A copolymer of phenylisocyanate (PhNCO) and epsilon-caprolactone (CL) was synthesized by the rare earth chloride systems lanthanide chloride isopropanol complex (LnCl(3)center dot 3iPrOH) and propylene epoxide (PO). Polymerization conditions were investigated, such as lanthanides, reaction temperature, monomer feed ratio, La/PO molar ratio, and aging time of catalyst. The optimum conditions were: LaCl3 preferable, [PhNCO]/[CL] in feed = I (molar ratio), 30 degrees C, [monomer]/[La] 200, [PO]/[La]=20, aging 15 min, polymerization in bulk for 6 h. Under such conditions the copolymer obtained had 39 mol % PhNCO with a 78.2% yield, M-n=20.3 x 10(3), and M-w/M-n = 1.60. The copolymers were characterized by GPC, TGA, H-1-NMR, and C-13-NMR, and the results showed that the copolymer obtained had a blocky structure with long sequences of each monomer unit. (c) 2006 Wiley Periodicals, Inc. I Appl Polym Sci 103: 2135-2140, 2007