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 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 ring-opened 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.