1,3-Diene Polymerization Mediated by Homoleptic Tetramethylaluminates of the Rare-Earth Metals

During the past two decades homoleptic tetramethylaluminates of the trivalent rare-earth metals, Ln(AlMe4)(3), have emerged as useful components for efficient catalyst design in the field of 1,3-diene polymerization. Previous work had focused on isoprene polymerization applying Ln(AlMe4)(3) precatalysts with Ln = La, Ce, Pr, Nd, Gd and Y, in the presence of Et2AlCl as an activator. Polymerizations employing Ln(AlMe4)(3) with Ln = La, Y and Nd along with borate/borane co-catalysts [Ph3C][B(C6F5)(4)], [PhNMe2H][B(C6F5)(4)] and [B(C6F5)(3)] were mainly investigated for reasons of comparison with ancillary ligand-supported systems (cf. half-sandwich complexes). The present study investigates into a total of eleven rare-earth elements, namely Ln = La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Y, Er and Lu. A full overview on the polymerization behavior of Ln(AlMe4)(3) in the presence of perfluorinated borate/borane cocatalysts and R2AlCl-type activators (R = Me, Et) is provided, probing the monomers isoprene and 1,3-butadiene (and preliminary ethylene). Virtually complete cis-1,4-selectivities are obtained for several catalyst/cocatalyst combinations (e.g., Gd(AlMe4)(3)/Me2AlCl, >99.9%). Insights into the black box' of active species are obtained by indirect observations via screening of pre-reaction time and cocatalyst concentration. The microstructure of the polydienes is investigated by combined H-1/C-13 NMR and ATR-IR spectroscopies. Furthermore, the reaction of [LuMe6(Li(thf)(x))(3)] with AlMe3 has been applied as a new strategy for the efficient synthesis of Lu(AlMe4)(3). The solid-state structures of Gd(AlMe4)(3) and Tb(AlMe4)(3) are reported.