Propylene polymerization primary insertion

Generally, metallocenes favor consecutive primary insertions as a consequence of their bent sandwich structures. Secondary insertion also occurs to an extent determined by the structure of the metallocene and the experimental conditions (especially temperature and monomer concentration). Secondary insertions cause an increased steric hindrance to the next primary insertion. The active center is blocked and therefore regarded as a resting state of the catalyst (138). The kinetic hindrance of chain propagation by another insertion favors chain termination and isomerization processes. One of the isomerization processes observed in metallocene-catalyzed polymerization of propylene leads to the formation of 1,3-enchained monomer units (Fig. 14) (139-142). The mechanism originally proposed to be of an elimination-isomerization-addition type is now thought to involve transition metal-mediated hydride shifts (143,144). 

Due to this chain-migration process ethylene is polymerized to macromolecules containing multiple branches - rather than to the linearly enchained polymer obtained with classical solid-state catalysts. In propylene polymerization with these catalysts 1,2-insertions give the normal methyl-substituted polymer chains, but after each 2,1-insertion the metal centre is blocked by the bulky secondary alkyl unit and can apparently not insert a further propylene. Instead the metal must then first migrate to the terminal, primary C atom before chain growth can continue by further propylene insertions. By this process, also called 1,CO-enchainment or polymer straightening, some of the methyl or (in the case of higher olefins) alkyl substituents are incorporated into the chain. 

We would like to emphasize here that the branching of polypropylene is controlled by different factors to that of polyethylene [29]. In the case of ethylene the primary/secondary insertion ratio is crucial, whereas in the propylene polymerization catalyzed by diimine catalysts, the ratio between the two alternative insertion pathways (1,2- and 2,1-) is more important [27]. As a result, an opposite temperature effect has been observed for ethylene (increase in branching number with T) and propylene (decrease in branching number with T). 

In propylene polymerization using titanium chloride catalysts, chain propagation takes place via primary (1,2-) insertion of the monomer. For isospecific propagation, there must be only one coordination vacancy and the active site must be chiral. Corradini and co-workers have demonstrated that the asymmetric... 

The polymerization of propylene and/or higher a-olefins with heterogeneous Ziegler-Natta catalysts proceeds by a primary insertion with occasional errors [9]. Vanadium complexes produce syndiotactic polypropylene by secondary insertion [10]. 

It was demonstrated by spectroscopic studies (IR, NMR) that in isotactic polymerization of propylene a so-called primary or type 1-2 insertion... 

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