Modulating Polymerization Thermodynamics of Thiolactones Through Substituent and Heteroatom Incorporation

Michael Addition-Elimination Ring-Opening Polymerization

A cyclic thioenone system capable of controlled ring-opening polymerization (ROP) is presented that leverages a reversible Michael addition-elimination (MAE) mechanism. The cyclic thioenone monomers are easy to access and modify and for the first time incorporate the dynamic reversibility of MAE with chain-growth polymerization. This strategy features mild polymerization conditions, tunable functionalities, controlled molecular weights (Mn), and narrow dispersities. The obtained polythioenones exhibit excellent optical transparency and good mechanical properties and can be depolymerized to recover the original monomers. Density functional theory (DFT) calculations of model reactions offer insights into the role of monomer conformation in the polymerization process, as well as explaining divergent reactivity observed in seven-membered thiepane (TP) and eight-membered thiocane (TC) ring systems. Collectively, these findings demonstrate the feasibility of MAE mechanisms in ring-opening polymerization and provide important guidelines toward future monomer designs.

Heteroatom Substitution Strategy Modulates Thermodynamics Towards Chemically Recyclable Polyesters and Monomeric Unit Sequence by Temperature Switching

In this paper, we proposed a heteroatom substitution strategy (HSS) in the δ-valerolactone (VL) system to modulate thermodynamics toward chemically recyclable polyesters. Three VL-based monomers containing different heteroatoms (M1 (N), M2 (S), and M3 (O)), instead of C-5 carbon, were designed and synthesized to verify our proposed HSS. All three monomers undergo organocatalytic living/controlled ROP and controllable depolymerization. Impressively, the resulting P(M1) achieved over 99 % monomer recovery under both mild solution depolymerization and high vacuum pyrolysis conditions without any side reactions, and the recycled monomers can be polymerized again forming new polymers. The systematic study of the relationship between heteroatom substitution and recyclability shows that introducing heteroatoms does change the thermodynamics of the monomers (ΔHp o, ΔSp o and Tc values), thereby adjusting the polymerizability and depolymerizability. DFT calculations found that the introduction of heteroatoms adjusts the ring strain by changing the angular strain of the monomers, and the order of their angular strain (M2>M1>M3) is consistent with the order of the experimentally obtained enthalpy change. Notably, the one-pot/one-step copolymerization of two of each of the three monomers enables the synthesis of sequence-controlled copolymers from gradient to random to block structures, by simply switching the copolymerization temperature.

Enabling Closed-Loop Circularity of "Non-Polymerizable" α, β-Conjugated Lactone Towards High-Performance Polyester with the Assistance of Cyclopentadiene

Chemical recycling of polymers to monomers presents a promising solution to the escalating crisis associated with plastic waste. Despite considerable progress made in this field, the primary efforts have been focused on redesigning new monomers to produce readily recyclable polymers. In contrast, limited research into the potential of seemingly "non-polymerizable" monomers has been conducted. Herein, we propose a paradigm that leverages a "chaperone"-assisted strategy to establish closed-loop circularity for a "non-polymerizable" α, β-conjugated lactone, 5,6-dihydro-2H-pyran-2-one (DPO). The resulting PDPO, a structural analogue of poly(δ-valerolactone) (PVL), exhibits enhanced thermal properties with a melting point (Tm) of 114 °C and a decomposition temperature (Td,5%) of 305 °C. Notably, owing to the structural similarity between DPO and δ-VL, the copolymerization generates semi-crystalline P(DPO-co-VL)s irrespective of the DPO incorporation ratio. Intriguingly, the inherent C=C bonds in P(DPO-co-VL)s enable their convenient post-functionalization via Michael-addition reaction. Lastly, PDPO was demonstrated to be chemically recyclable via ring-closing metathesis (RCM), representing a significant step towards the pursuit of enabling the closed-loop circularity of "non-polymerizable" lactones without altering the ultimate polymer structure.

