Enzymatic Synthesis of Polythioester by the Ring-Opening Polymerization of Cyclic Thioester

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.

Structural Metamorphoses of d-Xylose Oxetane- and Carbonyl Sulfide-Based Polymers In Situ during Ring-Opening Copolymerizations

Polymers constructed from copolymerizations of carbohydrates with C1 feedstocks are promising targets that provide transformation of sustainably sourced building blocks into next-generation, environmentally degradable plastic materials. In this work, the initial intention was to expand beyond polycarbonates prepared by the copolymerization of oxetanes derived from d-xylose with CO2 and incorporate sulfur atoms through the establishment of monothiocarbonates that would provide the ability to modulate the backbone compositions and result in unique effects upon the chemical, physical, and mechanical properties. Therefore, the syntheses of poly(1,2-O-isopropylidene-α-d-xylofuranose monothiocarbonate)s were investigated by ring-opening copolymerizations of 3,5-anhydro-1,2-O-isopropylidene-α-d-xylofuranose with carbonyl sulfide (COS) facilitated by (salen)CrCl/cocatalyst systems. Unexpectedly, when copolymerization temperatures exceeded 40 °C, oxygen/sulfur exchange reactions occurred, causing in situ dynamic backbone restructuring through a series of inter-related and complex mechanistic pathways that transformed monothiocarbonate monomeric repeating units into carbonate and thioether dimeric repeating units. These backbone structural compositional transformations were investigated through a combination of Fourier transform infrared and nuclear magnetic resonance spectroscopic techniques and were demonstrated to be easily tuned via temperature and catalyst/cocatalyst stoichiometries. Furthermore, the regiochemistries of these d-xylose-based sulfur-containing polymers revealed that monothiocarbonate monomeric repeating units had a head-to-tail connectivity, while the carbonate and thioether dimeric repeating units had dual head-to-head and tail-to-tail connectivities. These sulfur-containing polymers exhibited enhanced thermal stabilities compared to their oxygen-containing polycarbonate analogues and revealed variations in the effects upon glass transition temperatures, demonstrating the effect of sulfur incorporation in the polymer backbone. These findings contribute to the advancement of sustainable polymer production by using feedstocks of natural origin coupled with COS.

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.

S-Carboxyanhydrides: Ultrafast and Selective Ring-Opening Polymerizations Towards Well-defined Functionalized Polythioesters

Aliphatic polythioesters are popular polymers because of their appealing performance such as metal coordination ability, high refractive indices, and biodegradability. One of the most powerful approaches for generating these polymers is the ring-opening polymerization (ROP) of cyclic monomers. However, the synthesis of precisely controlled polythioesters via ROP of thiolactones still faces formidable challenges, including the minimal functional diversity of available thiolactone monomers, as well as inevitable transthioesterification side reactions. Here we introduce a hyperactive class of S-carboxyanhydride (SCA) monomers derived from amino acids that are significantly more reactive than thiolactones for ultrafast and selective ROP. Inclusion of the initiator PPNOBz ([PPN]=bis(triphenylphosphine)-iminium) with chain transfer agent benzoic acid, the polymerizations that can be operated in open vessels reach complete conversion within minutes (1-2 min) at room temperature, yielding polythioesters with predictable molecular weight, low dispersities, retained stereoregularity and chemical recyclability. Most fascinating are the functionalized SCAs that allow the incorporating of functional groups along the polythioester chain and thus finely tune their physicochemical performance. Computational studies were carried out to explore the origins of the distinctive rapidity and exquisite selectivity of the polymerizations, offering mechanistic insight and explaining why high polymerizability of SCA monomer is able to facilitate exquisitely selective ring-opening for enchainment over competing transthioesterification and backbiting reactions.

Polymers from sugars and CS2: ring opening copolymerisation of a d-xylose anhydrosugar oxetane

An oxetane derived from d-xylose has been copolymerised with CS₂ into sulfur-containing polymers which are chemically recyclable and degradable under UV light.

Evidence for Orientational Order in Nanothreads Derived from Thiophene

Nanothreads are one-dimensional sp³ hydrocarbons that pack within pseudohexagonal crystalline lattices. They are believed to lack long-range order along the thread axis and also lack interthread registry. Here we investigate the phase behavior of thiophene up to 35 GPa and establish a pressure-induced phase transition sequence that mirrors previous observations in low-temperature studies. Slow compression to 35 GPa results in the formation of a recoverable saturated product with a 2D monoclinic diffraction pattern along (0001) that agrees closely with atomistic simulations for single crystals of thiophene-derived nanothreads. Paradoxically, this lower-symmetry packing signals a higher degree of structural order since it must arise from constituents with a consistent azimuthal orientation about their shared axis. The simplicity of thiophene reaction pathways (with only four carbon atoms per ring) apparently yields the first nanothreads with orientational order, a striking outcome considering that a single point defect in a 1D system can disrupt long-range structural order.

Thioester functional polymers

Inspired by the uniqueness and ubiquity of thioesters in nature, much attention has been paid to thioester functionalized materials, yielding applications ranging from responsive polymers to bioconjugates and (bio)degradable polymers.

Enzymes as Green Catalysts for Precision Macromolecular Synthesis

The present article comprehensively reviews the macromolecular synthesis using enzymes as catalysts. Among the six main classes of enzymes, the three classes, oxidoreductases, transferases, and hydrolases, have been employed as catalysts for the in vitro macromolecular synthesis and modification reactions. Appropriate design of reaction including monomer and enzyme catalyst produces macromolecules with precisely controlled structure, similarly as in vivo enzymatic reactions. The reaction controls the product structure with respect to substrate selectivity, chemo-selectivity, regio-selectivity, stereoselectivity, and choro-selectivity. Oxidoreductases catalyze various oxidation polymerizations of aromatic compounds as well as vinyl polymerizations. Transferases are effective catalysts for producing polysaccharide having a variety of structure and polyesters. Hydrolases catalyzing the bond-cleaving of macromolecules in vivo, catalyze the reverse reaction for bond forming in vitro to give various polysaccharides and functionalized polyesters. The enzymatic polymerizations allowed the first in vitro synthesis of natural polysaccharides having complicated structures like cellulose, amylose, xylan, chitin, hyaluronan, and chondroitin. These polymerizations are "green" with several respects; nontoxicity of enzyme, high catalyst efficiency, selective reactions under mild conditions using green solvents and renewable starting materials, and producing minimal byproducts. Thus, the enzymatic polymerization is desirable for the environment and contributes to "green polymer chemistry" for maintaining sustainable society.

Lipase-catalyzed polyester synthesis--a green polymer chemistry

This article is a short comprehensive review describing in vitro polyester synthesis catalyzed by a hydrolysis enzyme of lipase, most of which has been developed for these two decades. Polyesters are prepared by repeated ester bond-formation reactions; they include two major modes, ring-opening polymerization (ROP) of cyclic monomers such as cyclic esters (lactones) and condensation polymerization via the reaction between a carboxylic acid or its ester group and an alcohol group. Polyester synthesis is, therefore, a reaction in reverse way of in vivo lipase catalysis of ester bond-cleavage with hydrolysis. The lipase-catalyzed polymerizations show very high chemo-, regio-, and enantio-selectivities and involve various advantageous characteristics. Lipase is robust and compatible with other chemical catalysts, which allows novel chemoenzymatic processes. New syntheses of a variety of functional polyesters and a plausible reaction mechanism of lipase catalysis are mentioned. The polymerization characteristics are of green nature currently demanded for sustainable society, and hence, desirable for conducting 'green polymer chemistry'.