arise due to the

replacement of the epoxide by CO2 and not only from the carbon dioxide recycling. Recent

research also demonstrates the potential to use epoxides derived from limonene and

vegetable oil to yield fully renewable polycarbonates.26,27 Poly(limonene carbonate) has been

qualified for various applications, including as a resistant and hard dry-powder coating

achieved by cross-linking the pendant alkene functional group introduced on the limonene

unit.12,26,28 By careful selection of the catalyst, it’s also possible to prepare highly crystalline

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stereocomplex poly(limonene carbonate) (a co-crystallite between polymer chains of

opposite chirality) which shows better thermal properties, including a higher degradation

temperature (Td = 265 C), compared to lower crystallinity analogues.29 These results

highlight the potential for future bioderived materials to deliver high impact, high-value

products, by taking advantage of naturally ‘rigid’ chemical functionalities and to deliver cost

efficiency by using wastes as raw materials. Nonetheless, there is not yet sufficient physical,

rheological or processing data to fully understand their potential.

2) From Plants to Plastics

The entire field of polymer science originates from studies of biopolymers such as

cellulose.30 Many commercial sustainable polymers are sourced from sugar- or starch-rich

plants, such as sugarcane, wheat or sugar beet. For example, bio-PET is a material where

there is partial substitution of petrochemicals: up to 30 wt% of the ethylene glycol monomer

is produced from starch. The process is quite complex and involves starch degradation,

glucose fermentation, ethanol dehydration, ethene oxidization and hydrolysis to the product.

Additionally, there are a number of active research programs, including some at pilot stage,

investigating fully bioderived PET which requires that the co-monomer terephthalic acid also

comes from biomass.31 LCA studies of the current generation bio-PET have shown 20-50%

reductions in greenhouse gas (GHG) emissions compared to the petro-PET. Polyethylene

(PE) can also be produced from sugarcane with the ethylene monomer being obtained from

ethanol dehydratation. This production method is somewhat controversial, as it requires a a

well-developed sugar industry and is primarily being explored in Brazil.32 Once ethylene is

produced, the polymerization process and polymer properties are identical to petro-PE.

Regardless of how it is produced polyethylene is usually environmentally pervasive and

polluting and it is unlikely to become economically viable to recycle it.

The advantages of developing ‘drop-in’ bioderived monomers — e.g. for PET and PE — are

that the processes and applications for the polymers remain identical, thereby simplifying

adoption and accelerating uptake. This is particularly important for PET which is one of the

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few polymers for which large-scale recycling infrastructures exist and where LCA studies

indicate positive contributions to its sustainability if it is recycled.33 Recently, highly efficient

catalytic methods have been pioneered enabling PET chemical recycling and showing

potential for subsequent ‘up-cycling’ to new polymers.34 Such methods may be a promising

alternative to well-established mechanical recycling. Generally, using renewable resources

to prepare existing polymers are front-runner technologies in the commercialization of

sustainable polymers.

One question is whether there could be a societal impact from using edible feedstocks to

prepare polymers? At first sight there appears something of an analogy to the controversy

surrounding some bio-fuels. However, on consideration of the scales, it is apparent that

overall polymer production is dwarfed in magnitude by fuels and that bioderived polymers

are still a niche in the polymer sector. A detailed study of bioderived polymer production in

the EU substantiates the possible land-use requirements.35 It envisages a 1-4% market

share for bio-derived polymers by 2020, dependent on various economic and growth

models. The authors considered a scenario in which wheat is the sole starch source and

estimated that just 1-5% of the land currently used to grow wheat would be needed. The

study concludes that “the land requirements for bio-derived polymers are modest and are not

expected to cause any particular strain within the EU”.

2a) From Terpenes to Elastics and Coatings

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Figure 3. Illustrations of the common sources of terpenes and terpenoids and some

materials prepared from them.

