δ-valerolactone anb ε-caprolactone were bulk polymerized employing different diphenylzinc/coinitiator systems. Water and butanone were used as coinitiators and experiments varying Ph2Zn/coinitiator ratiobetween 0.25-1.50 were carried out in order to determinate its effects on the polymerization of these monomers. Both systems are highly influenced by Ph2Zn/coinitiator ratio and a higher catalytic activity was observed for a Ph2Zn/coinitiator molar ratio ∼1. Experiments were carried out starting at 40ºC during one day and, after not being observed changes in the reaction mixture, they were continued for 7 days at 100ºC. This behavior indicates that studied Ph2Zn/coinitiator systems are less active than Ph2Zn alone.