Catalysis as an Enabling Science for Sustainable Polymers

Review pubs.acs.org/CR

Catalysis as an Enabling Science for Sustainable Polymers Xiangyi Zhang,† Mareva Fevre,‡ Gavin O. Jones,‡ and Robert M. Waymouth*,† †

Department of Chemistry, Stanford University, Stanford, California 94305-5080, United States IBM Research−Almaden, 650 Harry Road, San Jose, California 95120, United States

‡

ABSTRACT: The replacement of current petroleum-based plastics with sustainable alternatives is a crucial but formidable challenge for the modern society. Catalysis presents an enabling tool to facilitate the development of sustainable polymers. This review provides a system-level analysis of sustainable polymers and outlines key criteria with respect to the feedstocks the polymers are derived from, the manner in which the polymers are generated, and the end-of-use options. Specifically, we define sustainable polymers as a class of materials that are derived from renewable feedstocks and exhibit closed-loop life cycles. Among potential candidates, aliphatic polyesters and polycarbonates are promising materials due to their renewable resources and excellent biodegradability. The development of renewable monomers, the versatile synthetic routes to convert these monomers to polyesters and polycarbonate, and the different end-of-use options for these polymers are critically reviewed, with a focus on recent advances in catalytic transformations that lower the technological barriers for developing more sustainable replacements for petroleum-based plastics.

CONTENTS 1. Introduction 1.1. Definition 1.2. Scope of Review 1.3. General Principles 1.3.1. Feedstocks, Monomers, and Polymers 1.3.2. End-of-Use Options 1.4. Challenges 1.5. Metrics Used to Evaluate Sustainability 1.5.1. Green Design Metrics (GDM) 1.5.2. Life Cycle Assessment (LCA) 1.5.3. Relation Between the Two Metrics 1.6. Commercial Examples 1.7. Opportunities and Challenges for the Development of Sustainable Polyesters and Polycarbonates 2. Renewable Monomers 2.1. Diacids, Hydroxyl Acids 2.2. Polyols 2.3. Cyclic Esters 2.4. Carbonates 2.5. Epoxides 3. Synthesis of Polyesters and Polycarbonates 3.1. Step-Growth Polymerization (SGP) 3.2. Chain-Growth Polymerization 3.2.1. Ring-Opening Polymerization (ROP) 3.2.2. Ring-Opening Copolymerization (ROCP) 3.3. Microbial Fermentation 4. End-of-Use Options 4.1. Mechanical Recycling 4.2. Chemical Recycling 4.2.1. Solvolysis 4.2.2. Thermal Recycling (Pyrolysis) © 2017 American Chemical Society

4.3. Biological Recycling 4.3.1. Anaerobic Bacteria 4.3.2. Aerobic Bacteria 4.3.3. Cells Extracts 5. Conclusions and Outlook Author Information Corresponding Author ORCID Notes Biographies Acknowledgments References Note Added after ASAP Publication

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1. INTRODUCTION The development of petroleum-based plastics is one of the crowning achievements of the 20th Century. Eighty years of commercial development have led to a family of materials whose low cost, processing versatility, and range of mechanical properties have made them the materials of everyday life, from clothing, cutlery, automobiles to healthcare, electronics, and the materials of our modern infrastructure. From 1.65 million tons in 1950 to 311 million tons in 2014 worldwide,1 plastics usage is expanding and expected to grow at a steady pace of 3−4% per year.2 The relatively low costs of energy, abundance of waste disposal sites, and an economic and cultural focus on the convenience of disposable items of commerce have fostered an

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Special Issue: Sustainable Chemistry Received: June 8, 2017 Published: October 19, 2017 839

DOI: 10.1021/acs.chemrev.7b00329 Chem. Rev. 2018, 118, 839−885

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science, an effort to provide a coherent system-level analysis to define sustainability and to identify the appropriate goals, indices, and metrics to assess whether a particular activity is sustainable.7−10 In this context, the definition of sustainable polymers should be considered at a system level, encompassing considerations such as the resources from which they are made, the manner in which they are produced, stored, distributed, utilized, and either recovered or otherwise released into the environment. Concepts from industrial ecology3 and circular economy11 can provide a useful framework for the definition of a sustainable polymer, that is a class of materials that exhibit closed-loop life cycles, such that they are components of a system where “resources” and “waste” are undefined as the waste from one process provides the resources for the next, in analogy to highly evolved natural ecosystems (type III ecology).3 Of course, any such system will require energy as an input but should be optimized to keep energy inputs at a minimum and optimally maximize the use of renewable energy. It should be appreciated that the definition of a sustainable polymer as members of a class of materials that exhibit closedloop life cycles (Figure 1, dotted lines) is not universally accepted; few existing plastics today would meet this definition. We have adopted this definition as an aspirational goal in the spirit of sustainable development.

industrial ecology based on a linear model of resource utilization (Resource-Monomer-Polymer-Product-Waste, Figure 1, black lines).3 Petrochemical feedstocks that provide the

Figure 1. Different models of industrial ecology for plastics industry and innovation leverage points (ILP) for the development of sustainable polymers with closed-loop life cycles.

1.2. Scope of Review

This review is framed in the context of the opportunities and challenges for the development of plastic materials that would exhibit closed-loop life cycles as part of a future industrial ecology where all resources, products, and “waste” form a regenerable system sustained only by the input of energy.3,7,9 Several excellent reviews on sustainable polymers have been published in the past few years.12−23 The majority focus on the use of renewable resources for feedstocks and monomers, particularly biomass-derived monomers and processes for the generation of polymers derived from these monomers.12,13,15,16,24−27 Several other reviews discuss the topic from the end-of-use perspective, covering different families of recyclable and biodegradable polymers.20,23,28−31 Herein, we attempt a system-level analysis, where all aspects of the plastics life-cycle are considered, which include the natural resources that provide the feedstocks and monomers, as well as the means of converting these to polymer products, their life cycles, and potential for these to exhibit closed-loop life cycles. We have framed opportunities and challenges in terms of “innovation leverage points” (ILP, Figure 1)32 where the development of new science and catalytic technologies might lower the technological barriers to achieving closed-loop life cycles. To develop a more cyclical model of resource utilization (Figure 1, dotted lines), new economic and technological innovations are needed at all steps of the plastics life cycle. For reasons we describe in more detail below, we focus the review on two classes of materials, polyesters and polycarbonates, as representative materials to illustrate key principles for the generation of sustainable polymers. Since catalysis represents an essential facet of sustainability and continues to drive the innovation in sustainable polymers, we highlight recent advances in the catalytic transformations of renewable feedstocks to monomers, monomers to polyesters and polycarbonates, as well as the chemical and biological recycling/degradation of these polymers. Different synthetic routes and catalytic systems will be compared and discussed in

source of modern plastics are typically byproducts of oil and gas refining; extraordinarily sophisticated and efficient catalytic technologies have been developed for converting these low cost feedstocks to plastics on a vast scale. Nevertheless, once introduced into the marketplace, the majority of these materials end up in the environment as waste. While the economic benefits of this industrial model of resource utilization have been significant in the past century, the environmental consequences are sobering. More than 300 million metric tons of plastic waste is generated every year, with 4.8 to 12.7 million metric tons entering the ocean, which has negatively impacted the marine environment.4 The vast scale of manufacturing and the environmental consequences associated with disposal have illuminated the limits to which the planet can cope with our current “take, make and dispose” model of resources utilization. Furthermore, fossilized carbon remains an abundant but finite natural resource; in the long term, the laws of supply and demand will lead to increasing costs and volatility for fossilized natural resources that provide the fuels and the attendant feedstocks that form the basis of the current plastics economy. These considerations have stimulated efforts to develop alternative classes of materials that are both more sustainable and not so closely tied to petrochemical resources. 1.1. Definition

In 1987, the United Nations introduced the concept of sustainable development as “addressing the needs of the present without compromising the ability of the future generations to meet their needs”.5 The concept of sustainable development6 has grown out of an appreciation that our planet is small, its natural resources finite. The scale of human technological activity has had a significant impact on economic development worldwide, but the attendant environmental impacts are becoming increasingly apparent. These considerations have spawned increased attention to sustainability 840


