Organic Catalytic Copolymerization Of α-Methylene-δ-Valerolactone With Cyclic Latone To Prepare Modifiable Polyesters

Aliphatic polyesters have attracted increased attentions over the past decades considering their advantages of biorenewable origins,inherent biodegradability and biocompatibility as well as good mechanical properties.It have been widely used as biomaterials for absorbable sutures,tissue engineering scaffolds and drug delivery carriers.However,in order to satisfy the growing demands for material properties in different fields,efforts have been made to develop modifiable polyesters by introducing modifiable functional groups in the polyester backbone to further improve and modulate the physicochemical properties of polyester materials.Despite the recent advancements,the modifiable polyesters still face challenges including suffers from harsh conditions,possible chain scission and racemization,and therefore tedious protection and deprotection procedures were necessary.α-Methylene-δ-valerolactone（MVL）is a biorenewable and recyclable six-membered lactones bearing an exocyclic double bond,which can be easily synthesized by the reaction ofδ-valerolactone with ethyl formate,It shows great potential in the preparation of modifiable polyesters bearing functional groups in the side chains.Copolymerization is an effective strategy to tailor the thermal and mechanical properties of the resultant copolymer by adjusting their compositions and microstructures.However,almost no reports have been published on the preparation of modifiable copolyesters by ring-opening copolymerization（ROCP）of MVL with other commercial lactones and the recycling of modifiable copolyesters.copolymerization is an effective strategy to tailor the thermal and mechanical properties of the resultant copolymer by adjusting their compositions and microstructures,but almost no studies have been conducted on the preparation of modifiable copolyesters by copolymerization of MVL with other commercial lactones and the recycling of modifiable copolyesters.In this paper,we used organocatalytic MVL and other cyclic lactone ROCP to prepare modifiable recyclable copolyesters.The details are as follows:（1）The chemoselective ROCP of MVL with CL to prepare functional P（MVL-co-CL）copolyesters without formation of vinyl addition product was successfully achieved when 1-tert-butyl-2,2,4,4,4-penta（dimethylamino）-2Λ⁵,4Λ⁵-di（phosphonium）（t Bu-P₂）combined with a bisurea（U3）was used as the binary catalyst,the copolymerization of MVL and CL exhibited“living”/controlled polymerization behavior,and the structures of the copolyesters were characterized by ¹H NMR,¹³C NMR and 2D DOSY spectra,the prepared sample is pure P（MVL-co-CL）copolymer.The copolymerization showed higher controllability in nonpolar solvents at low-temperature.The reactivity ratios and kinetic studies both indicate that the copolyesters are gradient copolymers.A series of copolyesters with different MVL contents were prepared by varied molar feed ratios of monomers under optimal polymerization conditions.The thermal stability of the copolyester was found to decrease slightly with increasing MVL content and the glass transition temperature（T_g）of the copolyester increased from-61.2 to-41.2°C by DSC and TGA characterization.Copolyesters can be post-modified by an efficient thiol-olefin click reaction.The copolyesters can be prepared by an efficient thiol-olefin click reaction to easily achieve the modification of the double bonds in their side chains to obtain the thiolated polyester P（MVL-co-CL）-SR.（2）The chemoselective ring-opening copolymerization of MVL withδ-valerolactone（VL）by Base/Urea organocatalyst,and the structures of the copolyesters were characterized by ¹H NMR,¹³C NMR,2D DOSY and MALDI-TOF MS spectra.The reactivity ratios and kinetic studies both indicate that the P（MVL-co-VL）are random copolymers.The enthalpy change（ΔH_p^?）and entropy change（ΔS_p^?）for ROCP of MVL with VL was-24.0 k J mol^-1 and-63.9 J mol^-1 K^-1.The P（MVL-co-VL）was found to have high thermal stability by DSC and TGA characterization,and thermal properties vary with copolymer composition.The copolyesters can be post-modified by an efficient thiol-olefin click reaction.The native depolymerization of P（MVL-co-VL）was achieved at 130°C using Sn（Oct）₂as the catalyst with 95 wt%monomer recovery,and the recovered monomer can be polymerized again,therefore achieving a closed cycle of"monomer-modifiable copolymer-monomer".