Ring-opening polymerization of L-lactide using N-heterocyclic molecules: mechanistic, kinetics and DFT studies

We report here our investigations of the mechanism and kinetics of N-heterocyclic molecules as initiators for PLA synthesis. This is a step towards employing life saving drugs based on N-heterocyclic functional groups, to generate carrier-drug complexes in situ, via ring opening polymerization (ROP) of L-lactide. During administration, the drug may be released with well-defined release rate kinetics. With this motivation, five and six membered N-heterocyclic molecules such as 4-pyrrolidino-pyridine (PDP), 2-methyl-pyridine, pyridine, pyrrole and imidazole, are examined as initiators of bulk ROP of L-lactide, to yield metal-free poly(L-lactide) (PLA). NMR, IR and MALDI-TOF MS analyses, establish the reacting species in the polymerization. Side reactions such as formation of macrocycles, occur when permitted by lack of steric hindrance, as when polymerization is initiated by PDP + benzyl alcohol or by imidazole. Accordingly, we propose a PDP-based anionic polymerization mechanism, and then examine the effect of reaction parameters. The kinetics studies indicate an overall first order reaction with lactide, and low activation energy. Density functional theory (DFT) calculations of the initiators' proton affinities, provide a fundamental understanding, to enable selection of more efficient various N-heterocyclic drug molecules, which can be utilized as initiators, to generate PLA chains as carriers for drug administration. We find that PDP has the highest proton affinity, and that the order of the calculated proton affinities correlates broadly with the resulting PLA molecular weights. We also find that PDP appears to the most effective initiator, yielding Mw ∼28,000 Da in 90 min, which is in line with the correlation between initiator effectiveness and proton affinities, of the investigated five and six member N-heterocyclic molecules.