Kinetic and mechanism of alkene polymerization |
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Authors: | M Bochmann R D Cannon F Song |
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Institution: | (1) Wolfson Materials and Catalysis Centre, School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich, NR4 7TJ, UK |
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Abstract: | 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)
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−
as the counter anion. The titanium system CGCTiMe2/1/AlBu
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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
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+
...B(C6F5)
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] (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)
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compound is an outer-sphere ion pair (OSIP), a consequence of the relegation of the anion into the second coordination sphere
by the γ-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.
The text was submitted by the authors in English. |
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Keywords: | |
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