Terpenes and terpenoids are the components of essential plant oils and have a common

‘isoprene’ unit in their chemical structures.36 Perhaps the best known example of a

poly(terpene) is natural rubber, produced on >10 Mt/year scale and with the major

constituent being polyisoprene. Other terpenes also investigated for polymer production,

albeit on a very much smaller scale, include turpentine, extracted from pine trees and

primarily composed of α-pinene (45-97%) and β-pinene (0.5-28%), and limonene, which is

extracted from citrus fruit peel (Figure 3).37 The global production volumes of these

monomers are modest — in 2013 productions were estimated for turpentine at ~0.3 kt38 and

limonene ~0.7 kt.39 There are already some commercially available polymer resins from

these terpenes, more details are available in recent reviews.36,37,39,40

One limitation of using terpenes is the low polymer molecular weights and this, in turn, limits

their mechanical performance. Recent research shows potential to access significantly

higher molecular weights by using a cationic polymerization of β-pinene followed by post-

polymerization hydrogenation.41 The resulting polymer exhibits some thermal/mechanical

properties akin to poly(methyl methacrylate) and shows high optical transmittance. Another

option is to copolymerize terpenes with common petro-derived vinyl monomers, like

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methacrylates, via radical polymerization which allows materials with partial bio-based

content.40,42 Polymer recycling is a key aspect of sustainability and a recent work neatly

demonstrates a means to use limonene as both reagent and recycling solvent.43 A

polystyrene drinking cup was dissolved in limonene which itself was reacted via a cross-

linking thiol-ene chemistry to prepare a new elastomer which could be molded to produce a

mobile phone cover.

Another major limitation for commercialization is the relatively high cost of terpenes, and to

ameliorate this one option is to make higher value products such as thermoplastic

elastomers (TPE) (Figure 3). Current thermoplastic elastomers are mostly derived from

petrochemicals and are produced on a >3.5 Mt/year scale, for applications as diverse as car

suspensions, window seals, electronic coatings, running shoe soles and medical devices.44

Using terpene monomers derived from natural mint, Mentha arvesis, and the common tulip

plant, Tulipa gesneriana, together with controlled polymerization methods, has led to the

production of block copolymer thermoplastic elastomers (Figure 3).45,46 The best materials

showed a Young’s modulus >6.0 MPa, which is within the range observed for commercial

poly(styrene-butadiene-styrene) SBS. In contrast to SBS, however, the bio-derived

elastomer has a very high glass transition temperature (Tg = 170-190 oC) enabling it to retain

elasticity at elevated temperatures, a feature which might be desirable for harsh environment

applications. It should be noted that most bio-based elastomers show elongation at break

values below 1000% which is inferior to petro-polymers and needs to be improved in future

research efforts.

2b) Adding Value to Vegetable Oils

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Figure 4. Illustrations of the common sources of vegetable oils and some polymer products.

Triglycerides are harvested from the seeds of certain plants, with the top four, by volume,

being soy bean, palm, rape and sunflower. They are produced on a very large scale (156 Mt

in 2012), with the majority being used for food, ~30 Mt being used for bio-fuel and ~20 Mt

being used as a chemical feedstock.47 Nonetheless, they also represent an important and

long-standing raw material for polymer production (Figure 4). Indeed, linoleum and

epoxidized oils have a strong track record as resins, coatings and in paints. More recently,

commercial production of polyamides from castor oil yields bioderived Nylon 11, Nylon 6,10

and Nylon 4,10. Some of these bio-nylons show beneficial properties, including low water

absorption, good chemical resistance, high temperature stability and a lack of long-term

aging.47 They have been applied as toothbrush fibers, in pneumatic airbrake tubing and in

flexible oil and gas pipes. One important limitation is the reliance on castor oil which contains

a secondary hydroxyl group in the fatty acid chain facilitating its efficient transformation to

monomers and subsequent polymerizations. It is notable that castor oil is almost double the

cost of more common oils such as palm or rape.

