Entropically Driven Macrolide Polymerizations for the Synthesis of Aliphatic Polyester Copolymers Using Titanium Isopropoxide

Thermally Stable and Chemically Recyclable Poly(ketal-ester)s Regulated by Floor Temperature

Chemical recycling to monomers (CRM) offers a promising closed-loop approach to transition from current linear plastic economy toward a more sustainable circular paradigm. Typically, this approach has focused on modulating the ceiling temperature (Tc) of monomers. Despite considerable advancements, polymers with low Tc often face challenges such as inadequate thermal stability, exemplified by poly(γ-butyrolactone) (PGBL) with a decomposition temperature of ∼200 °C. In contrast, floor temperature (Tf)-regulated polymers, particularly those synthesized via the ring-opening polymerization (ROP) of macrolactones, inherently exhibit enhanced thermodynamic stability as the temperature increases. However, the development of those Tf regulated chemically recyclable polymers remains relatively underexplored. In this context, by judicious design and efficient synthesis of a biobased macrocyclic diester monomer (HOD), we developed a type of Tf -regulated closed-loop chemically recyclable poly(ketal-ester) (PHOD). First, the entropy-driven ROP of HOD generated high-molar mass PHOD with exceptional thermal stability with a Td,5% reaching up to 353 °C. Notably, it maintains a high Td,5% of 345 °C even without removing the polymerization catalyst. This contrasts markedly with PGBL, which spontaneously depolymerizes back to the monomer above its Tc in the presence of catalyst. Second, PHOD displays outstanding closed-loop chemical recyclability at room temperature within just 1 min with tBuOK. Finally, copolymerization of pentadecanolide (PDL) with HOD generated high-performance copolymers (PHOD-co-PPDL) with tunable mechanical properties and chemical recyclability of both components.

Polymers from Plant Oils Linked by Siloxane Bonds for Programmed Depolymerization

The increased production of plastics is leading to the accumulation of plastic waste and depletion of limited fossil fuel resources. In this context, we report a strategy to create polymers that can undergo controlled depolymerization by linking renewable feedstocks with siloxane bonds. α,ω-Diesters and α,ω-diols containing siloxane bonds were synthesized from an alkenoic ester derived from castor oil and then polymerized with varied monomers, including related biobased monomers. In addition, cyclic monomers derived from this alkenoic ester and hydrosiloxanes were prepared and cyclized to form a 26-membered macrolactone containing a siloxane unit. Sequential ring-opening polymerization of this macrolactone and lactide afforded an ABA triblock copolymer. This set of polymers containing siloxanes underwent programmed depolymerization into monomers in protic solvents or with hexamethyldisiloxane and an acid catalyst. Monomers afforded by the depolymerization of polyesters containing siloxane linkages were repolymerized to demonstrate circularity in select polymers. Evaluation of the environmental stability of these polymers toward enzymatic degradation showed that they undergo enzymatic hydrolysis by a fungal cutinase from Fusarium solani. Evaluation of soil microbial metabolism of monomers selectively labeled with 13C revealed differential metabolism of the main chain and side chain organic groups by soil microbes.

Chemically Recyclable Linear and Branched Polyethylenes Synthesized from Stoichiometrically Self-Balanced Telechelic Polyethylenes

High-density polyethylene (HDPE) is a widely used commercial plastic due to its excellent mechanical properties, chemical resistance, and water vapor barrier properties. However, less than 10% of HDPE is mechanically recycled, and the chemical recycling of HDPE is challenging due to the inherent strength of the carbon-carbon backbone bonds. Here, we report chemically recyclable linear and branched HDPE with sparse backbone ester groups synthesized from the transesterification of telechelic polyethylene macromonomers. Stoichiometrically self-balanced telechelic polyethylenes underwent transesterification polymerization to produce the PE-ester samples with high number-average molar masses of up to 111 kg/mol. Moreover, the transesterification polymerization of the telechelic polyethylenes and the multifunctional diethyl 5-(hydroxymethyl)isophthalate generated branched PE-esters. Thermal and mechanical properties of the PE-esters were comparable to those of commercial HDPE and tunable through control of the ester content in the backbone. In addition, branched PE-esters showed higher levels of melt strain hardening compared with linear versions. The PE-ester was depolymerized into telechelic macromonomers through straightforward methanolysis, and the resulting macromonomers could be effectively repolymerized to generate a high molar mass recycled PE-ester sample. This is a new and promising method for synthesizing and recycling high-molar-mass linear and branched PE-esters, which are competitive with HDPE and have easily tailorable properties.

