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

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 CH 2 O i Pr , PBPL CH 2 O t Bu and PBPL CH 2 OTBDMS ( M n,SEC up to 94 300 g mol −1 ; M = 1.06-1.18; TBDMS = Si t BuMe 2 ), with syndiotactic enrichment ( P r = 0.76-0.87) were successfully synthesized by stereoselective ROP of the corresponding functional racemic β-propiolactones, rac -BPL CH 2 O i Pr , rac -BPL CH 2 O t Bu and rac -BPL CH 2 OTBDMS , 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 = CH 2 O i Pr, CH 2 O t Bu, and CH 2 OTBDMS, 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 = CH 2 OMe, CH 2 OAllyl, CH 2 OBn, and CH 2 OPh. Overall, this study evidenced that, for the newly prepared rac -BPL CH 2 O i Pr , rac -BPL CH 2 O t Bu and rac -BPL CH 2 OTBDMS 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 = C H 2 OC H 2 X with X = H, CH = CH 2 and C 6 H 5 as in BPL CH 2 OMe , BPL CH 2 OAllyl , and BPL CH 2 OBn , 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 CH 2 OMe,OAll,OBn . ROP of bulky functional β-lactones mediated by an achiral diamino-bis(phenolate) yttrium catalyst affords syndio-enriched polyhydroxyalkanoates.