A Strategy of In Situ Catalysis and Nucleation of Biocompatible Zinc Salts of Amino Acids towards Poly(l-lactide) with Enhanced Crystallization Rate

The intrinsic drawback of slow crystallization rate of poly(l-lactide) (PLLA) inevitably deteriorates its final properties of the molded articles. In this work, we proposed a new strategy towards poly(l-lactide) with enhanced crystallization rate by ring opening polymerization (ROP) of l-lactide (l-LA) catalyzed by biocompatible zinc salts of amino acids. For the first time we developed a one-pot facile method of zinc salts of amino acids acting dual roles of catalysis of l-LA polymerization and in situ nucleation of the as-prepared PLLA. Nine zinc salts of different amino acids, including three kinds of amino acids ligands (alanine, phenylalanine, and proline) with l/d-enantiomers and their equimolar racemic mixtures, were first prepared and tested as catalysts of l-LA polymerization. A partial racemization was observed for zinc salts of amino acids whereas no racemization was detected for the reference stannous octoate. The polymerization mechanism study showed that the interaction of zinc salts of amino acids and benzyl alcohol forms the actual initiator for l-LA polymerization. Isothermal crystallization kinetics analysis showed that the residual zinc salts of amino acids exhibited a significant nucleation effect on PLLA, evidenced by the promotion of the crystallization rate, depending on the amino acid ligand and its configuration. Meanwhile, the residual zinc salts of amino acids did not compromise the thermal stability of the pristine PLLA.