Ring-opening polymerization of thiolactide by using thiol-amine combination

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.

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.

Lewis Acid Promoted Deprotection of Benzylidene Acetals and p-Methoxybenzyl Ethers with Mercaptoacid Acid as Scavenger

We demonstrate a new and high-yielding method for removal of benzylidene acetals and para-methoxybenzyl ethers under catalytic conditions (BF₃:OEt₂ orFeCl₃, 10 mol%) with mercaptoacetic acid as a scavenger. The reaction coproducts are converted to water-soluble molecules, which can be removed by aqueous extraction, thereby bypassing the need for chromatographic purification. The reaction was demonstrated on both multimilligram and multigram scale.

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.

Polythioesters Prepared by Ring-Opening Polymerization of Cyclic Thioesters and Related Monomers

Polyhydroxyalkanoates (PHAs) are biodegradable and biocompatible polyesters with a wide range of applications; in particular, they currently stand as promising alternatives to conventional polyolefin-based "plastics". The introduction of sulfur atoms within the PHAs backbone can endow the resulting polythioesters (PTEs) with differentiated, sometimes enhanced thermal, optical and mechanical properties, thereby widening their versatility and use. Hence, PTEs have been gaining increasing attention over the past half-decade. This review highlights recent advances towards the synthesis of well-defined PTEs by ring-opening polymerization (ROP) of cyclic thioesters - namely thiolactones - as well as of S-carboxyanhydrides and thionolactones; it also covers the ring-opening copolymerization (ROCOP) of cyclic thioanhydrides or thiolactones with epoxides or episulfides. Most of the ROP reactions described are of anionic type, mediated by inorganic, organic or organometallic initiators/catalysts, along with a few enzymatic reactions as well. Emphasis is placed on the reactivity of the thio monomers, in relation to their ring-size ranging from 4- to 5-, 6- and 7-membered cycles, the nature of the catalyst/initiating systems implemented and their efficiency in terms of activity and control over the PTE molar mass, dispersity, topology, and microstructure.

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.

Lithium achieves sequence selective ring-opening terpolymerisation (ROTERP) of ternary monomer mixtures

Heteroatom-containing degradable polymers have strong potential as sustainable replacements for petrochemically derived materials. However, to accelerate and broaden their uptake greater structural diversity and new synthetic methodologies are required. Here we report a sequence selective ring-opening terpolymerisation (ROTERP), in which three monomers (A, B, C) are selectively enchained into an (ABA'C) n sequence by a simple lithium catalyst. Degradable poly(ester-alt-ester-alt-trithiocarbonate)s are obtained in a M n range from 2.35 to 111.20 kDa which are not easily accessible via other polymerisation methodologies. The choice of alkali metal is key to achieve high activity and to control the terpolymer sequence. ROTERP is mechanistically compatible with ring-opening polymerisation (ROP) allowing switchable catalysis for blockpolymer synthesis. The ROTERP demonstrated in this study could be the first example of an entirely new family of sequence selective terpolymerisations.

UV degradation of poly(lactic acid) materials through copolymerisation with a sugar-derived cyclic xanthate

The copolymerisation of L-Lactide with a cyclic xanthate monomer derived from tri-O-acetyl-D-glucal has been used to incorporate thionocarbonate and thioester linkages into a polyester backbone. The poly(lactide-co-xanthate) copolymers show enhanced UV-degradability compared to PLA, with 40% mass loss within 6 hours of UV exposure (365 nm) for only 3% of sulfur-containing linkages.

Recent advances in the ring-opening polymerization of sulfur-containing monomers

Inspired by the broad range of applications for sulfur-containing polymers, this article presents an overview regarding various ROP technologies (ROP/rROP/ROMP) which cement the importance of sulfur-containing monomers in modern polymer chemistry.

O-to-S Substitution Enables Dovetailing Conflicting Cyclizability, Polymerizability, and Recyclability: Dithiolactone vs. Dilactone

Developing chemically recyclable polymers represents a greener alternative to landfill and incineration and offers a closed-loop strategy toward a circular materials economy. However, the synthesis of chemically recyclable polymers is still plagued with certain fundamental limitations, including trade-offs between the monomer's cyclizability and polymerizability, as well as between polymer's depolymerizability and properties. Here we describe the subtle O-to-S substitution, dithiolactone monomers derived from abundant feedstock α-amino acids can demonstrate appealing chemical properties different from those of dilactone, including accelerated ring closure, augmented kinetics polymerizability, high depolymerizability and selectivity, and thus constitute a unique class of polythioester materials exhibiting controlled molecular weight (up to 100.5 kDa), atactic yet high crystallinity, structurally diversity, and chemical recyclability. These polythioesters well addresses the formidable challenges of developing chemically recyclable polymers by having an unusual set of desired properties, including easy-to-make monomer from ubiquitous feedstock, and high polymerizability, crystallinity and precise tunability of physicochemical performance, as well as high depolymerizability and selectivity. Computational studies explain why O-to-S modification of polymer backbone enables dovetailing desirable, but conflicting, performance into one polymer structure.