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Tridentate Schiff base complexes of zinc and calcium were prepared and tested in the ring-opening polymeriza- tion of 3-caprolactone and rac-lactide to generate biodegradable polymeric materials from biocompatible metals. Alteration of the pendant donor arm attached to the imine backbone provides some control over catalyst composition and polymeriza- tion activity. Complexes of the formula (ONN)ZnN(SiMe3)2, where (ONN) = 2-(N-donor arm-imine)(4,6-di(tert-butyl)phen- oxide), were isolated with ethyldimethylamine, ethylpiperidine, and ethylmorpholine substituents, while disproportionation led to the isolation of (ONN)2Zn complexes with methylpyridine, quinoline, and ethyldiisopropylamine derivatives, two of which were crystallographically characterized. Calcium complexes were more stable and novel (ONN)CaN(SiMe3)2 com- plexes with ethylpiperidine and ethyldiisopropylamine substituents were reported. Zinc and calcium catalysts coordinated to a single tridentate ligand were effective at initiating the polymerization of 3-caprolactone, but did not control the polymeriza- tions, whereas the bis(ligand) complexes produced no polymer. These catalysts were effective at controlling the polymeriza- tion of rac-lactide. Coordinatively saturated complexes inhibit the polymerization, while initiation from either the amido or ligand alkoxide functionalities produces poly(lactic acid) with low polydispersities.

Ring-opening polymerization of rac-lactide and ε-caprolactone using zinc and calcium salicylaldiminato complexes