Isomerization of Linear to Hyperbranched Polymers: Two Isomeric Lactones Converge via Metastable Isostructural Polyesters to a Highly Branched Analogue.

We report here the Zn(II)-catalyzed convergence of two metastable and isostructural polyesters to an isomeric polymer having a hyperbranched architecture. Ring-opening transesterification polymerization (ROTEP) of 4-carbomethoxyvalerolactone () under Brønsted catalysis is known to give the linear polyester . We show here that this can be isomerized to the equilibrated (and highly branched) polyester . Analysis of the fragments obtained from eliminative degradation of were critical in the formulation of its structure. The isomerization of to is a direct consequence of the presence of the second ester functional group in the ester-lactone, a rarely studied class of monomer. Zn(II)-catalysis of the ROTEP of the isomeric β-lactone, 2-(2-carbomethoxyethyl)propiolactone (), as well as isomerization of the isostructural linear homopolymer derived from that isomeric monomer, led to the same . These results suggest a new strategy for the introduction of branching into various polyesters.