Mechanism of ring-opening polymerization of 1,5-dioxepan-2-one and L-lactide with stannous 2-ethylhexanoate. A theoretical study

A theoretical study of the ring-opening polymerization (ROP) mechanism of 1 ,5-dioxepana-2-one (DXO) and (L)-lactide (LLA) with stannous(IE) a-ethylhex anoate (Sn(Oct)(2)) is presented. The B3LYP density functional method has b een used for the quantum chemical calculations. Our results support a coord ination-insertion mechanism initiated by a tin-alkoxide species formed prio r to the ROP. The rate-determining step in the ROP was the nucleophilic att ack of the alkoxide on the carbonyl carbon of the monomer. The activation e nergy for the ROP of DXO with Sn(Oct ')(2) has been determined to be 19.8 k cal/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 carboxyli c acid hinders the coordination of monomer to the initiating/propagating co mplex.