New zinc beta-diiminate complexes are covalently immobilized on silica supports of different pore sizes and structures and the resulting catalysts are evaluated in the catalytic polymerization of lactide. The homogeneous complexes effectively polymerize lactide to produce high molecular weight polylactic acid (PLA) with a narrow polydispersity. The immobilized catalysts also readily convert lactide; however, only oligomeric products with moderate molecular weights are produced when the complexes are immobilized on porous silica. Control experiments indicate that surface silanols may play an important role in modulating the molecular weight of the PLA, with the surface hydroxyl groups acting as chain-transfer agents. High molecular weight, relatively monodisperse polymer can be obtained using a nonporous supported catalyst. (C) 2004 Elsevier Inc. All rights reserved.