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and infrastructure for recovery and reuse are not nearly as well developed as those for production of plastics. As described in section 4, new innovations are needed to provide more technology options for plastics recycling. New strategies are needed for the selective depolymerization of plastics either to their constituent monomers or other useful intermediates that could be used as resources for other chemical processes (Figure 1, ILP 5−6). This strategy preserves part of the chemical complexity and energy43 that was invested to create the plastics. In order for this process to be energetically efficient, plastic materials should optimally be derived from polymerization reactions that are not strongly exergonic to ensure that, once generated, the materials can be catalytically depolymerized or chemically recycled with minimal energy inputs (ΔGp° ∼ 0, such that ΔGrxn = ΔGp° + RT ln Q can be optimized for polymerization or depolymerization). This latter criterion motivates our focus on polyesters and polycarbonates. Additionally, an effective catalytic system is needed to favor the kinetics and the selectivity of the depolymerization reaction to the desired feedstock. Even under the most highly optimized circular materials economies, some fraction of the products will eventually end up in the environment. To mitigate the environmental impact of plastics that end up in the environment, these plastics should be biodegradable. This requires engineering of either the polymer composition/formulation or its degradation pathway so that it can be broken down by microorganisms to primarily CO2 and H2O in a reasonable time frame.20 CO2 and H2O will re-enter the life cycle through photosynthesis to grow plants that serve as the biofeedstock of sustainable polymers. Although there is no direct correlation between a plastic’s ability to biodegrade and the resource from which it is derived, many biomassderived polymers have oxygen-rich backbones and are thus more readily degraded than the all-carbon backbones of most petroleum-based polymers. Such bioderived and biodegradable polymers are therefore emerging as attractive alternatives to conventional petroleum-based polymers. However, while biodegradable plastics can mitigate some of the environmental consequences of plastics that find their way into the environment, it is only one of the several end-of-life strategies to minimize the environmental impact of plastics. It is arguably not the optimal end-of-life option in that the energy invested to create these materials is wasted and provides benefit only to the microbes. The end-of-life challenges and opportunities are described in section 4.

the context of sustainability (i.e., atom efficiency, the use of green catalytic systems, and mild polymerization conditions). 1.3. General Principles

1.3.1. Feedstocks, Monomers, and Polymers. One guiding principle to sustainable development is that the rate of natural resource consumption cannot exceed its rate of regeneration if the system is to be sustainable in the long term.3,7,9 This has encouraged a focus on renewable resources, including those derived from regenerable biomass,12,13,15,16,24−27 as sources of feedstocks for the next generation of plastics (Figure 1). For bioderived feedstocks, the rate and time scale of CO2 sequestration can, in principle, be in balance with the use and release, resulting in an overall “neutral” carbon footprint. This is in contrast to the carbon cycle for petroleum feedstocks, in which the rate and time scale of CO2 sequestration (millions of years) is much slower than the use and release (1−10 year time frame).2 Natural polymers such as cellulose, hemicellulose, and lignin are among the most abundant sources of renewably sourced carbon.14,33,34 The physical properties of these raw materials can be enhanced by physical or chemical modifications, making them useful materials for a variety of applications in our daily life.35−37 Small molecules including vegetable oils, terpenes, and CO2 are also useful renewable raw materials.15,16,33 These raw materials can be further converted to value-added chemicals in integrated biorefineries, in analogy to that currently practiced in petroleum refineries. Biorefinery products such as ethanol, lactic acid, and glycerol are important building blocks for sustainable polymers. 14 Among the different biomass sources for biorefinery, the lignocellulosic feedstock biorefinery38 is attractive because of the availability of feedstock (e.g., agrofood wastes, or even paper wastes) at competitive prices and because the use of these feedstocks does not compete with the food supply. As the chemical composition of renewable resources are quite different from those derived from petroleum-based resources, new scientific and technological innovations are needed for the development of efficient catalytic strategies for converting these resources into platform chemicals, monomers, polymers, and products that can be produced economically on a scale sufficient for commodity materials (Figure 1, ILP1−ILP4). This remains a formidable challenge, as discussed in sections 2 and 3. 1.3.2. End-of-Use Options. New innovations are also needed to develop strategies for the recapture and reutilization of plastic materials at the end of their useful life. In the United States in 2014, over 75% of plastics wastes were landfilled,39 with the remainder being either recycled (primarily HDPE and PET) or incinerated.39,40 However, the amount of landfill space available for discarded plastic wastes is finite. Incineration of recovered plastic provides a potential source of energy39 but generates CO2; moreover, rigorous purification and process controls are needed to ensure that the hazardous substances are not released into the atmosphere.41 Petroleum-based thermoplastics such as PET and HDPE are currently recycled on a large scale through a sorting, melting, reprocessing process into useful products.42 The current recycling system requires a complicated sorting step which limits the types and amounts of plastic being recycled. Thus, while many petrochemically based polymers are recyclable, the current rates of plastics recycling are exceedingly low ( 60 °C) can be prepared by using a rigid diol such as isosorbide.214 The crystallinity and biodegradation rate of poly(alkylene carbonates) can be adjusted by employing diols with different chain lengths.215,216 Moreover, the use of multifunctional monomers such as glycerol and citric acid enables a facile synthesis of biodegradable thermosets.217 However, the step-growth nature of SGP leads to several drawbacks, including the difficult control of molecular weights and the inaccessibility to block copolymers or other sequencespecific polymer architectures. In addition, the removal of byproducts, water or alcohol, from the polycondensation reactions necessitates the use of high temperatures and oftentimes vacuum conditions, which makes this approach very energy-intensive.218 This also precludes the synthesis of thermally unstable polymers as well as complicates the use of volatile monomers.219 Side-reactions and evaporation of monomers occurring at these harsh conditions can result in a stoichiometric imbalance of reactants, which makes the synthesis of high molecular weight polymers very difficult. To address this problem, a two-step procedure is employed for many polycondensation reactions.215,220−222 In the first step, the esterification or carbonation byproduct (water or alcohol) is distilled off at ambient pressure to form oligomers. The reaction between these oligomers is promoted in the second step by slowly reducing the pressure and/or increasing the temperature. The elimination of the small-molecule byproducts generated from the chain end coupling drives the increase in chain length. This two-stage procedure has enabled the synthesis of polyesters and polycarbonates with molecular weights over 50 kDa.215,223 Metal salts such as titanium alkoxides, tin alkoxides, and zinc or magnesium carboxylates are commonly used catalysts for the step-growth synthesis of polyesters and polycarbonates.224−229 These catalytic polycondensations are typically conducted at very high temperature (>200 °C) and high vacuum to reach high conversion. For example, Ti(OiPr)4 can mediate the

chemistries. Polyesters and polycarbonates can be synthesized through a variety of routes summarized in Scheme 4. These routes can be divided into two classes: step-growth and chaingrowth polymerizations. The chain-growth polymerization methods include the ring-opening polymerization of a cyclic monomer and the ring-opening copolymerization of two monomers. In addition, a specific class of polyesters, PHAs, can be synthesized through microbial fermentation. In this section, different synthetic routes and catalytic systems are compared and discussed in the context of sustainability (i.e., the use of green catalytic systems, mild polymerization conditions, and high product selectivity). The advantages and limitations of each method are highlighted. 3.1. Step-Growth Polymerization (SGP)

The traditional way of synthesizing polyesters and polycarbonates is by step-growth polymerization. Polyesters are synthesized from polycondensation reactions of diols and diacids (or diesters) or hydroxyl acids. Polycarbonates are prepared by polycondensations between diols and a carbonyl source, traditionally phosgene. Although the step-growth method has been known since the discovery of synthetic polymers in early 20th century,212 it is still the most frequently used method for the synthesis of polyesters and polycarbonates in industry. Due to the wide availability of diols, diacids or hydroxy acids, SGP provides easy access to polyesters and polycarbonates with diverse backbone structures and side-chain functionalities. Given that many diols and diacids can be readily obtained from biomass (sections 2.1 and 2.2), SGP provides a straightforward route to renewable polyesters and polycarbonates. One such commercial example is biopoly(butylene succinate). Unlike chain-growth polymerizations, SGP is not restricted by the size and substitution pattern of the monomers, and it is particularly useful for the synthesis of polyesters and polycarbonates with long alkyl segments (>8 carbons) between each ester/carbonate moiety. For example, SGP of plant oils derived diols and diacids with >20 methylene sequences generates polyesters with structures and thermal properties reminiscent of PE. 213 These polyesters are otherwise 851


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Scheme 5. Synthesis of Polycarbonates via Direct Polycondensations of Diols and CO2

Many biobased carboxylic acids and alcohols contain multiple hydroxyl groups. The use of a chemoselective catalyst is critical to eliminate the need for tedious and uneconomical protection chemistry239,240 and to enhance the product specificity. Takasu found that scandium triflate (Sc(OTf)3) selectively esterifies primary hydroxyl groups over the secondary or tertiary alcohol.241 They developed a chemoselective polycondensation of tartaric acid or malic acid with polyols such as glycerol and sorbitol under mild conditions [60 °C, 0.3−3.0 mmHg, 0.5 mol % Sc(OTf)3] to afford linear polyesters (Mn between 4 and 26 kDa) with negligible branching or cross-linking. The pendant hydroxyl groups not only enable postpolymerization functionalization but also provide a handle to tailor the crystallinity of these aliphatic polyesters.242 Recently, there has been a surge of interest in developing organocatalytic processes for the synthesis of polyesters and polycarbonates due to their wide applications in biomedical devices, food packaging, and electronic devices. Several classes of organocatalysts have been used to catalyze the SGP of diols with diacids or carbonates, including strong Brønsted acids such as H2SO4, triflic acid, p-toluenesulfonic acid (PTSA), bis(trifluoromethanesulfonyl)-imide (Tf2NH),243,244 and organic bases such as TBD,221,245−247 MTBD,248 and 1-n-butyl-3methylimidazol-2-carboxylate.249 Kobayashi developed a unique catalytic system250−252 using a surfactant-like acid, such as dodecylbenzenesulfonic acid (DBSA), to mediate the emulsion polycondensation of aliphatic carboxylic acids and alcohols in water. Since the water byproduct is expelled out of the hydrophobic interior of micelles, this reaction can proceed effectively at low temperature (40−80 °C) and 1 atm. This mild and environmentally benign system has been successfully applied to several fatty-acid type monomers.253,254 To date, the activity of organocatalysts for SGP and the molecular weights of the resulting polyesters and polycarbonates are not yet comparable to the most efficient metal catalysts. SGP mediated