Proton-triggered topological transformation in superbase-mediated selective polymerization enables access to ultrahigh-molar-mass cyclic polymers

The selective synthesis of ultrahigh-molar-mass (UHMM, >2 million Da) cyclic polymers is challenging as an exceptional degree of spatiotemporal control is required to overcome the possible undesired reactions that can compete with the desired intramolecular cyclization. Here we present a counterintuitive synthetic methodology for cyclic polymers, represented here by polythioesters, which proceeds via superbase-mediated ring-opening polymerization of gem-dimethylated thiopropiolactone, followed by macromolecular cyclization triggered by protic quenching. This proton-triggered linear-to-cyclic topological transformation enables selective, linear polymer-like access to desired cyclic polythioesters, including those with UHMM surpassing 2 MDa. In addition, this method eliminates the need for stringent conditions such as high dilution to prevent or suppress linear polymer contaminants and presents the opposite scenario in which protic-free conditions are required to prevent cyclic polymer formation, which is capitalized to produce cyclic polymers on demand. Furthermore, such UHMM cyclic polythioester exhibits not only much enhanced thermostability and mechanical toughness, but it can also be quantitatively recycled back to monomer under mild conditions due to its gem-disubstitution.

Stereoselective Ring-opening Polymerization of S-Carboxyanhydrides Using Salen Aluminum Catalysts: A Route to High-Isotactic Functionalized Polythioesters

Polythioesters are important sustainable polymers with broad applications. The ring-opening polymerization (ROP) of S-Carboxyanhydrides (SCAs) can afford polythioesters with functional groups that are typically difficult to prepare by ROP of thiolactones. Typical methods involving organocatalysts, like dimethylaminopyridine (DMAP) and triethylamine (Et3 N), have been plagued by uncontrolled polymerization, including epimerization for most SCAs resulting in the loss of isotacticity. Here, we report the use of salen aluminum catalysts for the selective ROP of various SCAs without epimerization, affording functionalized polythioester with high molecular weight up to 37.6 kDa and the highest Pm value up to 0.99. Notably, the ROP of TlaSCA (SCA prepared from thiolactic acid) generates the first example of a isotactic crystalline poly(thiolactic acid), which exhibited a distinct Tm value of 152.6 °C. Effective ligand tailoring governs the binding affinity between the sulfide chain-end and the metal center, thereby maintaining the activity of organometallic catalysts and reducing the occurrence of epimerization reactions.

Accelerated Scheme to Predict Ring-Opening Polymerization Enthalpy: Simulation-Experimental Data Fusion and Multitask Machine Learning

Ring-opening enthalpy (ΔHROP) is a fundamental thermodynamic quantity controlling the polymerization and depolymerization of an important class of recyclable polymers, namely, those created from ring-opening polymerization (ROP). Highly accurate first-principles-based computational methods to compute ΔHROP are computationally too demanding to efficiently guide the design of depolymerizable polymers. In this work, we develop a generalizable machine-learning model that was trained on experimental measurements and reliably computed simulation results of ΔHROP (the latter provides a pathway to systematically increase the chemical diversity of the data). Predictions of ΔHROP using this machine-learning model require essentially no time while the prediction accuracy is about ∼8 kJ/mol, approaching the well-known chemical accuracy. We hope that this effort will contribute to the future development of new depolymerizable polymers.

