Kinetic and mechanism of alkene polymerization

Triphenylmethyl salts of the very weakly-coordinating borate anions [CN{B(C6F5)(3)}(2)]-(1), [H2N{B(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/AlBu3i (SBI = rac-Me2Si(Ind)(2)) are 30-40 times more active in ethylene polymerizations at 60-100 degrees C than (SBI)ZrCl2/MAO. The quantification of anion effects on propene polymerization activity at 20 degrees C gives the order [CN{B(C6F5)(3)}(2)](-) > [H2N{B(C6F5)(3)}(2)](-) approximate to B(C6F5)(4)(-) >> [MeB(C6F5)(3)](-). The highest productivities were of the order of ca. 3.0 x 10(8) 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 CGCTiMe(2)/1/AlBu3i gave activities that were very similar 3 c, 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)(mu-PMe)B(C6F5)(3) and [(SBI)MCH2SiMe3+center dot center dot center dot 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 gamma-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.