polycondensation of several bioderived diesters (succinate, adipate, and sebacate) with 1,4-butanediol under high temperature (up to 240 °C) and in vacuo to generate aliphatic polyesters with Mn > 20 kDa.225,230,231 Melt condensation of ωhydroxyl fatty acids [e.g., OH(CH2)13COOH], catalyzed by Ti(OiPr)4 via a two-stage polymerization (200 °C under N2 then 220 °C under 0.1 mmHg) generate polyesters with molecular weights ranging from 50 to 110 kDa.232 Meier recently used Sn(Oct)2 to polymerize dimethyl itaconate and long chain diols in the presence of 0.5 wt % of 4methoxyphenol as a radical inhibitor to make linear unsaturated polyesters with Mn up to 11500 Da.233 This was the first time that a high-molecular-weight linear polyester was produced from itaconic acid without isomerization or cross-linking of the vinylic double bond. Further modification of the resulting unsaturated polyesters by Michael addition yielded polyesters with different substituents dangling off the backbone, which provided a means to tune the thermal property of the materials.233 The development of catalytic processes that can operate at lower temperatures will reduce the energy input and thus increase the sustainability of the step-growth synthesis. Metal triflates have become a class of popular polycondensation catalysts for this reason. Because of their strong Lewis acidity, they can catalyze polycondensation reactions at much milder temperatures ( 100 kDa) with extremely narrow polydispersities (Đ < 1.1).310,311 Other metal catalysts such as zirconium complexes with bispyrrolidine-salan ligand312 and zinc complexes with chiral or achiral auxiliary ligands313−316 (Figure 8) have also been used to generate iso-enriched stereoblock PLAs from racLA. Another important bioderived and biodegradable plastics on the market is poly(3-hydroxyalkanoates) (PHAs). PHAs are naturally produced by various bacteria (section 3.3) but only isotactic microstructures are made biologically.317 Syndiotactic poly(3-hydroxyalkanoate)s (PHAs) have been synthesized by stereospecific ROP of racemic β-lactones by several rare-earth metal catalysts.318 This provides new possibilities for engineering the properties and applications for these plastics. 855


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Scheme 8. Representative Examples of Using Switchable Metal Catalysts for Monomer Sequence Control319,320,322

Scheme 9. Different Catalytic Mechanisms for the ROP of Cyclic Esters

and polycarbonates. Diaconescu reported several redox-switchable group 4 metal complexes supported by ferrocenyl-based ligand (Scheme 8A).319 They found that the reduced form was much more active toward LA than CL while the oxidized form

Recently, the use of switchable metal catalysts to strategically modulate the rate and monomer selectivity of ROP is emerging.267 This innovative catalytic strategy has enabled the facile one-pot synthesis of block copolymers of polyesters 856


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The synthesis of well-defined linear polyesters or polycarbonates with molecular weights over 100 kDa has been difficult using organocatalysts due to the competitive catalyst-initiated process at low alcohol initiator concentrations. Coulembier developed a zwitterionic ammonium betaine catalyst that exhibited high selectivity for the ROP of lactide.333 PLA of Mn = 97 kDa (DP = 750) with molecular weight conformed to the monomer-to-alcohol ratios and Mw/Mn = 1.18 can be synthesized by this catalyst. They proposed a charged-assisted hydrogen-bonding mechanism for this catalyst where it activates the alcohol initiator/chain-end through a combination of hydrogen bonding between the aryloxide group and the H(δ+) of ROH and the Coulomb attraction between the ammonium group and the O(δ-) of ROH (Scheme 10). Despite the weak basicity of aryloxide, these catalysts showed good reactivity toward L-LA owing to the supplementary ionic activation.

had the opposite selectivity. This distinct substrate selectivity allowed the one-pot copolymerization of the two monomers by in situ switching of the redox state of the catalyst to synthesize poly[(LA-minor-CL)-block-(CL-minor-LA)]. In the similar vein, Byers developed a chemoselective copolymerization of LA and epoxides using a redox-switchable iron catalyst (Scheme 8B).320 Unlike the previous case, the two monomers exhibited orthogonal reactivity with one being completely inactive during the polymerization of the other. This allowed the formation of clean diblock copolymers. Similar copolymerization of LA and epoxides were obtained using a redoxswitchable zirconium complex.321 Williams developed a strategy using CO2 as a chemical switch as well as a comonomer for the one-pot chemoselective polymerization of epoxide and CL (Scheme 8C).322 They found that the dizinc acetate catalyst was an excellent catalyst for the ring-opening copolymerization of epoxide and CO2 but not for the ROP of CL. But as soon as CO2 was removed, the catalyst turned on the ROP of CL with no incorporation of epoxide. Thus, repetitive introduction and removal of CO2 resulted in the formation of multiblock poly(carbonate-block-ester). Recently, this approach was extended to a monomer mixture of epoxide, lactone, anhydride, and CO2 to synthesize a variety of block copolymers with great sequence control and predictable compositions in the polymer chains.323 The replacement of metal catalysts with organic catalysts has emerged for the synthesis of plastics, especially for those used in biomedical and microelectronic applications. Since the pioneering work of Hedrick in 2001,324 the application of organic catalysts to ROP has witnessed significant progress in the last 15 years and this field has grown to the point that it now provides a powerful and, in some cases, better alternative to the use of traditional metal-based catalysts.325,326 Several attributes of organic catalysts, including their low cost and wide availability, easy removal from polymers, low toxicity, and versatile catalytic mechanisms, make possible a greener and more versatile synthesis of polyesters and polycarbonates. Depending on the nature of the catalyst, organocatalysts can operate through a variety of mechanisms (Scheme 9).326 Organic bases such as N-heterocyclic carbenes (NHCs) and amidines can activate the alcohol initiator/chain ends through hydrogen bonding interactions. Alternatively, they can activate the monomer through nucleophilic addition. This has led to the development of zwitterionic ring-opening polymerization for the synthesis of high molecular weight polyesters327 and polycarbonates284 with cyclic topology. Organic acids such as sulfonic or phosphoric acid can catalyze the polymerization through electrophilic activation of monomers. Catalytic systems combining both H-bond donor and H-bond acceptor moieties, such as thiourea/amine, can simultaneously activate the monomer and alcohol initiator/chain end. These systems have proven especially selective for chain propagation over transesterification, affording narrowly dispersed polyesters and polycarbonates. The diversity in mechanistic pathways not only provides new opportunities for enhancing the rate and selectivity of polymerization but also enables the synthesis of polymer architectures that are difficult to access by metalmediated processes. Many review articles have covered different aspects of this topic.325−332 Herein, we aim to highlight some recent catalytic developments that feature improved productivity and/or higher level of control on the macromolecular or microstructural parameters of polymers.

Scheme 10. Proposed Interactions between Ammonium Betaine and Alcohol Initiator/Chain End333

Recently, N-heterocyclic olefins (NHOs) have emerged as a class of active and versatile catalysts for the ROP of cyclic esters and carbonates.334,335 These catalysts can function by different mechanisms: activating the alcohol initiator through hydrogenbonding (Scheme 11A), deprotonating monomers to form enolate-type initiator (Scheme 11B), or activating monomer through nucleophilic pathway (Scheme 11C). Their catalytic behavior largely depends on the ring size, (un)saturation of the backbone, and the steric environment of the exocyclic carbon. While nonsubstituted NHOs (at the exocyclic carbon, NHO1,3) were ineffective for ROP due to catalyst deactivation (Scheme 11C), NHO bearing dimethyl substituents (NHO-2) proved highly active for the ROP of LA and VL.334 In the presence of an alcohol initiator, the ROP of 50 equiv. L-LA by as low as 0.2 mol % NHO-2 reached quantitative conversion in less than a minute, resulting in PLAs with low molecular weight distributions (Đ < 1.2). Under the same conditions, the ROP of VL is fast but poorly controlled due to the more significant enolate-based initiation. Chen recently combined NHOs with a Lewis acid Al(C6F5)3 to develop a highly efficient catalytic system.335 The living polymerizations of VL and CL mediated by NHO-1/Al(C6F5)3 generated linear polyesters with high molecular weights up to 855 kDa (DP = 3200) and narrow molecular weight distributions (Đ = 1.03−1.63). End-group analysis revealed an imidazolium enol-like chain end of the final polymers, indicative of an intramolecular proton transfer within the zwitterionic intermediates generated from the nucleophilic addition of NHOs to the monomer (Scheme 11D). The high activity and the ability to operate through different polymerization mechanisms make NHO an attractive motif for future catalyst design and optimization. Typical nucleophilic organocatalytic ROP is subject to a trade-off between rate and selectivity: the most active catalysts (i.e., NHCs, TBD) tend to promote side reactions such as chain transfer and epimerization especially at high conversions 857


