Ziegler-Natta catalysis polymer chain growth

In an attempt to provide a model to explain the growth of the polymer chain satisfactorily, several hypotheses have been suggested. In Boor s book on Ziegler-Natta catalysis (179), he exhaustively reviewed the literature and addressed four mechanisms for chain growth. The four mechanisms are based on the description of the center where chain growth takes place ... 

Polymer Chain Growth. The essential characteristic of Ziegler-Natta catalysis is the polymerization of an olefin or diene using a combination of a transition-metal compound and a base-metal alkyl cocatalyst, normally an aluminum alkyl. The function of the cocatalyst is to alkylate the transition metal, generating a transition-metal-carbon bond. It is also essential that the active center contains a coordination vacancy. Chain propagation takes place via the Cossee-Arlman mechanism (23), in which coordination of the olefin at the vacant coordination site is followed by chain migratory insertion into the metal-carbon bond, as illustrated in Figure 1. 

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... 

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. 

---