Polypropylene stereoregulation

The symmetry of the metallocene and also the kind of procatalyst metal atom, the nature of the catalyst activator and the polymerisation temperature determine the polypropylene tacticity. The general stereoregulation behaviour of metallocene catalysts may be explained in terms of the local chirality, or chirotopicity, of the catalytic sites bonded to the same metal atom. For this analysis, the structure of metallocenes as catalysts should be considered. 

In reality various direct experimental evidences exist to support such an hypothesis first of which, in the case of polypropylene, is the well known heterogeneity according to stereospecific properties of centres Actually, the stereoregular polypropylene isotactic fraction and the amorphous one generally show broad MWD too, thus proving the probable existence of different active centres although similar in stereoregulating ability... 

As an additional example, benzoate or phthalate esters are used to modify the solid support and are incorporated into the solid-phase structure. Here, they become effective as internal bases [14], which are critical components in high-stereospecific polypropylene catalysts. As the range of suitable internal bases for stereoregulation increases, these compounds also find their way into reaction schemes involving Grignard reagents. 

Catalyst composition affects both stereoregulation and polymer yield. Thus, TP+ is a more active producer of isotactic polypropylene than TP+ or Ti, although an increase in the length of the associating alkyl group decreases the efficiency of stereoregular placements. Varying the transition metal and the associated aluminum compounds in the catalysts also influences the nature of the product. 

The following forms of polypropylene are differentiated in the stereoregulated polymerization of this polymer ... 

Reaction stereoregulation mechanisms have been reduced to mathematical, kinetic models in several cases. A one-parameter Bernoullian statistical model accounts for the NMR methyl pentad intensities of both syndiotactic and isotactic polypropylenes obtained at subambient temperatures with homogeneous, achiral catalyst precursors. The polypropylenes have. ..rrrrrmrrrrr... and. ..mmmmmrmmmmm... microstructures respectively. The m and r deffects are consistent with the chain-end configurations being responsible for stereoregulation. Two additional models of the chain-end control type are the first- and the... 

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