The ring-opening polymerization of omega-pentadecalactone (PDL), a 16-membered lactone, in the bulk and in solution using yttrium isopropoxide as an initiator was investigated. All isopropoxide groups of yttrium isopropoxide participated in the initiation and the polymerization took place via acyl-oxygen cleavage of the monomer. The molecular weight of the resulting polymers could be controlled effectively by varying the monomer/initiator ratio. An induction period was observed for the bulk polymerization conducted at 60 degrees C. DSC analyses indicated that poly(omega-pentadecalactone) is a highly crystalline polymer, having a high crystallization rate.