As an approach to control the molecular weight and polymer end groups of polyester obtained by A 2 + B 2 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 cisor 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.

End-functionalized linear polyesters were synthesized by means of base-catalyzed transesterification of a cyclic polyester, obtained by A2 + B2 polycondensation, with a symmetric functional diester as an exchange reagent.

Synthesis of B−A−B type triblock copolymers of aliphatic polyester (PEs) and polystyrene (PSt) was investigated by using cyclic unsaturated PEs prepared by conventional polycondensation of 4-octene-1,8-diol and sebacoyl chloride. The obtained cyclic PEs underwent cross-metathesis with PSt containing a carbon−carbon double bond (CC) at the central position, which was obtained by atom transfer radical polymerization (ATRP) of styrene with a bifunctional initiator containing a CC bond. PSt with a longer methylene spacer between the CC bond and PSt successfully afforded the PStb-PEs-b-PSt triblock copolymer. As another approach to obtain the triblock copolymer, cross-metathesis of cyclic PEs with 2butene-1,4-diol or 4-octene-1,8-diol bis(2-bromoisobutyrylate)s was conducted to afford linear PEs having ATRP initiation sites at both ends, followed by ATRP of styrene. The unsaturated PEs segment in the triblock copolymer obtained by the second approach was converted into a saturated PEs segment by treatment with tosyl hydrazide and tributylamine. DSC analysis of the triblock copolymer containing the saturated PEs segment showed crystallinity when the PEs content was ≥14 mol %.