Kinetics and mechanism of cyclic esters polymerisation initiated with tin(II) octoate, 4. Influence of proton trapping agents on the kinetics of ɛ-caprolactone andL,L-dilactide polymerisation

Abstract

Kinetics of the polymerisation of e-caprolactone and L,L-dilactide initiated with tin(II) 2-ethylhexanoate (tin octoate (Sn(Oct)2)) and carried out in the presence of 2,6-di-tert-butylpyridine (1) and/or 1,8-bis(dimethylamino)naphthalene (2), in THF as the solvent, at 80°C was studied. The rate of polymerisation of the cyclic ester in the presence of 1 or 2, known to be a “proton trap” or “proton sponge”, respectively, is either practically the same or even exceeds that of the polymerisation conducted in the absence of these hindered amines. Consequently, the proposed earlier mechanisms of polymerisation of cyclic esters coinitiated by Sn(Oct)2, with chain growth involving active species with “protons”, i. e. primary or secondary oxonium ions, have to be put on rest. This includes also the mechanism in which propagation was proposed to proceed within a ternary complex consisting of hydroxyl group terminated macromolecule, Sn(Oct)2, and a cyclic ester monomer. The observed final increase of the rate of polymerization is in agreement with the interconversion previously decribed by us: Sn(Oct)2 + ROH ⇌ OctSnOR + OctH since OctH (a carboxylic acid) is becoming complexed with a proton trap/sponge and the concentration of OctSnOR (the actual initiator) is effectively increased.