End-functionalized poly(ϵ-caprolactone) oligomers through heterogeneous catalysis in protic conditions: a mechanistic approach

Abstract

An heterogeneous system constituted of aluminium alkoxides grafted on a porous silica has been used for the ring opening polymerization of lactones. Adding alcohol in excess induces a rapid exchange reaction between the grafted active centres and these free alcohol molecules present in the medium, which competes with the propagation step. The polymer chains are functionalized by end-groups issued from the alcohol and are produced in a number larger than that of metal atoms used. In addition, the active centres can be re-used for further polymerizations which brings an innovative catalytic dimension to this process. This case was engaged in the polymerization of ϵ-caprolactone and some kinetic features are reported. The activation energy is 28 kJ mol −1. The catalytic activity is strongly dependent on the alcohol concentration and exhibits some optimum. A mechanistic approach based on the share of the aluminium vacancy between the Lewis bases present in the medium was proposed to account for the experimental results. Molecular geometry calculations have been used to study the possible arrangements of the monomer-aluminium alkoxide complex.