Ziegler polymerizations monometallic

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 most important characteristic of Ziegler-Natta catalysts is their ability to produce stereoregular polymers. On the basis of the monometallic mechanism of Cossee and Arlman, described above, stereoregulation of propylene polymerization can be explained as follows. 

The monometallic mechanism of Cossee and Arlman [10] for Ziegler-Natta polymerization has found favor in the literature because it is based upon quantum mechanical considerations rather than on agreement with the kinetic data [5]. According to this mechanism, as described earlier and shown in Figs. 9.3 and 9.4, the initiation process involves interaction of aluminum alkyl with an octahedral ligand vacancy around Ti which results... 

Although the exact mechanism of the polymerization of olefins by Ziegler-Natta catalysis is not delineated with the precision that would satisfy every one, it is possible to write a reasonable mechanism [1,2]. The first of these is a monometallic mechanism. The following mechanism is for a TiCl3/AlEt3 combination. We have seen above that titanium tetrachloride has been initially used in the polymerization of ethylene. It has been subsequently found that TiCls is better than TiCLj. 

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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