Exploring Electronic versus Steric Effects in Stereoselective Ring-Opening Polymerization of Lactide and beta-Butyrolactone with Amino-alkoxy-bis(phenolate)-Yttrium Complexes

A series of methoxy-amino-bis(phenol)s (ONOOR1,R2)H-2 possessing on the phenol rings R-1 ortho substituents with variable steric and electronic properties (R-1 = CMe2Ph, 1; CMe(2)tBu, 3; CMe2(4-CF3C6H4), 5; CPh3, 9; CI, 10) has been synthesized and further reacted with [Y{N(SiHMe2)(2)}(3)](THF)(2) to give cleanly the corresponding yttrium compounds [Y(ONOOR1,R2){N-(SiHMe2)(2)}(thf)(n)] (Y-x); the solid-state structures of Y-3 and Y-10 have been determined. These amido complexes have been used as initiators for the ring-opening polymerization (ROP) of rac-lactide (LA) and rac-beta-butyrolactone (BBL) to provide heterotactically enriched poly(lactic acid)s (PLAs) and syndiotactically enriched poly(3-hydroxybutyrate)s (PHBs), respectively, by means of a chain-end control mechanism. Most of these polymerizations proceeded in a controlled fashion, giving polymers with narrow polydispersities and experimental molecular weights in good agreement with calculated values. The nature of the R-1 ortho substituents has a profound impact on the rates and, more spectacularly, on the stereocontrol of the polymerizations. The heterotactic stereocontrol in the ROP of rac-LA appears to be governed essentially by steric considerations; the larger the substituent, the higher the heterotacticity: R-1 = Cl (P-r = 0.56) << CMe3 (P-r = 0.80) << CMe2Ph (P-r = 0.90) < CMe2(4CF(3)-Ph) (P-r = 0.93-0.94) <= CMe(2)tBu (P-r = 0.94-0.95) <= CPh3 (P-r = 0.95-0.96). On the other hand, the syndiotactic stereocontrol in the polymerization of rac-BBL follows a quite different trend: R-1 = Cl (P-r = 0.42-0.45) << CMe(2)tBu (P-r = 0.62-0.70) < CMe3 (P-r = 0.80) <= CMe2(4CF(3)-Ph) (P-r = 0.82-0.84) < CMe2Ph (P-r = 0.89) < CPh3 (P-r = 0.94), which suggests the involvement of electronic interactions. DFT computations on model intermediates confirmed a stabilizing C H center dot center dot center dot pi interaction between a methylene C H of the ring-opened BBL unit and the It system of one of the ortho-aryl substituents of the ONOOR1 ligand; by contrast, for model intermediates in the ROP of LA, no such C-H center dot center dot center dot pi interaction involving the methyl group of lactate was observed.