Rare-earth alkyl complexes supported by formamidinate ligands: synthesis, structure, and catalytic activity for isoprene polymerization. 
A series of rare-earth metal monoalkyl complexes supported by N,N'-di(2,6-dialkylphenyl)formamidinate ligand (L)2RECH2SiMe3·thf [L(1) = HC(N-2,6-Me2C6H3)2, RE = Y (1), L(2) = HC(N-2,6-(i)Pr2C6H3)2, RE = Y (2), Er (3), Dy (4), Sm (5), and Nd (6)] were synthesized by alkyl elimination reaction or by salt metathesis reaction in good yields.  All complexes were characterized by elemental analyses, FT-IR spectroscopy and single crystal X-ray diffraction.  In combination with [Ph3C][B(C6F5)4] and alkylaluminium, these complexes displayed a good activity towards isoprene polymerization to give polyisoprenes with high molecular weight (M(n) > 10(4)) and narrow molecular distribution (PDI < 2.0).  The influence of alkylaluminium, central metal, temperature, sequence of addition of alkylaluminium and [Ph3C][B(C6F5)4] on the polymerization of isoprene was studied.  It was interesting to find that addition of the cocatalysts sequence has a great influence on the regioselectivity of the polymerization.  High 1,4-regioselectivity polymerizations of isoprene (as high as 98%) were observed when the catalysts were added in the order [RE]/[alkylaluminum]/[borate].