A series of substituted benzaldehydes were investigated as initiators for the living ring-opening polymerization (LROP) of epsilon-caprolactone (CL) mediated by titanium alkoxides obtained from the Cp2TiCl-catalyzed single electron transfer (SET) reduction of the carbonyl group following the in situ reduction of Cp2TiCl2 with Zn. The aldehyde initiation was demonstrated (NMR) by the presence of the initiator derived fragment on the polycaprolactone (PCL) chain end. The effect of the nature of the aldehyde functionality (R-Ph-CHO, R = H, Cl, PhCH2O, NMe2, CH3O, NO2, and CHO), reagent ratios ([CL]/[aldehyde] = 50/1 to 400/1, [aldehyde]/[Cp2TiCl2] = 1/1 to 1/4, and [Cp2TiCl2]/[Zn] = 1/0.5 to 1/2), and temperature (T = 75-120 degrees C) was investigated over a wide range of values to reveal a living polymerization in all cases with an optimum observed at 90 degrees C with typical stoichiometric ratios of [CL]/ [aldehyde]/[Cp2TiCl2]/[Zn] = 100/1/1/2. (C) 2008 Wiley Periodicals, Inc.