Chemically Recyclable Dithioacetal Polymers via Reversible Entropy-Driven Ring-Opening Polymerization

In a sustainable circular economy, polymers capable of chemical recycling to monomers are highly desirable. We report an efficient monomer-polymer recycling of polydithioacetal (PDTA). Pristine PDTAs were readily synthesized from 3,4,5-trimethoxybenzaldehyde and alkyl dithiols. They then exhibited depolymerizability via ring-closing depolymerization into macrocycles, followed by entropy-driven ring-opening polymerization (ED-ROP) to reform the virgin polymers. High conversions were obtained for both the forward and reverse reactions. Once crosslinked, the network exhibited thermal reprocessability enabled by acid-catalyzed dithioacetal exchange. The network retained the recyclability into macrocyclic monomers in solvent which can repolymerize to regenerate the crosslinked network. These results demonstrated PDTA as a new molecular platform for the design of recyclable polymers and the advantages of ED-ROP for which polymerization is favored at higher temperatures.

Modulating Polymerization Thermodynamics of Thiolactones Through Substituent and Heteroatom Incorporation

A central challenge in the development of next-generation sustainable materials is to design polymers that can easily revert back to their monomeric starting material through chemical recycling to monomer (CRM). An emerging monomer class that displays efficient CRM are thiolactones, which exhibit rapid rates of polymerization and depolymerization. This report details the polymerization thermodynamics for a series of thiolactone monomers through systematic changes to substitution patterns and sulfur heteroatom incorporation. Additionally, computational studies highlight the importance of conformation in modulating the enthalpy of polymerization, leading to monomers that display high conversions to polymer at near-ambient temperatures, while maintaining low ceiling temperatures (T_c). Specifically, the combination of a highly negative enthalpy (-19.3 kJ/mol) and entropy (-58.4 J/(mol·K)) of polymerization allows for a monomer whose equilibrium polymerization conversion is very sensitive to temperature.

Catalytically Controlled Ring-Opening Polymerization of 2-Oxo-15-crown-5 for Degradable and Recyclable PEG-Like Polyesters

Poly(ethylene glycol) (PEG) has been extensively used in diverse applications. However, it is not biodegradable and shows abnormal immune responses. Herein, a fast, controlled, ring-opening polymerization (ROP) of 2-oxo-15-crown-5 (O-15C5) is reported to prepare well-defined PEG-like polyesters, poly(O-15C5). This approach relies on a coordination between the macrocyclic monomer and Na⁺ that increases the electrophilicity of the carbonyl group of O-15C5 and leads to a fast controlled ROP (dispersity, Đ_M < 1.2). Both computational and mechanistic studies show that the selective Na⁺ binding to the monomer over poly(O-15C5) allows the ring-opening initiation and propagation to be more energetically favorable than side transesterifications. This is the key to control the challenging entropy-driven ROP of O-15C5. Moreover, with the aid of Na⁺ and organic base, poly(O-15C5) depolymerized readily into O-15C5 in 2 h. Also, it degraded in a buffer of pH 7.4 by hydrolysis.

Highly Reactive Cyclic Carbonates with a Fused Ring toward Functionalizable and Recyclable Polycarbonates

Monomer design plays an important role in the development of polymers with desired thermal properties and chemical recyclability. Here we prepared a class of seven-membered ring carbonates containing trans-cyclohexyl fused rings. These monomers showed excellent activity for ring-opening polymerization (ROP) with turnover frequency (TOF) up to 6 × 10⁵ h^-1 and catalyst loading down to 50 ppm, which yielded high-molecular-weight polycarbonates (M_n up to 673 kg/mol) with great thermostability (T_d > 300 °C). Ultimately, the resulting polycarbonates can completely depolymerize into their corresponding cyclic dimers that can repolymerize to synthesize the starting polymers in moderate yields, demonstrating a potential route to achieve chemical recycling. Postfunctionalization of the unsaturated polycarbonate was conducted through cross-linking reaction and "click" reaction under UV irradiation.

Defining the Macromolecules of Tomorrow through Synergistic Sustainable Polymer Research

Transforming how plastics are made, unmade, and remade through innovative research and diverse partnerships that together foster environmental stewardship is critically important to a sustainable future. Designing, preparing, and implementing polymers derived from renewable resources for a wide range of advanced applications that promote future economic development, energy efficiency, and environmental sustainability are all central to these efforts. In this Chemical Reviews contribution, we take a comprehensive, integrated approach to summarize important and impactful contributions to this broad research arena. The Review highlights signature accomplishments across a broad research portfolio and is organized into four wide-ranging research themes that address the topic in a comprehensive manner: Feedstocks, Polymerization Processes and Techniques, Intended Use, and End of Use. We emphasize those successes that benefitted from collaborative engagements across disciplinary lines.

