Neodymium alkoxides: synthesis, characterization and their combinations with dialkylmagnesiums as unique systems for polymerization and block copolymerization of ethylene and methyl methacrylate.

ABSTRACT

The synthesis and characterization (NMR and X-ray) of a variety of neodymium alkoxides derived from simple and functionalized tertiary monoalcohols, and their application as inorganic precursors in combination with dialkylmagnesium reagents for ethylene and methyl methacrylate (MMA) (co)polymerization have been investigated. Salt metathesis reactions between NdCl(3) and sodium alkoxides in THF led to the formation of trinuclear complexes [Nd(3)(mu(3)-OR)(2)(mu(2)-OR)(3)(OR)(4)(thf)(2)] with R=tBu (1), tAm (2), while aggregate structure [Nd(12)(OtAm)(26)(HOtAm)(2)Cl(11)Na].(OEt(2))(2) (3) was obtained when the synthesis was performed in Et(2)O. [Nd(3)(mu(3)-OtBu)(2)(mu(2)-OtBu)(3)(OtBu)(4)(HOtBu)(2)] (4), prepared by aminolysis of Nd[N(SiMe(3))(2)](3) in hexane, slowly decomposed in toluene into oxo complex [Nd(5)(mu(5)-O)(mu(3)-OtBu)(4)(mu(2)-OtBu)(4)(OtBu)(5)] (5). Finally, the dimer [Nd(2)(mu(2),eta(2)-OR)(2)(eta(2)-OR)(2)(eta(1)-OR)(2)] (OR=OCMe(2)CH(2)CH(2)OMe) (6) was synthesised by aminolysis reaction from the corresponding gamma-donor-functionalized alcohol. Some of these neodymium alkoxides, in particular homoleptic complex 1, when associated in situ to one equivalent of a dialkylmagnesium, allow the formation of an active catalyst for ethylene polymerization. Under mild conditions (0 degrees C, 1 bar), the latter catalyst system exhibited a moderate activity (5-10 kg mol(-1) h(-1) bar(-1)). Effective transfer reactions were observed in the presence of H(2) or PhSiH(3) and renewal/improvement of activity occurred upon extra addition of dialkylmagnesium. The most outstanding feature of this catalytic system lies in the precipitation of the active "Nd-polyethylenyl" species during the ethylene polymerization course as solid S which could be isolated. This heterogeneity was turned to good account, enabling to achieve heterogeneous solid-gas ethylene polymerization and to prepare diblock PE-PMMA copolymers with high diblock efficiency and high molecular weights (M(n) > 200 000). A catalytic cycle for this unique system is proposed based on the isolation of a transmetallation product (7) from a neodymium alkoxide/dialkylmagnesium combination and NMR studies of the latter.