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Scheme 11. Structures of NHOs and Their Different Catalytic Pathways for the ROP of Lactones334

while the most selective systems, such as TU/amines, suffer from slow kinetics. Recently a class of (thio)urea anions catalysts were disclosed that exhibit both high activity and selectivity for the ROP of a variety of cyclic esters and carbonates (Scheme 12).321,336 These ionic catalysts are

prepared by simple deprotonation of the neutral ureas or thioureas. The rate of the polymerization can be tuned over several orders of magnitude by modifying the urea or thiourea substituents. The urea anions are more active than the corresponding thiourea anions, with the ability to polymerize all tested monomers in just seconds (70%) of recycled PET (rPET) is used for production of lower-grade products, mainly polyester fibers, and only a small fraction (∼10%) is used for closed-loop bottle-to-bottle recycling.394 This is due to a common downgrading problem of mechanical recycling, that is, the recovered plastics tend to lose some quality attributes of the virgin plastics, such as color, clarity, or mechanical properties.395 Since closed-loop mechanical recycling requires the polymers to be effectively separated from sources of contaminations (other plastics, metals, pigments, adhesives, etc.) and stable during reprocessing (grinding and melting), the only postconsumer plastic wastes that have routinely been recycled in a closed-loop fashion are clear PET bottles and more recently HDPE bottles.388 Mechanical recycling of PET comprises several key steps: collection, sorting, size reduction and cleaning, further separation, and drying.388 Sorting of mingled recyclables are

4.2. Chemical Recycling

Chemical recycling has attracted increasing scientific and commercial attention as an alternative with the potential to absorb and circulate very large amounts of plastic wastes. The application of catalysis is a key to the success of chemical recycling and is expected to put this technology at the forefront of plastic management for sustainable polymers. 4.2.1. Solvolysis. Solvolysis using water, methanol, and glycol has been the most common chemical recycling strategy for step-growth polyesters and polycarbonates.41,397 A number of companies have implemented PET depolymerization at pilot or commercial level, including Loop Industries,398 Carbios,399 Ioniqa,400 and Gr3n Project.401 Different solvolysis processes of PETs and bisphenol A (BPA)-type PCs have been developed to recover suitable monomers for the production of new PETs and PCs.402,403 However, the industrial applications of this recycling approach are quite limited due to the higher cost of recycled monomers compared to virgin feedstocks. The 863


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Scheme 18. One-Pot Chemical Recycling and Repurposing of PET

Scheme 19. One-Pot Chemical Repurposing of PC

strategy was applied to BPA-PC.416 Postconsumer compact discs, mainly composed of PC, was depolymerized with ethylene carbonate and repolymerized with isosorbide and succinic acid to generate copolyesters for coating applications. Recently, a quantitative one-step transformation of PC into high-value poly(aryl ether sulfone)s in the presence of a carbonate salt was developed as another strategy for repurposing these plastic wastes.417 This approach uses a cascade reaction involving the depolymerization of PC by K2CO3 and formation of reactive phenoxides, which are then polycondensed in situ with bis-fluorinated aryl sulfones to form poly(aryl ether sulfone)s (Scheme 19). Computational studies revealed that this reaction proceeds by the nucleophilic attack of the metal carbonate on the PC and the evolution of CO2 gas as the byproduct. Both the metal carbonate and PC are degraded in this process, which leads to the formation of a metal phenoxide salt that is activated for subsequent reaction with the aryl fluoride. The computational investigation showed that stoichiometric amounts of metal carbonate are needed for the reaction to proceed to completion, but insights provided by investigating the mechanism also led them to develop a catalytic process using 12 mol % K2CO3 with lower reaction efficiency. The acid- or base-catalyzed hydrolysis of aliphatic polyester and polycarbonates has also been widely explored, the results of which have been used to guide the design and application of these polymers for therapeutic delivery and medical implants.418 The overall degradation process is a complicated interplay between the intrinsic degradation kinetics and the diffusive rate of water to the polymer.419,420 The rate of water diffusion depends on the hydrophobicity of the polymer and its shape or surface area while the intrinsic degradation kinetics is affected by the pH421 and temperature of the medium422 as well as the molecular weight423 and microstructures of the polymer.424 The degradation of polyesters and polycarbonates is in general much faster under basic conditions than acidic conditions and under acidic conditions than neutral con-

depolymerization is typically catalyzed by acids or bases. For example, the alkali-catalyzed methanolysis of PC in THF at 40 °C provides BPA and dimethyl carbonate in over 95% yield within 35 min.404 Achilias developed an efficient depolymerization method of PET and PC into their monomers and oligomers under microwave irradiation catalyzed by NaOH.405,406 Glycolysis of PET by a variety of metal or organic catalysts yields bis(2-hydroxyethyl)terephthalester (BHET) as the major product.403,407 Computational chemistry provides a useful tool to study the depolymerization mechanism, which can in turn guide the development of new recycling strategies. In one example of glycolytic depolymerization of PET by TBD or DBU, Horn et al.408 showed that the rate-determining step for the depolymerization is the ethylene glycol involved nucleophilic attack. The calculations suggest that these reactions likely involve a bifunctional mechanism in which the alcohol reactant activates the carbonyl group of these polyesters while the organocatalyst activates the reacting alcohol. The insights derived from this study led to the development of a variety of amine-based catalytic systems for PET depolymerization.409 The catalyst-free depolymerization of PET and PC in supercritical alcohol410,411 or high-pressure steam412,413 has also proven efficient for recovering high-quality monomers equivalent to the virgin monomers produced from petroleum. In addition to monomer recovery, repurposing of waste plastics into other valuable feedstocks or into value-added polymeric materials is an attractive option for plastic recycling. Colonna reported a two-step one-pot strategy for converting PET to new polyesters using monomers available from renewable sources.414 In the first step, PET was depolymerized using isosorbide to oligomers, which were chain extended by succinic acid in the second step to form a novel copolyester (Scheme 18). Monobutyltin oxide was identified as the most active catalyst for both steps and resulted in minimal discoloration. The obtained copolyesters demonstrate properties suitable for powder coating applications.415 A similar 864


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Scheme 20. Different Chain-Scission Mechanisms for the Hydrolysis of PLA Catalyzed by an Acid or Base

Scheme 21. Catalytic Alcoholysis of PLA to Generate Value-Added Lactate-Type Esters

ditions.421 Jung compared the hydrolytic rate of poly(D,L-lactic acid) (PLA), poly(ε-caprolactone) (PCL), and poly(propylene carbonate) (PPC) at various pHs (1−13).425 They found that PPC was less susceptible to degradation in acidic medium than PCL and PLA, with increased degradation rate as PPC ≪ PLA < PCL. In contrast, PPC was more readily hydrolyzed in basic conditions, with increased degradation rate being PCL ≪ PLA < PPC. The mechanism of chain scission has been debated for a long time. A random backbone scission and a chain-end scission (“unzipping”) mechanism were proposed for these polymers. Although the two mechanisms operate simultaneously during hydrolysis, several studies have found that the chain-end scission is significantly faster than the random backbone cleavage.424,426,427 This is attributed to the hydrophilic nature of the COOH and OH chain ends compared to the hydrophobic backbone. A detailed chain-scission mechanism for PLA has been established based on model studies of PLA oligomers.421,424,428 Interestingly, the exact chain-end scission mechanism depends on the pH of the medium (Scheme 20). While at acidic pH, the hydrolysis proceeds through a preferential scission of the end groups and spits out one lactic acid unit at a time; the base catalyzed hydrolysis proceeds preferentially via a backbiting process from the OH chain end leading to the formation of lactide, which is eventually hydrolyzed to lactate. These mechanisms are supported by the fact that when the OH chain end is capped via acetylation, the degradation rate significantly decreases and the relatively slow random chain scission becomes dominant.428 The conversion of aliphatic polyester wastes to value-added small molecules and polymer precursors has been welldemonstrated in the alcoholysis of polyesters, commodity PLAs in particular. It provides a strategy for the production of industrially useful small esters (Scheme 21). For example, alkyl lactates, the alcoholysis products of PLAs, are green substitutes for petroleum-derived esters in applications such as solvents429

or diluents for polymeric resins,430 low-temperature lubricants,431 and emulsifiers or dispersants in pharmaceutical compositions.432 The alcoholysis of PLAs have been achieved under different conditions,433−436 and the efficiency of these processes is highly condition-dependent. In the early 1990’s, Du Pont developed a method using 2−3 equiv of alcohol (ROH for R = Me, Et, Bu) per polymer unit at 150−190 °C with H2SO4 as the catalyst. 69−87% conversion of highmolecular-weight PLA to lactates is achieved within 2 h.437,438 Leibfarth reported a very efficient way to recycle PLAs and PGA to small esters using TBD as the catalyst.439 With only 1.5 equiv of alcohol per ester bond, quantitative alcoholysis to small esters was observed in only 2 min at room temperature with nearly complete retention of stereochemistry. The functional group tolerance of TBD allows the introduction of different polymerizable groups (e.g., olefin) to the ester products (Scheme 21), which provides a number of opportunities in the production of new polymer materials. The alcoholysis of PLA under catalyst-free, solvothermal conditions (4 equiv of ROH, 200−260 °C) or catalyzed by metal salts (2 equiv of ROH, 140−200 °C) were systematically investigated by Sobota.440 The presence of catalyst enables milder reaction conditions and among the screened metal catalysts, magnesium and calcium alkoxides generated in situ from the metallic precursors and alcohols gave the best yields (>90%) and negligible epimerization. Compared to the traditional bulk production of lactate esters based on the esterification of lactic acid,441,442 these catalytic alcoholyses of PLAs provide a number of advantages, including starting materials derived from PLA wastes, reduced reaction time and high yield, simple purification, and retention of stereochemistry. In addition to the production of small-molecule esters, alcoholysis of PLAs by a substoichiometric amount of multifunctional alcohols or amines can degrade the polymer chains to lower-molecular-weight telechelic polyols that can be 865


