ε-Thionocaprolactone: an accessible monomer for preparation of degradable poly(vinyl esters) by radical ring-opening polymerization†
The 7-membered cyclic thionolactone, ε-thionocaprolactone (TCL), undergoes radical copolymerization with vinyl esters to form degradable copolymers. While most radical ring-opening monomers require laborious and low-yielding syntheses, TCL can be prepared in one step from caprolactone (75% yield). TCL copolymerizes readily with vinyl esters, but is unreactive towards more activated monomers and does not homopolymerize under radical conditions. The molecular weight and to some extent the dispersity of the TCL/vinyl ester copolymers can be controlled via xanthate-mediated RAFT polymerization in the range of 5–20 kg mol−1. Analysis of the copolymer structure shows a high level of TCL–TCL dyads, suggesting that the terminal copolymerization model does not accurately describe TCL copolymerization.