Mechanism of Ring-Opening Polymerization of 1,5-Dioxepan-2-one and l-Lactide with Stannous 2-Ethylhexanoate. A Theoretical Study

Abstract

A theoretical study of the ring-opening polymerization (ROP) mechanism of 1,5-dioxepan-2-one (DXO) and l-lactide (LLA) with stannous(II) 2-ethylhexanoate (Sn(Oct)2) is presented. The B3LYP density functional method has been used for the quantum chemical calculations. Our results support a coordination−insertion mechanism initiated by a tin−alkoxide species formed prior to the ROP. The rate-determining step in the ROP was the nucleophilic attack of the alkoxide on the carbonyl carbon of the monomer. The activation energy for the ROP of DXO with Sn(Oct‘)2 has been determined to be 19.8 kcal/mol and for l-lactide 20.6 kcal/mol. At normal reaction temperatures, a ligand may dissociate as Oct‘H during propagation. An excess of carboxylic acid hinders the coordination of monomer to the initiating/propagating complex.