Six Sm(III) phenolates were firstly synthesized with un-substituted phenol (P), 2-tertbutylphenol (213), 4-tertbutylphenol (413), 4-methylphenol (4M), 2-tertbutyl-4-methylphenol (2B4M) and 2,6-ditertbutyl-4-methylphenol (26B4M) separately. All the phenolates were used as single component catalyst for the ring-opening polymerization (ROP) of epsilon-caprolactone (CL). The experimental results showed that, with phenols substituted by single or double tertbutyls at different sites, Sm(III) phenolates resulted in ordered variations both in their catalytic activities and in ROP characteristics. In order to find the correlation between these ordered variations and tertbutyl substituents' electronic and steric effects, data describing phenols' geometric and charge distribution parameters were obtained with quantum chemical (QC) calculation and discussed with experimental results in organized groups. It was found that, single tertbutyl on phenol, especially ortho one, would induce mixed electronic effect but positive steric effect, latter of which induce easier ROP under mild conditions, increased catalytic activity while more inter- and intramolecular transesterifications, resulting in PCL with wider molecular weight distribution (MWD). Two ortho tertbutyls induce not only positive steric effect but also positive electronic effect, which induce the highest catalytic activity of Sm(26B4M)3, Most transesterifications and the resultant widest MWD of PCL. (c) 2004 Elsevier B.V. All rights reserved.