The ability of various rare earth borohydride and chloride complexes/n-butylethylmagnesium systems to operate styrene chain transfer polymerization in mild conditions has been assessed. Thirteen precatalysts have been considered: the rare earth trisborohydrides Ln(BH4)(3)(THF)(x) (x = 3, Ln = Nd (1), La (2), Sm (3), x = 2, Ln = Y (4), Sc (5)), the rare earth chlorides LnCl(3)(THF)(x) (x = 3, Ln = Nd (6), La (7), Sm (8), Y (9), x = 2, Ln = Sc (10)), the mixed La(BH4)(2)CI(THF)(2.6) (11) and the half-lanthanidocenes Cp*Ln(BH4)(2)(THF)(2) (Ln = Nd (12), La (13)). Six systems were found to be active precatalysts for the polymerization of styrene. 1, 2, and 11 led to an efficient transmetalation of the growing polystyrene chain with the simultaneous occurrence of beta H elimination, whereas 7, 12, and 13 led to catalyzed chain growth behavior. It is noteworthy that the catalyzed chain growth obtained with 12 and 13 occurs with significant stereoselectivity. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 802-814, 2010