Bimetallic and Monometallic Mechanisms

While a limited amount of experimental evidence does lend support to the bimetallic concept, major objections were voiced by Ziegler, who was of the opinion that like dimeric aluminum alkyls the Ti-Al complex is not likely to be the effective catalyst agent. Other more recent work also favors the second and simpler alternative, the monometallic mechanism. 

Majority opinion now favors the concept that the d-orbitals in the transition element are the main source of catalytic activity and that chain growth occurs at the Ti-alkyl bond, which acts as the polymerization center, the function of the aluminum alkyl being only to alkylate TiCls. The monometallic mechanism presented below are mainly based on the ideas of Cossee and 

Arlraan [10], The monometallic mechanism was put on a sound theoretical basis by the quantum theory developed by Cossee [11). 

After insertion of the monomer between the Ti—alkyl bond, the polymer chain migrates back into its original position and consequently the vacant site also migrates to its original position ready for a further complexing reaction. This migration is necessary, else an alternating position is offered to the monomer and a syndiotactic polymer would thus result 

The main features of the monometallic mechanism are (1) an octahedral vacancy on the Ti is available to complex the olefin (2) the presence of an alkyl to transition metal bond at this site is required and (3) the growing polymer chain is always attached to the transition metal. 

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