Synthesis and characterization of new Al and Zn complexes for the ring opening polymerization of lactide

Aliphatic polyesters are a class of technologically important biodegradable and/or biocompatible polymers. Among them, polylactide (PLA) has unique mechanical and physical properties that make it useful not only in a wide range of commodity applications but also in the biomedical area. Therefore, there is increasing interest in methods that allow for the preparation of PLA in a reproducible and controlled fashion. Nowadays, the best way to obtain PLA is by far the ring-opening polymerization (ROP) of the corresponding lactide with well-defined metal complexes. For this reason, a large number of investigations focused on the development of new catalysts in order to achieve better control, activity and selectivity of the polymerization reaction. The first part of this project consists in the synthesis and characterization of two different metal amides, particularly 2,2,6,6-tetramethylpiperidinates, while the second part consists in the study of their activity. In particular, our investigation resulted in the synthesis of 2,2,6,6-tetramethylpiperidide ethyl aluminium chloride (TMPAlClEt) and 2,2,6,6-tetramethylpiperidide zinc chloride lithium chloride (TMPZnCl*LiCl). These metal complexes were characterized by 1H, 13C{1H}, 13C{1H} DEPT, 27Al and 7Li NMR experiments. After reaching this goal, we were interested in the ring-opening polymerization of rac-lactide using our freshly synthetized catalysts. In order to activate our catalysts and increase the polymerization reaction rate, we used [PPN]Cl (bis(triphenylphosphine)iminium chloride) as a co-catalyst. All the polymerization reactions were performed in propylene oxide to generate the alkoxide species, which is supposed to be our initiator. Particularly, our zinc catalyst proved to be very active and full conversion of rac-lactide was achieved in less than one hour at 30°C, even if the polymerization reaction is not well-controlled.