Lanthanide tris(2,4,6-tri-tert-butylphenolate)s are highly active initiators for ring-opening polymerization of E-caprolactone to give polycaprolactone with number average molecular weight as high as 8.3 x 104 at 20 degrees C in 20 min in toluene. The effects of solvent, rare earth element, monomer/initiator molar ratio, monomer concentration, as well as polymerization temperature and time have been investigated. The kinetic studies of epsilon-caprolactone polymerization have indicated that the polymerization rate is first-order with respect to both monomer and initiator concentrations. The overall activation energy amounts to 39.3 kJ/mol. Mechanistic studies show that the monomer inserts into the growing chain with the acy-loxygen bond scission. (c) 2008 Elsevier B.V. All rights reserved.