The ring‐opening polymerization of D,L‐lactide catalyzed by new complexes of Cu, Zn, Co, and Ni Schiff base derived from salicylidene and L‐aspartic acid

Abstract D , L ‐lactide (LA) was first successfully ring‐opening polymerized in melt by Schiff base complexes K[ML] n H 2 O [M = Cu(II), Zn(II), Co(II), Ni(II); n = 2, 2, 3, 3.5; H 3 L = L ‐aspartic acid‐salicylidene Schiff base], which were prepared by Schiff base ligand derived from salicylidene and L ‐aspartic acid and corresponding acetates. The effects of various complexes, the molar ratio of K[ML] n H 2 O/LA, the polymerization temperature, and time were studied in detail. The results show that all complexes studied have the ability to initiate the ring‐opening polymerization of D , L ‐lactide in melt. More than 90% high polymerization conversion and narrow molecular weight distribution (MWD) can be obtained very easily. However, the Ni(II) complex shows better catalytic property than other complexes on the polymerization and the molecular weight (MW) of poly( D , L ‐lactide) (PLA) produced. With a rise in temperature and a prolongation of time, the MW of PLA decreased remarkably. The MW of PLA prepared by all complexes is not very high, which might be related to the crystalline water of complexes. X‐ray study indicated that PLA produced by Ni(II) complex is an amorphous polymer. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3312–3315, 2002