Ring opening copolymerization of δ-valerolactone with 2-methyl-1,3-dioxane-4-one towards poly(3-hydroxypropionate-co-5-hydroxyvalerate) copolyesters

A straightforward strategy of ring-opening copolymerization (ROCOP) toward the synthetic copolyesters is gaining increasing interest. In present work, a novel series of poly(3-hydroxypropionate-co-5-hydroxyvalerate) P(3HP-co-5HV) copolyesters are obtained through ROCOP of...

A cascade strategy towards the direct synthesis of green polyesters with versatile functional groups

The cascade coupling of ROP and CP enables the facile synthesis of high functional group content biodegradable polyesters.

Use of pyrazoles as ligands greatly enhances the catalytic activity of titanium iso-propoxide for the ring-opening polymerization of l-lactide: a cooperation effect

Using TiOⁱPr₄ with a pyrazole ligand for one-pot LA polymerization improved catalytic activity compared with using TiOⁱPr₄ only. At 60 °C, TiOⁱPr₄ with ^furPz exhibited a higher catalytic activity (approximately 3-fold) than TiOⁱPr₄. At room temperature, TiOⁱPr₄ with ^BuPz exhibited a higher catalytic activity (approximately 17-fold) than TiOⁱPr₄. High molecular mass PLA (M _nGPC = 51 100, and Đ = 1.10) could be produced by using TiOⁱPr₄ with ^furPz in melt polymerization ([TiOⁱPr₄] : [^furPz] = 1000 : 1 : 1 at 100 °C, 240 min). The crystal structure of ^MePz₂Ti₂OⁱPr₇ revealed the cooperative activation between two Ti atoms during LA polymerization.

Ring-Opening Alternating Copolymerization of O-Carboxyanhydrides of Lactic Acid and Malic Acid Using Hafnium Alkoxide Initiators with Different Stereo Selectivities and Activities

Monomer sequence controllable syntheses of copolymers, including copolyesters, remain a challenge in polymer science. Although alternating sequence-controlled copolymerization of O-carboxyanhydrides (OCAs) can be achieved via using syndioselective initiators, the alternating copolymerization of lactic acid-derived O-carboxyanhydride (LacOCA) with other monomers still suffers from a lack of highly syndioselective initiators. In this work, a highly syndioselective system for the ring-opening polymerization (ROP) of LacOCA was achieved using a bulky amine tris(phenolate) hafnium alkoxide initiator with a high P_r value of 0.91. However, the stereoselectivities of amine tris(phenolate) hafnium alkoxide initiators for the ROP of malic acid O-carboxyanhydride (MalOCA) change to be modestly or lowly isoselective. Interestingly, despite the different stereoselectivities of this system for the two different monomers, the high syndioselectivity of the initiator for the ROP of LacOCA and the low activity of MalOCA in the ROP allow comparatively high rates of cross-propagation; consequently, the ring-opening alternating copolymerization (ROAP) of LacOCA and MalOCA was achieved successfully.

Readily Degradable Aromatic Polyesters from Salicylic Acid

Polyesters constitute around 10% of the global plastic market with aromatic polyesters, such as poly(ethylene terephthalate) (PET), being the most prevalent because of their attractive properties. As for most commercial plastics, polyesters are primarily derived from fossil resources and are not readily degradable, which raises a number of sustainability concerns. Designing polymers with competitive properties from sustainable feedstocks that rapidly degrade under mild conditions is an attractive strategy for addressing the current plastic waste problem. Here, the detailed synthesis and characterization of degradable, high molar mass aromatic polyesters derived from salicylic acid, poly(salicylic glycolide) (PSG), and poly(salicylic methyl glycolide) (PSMG) are described. The synthesis of polymers was investigated through mechanistic experiments and complementary computational studies. The glass transition temperature (T_g ≈ 85 °C) and Young's modulus (E ≈ 2.3 GPa) of these polyesters are comparable to those of PET. In contrast to the poor hydrolytic degradability of PET, both PSG and PSMG are readily degradable in neutral aqueous solutions (e.g., complete degradation in seawater at 50 °C in 60 days). These aromatic polyesters derived from salicylic acid have potential as future high-performance, sustainable, and degradable plastics.

Ring opening polymerization of macrocyclic oligoesters derived from renewable sources

A review about the synthesis of renewable alkylene furanoate and dicarboxylate MCOs, their ROP to polyesters and their recycling by cyclodepolymerization.

Polycondensation of bis(α-diazo-1,3-dicarbonyl) compounds with dicarboxylic acids: an efficient access to functionalized alternating polyesters

We report an efficient and mild approach to functionalized alternating polyesters on the basis of O–H insertion polycondensation of bis(α-diazo-1,3-dicarbonyl) compounds with dicarboxylic acids.