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

Reported herein is the first coordination insertion ring-opening copolymerization of alpha-methylene-gamma-butyrolactone (MBL) and epsilon-caprolactone (epsilon-CL) catalyzed by f-block lanthanide (Ln) catalysts, Ln[N(SiMe3)(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 gamma-butyrolactone (gamma-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 degrees C), effectively copolymerizes MBL and epsilon-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 gamma-BL by the Ln catalyst has also been realized at -78 degrees C under ambient pressure, producing the polyester PBL with a T-m of 63.0 degrees C and a T-g of -53.7 degrees 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 ring-opened MBL (unsaturated) polyester incorporated in the random copolymer is noncrystallizable and disrupts the crystallization process of the crystallizable, ring-opened epsilon-CL (saturated) polyester segment. Mechanistic studies provide key evidence for a coordination-insertion ring-opening copolymerization mechanism.