Coordination Ring-Opening Copolymerization of Naturally Renewable α?Methylene-γ-butyrolactone into Unsaturated Polyesters

摘要

Reported herein is the first coordination-insertion ring-opening copolymerization of α-methylene-γ-butyrolactone (MBL) and ε-caprolactone (ε-CL) catalyzed by f-block lanthanide (Ln) catalysts, Ln[N(SiMe_3)_2]_3, that produce exclusively an unsaturated copolyester PMBL-co-PCL without coproducing any homopolymer PMBL. Accomplishing such synthesis requires effective strategies to meet two key challenges: ring-opening of the γ-butyrolactone (γ-BL) ring in MBL--the five-membered lactone well recognized for its nonpolymerizability--and shutting down the vinyl-addition pathway via conjugate addition across the double bond--the exocyclic CC moiety in MBL known for its high reactivity toward vinyl addition. Remarkably, the current Ln coordination catalyst system, coupled with judiciously chosen reaction conditions (relatively nonpolar solvent and low temperature, 0 or -20 °C), effectively copolymerizes MBL and ε-CL to produce the ring-opening copolyester PMBL-co-PCL, with the MBL incorporation up to 40 mol % and without any detectable PMBL formation, even when employing a large excess of MBL in feed. Successful ring-opening homopolymerization of γ-BL by the Ln catalyst has also been realized at -78 °C under ambient pressure, producing the polyester PBL with a T_m of 63.0 °C and a T_g of -53.7 °C. Investigation into the thermal property of the resulting copolyester reveals an overall depression of T_m of the copolyester as increasing the MBL incorporation, indicating that the ringopened MBL (unsaturated) polyester incorporated in the random copolymer is noncrystallizable and disrupts the crystallization process of the crystallizable, ring-opened ε-CL (saturated) polyester segment. Mechanistic studies provide key evidence for a coordination-insertion ring-opening copolymerization mechanism.