Emerging Trends in the Chemistry of End-to-End Depolymerization

Over the past couple of decades, polymers that depolymerize end-to-end upon cleavage of their backbone or activation of a terminal functional group, sometimes referred to as "self-immolative" polymers, have been attracting increasing attention. They are of growing interest in the context of enhancing polymer degradability but also in polymer recycling as they allow monomers to be regenerated in a controlled manner under mild conditions. Furthermore, they are highly promising for applications as smart materials due to their ability to provide an amplified response to a specific signal, as a single sensing event is translated into the generation of many small molecules through a cascade of reactions. From a chemistry perspective, end-to-end depolymerization relies on the principles of self-immolative linkers and polymer ceiling temperature (Tc). In this article, we will introduce the key chemical concepts and foundations of the field and then provide our perspective on recent exciting developments. For example, over the past few years, new depolymerizable backbones, including polyacetals, polydisulfides, polyesters, polythioesters, and polyalkenamers, have been developed, while modern approaches to depolymerize conventional backbones such as polymethacrylates have also been introduced. Progress has also been made on the topological evolution of depolymerizable systems, including the introduction of fully depolymerizable block copolymers, hyperbranched polymers, and polymer networks. Furthermore, precision sequence-defined oligomers have been synthesized and studied for data storage and encryption. Finally, our perspectives on future opportunities and challenges in the field will be discussed.

Precise construction of weather-sensitive poly(ester-alt-thioesters) from phthalic thioanhydride and oxetane

We report the selective ring opening copolymerisation (ROCOP) of oxetane and phthalic thioanhydride by a heterobimetallic Cr(III)K catalyst precisely yielding semi-crystalline alternating poly(ester-alt-thioesters) which show improved degradability due to the thioester links in the polymer backbone.

Precise cooperative sulfur placement leads to semi-crystallinity and selective depolymerisability in CS2/oxetane copolymers

CS₂ promises easy access to degradable sulfur-rich polymers and insights into how main-group derivatisation affects polymer formation and properties, though its ring-opening copolymerisation is plagued by low linkage selectivity and small-molecule by-products. We demonstrate that a cooperative Cr(III)/K catalyst selectively delivers poly(dithiocarbonates) from CS₂ and oxetanes while state-of-the-art strategies produce linkage scrambled polymers and heterocyclic by-products. The formal introduction of sulfur centres into the parent polycarbonates results in a net shift of the polymerisation equilibrium towards, and therefore facilitating, depolymerisation. During copolymerisation however, the catalyst enables near quantitative generation of the metastable polymers in high sequence selectivity by limiting the lifetime of alkoxide intermediates. Furthermore, linkage selectivity is key to obtain semi-crystalline materials that can be moulded into self-standing objects as well as to enable chemoselective depolymerisation into cyclic dithiocarbonates which can themselves serve as monomers in ring-opening polymerisation. Our report demonstrates the potential of cooperative catalysis to produce previously inaccessible main-group rich materials with beneficial chemical and physical properties.

Chemically Recyclable Polymer System Based on Nucleophilic Aromatic Ring-Opening Polymerization

The development of chemically recyclable polymers with desirable properties is a long-standing but challenging goal in polymer science. Central to this challenge is the need for reversible chemical reactions that can equilibrate at rapid rates and provide efficient polymerization and depolymerization cycles. Based on the dynamic chemistry of nucleophilic aromatic substitution (S_NAr), we report a chemically recyclable polythioether system derived from readily accessible benzothiocane (BT) monomers. This system represents the first example of a well-defined monomer platform capable of chain-growth ring-opening polymerization through an S_NAr manifold. The polymerizations reach completion in minutes, and the pendant functionalities are easily customized to tune material properties or render the polymers amenable to further functionalization. The resulting polythioether materials exhibit comparable performance to commercial thermoplastics and can be depolymerized to the original monomers in high yields.

Insights into substitution strategy towards thermodynamic and property regulation of chemically recyclable polymers

The development of chemically recyclable polymers serves as an attractive approach to address the global plastic pollution crisis. Monomer design principle is the key to achieving chemical recycling to monomer. Herein, we provide a systematic investigation to evaluate a range of substitution effects and structure-property relationships in the ɛ-caprolactone (CL) system. Thermodynamic and recyclability studies reveal that the substituent size and position could regulate their ceiling temperatures (T_c). Impressively, M4 equipped with a tert-butyl group displays a T_c of 241 °C. A series of spirocyclic acetal-functionalized CLs prepared by a facile two-step reaction undergo efficient ring-opening polymerization and subsequent depolymerization. The resulting polymers demonstrate various thermal properties and a transformation of the mechanical performance from brittleness to ductility. Notably, the toughness and ductility of P(M13) is comparable to the commodity plastic isotactic polypropylene. This comprehensive study is aimed to provide a guideline to the future monomer design towards chemically recyclable polymers.