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20 ppm, the degradation proceeded mainly via random intraand intermolecular ester exchange, generating a significant amount of diastereomers and cyclic oligomers.448 In contrast, for samples with Sn > 480 ppm, the degradation kinetics indicated an unzipping depolymerization mechanism along with minor bimolecular transesterification reactions, leading to the predominant production of L-LA.448,449 Acetylation of the OH chain end significantly increased the polymer stability and impeded the thermal recycling, consistent with an unzipping mechanism initiated from the OH chain end.444 Other transesterification catalysts have been used for the depolymerization of PLA.450−453 Noda compared the catalytic efficiency of a series of metal alkoxide, enolate, halide, and oxide salts for the monomer recycling of PLLAs at 190−245 °C under 4−5 mmHg. They found that the yield and selectivity for L-LA were dependent on the nature of the metal and increased in an order of Al < Ti < Zr < Zn < Sn. In the presence of 1 wt % Sn(Oct)2 or zinc naphthenate, L-LA was recovered within 2 h in ∼90% yield and over 95% selectivity.450 Selective thermal recycling of other thermodynamically stable polyesters and polycarbonates has also been reported.454 For example, the thermal depolymerization of PCL and poly(2,2dimethyl trimethylene carbonate) can be achieved in bulk at temperature around 250 °C in the presence of a transesterification catalyst. The use of Bu2Sn(OMe)2 or NaOH facilitated a nearly quantitative depolymerization to CL, while Ti(OiPr)4 gave almost equal amounts of CL and its cyclic dimer. When the depolymerization of PCL was performed in toluene with Bu2Sn(OMe)2 at 110 °C, the products were, however, a mixture of cyclic oligomers with little formation of CL.455 These studies once again highlight the importance of the choice of catalyst and reaction conditions to achieve selective depolymerization to monomer. The recycling of long chain polyesters derived from stepgrowth polymerization of ω-hydroxy fatty acids is an important aspect of increasing the degree of sustainability of these renewable polymers. In addition to recovering the linear repeat units by hydrolysis, the depolymerization of these polymers through either thermal depolymerization or solution depolymerization to specific cyclic oligomers provide an attractive option since the resulting cyclic oligomers are difficult to synthesize through traditional cyclization methods and they can be repolymerized to the original polymers through entropy-driven ROP180 in a more controlled fashion than the step-growth method. Back in 1939, Carothers has described the selective depolymerization of polydecanoate, polytridecanoate and polytetradecanoate at 270 °C and 1 mmHg pressure using MgCl2 or SnCl2 as the catalyst.456,457 Depending on the starting polyesters, either cyclic monomer or dimer were obtained in moderate yield (50−70%) and selectivity (50−90%). Various families of macro-lactones have later been synthesized via the depolymerization of poly(hexamethylene succinate), poly(tetraethylene glycol succinate), poly(ω-hydroxy acids) ([−(CH2)x−1-CO2-]n, x = 8, 10, 11, and 12).458,459 Hodge showed that the depolymerization of polyundecanoate in 2% w/v chlorobenzene in the presence of a tin catalyst produced 90% yield of cyclic oligomers with cyclic dimer being the major product (56%). The isolated cyclic dimer can undergo ROP in neat with a tin catalyst to regenerate polyundecanoates. The molecular weights of these polymers can be significantly higher than the original step-growth polymers.459 Recently, Chen discovered appropriate thermodynamic and catalytic conditions to polymerize the low-strain γ-BL into high

used for a variety of industrial processes. Plichta showed that the degradation of PLA with less than 10 wt % diols or diamines occurred readily in xylene at reflux or in bulk at 180− 200 °C in the presence of Sn(Oct)2, affording hydroxylterminated PLAs with molecular weights significantly lower than the pristine samples.443 The diols or diamines were quantitatively incorporated into the degradation products. This strategy provides a facile access to polymers with various architectures. For example, the use of multihydroxyl compounds such as dipentaerythritol results in star polymers with six short arms of PLA. When macrodiols such as PEG are used, a variety of triblock copolymers can be synthesized. However, this method exerts little control over the chain length of each block as the insertion of the diols to the PLA is random. In all cases, the products obtained are not pure triblock structures but contain some molar fraction of homopolymers of PLA. Nevertheless, this study presents a practical method of chemical repurposing of PLA to polymers and oligomers that can be useful precursors for other processes. 4.2.2. Thermal Recycling (Pyrolysis). While monomer recovery cannot be achieved through direct pyrolysis for stepgrowth polyesters and polycarbonates, this is an attractive recycling option for chain-growth polymers. An emerging frontier in sustainable polymers is the design and synthesis of thermally recyclable polymers that can be depolymerized to their monomers in high yields, which can then be reused to produce virgin quality polymers.23 However, controlled depolymerization through chain-end unzipping (the exact reverse process of chain-growth) depends on the thermodynamics of the polymerization: for polymerization reactions that are highly exergonic (ΔG°p ≪ 0) at normal operating temperatures, the reverse processes are energy intensive under similar conditions. Even so, in many cases the depolymerization may be kinetically slow or unselective, requiring the use of a catalyst. Catalysts developed for polymerization can also be used for depolymerization as they reduce the kinetic barrier of both polymerization and depolymerization to the same extent. It will be seen from the examples below that the use of a proper catalyst not only allows milder depolymerization conditions but also can improve the selectivity of depolymerization reactions. For polyesters and polycarbonates with large polymerization enthalpies, a simple pyrolysis process leads to undesired products. For example, pyrolysis of PLLA at ambient pressure leads to uncontrolled degradation through various nonradical and radical pathways that yield a mixture of products consisting of L-LA, meso-LA, cyclic oligomers, acetaldehyde, acrylic acid, CO, and CO2.444−446 To drive the equilibrium toward the formation of monomer and suppress other unwanted degradation reactions, the thermal recycling is often performed at lower temperature and reduced pressure to distill the volatile monomer from the polymer melt. The use of a transesterification catalyst is a key factor in promoting the selective depolymerization under milder conditions. Kopinke et al. compared the thermal degradation of PLAs in the absence and presence of Sn(Oct)2.446 While the degradation without the assistance of catalyst was slow and uncontrolled, the presence of residual Sn (from the polymerization process) reduced the depolymerization temperature by 60 °C and increased the selectivity for lactide recovery. Several groups further studied the effect of the Sn concentration on the depolymerization efficiency. It was shown that both the depolymerization rate and selectivity increased with Sn concentration.447−449 For Sn < 866


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ized by a highly active catalyst TBD in bulk at room temperature to obtain a rubbery polyester,461 which can then be selectively depolymerized back to MVL in 97% yield at 225 °C, 100 mTorr.161 On the basis of this chemistry, they developed chemically recyclable thermoplastic polyurethanes161 and novel polyester elastomers.162 The former case exploited the reversibility of the urethane bond to generate the OH end group to initiate the depolymerization of PMVL block while in the latter case a small amount of alcohol is intentionally added to the elastomers to form the OH chain end for recycling. In both cases, the thermal depolymerization of the materials leads to complete recovery of MVL. Another monomer, benzodioxipinone (BDP, Figure 11), was utilized by Shaver to establish a clean and selective polymerization and recycling process using an aluminum-salen catalyst.462 The resulting polyesters with integrated aromatic groups are potential recyclable alternatives for commodity polyesters like PET. These examples clearly demonstrate the advantage of using monomers with low polymerization enthalpies for the feedstock-recycling process. However, their exceptional recyclability comes with several intrinsic drawbacks. First, because of the low polymerization enthalpy of the monomer, the polymerization reactions and formulation/processing have to be performed at low temperature (often below room temperature) in order to achieve decent yields. This increases the energy cost of the products. Second, the polymers are intrinsically unstable and tend to degrade in the environmental stresses such as heat or moisture. The low thermal stability of these materials could limit their practical use and shelf life. While the closed-loop recycling of PHA can be realized by several biotic pathways,463,464 the chemical recycling of PHAs to the constituent monomers can provide a time-efficient option.375 Thermal recycling is the most common approach to recycle PHAs. While uncatalytic pyrolysis performed by heating the polymer to 500 °C under vacuum or N2 leads to the formation of PHA oligomers, crotonic acid, and several volatile side products (Scheme 22A),465 catalytic pyrolysis provides a more efficient and selective degradation behavior. For example, the use of MgO or Mg(OH)2 as the catalyst enables a selective thermal degradation of PHB to trans-crotonic acids (Scheme 22B).466 The presence of catalyst not only allowed the pyrolysis to be conducted at lower temperature than without a catalyst but also suppressed the formation of oligomeric species. Similar trend was observed for Ca2+ containing PHB.467 It is proposed that the Lewis acidic nature of Ca2+ and Mg2+ facilitates the activation of the six-membered transition state in the unzipping depolymerization of PHA (Scheme 23), and thus leads to a more selective degradation process than in the absence of

molecular weight poly(γ-BL)s with cyclic or linear topologies (Figure 11).278,460 At −40 °C and ambient pressure, the ROP

