Higher Order Speciation Effects on Plutonium L3 X-Ray Absorption near Edge Spectra.

Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe

Kinetic and mechanism of alkene polymerization

Triphenylmethyl salts of the very weakly-coordinating borate anions [CN{B(C6F5)3}2]- ( 1 ), [H2N{(C6F5)3}2]- and [M{CNB(C6F5)3}4]2- (M = Ni, Pd) have been prepared in simple one-pot reactions. Mixtures of (SBI)ZrMe2/ 1 /AlBu 3 i (SBI = rac-Me2Si(Ind)2) are 30–40 times more active in ethylene polymerizations at 60–100°C than (SBI)ZrCl2/MAO. The quantification of anion effects on propene polymerization activity at 20°C gives the order [CN{B(C6F5)3}2]- > [H2N{(C6F5)3}2]- ˜ B(C6F5) 4 - » [MeB(C6F5)3]-. The highest productivities were of the order of ca. 3.0 × 108 g PP (mol Zr)-1 h-1 [C3H6]-1, about 1.3–1.5 times higher than with B(C6F5) 4 - as the counter anion. The titanium system CGCTiMe2/ 1 /AlBu 3 i gave activities that were very similar to the zirconocene catalyst. The concentration of active species [C*] as determined by quenched-flow kinetic techniques indicates typical values of around 10%, independent of the counter anion, for both the borate and MAO systems. Pulsed field-gradient spin echo and nuclear Overhauser effect NMR experiments on systems designed to be more realistic models for active species with longer polymeryl chains, (SBI)M(CH2SiMe3)(µ-Me)B(C6F5)3 and [(SBI)MCH2SiMe 3 + ...B(C6F5) 4 - ] (M = Zr, Hf), demonstrated the influence of bulky alkyl chains on the ion pair solution structures: while the MeB(C6F5)3 compound exists as a simple inner-sphere ion-pair, the B(C6F5) 4 - compound is an outer-sphere ion pair (OSIP), a consequence of the relegation of the anion into the second coordination sphere by the ?-agostic interaction with the alkyl ligand. The OSIP aggregates to ion hextuples (10 mM) or quadruples (2 mM). Implications for the polymerization mechanism are discussed; the process follows an associative interchange (I a) pathway