Control of Molecular Weight and End-Functional Groups of Polyester from A₂ + B₂ Polycondensation via Cross-Metathesis of Cyclic Unsaturated Polyester with Difunctional Olefin

As an approach to control the molecular weight and polymer end groups of polyester obtained by A₂ + B₂ polycondensation, we investigated cross-metathesis of cyclic polyesters containing carbon–carbon double bonds in the backbone with a symmetric olefin having two functional groups as an exchange reagent (ExR). Polycondensation of cis-2-butene-1,4-diol (1) and sebacoyl dichloride (4) did not selectively afford cyclic polyester, but when cis- or trans-4-octene-1,8-diol, instead of 1, was reacted with 4 or isophthaloyl dichloride, we found that cyclic unsaturated polyesters were formed selectively. The obtained cyclic polyesters successfully underwent cross-metathesis reaction with cis-1,4-diacetoxy-2-butene (6) in the presence of second-generation Grubbs catalyst to afford linear polyester bearing acetoxy groups at both ends. The molecular weight of the linear polyester decreased with increasing amount of 6 regardless of the molecular weight of the starting cyclic polyester; the molecular weight of the resulting linear polyester was governed by the molar ratio of 6 to carbon–carbon double bonds in the cyclic ester. Cross-metathesis using other ExRs enabled the introduction of tert-butoxycarbonyl (Boc) amino, bromophenyl, and tert-butyl carboxylate groups at both ends of polyester.