Figure 11. Chemical recycling of monomers with low enthalpies of polymerization.

of γ-BL proceeded smoothly in the presence of a catalyst (La[N(SiMe3)2]3 or tert-Bu-P4) to high conversions within 24 h, generating poly(γ-BL) with Mn up to 30 kDa and Đ ∼ 2.0. The topologies of the products can be controlled by the La[N(SiMe3)2]3/ROH ratios, with excess ROH favoring the formation of linear chains. A clean and quantitative thermal recycling of these polymers was achieved by simply heating the bulk material at 220 °C (for the linear polymer) or 300 °C (for the cyclic polymer). Alternatively, the polymer can readily be depolymerized back to the monomer at room temperature in the presence of a metal or organic catalyst. Chen later extended this recycling strategy to α-methylene-γ-butyrolactone (MBL), which is even more challenging to polymerize than γ-BL due to the additional complication of controlling the chemoselectivity between a reactive exocyclic olefin and a stable five-membered lactone.177 Using similar conditions (−60 °C, Ln/ROH = 1/3) to that of γ-BL, they were able to achieve the selective ROP of MBL to produce poly(MBL) with Mn up to 21 kDa, which can be facilely recycled back to the monomer MBL by heating the polymer ≥100 °C for 1 h in the presence of catalysts. These olefin containing polyesters present a class of useful polymer precursors for cross-linking or thiol-addition to make functional thermoplastics and thermosets. Hillmyer and Albertsson have independently measured the thermodynamics of a variety of substituted lactones and elucidated the effect of the sterics and position of the substituent on the thermodynamics of ringopening.274,275 In general, substituted lactones have relatively low polymerizability compared to the unsubstituted ones and are thus potential candidates for thermal recycling. Hillmyer found that β-methyl-δ-valerolactone (MVL) can be polymer-

Scheme 22. Thermal Degradation of PHBs in the Absence and Presence of a Catalyst

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Scheme 23. (A) Chain-End Unzipping Mechanism for Selective Thermal Degradation of PHB and (B) Influence of Mg2+ on the Unzipping of PHB

catalysts. This catalytic pyrolysis strategy (using Mg(OH)2 as the catalyst) was used to recycle a variety of short- and medium-chain length PHAs and further proved the utility of the recovered monomers as feedstocks for the microbial synthesis of PHAs.375,381 In this way, a time- and energyefficient recycling between monomers (hydroxyalkanoate or alkenoic acids) and PHAs is established. 4.3. Biological Recycling

Microbial degradation has attracted considerable interest, as bacteria are responsible for any degradation occurring to plastics that end up in the environment,75,76 as well as in industrial composting settings. The first report of microbial degradation of polyesters dates back to the early 1960’s. Merrick and Doudoroff et al. noticed that polyhydroxybutyrate (PHB) “granules” isolated from microbial cell extracts could be hydrolyzed to β-hydroxybutyric acid while the polymer was incubated with part of the cell extracts it had been purified from.468,469 Since then, numerous studies have focused on the degradation of aliphatic polyesters and polycarbonates by microbes (mainly bacteria, yeast, and fungi) or cell extracts from these organisms, in laboratory settings or in the environment (e.g., marine470 or soil degradation471). The occurrence of polyesters-degrading microorganisms in the environment has been classified as follows: PCL = PHB > PES (polyethylene succinate) > PBS (polybutylene succinate) > PLA.472 The structure of the aliphatic polyesters and polycarbonates which have been shown to be susceptible to biodegradation are depicted in Figure 12. Most experiments have been performed in aqueous settings (i.e., aqueous solution, sludge, or soil) with water-insoluble polymers, even though organic solvents have also been used. In all cases, enzymes, which are naturally occurring in microbes (i.e., hydrolases), catalyze the material’s hydrolysis. These enzymes are either esterases, which hydrolyze esters bonds, or proteases, which are used by microbes to cleave peptidic bonds. The most commonly studied protease is proteinase K, which uses serine as a nucleophile. Esterases relevant to the field of biodegradation of polyesters and polycarbonates, on the other hand, can be mainly categorized into lipases, which cut esters of fats or PHA-depolymerases, which hydrolyze PHAs. These enzymes were shown to exhibit different efficacies depending on the nature of the ester/ carbonate bonds and are summarized in Figure 13.471,473 Since most aliphatic polyesters and polycarbonates are hydrophobic,

Figure 12. Aliphatic polyesters and polycarbonates amenable to depolymerization or mineralization by microbial systems.

Figure 13. Classification of the enzymes that trigger the depolymerization of aliphatic polyesters and polycarbonates and polymers against which activity has been demonstrated.

enzymes exhibit a substrate binding domain (SBD) in addition to their catalytic domain, which facilitates the adsorption of the enzyme on the water-insoluble polymers. For the same reason, the degradation of polyesters and polycarbonates by enzymes generally proceeds through surface erosion: the bulk of the material is not affected and the properties (e.g., molar mass, crystallinity, etc.) are maintained, while lower molar mass fragments are released from the surface. 868


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soil bacteria in aquatic conditions (up to 90% after 60 days for PLA and 50% after 40 days for PLLA).477−479 Moreover, faster mineralization of PLLA was demonstrated in thermophilic conditions because of the close proximity of experimental temperatures to the Tg of PLLA.479 PCL and PHB could also be biodegraded in similar settings (up to 90% degradation after 14 days for PHB), while PBS remained mostly intact (3% degradation after 96 days).478 In contrast, Massardier-Nageotte et al. reported that their inoculum was inefficient to biodegrade both PLA and PCL in anaerobic conditions, evidencing that the experimental settings, as well as the origin of the inoculum, have a tremendous impact on the rate of degradation.480 Overall, anaerobic microbial degradation of aliphatic polyesters and polycarbonates remains poorly studied, despite several advantages such as high efficiency, especially for PLA and PHAs, and fast rates. 4.3.2. Aerobic Bacteria. Aerobic degradation tests are either performed in composting or aquatic conditions, mostly using soil inocula. Because the microorganism has access to O2 in aerobic conditions, the end-products of mineralization are CO2 and H2O, instead of CO2 and CH4 in anaerobic conditions. The mineralization of polyesters has been evaluated using soil (i.e., compost) or diluting the inoculum in growing medium enriched with minerals and the rates decreased as follows: PHAs > PCL > PLA.481−484 Similarly to anaerobic conditions, the nature and provenance of the inoculum, as well as the experimental setup, considerably influence the biodegradation’s efficiency. For instance, Enoki et al. isolated Pseudomonas, Alcanivorax, and Tenacibaculum, which were able to partially degrade PCL and PHB fibers in deep seawater. They found that the bacteria, after isolation, were more efficient in experimental conditions close to their natural occurrence [i.e., low temperature (4−10 °C) and high pressure (30 MPa)].485 A variety of soil microorganisms have also been isolated and proved efficient for the mineralization of PLLA on agar plates or in growing medium.486,487 Depending on the experimental conditions, the isolation of the microorganisms/enzymes that are thought to be responsible for the polyesters degradation can be beneficial or detrimental. For instance, the mineralization of PHVB was shown to be faster in fertile soil versus isolated enzymes, owing to a more diverse population of degrading enzymes in the soil.484 In contrast, Fukushima et al. found that the isolation of microorganisms was advantageous to the mineralization of PLA and PLA nanocomposites.488 Moreover, the rate of degradation was found to be faster for PLA nanocomposites than for pure PLA, owing to the presence of hydroxyl moieties at the surface of nanoclay that can act as nucleophiles. Finally, even though most polyesters and polycarbonates are mineralized by microorganisms, Lee et al. showed that enantiomerically pure (R)-(−)-3-hydroxycarboxylic acids could be produced by in vivo depolymerization of P3HB, both in anaerobic and aerobic conditions, in Alcaligenes latus.489 They did so by suppressing the intracellular activity of (R)(−)-3-hydroxybutyric acid dehydrogenase, which allows metabolizing (R)-(−)-3-hydroxycarboxylic acids to acetoacetate. A higher yield was observed when the bacterium was deprived of nutrient and of light in order to block normal pathways. This contribution elegantly showed that useful chemicals that could be reused as monomers or in other chemical industries could be the end-products of microbial degradation. Ren et al. later extended this concept to a broader

Microorganisms degrade polymers to use the degradation products as carbon and/or energy source(s). High molar mass polymers are unable to penetrate the membrane of microbial organisms and so are not as susceptible to microbial attack by intracellular enzymes. Therefore, microbes first express extracellular enzymes, in order to decrease the polymer’s molar mass and allow for solubilization and assimilation of the oligomers through their cellular membrane. When the chains are short enough to be soluble in the media and cross the membrane, further degradation/digestion might take place with the help of intracellular enzymes. If the degradation stops at the monomeric or oligomeric stage, the process is then called depolymerization, while mineralization is a process that further transforms the monomeric units into CO2, H2O, and/or CH4 as end-products. The working mechanism of microorganisms on hydrophobic polyesters and polycarbonates is summarized in Figure 14. Noteworthy, when cell extracts are used to depolymerize water-insoluble polymers, only steps 2 and 3 are observed (see Figure 14).

