The polyaddition process involving a hydroxy-terminated linear polyester and an organic diisocyanate in the presence of a series of metal acetylacetonates has been investigated. The method involved kinetic analysis of plasticity build-up in a Brabender Plastograph. Catalytic orders and relative catalytic constants (kc/k0) were determined for Mn(Aa)3, VO(Aa)2, V(Aa)3, Fe(Aa)3, Cu(Aa)2, Co(Aa)3, and Cr(Aa)3; the constants decrease in that order. Catalytic orders are unity for all complexes save manganese, which is 1.75. No correlation between dissociation potential and catalytic coefficient has been found, and it is believed that catalysis may be associated with the paramagnetic properties of the metal.