Exploring electronic versus steric effects in stereoselective ring-opening polymerization of lactide and β-butyrolactone with amino-alkoxy-bis(phenolate)-yttrium complexes.

A series of methoxy-amino-bis(phenol)s (ONOO(R(1),R(2)))H(2) possessing on the phenol rings R(1) ortho substituents with variable steric and electronic properties (R(1)=CMe(2)Ph, 1; CMe(2)tBu, 3; CMe(2)(4-CF(3)C(6)H(4)), 5; CPh(3), 9; Cl, 10) has been synthesized and further reacted with Y{N(SiHMe(2))(2)}(3)(2) to give cleanly the corresponding yttrium compounds [Y(ONOO(R(1),R(2))){N(SiHMe(2))(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 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)≪CMe(3) (P(r)=0.80)≪CMe(2)Ph (P(r)=0.90)<CMe(2)(4 CF(3)-Ph) (P(r)=0.93-0.94)≤CMe(2)tBu (P(r)=0.94-0.95)≤CPh(3) (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)<CMe(3) (P(r)=0.80)≤CMe(2) (4 CF(3)-Ph) (P(r)=0.82-0.84)<CMe(2)Ph (P(r)=0.89)<CPh(3) (P(r)=0.94), which suggests the involvement of electronic interactions. DFT computations on model intermediates confirmed a stabilizing C-H···π interaction between a methylene C-H of the ring-opened BBL unit and the π system of one of the ortho-aryl substituents of the ONOO(R(1)) ligand; by contrast, for model intermediates in the ROP of LA, no such C-H···π interaction involving the methyl group of lactate was observed.