Figure 14. Biodegradation of a water-insoluble polymer by a bacterium (if purified enzymes are used, only steps 2 and 3 are observed).

Various test methods have been developed to assess the efficacy of the biodegradation, including, just to name a few, weight loss measurements, quantification of the amount of produced CO2 and/or CH4, analysis of the surface of the polymer after exposure to microorganism/enzyme by microscopy, and analysis of the supernatant by spectroscopy or chromatography.474,475 The efficiency of microorganisms at biodegrading plastics have been tested in anaerobic (no O2) or aerobic conditions (i.e., composting), as well as mesophilic (20−45 °C) or thermophilic (≈ 55 °C) conditions. Most of these experiments have been performed on a lab scale using soil or sludge collected from waste treatment plants. 4.3.1. Anaerobic Bacteria. PLA, PCL, and polyhydroxyalkanoates (PHAs) have been shown to be susceptible to mineralization in anaerobic conditions. The products of anaerobic bacterial digestion are energy, which is used by the microorganism, and CO2, CH4, and biomass. Kim et al. have studied the biodegradation of PLA with different mesh sizes with an initial weight content of PLA versus media of 10 wt % and reported that the production of CO2 and CH4 could go up to 0.21 mmol/day m2 and 0.24 mmol/day m2, respectively, for the smaller PLA particles.476 Kunioka et al. and Itävaara et al. showed that PLA and PLLA, respectively, could be degraded by 869


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4.3.3.2. Lipases. Lipases are the most-studied enzymes for the hydrolysis of polyesters and polycarbonates as they degrade a broad spectrum of polymeric structures. Commercially available lipases mainly originate from porcine pancreas, Pseudomonas, or the yeast Candida antarctica. Lipase B from Candida antarctica was expressed in Aspergilus niger and immobilized on acrylics beads and is commercialized as Novozym 435. Lipases were shown to degrade PCL,504−508 PLA,504,505 PLLA,509 poly(γ-butyrolactone),507 PHB,508,510 poly(butylene succinate adipate),511 PCL/PBS blend,511 and poly(propylene carbonate)508 in PBS at 37 °C, producing mainly short oligomers. The rates of hydrolysis varied tremendously with the microorganisms/tissue the lipase was extracted from and the polymer.507,512 Overall PCL degrades the fastest with quantitative depolymerization obtained with Pseudomonas lipase after 4 days in aqueous media, and the addition of PLA or PPC to PCL greatly decreases this rate. Noteworthy, Vert et al. showed that Pseudomonas lipase could break down the crystalline domains of PCL within days in contrast to hydrolytic degradation that would take several years to do the same.504 Finally, the ability of lipases to degrade polycarbonates in buffer has been linked to the glass transition temperature of the polymer.513 The influence of organic solvents on the degradation of aliphatic polycarbonates and polyesters with lipases has been investigated. The addition of organic solvents is advantageous to the solubilization of the polymers but can have a detrimental effect on the stability of the enzyme. Lipases are thought to only trigger esterification and transesterification reactions in dry organic solvents, while the presence of water allows for hydrolysis.514 In early studies, Kobayashi et al. showed that the water/organic solvent ratio and the chosen organic solvent had a strong influence on the degradation products of PCL: macrocycles were obtained in toluene and in dry isopropyl ether, while the addition of ≈0.2% H2O in isopropyl ether led to the formation of linear oligomers.515 After solvent evaporation, these oligomers could be repolymerized in bulk using the same lipase catalyst. The same strategy was applied to PBA and polyesters made from a 13-membered lactone with lower yields. The specific chain cleavage of lipases, as compared to random scission for uncatalyzed hydrolysis, was demonstrated. Madras et al. reported the coefficients of enzyme deactivation and of polymer degradation, kd and ks, respectively, for PLA, PCL, PGA, and their copolymers using Novozym 435 and Lipolase, an engineered lipase from fungus, in several organic solvents and at different water contents.516,517 Acetone allowed for the faster degradation and DMSO was the less efficient solvent, evidencing that overall, the depolymerization rate increased with higher solvent polarity and lower solvent viscosity. PTMC could be depolymerized to TMC in acetonitrile at 70 °C with recovery yields up to 80%.518 Matsumura et al. investigated the degradation of PHB, PCL, and PBA in mixtures of supercritical CO2 (sCO2) and organic solvents or water with lipase from Candida antarctica. In flow conditions (15 MPa) with 80% sCO2 and 20% toluene at 40 °C, more than 99% of the products were cyclic oligomers (210 < Mn < 1010 g/mol).519 In contrast, traces of H2O in sCO2 (18 MPa) triggered the formation of dicaprolactone (i.e., 1,8dioxacyclotetradecane-2,9-dione) from PCL in yields higher than 90%, therefore allowing for a full recycling cycle.520 Using the same lipase, Joly et al. demonstrated that PBS could be transformed to succinic acid (44% yield) by reactive extrusion at 120 °C (30 min, 10 wt % lipase).521 Finally, lipases are also

range of PHAs, allowing for the synthesis of a variety of Rhydroxyalkanoic acids, the structures of which depend on the parent PHA.490 4.3.3. Cells Extracts. 4.3.3.1. Depolymerases. Merrick and Doudoroff et al. showed that a mixture of trypsin, a serine protease, and depolymerase or cells’ extracts from Rhodospirillum rubrum composed of an “activator”, “depolymerase”, and “esterase” could digest the inclusion bodies of Bacillus megaterium containing PHB to produce D(−)-β-hydroxybutyric ester, with up to 90% hydrolysis after 100 min.469 Since this first report, several PHA depolymerases have been isolated on agar plates, mainly from Alcaligenes, Pseudomonas, and Comamonas, and their structures and properties have been extensively reviewed.491,492 PHA depolymerases usually exhibit a catalytic domain containing an esterase sequence, often similar to lipase, and a substrate-binding domain (SBD) to enable adsorption of the enzyme at the surface of hydrophobic PHAs.493 Several studies have been dedicated to the substratebinding domain of PHA depolymerases. For instance, Maeda et al. showed by AFM that the adhesive force of the SBD of PhaZRpiT1 was around 100 pN and that single mutations in the SBD to more hydrophobic amino acids allowed for faster rates of hydrolysis.494,495 Moreover, the polymerase’s SBD and insoluble PHA are thought to interact through hydrophobic effects but also molecule-specific contacts.496 Because of this mechanism of action, adding a higher amount of enzyme increases the efficiency of the degradation up to an inhibitory concentration that is detrimental to the degradation rates, owing to saturation of the substrate that prevents the catalytic site from interacting with the polymer.497 The efficiency of depolymerases was mostly evaluated in Tris or PBS buffer solutions with pH ranging from 7.5 to 9.8, to ensure stability of the enzyme. In a systematic study, Doi et al. demonstrated that PHB-depolymerase of Alcaligenes faecalis, Pseudomonas stutzeri, and Comamonas acidovorans could only degrade polyesters with 2 or 3 carbon atoms between each ester bond and only −H or −CH3 substituents on their backbone [i.e., poly-3-hydroxybutyrate (P3HB), poly-3-hydroxypropionate (P3HP), and poly-4-hydroxybutyrate (P4HB)], or AA/BB polyesters exhibiting a −O−CH2−CH2−O− motif (i.e., polyethylene succinate and polyethylene adipate).493 The rates for the best depolymerase/polyester pairs were up to 0.28 g/h/cm2. Depending on the provenance of the PHB depolymerase, either monomer and dimer of 3-hydroxybutyric acid497 or oligomers can be obtained.498−500 Doi et al. also reported that the surface erosion of P3HB and P3HB-co-P4HB was faster while carrying surface erosion with PHB-depolymerase at 37 °C than by uncatalyzed hydrolytic bulk erosion at 55 °C.498 Several reports mentioned that the purity and crystallinity of the PHAs had an influence on the efficacy of the degradation.469 PHB depolymerase hydrolyses PHB chains both by exo (chain ends) and endo (middle of the chain) attacks, as both linear and cyclic PHB could be hydrolyzed by this enzyme. Interestingly, PHB depolymerase can disturb the molecular packing of P3HB in crystals, thus facilitating hydrolysis.501,502 Lenz et al. studied the effect of tacticity and crystallinity of PHB on the efficiency of PHB depolymerase and showed that the rates of degradation were decreasing following the trend: atactic> isotactic> syndiotactic. They also concluded that PHB-depolymerases needed at least a [R]-unit diad in order to catalyze hydrolytic cleavage.503 870


