Propagation on the transition metal-carbon bond

Propene and the higher 1-alkenes can be polymerized to chains with the required degree of tacticity from almost atactic up to very highly tactic structures. However, a syndiotactic polymer can only be obtained from propene, mostly on soluble catalysts. The main factors determining controlled tactic addition are complexation, cis or trans addition, and primary or secondary addition. Most authors agree on the point that the interaction of the alkene molecule with the transition metal atom of the active centre leads to complex formation immediately before monomer insertion into the metal—polymer bond. The assumed existence of the complex is based on indirect experimental evidence and on theoretical considerations. 

A direct proof has not so far been presented let alone a structural description of the complex. 

Evidently 1-alkenes can be inserted into the metal-polymer bond in two ways by primary addition 

The model requires hindered rotation about the V—R bond. Considering the force fields of the Me of the last added unit and of the X ligands, this requirement is fulfilled at low temperatures. 

The presence of two double bonds greatly increase the number of the interconnected effects determining the configuration of an added monomeric unit. In this case the stereochemistry may already be established during the transition complex formation between the monomer and the centre or after the insertion of the diene into the metal—carbon bond. The realization of either of these possibilities is determined by the specific driving force of addition, composed of the electronic, steric, and isomeric states of the diene. 

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