Although they are present in almost all plants, the quantity of triglycerides available is

variable and even common crops such as soy bean are estimated to yield only

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approximately 20 wt% of triglycerides. Another challenge is that their chemical compositions

are variable both between and within a particular crop. Triglycerides are composed of three,

often distinct, fatty acid groups linked together through ester bonds to a glycerol unit. They

are commonly processed by transesterification reactions to produce fatty esters and

glycerol. Glycerol can be used as a cross-linking agent and as a precursor to existing

monomers such as epichlorohydrin or lactic acid.48,49 Nonetheless, the major opportunity

probably lies with the fatty esters which feature long alkyl chains (from C12-C22) and include a

significant portion of internal alkene functional groups.50 When polymerized they can show

properties intermediate between polyolefins, such as polyethylene, and more polar short-

chain polyesters. A common set of polymerization methods apply the alkene functional

groups present in unsaturated fatty esters (Figure 4). Indeed, the plant oils produced in

largest volume worldwide all have significant compositions (~20-60 wt%) of such

unsaturated fatty acids. Recently, crop engineering has produced a strain of soy bean

yielding >75% of mono-unsaturated oleic acid – a particularly useful monomer.51 There are

many methods to transform the alkene groups to polymers, including by the thiol-ene

reaction, acyclic diene methathesis, epoxidation and radical/thermal cross-linking reactions

(Figure 4). 50,52 One area of considerable potential is to react the alkene to produce -

diesters or diols, these monomers undergo conventional condensation polymerizations to

yield bioderived polyesters or nylons (in that case using diamides). One limitation is that -

difunctionalized monomers are usually produced reactions such as olefin metathesis,

ozonolysis or C=C bond oxidative cleavage and in all cases only about half of the fatty acid

is used and there are several by-products,53 An elegant solution involves the use of selective

chemical catalysis to isomerize the internal alkene group to the chain end and, after an

alkoxy carbonylation process, allows near quantitative production of the desired -

difunctionalized monomers. 53-55 The desired diesters are produced with an impressive >95%

selectivity, with high conversions and without significant by-products.53-55 Polyesters can be

prepared by conventional polycondensations and the materials show thermal properties,

solid state crystalline structures and tensile properties which mimic those of polyethylene.56

Enzyme catalysts allowing similar polycondensations have also been developed, showing

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efficient activity over a range of temperatures, solvents and substrates.57 Despite the facility

to process the long-chain polyester products using existing industrial methods such as

injection molding or film extrusion,54 they are so far not cost-competitive with polyethylene.

Nonetheless, the high crystallinity, thermal/chemical resistance and degradability are

valuable and applications as compatibilizers in blends of petrochemicals or as macro-

monomers could be feasible.

Polycondensation requires very high purity monomers and balancing of stoichiometry in

order to successfully produce polymers. An alternative method applies macrolactones,

derived from fatty acids, which can undergo ring-opening polymerization to produce similar

‘long alkyl chain’ polyesters. It allows access to high molecular weight polymers and is

compatible with block copolymerization. Bioderived macrolactones comprising up to 23

atoms have been polymerized,58 furthermore some macrolactones such as

pentadecalactone (PDL) or ambrettolide are themselves natural biochemicals. The

polyesters show thermal and rheological properties akin to those of linear low density

polyethylene.59 Using an alternative bio-catalytic approach, mixtures of glucose and oleic

acid were fermented to efficiently (>200 g/L) produce a macrolactone, lactonic sophorolipid

(LSL), which features both disaccharide and alkene functional groups. Its ring-opening

methathesis polymerization leads to the production of carbohydrate functionalized

polyesters.60

Given that many triglycerides are also applied as foods, there may be concerns regarding

land use. One solution may be to engineer algae to bio-synthesize unsaturated fatty acids –

an attraction would be that algae can be grown on non-arable land and may even flourish in

brackish water. They also only require sunlight and carbon dioxide as energy sources. Using

algal bio-synthesis has enabled 20-50% dry weight yields of triglycerides which is higher

than from many crops.61

Triglycerides have also been used to prepare thermoplastic elastomers, including even an

example of a self-healing and thermo-reversible elastomer.62 Its Young’s modulus is

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comparable to petro-polymer SBS (see terpene section) and its maximum strain exceeds

500%. By using reversible supramolecular hydrogen bonding interactions for cross-linking,

its processing is facilitated and it may even be possible to recycle it, something that is not

typically possible for conventional elastomers. Although mechanical creep and other

rheological properties are not yet reported, the diversity of available fatty acids and the

novelty of the physical property tuning methods are attractive. Fatty acids can also make

vitrimers, which are polymers showing reversible temperature-induced thermoset to

thermoplastic transitions, for example by thermally controllable transesterification

reactions.63,64

2c) Sugars: Sweet and Sustainable Materials

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Figure 5. Illustrations of the common sources of polysaccharides and different types of

polymers prepared from them.

