Ziegler polymerizations bimetallic

Fr . 1. Mechanisms of Ziegler polymerization (a) Bimetallic mechanism, (b) Monometallic mechanism [after Cossee]. (c) Monometallic Cossec mechanism (5). Crystal structure of a-TiC. la with active center in the surface. A propylene rnolocule is inserted in the Cl v acancy forming a 7r-bond with Ti ion A. Figures lb and Ic wore reprinted with the permission of the Faraday Society. 

The nature of the cocatalyst in the Ziegler-Natta systems exerts a significant effect on the polymerization rate and isotacticity of the polymer product Interesting data were obtained by Keii et al. when the cocatalyst was changed during polymerization. The pronounced effect of the cocatalyst is the basis of the concept on the bimetallic composition of ACg according to which the latter are represented, for example, as binuclear complexes including titanium and aluminium. 

FIGURE 22.5 A bimetallic mechanism for coordination (Ziegler-Natta) polymerization. 

In recent years, there has been an upsurge of interest in bimetallic mixed M-Al complexes (where M is an early transition metal) with structures related to 58, because of their activity in Ziegler-Natta or Kaminsky-type alkene polymerizations."" ... 

The original discovery of this type of catalysts is,of course, due to K. Ziegler. G. Natta having contributed ingenious applications, particularly to stereospecifle polymerization. However we use the expression Ziegler-Natta catalysts" in order to differentiate the bimetallic transition metal—aluminum alkyl catalysts from the monometallic aluminum alkyl catalyst of the Aufbau" reaction, also due to Ziegler. 

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