Ziegler-Natta catalysis chain termination

Many examples of such eliminations have now been seen for the f-block and for d metals. This type of /3-aIkyl elimination is recognized as an important chain transfer step in Ziegler-Natta and metallocene polymerization catalysis. When it occurs the polymer chain terminates in a C=C bond (equation 2) and in certain cases the aUcene chain end can undergo reinsertion and get back into the polymer growth... 

Much effort has been devoted during the last 30 years toward understanding the mechanisms operative in the coordination catalysis of ethylene and a-olefin polymerization using Ziegler-Natta systems (metal halide and aluminum alkyl, sometimes with Lewis base modifiers). Aspects of the complex heterogeneous reactions have been elucidated (jL- ) but the intimate mechanistic detail - for example the role of inhibitors and promoters, kinetics and thermodynamics of chain growth, modes of chain transfer and termination - comes primarily from studies of homogeneous catalysts ... 

When a polymer chain stops its growth after chain transfer, an active center is vacated to allow the formation of a new polymer chain. The chain transfer by the elimina tion of the p-H group is not important for most Ziegler-Natta catalysts, but it is the major chain termination reaction for most metallocene catalysts. The elimination of the p-methyl group does not occur in multiphase catalysis, but is the most important chain termination mechanism for the metallocene catalysts containing CpzMClz-MAO, where M is zirconium (Zr) or hafnium... 

All three reactions rely on repeated alkene insertion into an M—C bond to fomn new C—C bonds via the Cossee mechanism. The three types differ in the relative rates of chain growth (kg) by insertion to termination (k,), normally by elimination. If chain termination is very efficient, alkene dimerization may be seen. If it is very inefficient, a polymer will result, as in Ziegler-Natta and metallocene catalysis. In the intermediate case, oligomeric a olefins can be formed (Fig. 12.1), as in the SHOP process. Even though we discnss these reactions separately, they are nevertheless closely related. 

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