It is estimated that nature produces more than 150 billion tons of polysaccharides annually,

with human consumption only accounting for ~1%. To make new synthetic polymers, these

biopolymers must be separated and depolymerized so as to obtain pentoses and hexoses.

The most abundant is glucose: today accessed by starch saccharification but in future from

lignocellulosic sources. Glucose is transformed to ‘building block’ chemicals, like lactic or

succinic acid, and these are either directly polymerized or are further reacted, by either

chemical or enzymatic routes, to produce monomers (Figure 5).65,66 In 2004 the US

Department of Energy published a landmark report highlighting the ‘top-10 chemicals from

biomass’, selected on the basis of available volumes and facility for transformation into

useful products.67 These included lactic acid, succinic acid and hydroxyl methyl

furfuraldehyde (HMF) which have already delivered useful polymers (Figure 5). It is also

important to consider that in some cases new polymerization methods and processes have

to be used as the monomers are more polar and highly functionalized (oxygenated)

compared to fossil raw materials.

One leading example is commercially available polylactide (PLA) which is produced from

starch-rich crops such as maize (Figure 5). It is produced by starch fermentation to lactic

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acid and its polymerization. Polylactide has properties suitable for petro-plastics replacement

in some types of packaging and fibers.68,69 The process requires efficient lactic acid, and

lactide, production which is current achieved commercially by microbial fermentations.70,71

Recent advances include using cheaper fermentation substrates like glycerol, agricultural

wastes and even by using algae to produce the carbohydrates.70 Chemical catalysis may be

also of interest as a means to produce racemic mixtures of lactic acid from sugars.72 The

selective polymerization of racemic mixtures may increase the thermal resistance and range

of applications for PLA.73 In terms of its end-of-life fate, PLA can be recycled, degraded and

is even compostable, under high temperature conditions, degrading to lactic acid which is

naturally metabolized. Recent LCA studies have shown significant reductions in GHG

emissions (at best ~40%) and non-renewable energy use (at best ~ 25%) compared to

petro-polymers, such as PE or PET.74,75 Nonetheless, PLA may have other environmental

impacts, e.g. water and potentially fertilizer usage, which would be more difficult to compare

against the impacts of fossil fuel extraction, purification and storage. Another significant

hurdle is to replace the virgin crops (such as corn or sugarcane) with ligno-cellulosic or

waste biomass.76

Another group of renewably sourced polyesters, obtained by sugar fermentation, are the

polyhydroxyalkanoates (PHA) (Figure 5).77 These are, in fact, naturally occurring and they

can be harvested, in excellent yields, directly from micro-organisms without the need for

intermediate monomer isolation. Biosynthesis is achieved by culturing bacteria under growth

limiting conditions and results in the accumulation of significant quantities of the polymer in

the cytoplasm. The most promising polyhydroxalkanoates show physical properties similar to

polyolefins, such as polypropylene, but offer the advantage of degradability. Currently

bacterial production is not cost-effective for commodity applications but small-scale

production is being explored and may be more suitable for higher value medical

applications.65

Poly(ethylene furanoate) is another attractive example of a fully bioderived material showing

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properties suitable to substitute PET in some applications. It is not yet commercialized but

pilot scale production appears underway and there are a number of studies outlining its

properties. It is produced by the transformation of fructose or glucose, via acidification and

dehydration reactions, to 5-hydroxymethylfurfuraldehyde (HMF).78 One problem is that HMF

is rather unstable which limits process efficiency, resulting in side-products such as levulinic

acid and furanic acid. An improved route to HMF-ethers which are more stable and are

subsequently oxidized to furan dicarboxylic acid (FDCA) has been reported (Figure 5).79 The

dicarboxylic acid is copolymerized by polycondensation, with bio-derived ethylene glycol, to

yield fully bioderived PEF. Importantly, both the polymerization and oxidation reactions are

claimed to be compatible with PET manufacturing and the potential to apply existing

infrastructure may accelerate translation and uptake.79 In terms of its properties, PEF has a

higher glass transition temperature and improved barrier properties, especially towards

oxygen permeability, compared to PET (Table 1),79 although unlike PET appears less likely to

undergo cold-crystallization. A recent LCA benchmarked biobased PEF against fossil-based

