Synergetic Organocatalysis for Eliminating Epimerization in Ring-Opening Polymerizations Enables Synthesis of Stereoregular Isotactic Polyester

Abstract

Ring-opening polymerization of O-carboxyanhydrides (OCAs) can furnish polyesters with a diversity of functional groups that are traditionally hard to harvest by polymerization of lactones. Typical ring-opening catalysts are subject to unavoidable racemization of most OCA monomers, which hampers the synthesis of highly isotactic crystalline polymers. Here, we describe an effective bifunctional single-molecule organocatalysis for selective ring-opening polymerization of OCAs without epimerization. The close vicinity of both activating groups in the same molecule engenders an amplified synergetic effect and thus allows for the use of mild bases, thereby leading to minimal epimerization for polymerization. Ring-opening polymerization of manOCA monomer (OCA from mandelic acid) mediated by the bifunctional single-molecule organocatalyst yields highly isotactic poly­(mandelic acid) (PMA) with controlled molecular weights (up to 19.8 kg mol–1). Mixing of the two enantiomers of PMA generates the first example of a crystalline stereocomplex in this area, which displayed distinct T m values around 150 °C. Remarkably, the bifunctional catalysts are moisture-stable, recyclable, and easy to use, allowing sustainable and scalable synthesis of a stereoregular functional polyester.

Synthesis and stereocomplex formation of enantiomeric alternating copolymers with two types of chiral centers, poly(lactic acid-alt-2-hydroxybutanoic acid)s

“…1), such as poly(2-hydroxybutanoic acid) [P(2HB)] 18 and poly(2-hydroxy-3-methylbutanoic acid) [P(2H3MB)], 18 and poly(mandelic acid). 60 Hetero SC formation occurred between PLA and P(2HB), P(2HB) and P(2H3MB) with the different chemical structures and opposite congurations. Ternary SC formation took place in the blends of L-and D-congured P(2HB)s and L-or D-congured PLA, 18 L-and D-congured P(2H3MB)s and L-or D-congured P(2HB), 61 and D-congured PLA, L-congured P(2HB), and D-congured P(2H3MB) [or L-congured PLA, D-congured P(2HB), and L-congured P(2H3MB)], 62 whereas quaternary SC occurred in the blend of L-and D-congured P(2HB)s and L-and D-congured P(2H3MB)s. 18 Also, SC formation took place in enantiomeric random copolymer blends of poly(L-lactic acid-co-L-2-hydroxybutanoic acid) [P(LLA-co-L-2HB)] (56/44) and poly(D-lactic acid-co-D-2hydroxybutanoic acid) [P(DLA-co-D-2HB)] (52/48) 63 and of poly(L-lactic acid-co-L-2-hydroxy-3-methylbutanoic acid) [P(LLA-co-L-2H3MB)] (47/53) and poly(D-lactic acid-co-D-2-hydroxy-3methylbutanoic acid) [P(DLA-co-D-2H3MB)] (47/53), 64 and the staggered random copolymers, L-congured P(LLA-co-L-2HB) (50/50) and D-congured poly(D-2-hydroxybutanoic acid-co-D-2hydroxy-3-methylbutanoic acid) (50/50).…”

Photocatalyst-Independent Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides: Stereocontrol and Mechanism

“…S7-10). Note that considerable epimerization of L-5, which has an acidic αmethine proton, is often observed during ROP, 12,13,53 but this was not so in our system (Fig. S10).…”

Thiazolium as Single‐Group Bifunctional Catalyst for Selectively Bulk Melt ROP of Cyclic Esters

“…Benefitting from the diversity of their chemical structures, ingeniously designed bifunctional hydrogen‐bonding catalysts enabled to be stereoselective, switchable, and reaction process controllable . Bifunctional single‐molecule hydrogen‐bonding catalyst (Scheme B) which integrating hydrogen‐bonding donor and acceptor group together in one molecule provided a strategy to amplify the synergetic effect between donor and acceptor of catalyst, and thus allows for the use of mild bases as acceptor …”

“… (A) bifunctional hydrogen‐bonding catalyst, (B) bifunctional single‐molecule hydrogen‐bonding catalyst, (C) single‐group bifunctional thiazoliums. (D) Chemical structure of the catalysts: the cations were represented by I–VII , the fluorine‐containing anions were represented by a–c .…”