Click Step-Growth Polymerization and E/Z Stereochemistry Using Nucleophilic Thiol–yne/–ene Reactions: Applying Old Concepts for Practical Sustainable (Bio)Materials

Abstract

Conspectus Polymer sustainability is synonymous with “bioderived polymers” and the zeitgeist of “using renewable feedstocks”. However, this sentiment does not adequately encompass the requirements of sustainability in polymers. In addition to recycling considerations and mechanical performance, following green chemistry principles also needs to be maximized to improve the sustainability of polymer synthesis. The synthetic cost ( i.e. , maximizing atom economy, reducing chemical hazards, and lowering energy requirements) of producing polymers should be viewed as equally important to the monomer source (biomass vs petrol platform chemicals). Therefore, combining the use of renewable feedstocks with efficient syntheses and green chemistry principles is imperative to delivering truly sustainable polymers. The high efficiency, atom economy, and single reaction trajectories that define click chemistry reactions position them as ideal chemical approaches to synthesize polymers in a sustainable manner while simultaneously expanding the structural scope of accessible polymers from sustainably sourced chemicals. Click step-growth polymerization using the thiol–yne Michael addition, a reaction first reported over a century ago, has emerged as an extremely mild and atom-efficient pathway to yield high-performance polymers with controllable E / Z stereochemistry along the polymer backbone. Building on studies of aromatic thiol–yne polymers, around 10 years ago our group began investigating the thiol–yne reaction for the stereocontrolled synthesis of alkene-containing aliphatic polyesters. Our early studies established a convenient path to high-molecular-weight (>100 kDa) E- rich or Z -rich step-growth polymers by judiciously changing the catalyst and/or reaction solvent. This method has since been adapted to synthesize fast-degrading polyesters, high-performance polyamides, and resilient hydrogel biomaterials. Across several systems, we have observed dramatic differences in material properties among polymers with different alkene stereochemistry. We have also explored the analogous thiol–ene Michael reaction to create high-performance poly(ester-urethanes) with precise E / Z stereochemistry. In contrast to the stereoselective thiol–yne polymerization, here the use of monomers with predefined E / Z (geometric) isomerism (arising from either alkenes or the planar rigidity of ring units) affords polymers with total control over stereochemistry. This advancement has enabled the synthesis of tough, degradable materials that are derived from sustainable monomer feedstocks. Employing isomers of sugar-derived isohexides, bicyclic rigid-rings possessing geometric isomerism, led to degradable polymers with fundamentally opposing mechanical behavior ( i.e. , plastic vs ...

Regio- and stereospecific thiol-thioalkyne reaction facilitated by organic base

“…Other phenylthiols bearing different electron-donating or electron-withdrawing groups, as well as 2-naphthalenethiol, were tolerated by this protocol in an excellent manner, allowing the transformation to be completed in 10 minutes and the formation of corresponding adducts 2-6 in nearly quantitative yields. This system also exhibited outstanding efficiency in the treatment of thiols with diverse alkyl groups, such as 2-phenylethyl, triphenylmethyl, nbutyl group, as well as alkyl chains with hydroxyl or ester functional group (7)(8)(9)(10)(11)(12). Encouraged by the high feasibility and tolerance of this strategy, we next turned our attention to thiols involving N-heterocycles, which hold similar chemical properties with the DBU additive, thus possibly participating in this reaction process in a competitive fashion and leading deteriorated results.…”

“…1,2 Attributing to its simple, mild condition and high efficiency, in recent two decades this old fashion has evolved as one significant component of click chemistry, and been employed in various realms, such as the synthesis of diverse functional macromolecules, modification of peptides and proteins, and interface functionalization, etc. [3][4][5][6][7][8][9][10][11] However, in comparison with the wide use of terminal alkynes, challenges remain in the treatment of internal alkynes, which is chiefly due to the relatively low reactivity of their C-C triple bond as well as the difficult regulation of regio-and stereoselectivity in the addition process (Scheme 1a). [12][13][14] Introduction of electronwithdrawing groups (EWG) could lead  addition in a nucleophilic manner, howbeit achieving stereospecificity, which has demonstrated to be critical in determining material behaviours, [15][16][17] is still an issue (Scheme 1b).…”

Elucidation of Substantial Differences in Ring-Opening Polymerization Outcomes from Subtle Variation of Glucose Carbonate-Based Monomer Substitution Patterns and Substituent Types

“…Increasing efforts are being devoted toward the development of polymers derived from sustainably sourced feedstocks, − which have environmental and technological benefits, while also allowing for an in-depth study of unique polymerization chemistries, leading to highly functional polymer materials. Among these eco-friendly macromolecules, polymers based upon monosaccharides, − oligosaccharides, , and polysaccharides − have gained significant attention due to the great abundance and relatively low cost of natural carbohydrate resources.…”

cis-Selective Acyclic Diene Metathesis Polymerization of α,ω-Dienes

“…Homogeneous and heterogeneous catalysts have been developed for the coordination–insertion polymerization of 1,3-dienes with selective formation of either trans or cis linkages, but these catalytic systems are notoriously intolerant to polar functional groups and can lead to the formation of vinyl defects through competitive 1,2-insertions . Recently, several elegant approaches have been implemented to deliver polymers with predictable cis / trans contents either through thiol–yne click chemistry − or via a metal-free ring-opening metathesis polymerization (ROMP) mediated by light . However, the scope of these processes is limited, and high cis contents are generally more challenging to access because of thermodynamic penalties.…”