A series of lanthanum and neodymium complexes comprising the half-lanthanidocenes Cp*La(BH4)(2)(THF)(2) (Cp* = C5Me5) (1) and Cp*Nd(BH4)(2)(THF)(2) (2), the trisborohydrides La(BH4)(3)(THF)(3) (3) and Nd(BH4)(3)(THF)(3) (4), the trichlorides LaCl3(THF)(3) (5) and NdCl3(THF)(3) (6), the triisopropoxides La((OPr)-Pr-i)(3) (7) and Nd((OPr)-Pr-i)(3) (8), and the triaryloxide Nd(OC6H3-Bu-t(2)-2,6)(3) (9) has been assessed for the chain transfer polymerization of isoprene. A transmetalation process is occurring efficiently with the borohydride complexes in the presence of magnesium dialkyl. A gradual decrease of the 1,4-trans stereoselectivity of the reaction is observed at the benefit of 3,4-selectivity with increasing quantities of magnesium dialkyl. This can be at least partially attributed to the growth of 3,4 polyisoprene units onto the magnesium atom. By combining dialkylmagnesium and trialkylaluminum, a 1,4-trans stereospecific reversible coordinative chain transfer polymerization of isoprene is reached using the half-lanthanocene Cp*La(BH4)(2)(THF)(2). (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4640-4647, 2010