The Cp2TiCl-catalyzed SET reduction of aliphatic, alpha,beta-conjugated, and aromatic aldehydes generates reactive alpha-titanoxy radicals which add readily to styrene and initiate a radical polymerization. The linear dependence of molecular weight on conversion in conjunction with the relatively narrow molecular weight distributions and linear kinetics support the living character of the process which is mediated by the reversible termination of the growing chains with Cp2TiCl. Investigations of the reaction variables reveal a living polymerization in all cases, but with progressively narrower molecular weight distributions (M-w/M-n similar to 1.2) and larger initiator efficiencies (0.7-0.9) with increasing the Cp2TiCl2/aldehyde, aldehyde/styrene, and Zn/Cp2TiCl2 ratios at 70-90 degrees C for a wide variety of aldehyde structures. Aldehydes are thus introduced as a new class of initiators for living radical polymerizations and may also provide novel avenues for the synthesis of block and graft copolymers.