Sustainable Polythioesters via Thio(no)lactones: Monomer Synthesis, Ring-Opening Polymerization, End-of-Life Considerations, and Industrial Perspectives

As the environmental effects of plastics are of ever greater concern, the industry is driven towards more sustainable polymers. Besides sustainability, our fast-developing society imposes the need for highly versatile materials. Whereas aliphatic polyesters (PEs) are widely adopted and studied as next-generation biobased and (bio)degradable materials, their sulfur-containing analogs, polythioesters (PTEs), only recently gained attention. Nevertheless, the introduction of S atoms is known to often enhance thermal, mechanical, electrochemical, and optical properties, offering prospects for broad applicability. Furthermore, thanks to their thioester-based backbone, PTEs are inherently susceptible to degradation, giving them a high sustainability potential. The key route to PTEs is through ring-opening polymerization (ROP) of thio(no)lactones. This Review critically discusses the (potential) sustainability of the most relevant state-of-the-art in every step from sulfur source to end-of-life treatment options of PTEs, obtained through ROP of thio(no)lactones. The benefits and drawbacks of PTEs versus PEs are highlighted, including their industrial perspective.

Easy Access to Diverse Multiblock Copolymers with On-Demand Blocks via Thioester-Relayed In-Chain Cascade Copolymerization

Multiblock copolymers are envisioned as promising materials with enhanced properties and functionality compared with their diblock/triblock counterparts. However, the current approaches can construct multiblock copolymers with a limited number of blocks but tedious procedures. Here, we report a thioester-relayed in-chain cascade copolymerization strategy for the easy preparation of multiblock copolymers with on-demand blocks, in which thioester groups with on-demand numbers are built in the polymer backbone by controlled/living polymerizations. These thioester groups further serve as the in-chain initiating centers to trigger the acyl group transfer ring-opening polymerization of episulfides independently and concurrently to extend the polymer backbone into multiblock structures. The compositions, number of blocks, and block degree of polymerization can be easily regulated. This strategy can offer easy access to a library of multiblock copolymers with ≈100 blocks in only 2 to 4 steps.

Ring-Opening Polymerization for the Goal of Chemically Recyclable Polymers

A crucial modern dilemma relates to the ecological crisis created by excess plastic waste production. An emerging technology for reducing plastic waste is the production of "chemically recyclable" polymers. These polymers can be efficiently synthesized through ring-opening polymerization (ROP/ROMP) and later recycled to pristine monomer by ring-closing depolymerization, in an efficient circular-type system. This Perspective aims to explore the chemistry involved in the preparation of these monomer/polymer systems, while also providing an overview of the challenges involved, including future directions.

Toward Recyclable Polymers: Ring-Opening Polymerization Enthalpy from First-Principles

Ring-opening polymerization (ROP) enthalpy ΔH^ROP is an important thermodynamic property controlling the polymerization of cyclic monomers. While ΔH^ROP can be measured, computing ΔH^ROP for realistic polymer systems with an error of ≃5-10 kJ/mol is critical for designing new monomer systems for depolymerizable polymers. We have developed a first-principles computational scheme in which multiple challenges in computing ΔH^ROP are resolved definitively including extensive exploration of conformational states and adequately addressing finite size effects. This scheme is validated on a diverse benchmark set of 42 ROP polymers for which reliable experimental values of ΔH^ROP are available. For this set, the ΔH^ROP root-mean-square error is ≃7 kJ/mol, about 3-times smaller than conventional approaches. This development opens up new pathways to build up a high-quality database of ΔH^ROP for downstream predictive machine-learning models and ultimately to accelerate the design of depolymerizable polymers with desired properties.