Rare earths/main group metal alkyls catalytic systems for the 1,4-trans stereoselective coordinative chain transfer polymerization of isoprene

摘要

A series of lanthanum and neodymium complexes comprising the half-lanthanidocenes Cp*La(BH_4)_2(THF)_2 (Cp* = C_5Me_5) (1) and Cp*Nd(BH _4)_2(THF)_2 (2), the trisborohydrides La(BH _4)_3(THF)_3 (3) and Nd(BH_4) _3(THF)_3 (4), the trichlorides LaCl_3(THF) _3 (5) and NdCl_3(THF)_3 (6), the triisopropoxides La(O~iPr)_3 (7) and Nd(O~iPr)_3 (8), and the triaryloxide Nd(OC_6H_3-~tBu _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(BH _4)_2(THF)_2.