Stereo-electronic contributions in yttrium-mediated stereoselective ring-opening polymerization of functional racemic β-lactones: ROP of 4-alkoxymethylene-β-propiolactones with bulky exocyclic chains

Abstract

Stereoselective ring-opening polymerization (ROP) of cyclic esters is the privileged strategy to access stereoregular polyesters that are widely applied in various domains, such as in particular the biomedical and packaging fields. The production of synthetic stereo-enriched polyhydroxyalkanoates (PHAs) derived from racemic β-lactones by ROP is still a challenge. In this context, linear, high molar mass, narrowly dispersed PHAs, namely PBPL^CH₂OiPr, PBPL^CH₂OtBu and PBPL^CH₂OTBDMS (M_n,SEC up to 94 300 g mol⁻¹; Đ_M = 1.06–1.18; TBDMS = SitBuMe₂), with syndiotactic enrichment (P_r = 0.76–0.87) were successfully synthesized by stereoselective ROP of the corresponding functional racemic β-propiolactones, rac-BPL^CH₂OiPr, rac-BPL^CH₂OtBu and rac-BPL^CH₂OTBDMS, respectively, which are promoted by diverse achiral diamino-bis(phenolate) yttrium complexes featuring various R′/R′′ substituents (Y{ONNO^R′,R′′}, 2a–d). The influence of the steric hindrance of the BPL^FG side-functionality, with FG = CH₂OiPr, CH₂OtBu, and CH₂OTBDMS, on the ROP kinetics, stereoselectivity and thermal properties of the resulting PHAs, as a function of 2a–d catalysts, was compared to that of the previously reported similar but less hindered BPL^FG monomers, with FG = CH₂OMe, CH₂OAllyl, CH₂OBn, and CH₂OPh. Overall, this study evidenced that, for the newly prepared rac-BPL^CH₂OiPr, rac-BPL^CH₂OtBu and rac-BPL^CH₂OTBDMS monomers, due to steric constraints induced by the monomer alkoxy/silyloxy side-functionality, all ROPs afforded syndio-enriched polyesters, regardless of the catalyst used. Conversely, only combinations of a BPL^FG monomer containing two sets of methylene hydrogens within the side-functionality, i.e. with FG = CH₂OCH₂X with X = H, CH = CH₂ and C₆H₅ as in BPL^CH₂OMe, BPL^CH₂OAllyl, and BPL^CH₂OBn, with a yttrium catalyst bearing ortho/para-chloro substituents (2a), gave isotactic functional PHAs. With the latter three monomers, a catalyst with highly sterically crowded substituents on the ligand platform (2a,b) was necessary to recover syndio-enriched PBPL^{CH₂OMe,OAll,OBn}.