PET: it showed a reduction in GHG emissions by up to 55%.78,80 It is difficult to compare the

costs of the two materials, due to the disparity in scales of production, nonetheless, it is

envisaged that larger scale production of PEF will reduce its cost.78

Another important monomer derived by a highly efficient fermentation of glucose and

produced on 170 kt/annum scale is succinic acid.81 It can be reacted via polycondensation

with bioderived 1,4-butanediol to produce polybutylene succinate and commercial production

is at ~ 40 kt/annum scale. It is a semi-crystalline polymer, with a high melting temperature

(Tm = 115 oC) and is processable by some conventional techniques, although there are

rheological limitations to blown films.81 It has been used in packaging as a barrier and in

blends. An alternative method to produce related polyesters, featuring succinic acid repeat

units, is to copolymerize epoxides and succinic anhydride.82 The method is attractive as it is

controlled, may obviate the need for precise control of reagent stoichiometry and it yields

materials with predictable molecular weights. Very recently, new stereocomplexes of

poly(propylene succinate) which are crystalline and thermally resistant materials (Tm = 120

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C) have been reported.83 Succinic acid can also be dehydrated at elevated temperatures to

produce γ-butyrolactone. Historically, this five-membered ring lactone was considered

‘unpolymerizable’ due to its low ring-strain. Nonetheless, by using optimized low-temperature

processes involving in situ polymer precipitation it is possible to produce some polymer.84

Although the results are unlikely to be commercially deployed yet due to the production

methods, polymer stability and issues associated with the monomer being a controlled

substance,85 it does illustrate the potential for controlled catalysis and recycling of bioderived

monomers/polymers.

Thermoplastic elastomers have already been mentioned as a promising application area for

biobased polymers and using carbohydrates could be the most cost-competitive option (vs.

terpenes). Recent research shows the potential for block copolyester elastomers.

Engineered E.Coli was used to prepare a functionalized lactone in high efficiency (88 g/L in

semi-batch mode), with monomer cost being estimated at ~$2/kg, which is within the range

for some commodity applications.86 The lactone was polymerized using controlled ring-

opening polymerization to produce an elastomer (Tg = -50 C), and its copolymerization with

polylactide yielded a thermoplastic elastomer that could be stretched to 18 times its original

length without breaking.

The functionalization of cellulose to produce commercial polymers like cellophane or

cellulose acetate has long been known. Cellulose fibres are also natural re-enforcements in

polymer composites which could be attractive as resins or engineering materials.87-89

Recently, semi-renewable hydrogels were prepared by polymerizing from the hydroxyl

groups of hemicellulose which was harvested from Picea Abies.90 The method is attractive

due to the ease of synthesis, tolerance to conditions and the ability to control the crosslinking

density so as to tailor the ultimate mechanical properties of the polymer. In an alternative

approach, cellulose nanofibrils, derived from wood, were used as replacements for PET as

the flexible substrate in electronics manufacture. The cellulose fibrils showed a high

electrical breakdown tolerance (up to 1100 V) and paper product undergoes fungal

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biodegradation without adverse environmental effects.91

Outlook and Future Prospects

Overall, sustainable polymers from renewable resources are important today and society

wants and needs materials with an improved ecological footprint tomorrow. There are

already early successes in ‘greening’ polymers, with current commercial products mostly

used in packaging and as fibres. A key future challenge is to identify new platform chemicals

or ‘building blocks’ that are easily prepared from abundant feedstocks and that don’t

compete with food or alter the eco-system.

