The Effect of Al-alkyls

Pino 109), in turn, with the catalyst system TiCl4/MgCl2 TIB A, noticed a progressive decrease in the induction period with a simultaneous increase in the maximum polymerization rate and in the activity decay. 

From the results reported it seems clear that the effect the Al-alkyl has on the kinetics is rather complex and difficult to generalize. However, there are two phenomena most frequently described. The first is the increase of the polymerization rate, often accompanied by a considerable kinetic stabilization, upon an increase of the Al-alkyl concentration, in the range of low concentrations. The rate increase may be attributed to the progressive activation of the potential catalytic sites by the 

The second relevant phenomenon is the decrease of the overall rate or the increase of the decay rate which is often observed when the alkyl concentration increases beyond certain limits. According to some authors 88 11this may be due to the adsorption of the Al-alkyl on catalytic sites in competition with the monomer. Still others 92,95 107) attribute it to an overreduction of titanium. This seems plausible when considering the results obtained by Kashiwa78) who showed that Ti2+ is less active than Ti3+ or Ti4+ in ethylene polymerization and completely inactive in the polymerization of propylene. Keii98), in turn, based on the results of Fig. 32, hypothesizes that the decay rate is due to a bimolecular disproportionation of the Ti—R bonds, favored by Al-alkyl reversibly adsorbed on the catalyst surface. 

In the case of propylene, stereospecificity, as well as kinetics, are influenced by the nature and concentration of the Al-alkyl. As observed by many Authors 5°, si, 107,110, U1) besides activity, the stereospecificity is also higher with Al-trialkyls than with DEAC, contrary to what happens with conventional catalysts. According to TaitU1) this would seem to suggest that in MgCl2 catalysts AlEt2Cl, which is less alkylating 

In TiCl3, on the other hand, AlEt2Cl activates only the predominantly stereo-specific surface sites, while AlEt3 can disrupt the crystalline lattice of the catalyst thus forming non-stereospecific centers. 

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