Activated monomer mechanism in the cationic polymerization of L,L-lactide

Abstract

Cationic polymerization of L,L-lactide (LA) in the presence of trifluoromethanesulfonic acid (TfA) has been studied. It was found that propagation proceeds mainly according to the activated monomer (AM) mechanism. Hydroxyl groups required for this type of propagation are formed as a result of the ring opening of protonated lactide. Thus, part of the acid (acting as an initiator) is consumed for the generation of hydroxyl groups, and part (acting as a catalyst) is involved in the protonation of monomer molecules forming secondary oxonium ions which are then able to react with the hydroxyl groups. A dual role of the protic acid is reflected in the kinetic results and in the dependence of experimental degree of polymerization on theoretical values. The structure of active species responsible for polymer chain growth was determined by phosphorus ion-trapping method. The evidence that in the cationic ring-opening polymerization (ROP) of LA initiated by protic acids, both hydroxyl groups and secondary oxonium ions are present throughout the polymerization (as required for polymerization proceeding by the AM mechanism) was found on the basis of changes of the averaged proton chemical shift in 1H NMR spectra of LA polymerizing mixture.