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Catalysis is an enabling science that impacts every step of the plastics life cycle. Many catalytic approaches have been developed to transform renewable raw materials to versatile monomers and to further convert these monomers to existing or new polymer materials. The development of renewable monomers, the versatile synthetic routes to convert these monomers to polyesters and polycarbonate, and the different end-of-use options for these polymers are critically reviewed, with a focus on recent advances in catalytic transformations that lower the technological barriers for developing more sustainable replacements for petroleum-based plastics. Among the potentially sustainable alternatives to current commodity plastics, aliphatic polyesters and aliphatic polycarbonates are highlighted as they can be derived from renewable resources, they can be produced by a variety of versatile synthetic methods, they exhibit a wide range of useful properties, and they can be repurposed to versatile synthetic intermediates or environmentally degraded if they end up in the environment. The different catalytic routes to synthesize these polymers from renewable monomers are surveyed in this review. Significant progress has been made in increasing the catalytic activity (turnover frequency), productivity (turnover number), and selectivity toward desired product formation over side reactions and regio- or stereoselectivity. Recently, catalytic systems that enable the control of polymerization on a more sophisticated level such as architectures and monomer sequences are emerging. While metal-based catalysts are in general more competent in tackling the aforementioned challenges, organocatalysts have emerged as an efficient alternative. While a variety of active and selective catalysts are available for converting renewable monomers into well-defined polymers, data to assess their environmental impacts or sustainability are incomplete. Nevertheless, some sustainability metrics and indicators such as green design principles can be used to assess different catalytic methods based on their adherence to green design principles. The end-of-life options of plastics are critical for the development of circular plastics economies, but economic disincentives have contributed to the modest rates of recycling and material recovery. More efficient chemical and biochemical processes are needed to provide better technology options for preserving and recapturing the value of plastics at the end of their useful life. Considerable efforts have been focused on biodegradable polymers that can be decomposed naturally or in composting sites by microorganisms. The end products are primarily CO2, H2O, and other metabolites that can re-enter the life cycle. Although this end-of-life option mitigates some of the issues associated with indiscriminate disposal of plastics in the environment, it is not the only or even best option for capturing value. Further developments in the generation of polymers that can be selectively depolymerized to the original monomers or other building blocks are needed to provide more energetically efficient means of preserving the economic and chemical investments made in generating these materials. Materials that are designed to be both chemically recyclable and environmentally biodegradable are excellent candidates for circular material economies and represent the ideal end-of-life option for sustainable polymers. For such system to work at scale, we envision the need for an integrated infrastructure that takes on waste collection and sorting, depolymerization, and purification of recovered feedstocks. Similar to mechanical recycling, plastic sorting and tolerance to contaminants in the plastics feeds presents a significant challenge for chemical

thought to catalyze the hydrolysis of lipid ester groups in water insoluble aggregates like membranes and vesicles.522 4.3.3.3. Proteases. Proteases which use serine as the nucleophile for the hydrolysis of peptidic bonds, also called serine proteases, also showed activity for the depolymerization of aliphatic polyesters and polycarbonates. The most potent one, proteinase K, was first extracted from the fungus Tritirachium album and is a broad-spectrum protease known to hydrolyze keratin. Proteinase K is stable in the 8−8.6 pH range and is particularly efficient to degrade PLA.523,524 In contrast, the depolymerization of poly(glycolic acid),525 PCL,523,524 PHB,523 or poly(trimethylene carbonate)525 with proteinase K is slower than with the aforementioned enzymes. As for most enzymes, the efficiency of proteinase K at degrading semicrystalline polymers increases with decreasing crystallinity contents. For instance, crystalline PLA could not be degraded by proteinase K.525,526 However, Wei and Li et al. showed that proteinase K degraded PLLA faster than PDLA, mostly owing to higher adsorption of the enzyme at the surface of PLLA as compared to PDLA.527 In a thorough study on the influence of the chirality of PLA on the degradation activity of proteinase K, they also evidenced that LL, DL, and LD diads could be cleaved but not DD diads.

5. CONCLUSIONS AND OUTLOOK In this review, we describe the challenges and opportunities in the design and development of sustainable polymers that exhibit closed-loop life cycles. These challenges were discussed in the context of polyesters and polycarbonates as representative materials to illustrate key principles for the generation of sustainable polymers. “Innovation leverage points” (Figure 1)44 were introduced to describe where new science and catalytic technologies might lower the technological barriers to achieving closed-loop life cycles in a systems-level analysis where all aspects of the plastics life-cycle are considered, which include the natural resources that provide the feedstocks and monomers, as well as the means of converting these to polymer products, their life cycles, and the end-of-use options. One of the most formidable challenges for the development of sustainable polymers is the economic vitality and success of the current plastics industry, whose development over the past half century was spawned by access to relatively cheap energy and feedstocks derived from petrochemical refining and 60 years of breathtaking scientific and technological developments.528 The evolution of the successful socioeconomic ecosystem that has led to our current model of resource utilization to one that provides incentives both for environmentally sustainable as well as economically sustainable industrial processes and materials will require systems-level changes at all levels of society, including academic and industrial scientists and engineers, industrial ecologists, economists, social scientists, policy makers, and consumers. Critical to that evolution are technology options for more sustainable alternatives to our current practices. Additional challenges for sustainable polymers are ongoing developments to establish quantifiable goals, indices, and metrics to assess whether a particular activity or plastic is more sustainable than existing processes or materials.7−10 This is an important and active area of research and will be critical to guide future research and industrial development.3 The development of materials that exhibit closed-loop life cycles will require scientific and economic innovations at every stage of a plastics life cycle. 871


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recycling. Ideally, mixed chemical recycling facilities that are able to handle multiple depolymerizations from mixed materials flows are preferable. The development of more sustainable alternatives to our current model of linear resource utilization is a crucial but formidable challenge for the development of sustainable industrial processes, including those involving the production, use, and recycling of plastics. The innovations over the past 60 years that have provided the economic engine for our current plastics economies have been breathtaking. Future innovations are needed to ensure that the materials of the next century will not only be economically sustainable, but will be sourced, produced, utilized, and repurposed such that they can address the needs of current and future generations.

Grubbs. Following a year of postdoctoral research with the late Professor Pino at the ETH in Zurich, he joined the faculty at Stanford University in 1988, where he is now the Robert Eckles Swain Professor of Chemistry. His research interests are in homogeneous catalysis and polymer chemistry.

ACKNOWLEDGMENTS We thank Professor Geoffrey W. Coates and Dr. Scott Allen for their insights and discussions in framing the content of this review. We also acknowledge the Ellen MacArthur Foundation for their constructive comments in revising the manuscript. The authors thank the National Science Foundation (GOALI CHE1607092) and the Department of Energy (DE SC0005430) for financial support. X.Z. acknowledges a Stanford Graduate Fellowship and a LAM Graduate Fellowship.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

REFERENCES

ORCID

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Xiangyi Zhang: 0000-0003-4290-1600 Mareva Fevre: 0000-0001-6460-9227 Robert M. Waymouth: 0000-0001-9862-9509 Notes

The authors declare no competing financial interest. Biographies Xiangyi Zhang received her Bachelor’s Degree in 2008 from Nanjing University, China, where she did her undergraduate research with Professor Jing-lin Zuo on electroactive diruthenium complexes for molecular electronics applications. She obtained her Ph.D. in chemistry from Stanford University in 2017 under the supervision of Professor Robert M. Waymouth. Her doctoral research focuses on the development of organocatalytic approaches for the synthesis of functional polyesters and polycarbonates as sustainable alternatives to petroleum-based plastics. She is currently working as a Senior Chemist at the Dow Chemical Company in Midland, MI. Mareva Fevre received her Ph.D. in Chemistry 2012 from the University of Bordeaux, France, working on organocatalyzed polymerization reactions and the precise synthesis of polymeric ionic liquids, under the supervision of Prof. Daniel Taton. In 2012, she moved to a postdoctoral position at Duke University to study elastin-like polypeptides-containing recombinant proteins for anticancer applications. Since joining IBM Almaden Research Center in 2014, she’s been part of some of IBM’s programs leveraging polymer science for the preparation of high performance materials, as well as for biomedical applications, including the design of new antimicrobial and antiviral materials. Gavin O. Jones is a research staff member in the Computational Chemistry and Materials Research group at IBM Research−Almaden. He earned his Bachelor’s degree at Bard College and completed his Ph.D. in theoretical/computational organic chemistry at the University of California Los Angeles with Prof. Kendall Houk. He performed postdoctoral research with Prof. Stephen Buchwald at the Massachusetts Institute of Technology. He has interests in mechanisms, catalysis, molecular properties, and polymer formation and degradation. He was awarded with Foreign Policy Magazine’s Global Thinker Award (Innovator Category) in 2016. Robert M. Waymouth received bachelor’s degrees in mathematics and chemistry from Washington and Lee University and his Ph.D. from the California Institute of Technology in 1987 with Professor Robert H. 872


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NOTE ADDED AFTER ASAP PUBLICATION This paper was published to the Web on October 19, 2017, with errors in Figure 3 and Scheme 3. These were corrected in the version published to the Web on October 24, 2017.

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Catalysis as an Enabling Science for Sustainable Polymers

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