Here we have highlighted some new sustainable polymers derived from carbon dioxide or

plants, such as terpenes, vegetable oils and sugars. Clearly, improvements to agricultural

methods for crop production and harvesting, for example to optimize yields, are likely to

improve both economic and environmental impacts for bio-derived polymers. Future

research must target raw materials which are non-competitive with food crops and should

make better use of agricultural and industrial wastes as monomers, for example by using

corn-stover, fruit pulp, forestry wastes and carbon dioxide emissions. Another significant

future opportunity lies in taking advantage of the large worldwide availability of lignocellulosic

biomass – in order to apply it we still need better biopolymer separation, degradation and

transformation (bio)chemistry to optimize yield and cost of the monomers. Carbohydrates

are the most abundant and processible sources of future monomers and already a number

of very interesting processes and polymers are being developed. Future research needs to

exploit the fascinating high degree of natural functionality, including taking advantage of rigid

carbohydrate ring structures, extensive opportunities for non-covalent interactions and

stereoregularity to prepare high value products.92,93 In tandem, the transformation of lignin to

polymers is under-developed due to its highly complex and changeable structures.94 We still

need more methods to selectively transform lignin into monomers, indeed recent research

highlights the potential for catalysis to deliver new monomers, although so far these studies

tend to focus on using model compounds rather than native lignin.95,96 Another interesting

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option is to apply ferulic acid, which can be derived from either lignin or agricultural wastes,

to yield interesting polyesters and resins suitable to substitute petro-polymers.97,98 A critical

future design criterion, which is sometimes under-estimated, is the need to prioritize routes

to monomers and polymers that are compatible with existing industrial facilities and assets.

Low-cost and efficient raw material and product purifications also require much more

optimization for the highly oxygenated bio-based materials. The ability to ‘retro-fit’

sustainable polymer production using current manufacturing infrastructure will continue to be

a key driver in reducing costs and accelerating implementation.

In the future, the continued use of ‘green polymers’ in disposable applications, such as

packaging, will result in the end-life fate exerting a significant influence on sustainability. New

recycling, degradation or disposal options are likely to become even more important to

prevent new materials from contributing to existing ‘plastic waste issues’ and there may be

an opportunity for supporting policy and legislation here. Although the direct quantification

and comparison of sustainable polymers is at an early stage, there have been sufficient

studies to demonstrate that in many cases there are reductions in impacts, particularly in

GHG emissions and fossil resource depletion. Future studies should consider the life of the

product beyond the manufacturing and consider impacts associated with disposal. So far,

there are fewer polymers designed to be both fully bioderived and degradable, although

aliphatic polyesters such as polylactide are notable successes.

Although packaging applications are an important current opportunity for bio-based polymers

competing economically with petro-polymers is very challenging. In future, new bio-based

polymers should seek to compete in the higher-value and higher-performance application

areas, some of which were highlighted such as thermoplastic elastomers, engineering

plastics or composite materials. Key to success will be to tailor and improve polymer

properties. For example, an important future challenge is to prepare higher thermal

resistance polymers such that the temperature windows allow them to compete with existing

semi-aromatic polyesters/nylons. Another opportunity lies in developing elastomers showing

20

greater elongations at break competitive with petro-polymers. Yet another future challenge

lies in understanding and engineering the materials’ degradation profiles, for example

combining long-term durability with triggered degradation.

The task of widening the scope and range for sustainable polymers is significant; in order to

solve such complex problems we will need to work together across conventional disciplines

of agriculture, biology, biochemistry, catalysis, polymer chemistry, materials science,

engineering, environmental impact assessment, economics and policy. In future we will need

more materials made efficiently from natural wastes, suitable for recycling or biodegradation.

Acknowledgements

The Engineering and Physical Sciences Research Council EP/K035274/1, EP/M013839/1,

EP/L017393/1, EP/K014070/1) and CSC-Imperial College Scholarship (YZ) are

acknowledged for funding.

Authors Contribution

YZ and CR contributed equally to the review, CKW is the corresponding author.

Authors Information

Reprints and permissions information is available at www.nature.com/reprints

CKW declares she is a director of econic technologies, YZ and CR have no competing financial

interests.

Correspondence and requests for materials should be addressed to c.k.williams@Imperial.ac.uk

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