A series of lanthanum and neodymium borohydrido complexes comprising the trisborohydrides Nd(BH4)(3)(THF)(3) (1a) and La(BH4)(3)(THF)(3) (1b) and the half-lanthanidocenes Cp*Nd(BH4)(2)(THF)(2) (2a) (Cp* = C5Me5) and Cp*La(BH4)(2)(THF)(2) (2b) has been assessed for the chain transfer copolymerization of isoprene and styrene. A transmetalation process is occurring efficiently with the borohydride complexes in the presence of magnesium dialkyl. The transmetalation is accompanied by (i) a gradual decrease of the 1,4-trans stereoselectivity of the reaction at the benefit of 3,4-selectivity and (ii) an increase in the quantity of styrene inserted in the copolymer. This can be at least partially attributed to a magnesium induced co-oligomerization of isoprene and styrene. By combining dialkylmagnesium and trialkylaluminum, a 1,4-trans stereospecific reversible coordinative chain transfer copolymerization of isoprene and styrene is observed when the half-lanthanocene 2b is used as precatalyst. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 1615-1620, 2011