Phenomenological Aspects of the Reaction

catalysts are the only ones able to polymerize C2H4 and C3H6, two basic products of petrochemistry, into (stereo) regular high M.W. polymers. That is an extremely important industrial achievement, as illustrated by the (approximate) data below which mean that, taking into account polydienes (see Part II) and a few experimental polymers, Z.N. catalysis alone is not responsible for a production of ca. 10 miUion tons a year of excellent products (obtained in one step from basic petrochemicals) that is a unique case in industrial organic chemistry, particularly if one considers the extreme diversification achieved in their apphcations. 

Insoluble (cristalline) Z.N. catalysts are the only ones to control properly propylene conversion into the desirable isotactic polymer. They are essentially based on a mixture of TiCl3 and Al-alkyl. 

These a and y forms are violet, and have a structure composed of layers of Ti-atoms sandwiched between 2 chloride ion layers. The Ti atoms, in an octahedral coordination geometry, occupy positions in the interstices formed by close packed Cl ions. That packing is hexagonal for the a form and compact cubic for the y form (One can also obtain a chaotic d variety by grinding the a or y forms, possible with AICI3). 

The influence of that crystalline structure on the course of the reaction is decisive while remarkable stereoselectivity can be achieved (up to 98—99%) with a and y forms, only limited control (ca. 50% isotacticity) is exerted by the variety. 

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