Transesterification by air/moisture-tolerant bifunctional organocatalyst to produce 'nonstrained' γ-butyrolactone-based aliphatic copolyesters: Turning a bane into a boon

It has been a challenge in effective ring-opening copolymerization (ROC) of nonstrained gamma-butyrolactone (BL) with high-strained cyclic esters or lactones, due to their large difference in reactivity. Transesterification (TEster) would be a promising alternative strategy to ROC for the synthesis of BL-based copolyesters if advanced catalysts could be rationally designed by enabling air/moisture tolerance and preference toward BL ring-opening over chain breakage. Here we report the development of air/moisture-tolerant bifunctional oganocatalyst based on 1,5,7-triazabicyclo[4.4.0]clec-5-ene (TBD)/thiourea(urea) system for efficient TEster reaction of poly(delta-valerolactone) (PVL)/BL or poly(epsilon-caprolactone) (PCL)/BL, successfully affording BL-based copolyesters, especially poly(BL-co-VL)s, with relatively high BL incorporation (up to 38.3%) and high molecular weight (M-n, up to 40.6 kg/mol). Mechanistic study reveals that the enhancement of BL incorporation in TBD/TU(U)-mediated TEster reaction is presumably originated from the preferential activation of BL monomer over the polymer chain, rather than the enhanced bifunctional activation. Thanks to more homosequences present in the copolyesters produced by TEster, the resulting poly(BL-co-VL)s exhibit higher melting temperature (T-m) over those obtained by ROC at similar level of BL incorporation.