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

摘要

A series of methoxy-amino-bis(phenol)s (ONOO)H₂ possessing on the phenol rings R1ortho substituents with variable steric and electronic properties (R1=CMe₂Ph, 1; CMe₂tBu, 3; CMe₂(4-CF₃C₆H₄), 5; CPh₃, 9; Cl, 10) has been synthesized and further reacted with [Y{N(SiHMe₂)₂}₃](THF)₂ to give cleanly the corresponding yttrium compounds [Y(ONOO){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-β-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 R1ortho 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: R1=Cl (P_r=0.56)≪CMe₃ (P_r=0.80)≪CMe₂Ph (P_r=0.90)2(4 CF₃-Ph) (P_r=0.93–0.94)≤CMe₂tBu (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: R1=Cl (P_r=0.42–0.45)≪CMe₂tBu (P_r=0.62–0.70)3 (P_r=0.80)≤CMe₂(4 CF₃Ph) (P_r=0.82–0.84)2Ph (P_r=0.89)3 (P_r=0.94), which suggests the involvement of electronic interactions. DFT computations on model intermediates confirmed a stabilizing CHπ interaction between a methylene CH of the ring-opened BBL unit and the π system of one of the ortho-aryl substituents of the ONOO ligand; by contrast, for model intermediates in the ROP of LA, no such CHπ interaction involving the methyl group of lactate was observed.