This article discusses a new borane chain transfer reaction in olefin polymerization that uses trialkylboranes as a chain transfer agent and thus can be realized in conventional single site polymerization processes under mild conditions. Commercially available triethylborane (TEB) and synthesized methyl-B-9-borabicyclononane (Me-B-9-BBN) were engaged in metallocene/MAO [depleted of trimethylaluminum (TMA)]-catalyzed ethylene (Cp2ZrCl2 and rac-Me2Si(2-Me-4-Ph)(2)ZrCl2 as a catalyst) and styrene (Cp*Ti(OMe)(3) as catalyst) polymerizations. The two trialkylboranes were found-in most cases-able to initiate an effective chain transfer reaction, which resulted in hydroxyl (OH)-terminated PE and s-PS polymers after an oxidative workup process, suggesting the formation of the B-polymer bond at the polymer chain end. However, chain transfer efficiencies were influenced substantially by the steric hindrances of both the substituent on the trialkylborane and that on the catalyst ligand. TEB was more effective than TMA in ethylene polymerization with Cp2ZrCl2/MAO, whereas it became less effective when the catalyst changed to rac-Me2Si(2-Me-4-Ph)(2)ZrCl2. Both TEB and Me-B-9-BBN caused an efficient chain transfer in the Cp2ZrCl2/MAO-catalyzed ethylene polymerization; nevertheless, Me-B-9-BBN failed in vain with rac-Me2Si(2-Me-4-Ph)(2)ZrCl2/MAO. In the case of styrene polymerization with Cp*Ti(OMe)(3)/MAO, thanks to the large steric openness of the catalyst, TEB exhibited a high efficiency of chain transfer. Overall, trialkylboranes as chain transfer agents perform as well as B H-bearing borane derivatives, and are additionally advantaged by a much milder reaction condition, which further boosts their applicability in the preparation of borane-terminated polyolefins. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3534-3541, 2010