Transesterification:An Efficient Strategy For The Preparation Of ’Nonstrained’ γ-Butyrolactone-based Aliphatic Copolyesters

Aliphatic polyesters,a technologically important polymeric material,are widely used in packaging,biomedicine and tissue engineering due to their degradability and biocompatibility.Ring-opening polymerization（ROP）of cyclic esters has proven to be a powerful synthetic methodology for the chemical synthesis of high-molecular-weight aliphatic polyesters,but it is limited by the type of cyclic ester monomers.Ring-opening copolymerization（ROC）of distinct cyclic esters provide an effective strategy for facile tuning of properties and functions of the resultant copolyesters.γ-butyrolactone（γ-BL）would be a very potential comonomer,because of its green source derived from biomass and readily available in large quantities.However,it has been a challenge in effective ROC of‘nonstrained’γ-BL with high-strained cyclic esters,due to their large difference in reactivity.Aiming at this challenge,a series of aliphatic co-polyesters based onγ-BL were successfully prepared by developing transesterification（TEster）strategy:First,we found that copolymer poly（glycolic acid-co-BL）[poly（GA-co-BL）]were prepared efficiently though the Tester between poly（glycolic acid）（PGA）andγ-BL using metal complex La[N（TMS）₂]₃ as the catalyst.Compared with other methods,the sequence structures of copolymer synthesized by TEster are more controllable,and the resulting polymers exhibit much-enhanced thermal stability（≥44℃）.DFT calculation indicated that the enhanced thermal stability is attributed to the isolated glycolic sequence in the copolymer created uniquely by Tester,and this upcycling copolymer is 100%chemically recyclable,enabling a complete recovery of pure glycolic acid andγ-BL monomer.Second,in order to develop more green catalytic systems,we used air/moisture-tolerant 1,5,7-triazabicyclo[4.4.0]dec-5-ene（TBD）/thiourea（TU）or urea（U）as bifunctional organocatalyst to achieve the efficient Tester reaction of poly（δ-valerolactone）（PVL）/γ-BL or poly（ε-caprolactone）（PCL）/γ-BL,resulting in their correspondingγ-BL-based copolymers,in which poly（VL-co-BL）possess relatively high γ-BL incorporation（up to 38.3%）and high molecular weight（M_n up to 40.6kg/mol）.The enhancement ofγ-BL incorporation is presumably originated from the preferential activation ofγ-BL monomer over the polymer chain.Thanks to more homosequences in the copolyesters produced by TEster,the resulting poly（BL-co-VL）exhibit higher melting temperature（T_m）over those obtained by ROC at similar level ofγ-BL incorporation.