The polymerization of caprolactone (epsilon-CL) was initiated by yttrium triisopropoxide {Y(OPri)(3)} bimetallic isopropoxide of yttrium and aluminum {Y[Al(OPsi)(4)](3)}, yttrium and tin(II) {Y[Sn(OPri)(3)](3)}, and tin(II) and yttrium {Sn[Y(OPri)(4)](3)}, respectively. The polymerization was carried out through coordinative insertion of a monomer into the free metal-oxygen bond. The molecular weight and yield of poly(epsilon-caprolactone) (PCL) were affected drastically by the mol % of the initiator to the monomer (C-o/M-o). The results showed that Y(OPri)(3) and Sn[Y(OPri)(4)](2) were more effective than were {Y[Al(OPri)(4)](3)} and {Y[Sn(OPri)(3)](3)} for the polymerization of epsilon-CL. When polymerization was conducted at 5 degrees C using Y(OPri)(3) as the initiator or at 10 degrees C using Sn[Y(OPri)(4)](4) as the initiator, polymers with a molecular weight of 45.9 x 10(3) or 54.0 x 10(3) and high yield resulted in 30 or 5 min, respectively. The polymer was characterized by FTIR, H-NMR, and GPC.