Stereochemistry of polymerization

In addition to the effects of skeletal structure and of the chemical composition of the repeat units, the properties of a polymer are strongly influenced by its molecular microstructure. Variations in the geometric and configurational arrangements of the atoms in the repeat unit, and the distribution of these different spatial arrangements for the repeat units along the chain, are of particular importance. 

Different molecular microstructures arise from there being several possible modes of propagation. The possibility of head-to-tail and head-to-head placements of the repeat units has been encountered already, with the observation that for both steric and energetics reasons the placement is almost exclusively head-to-tail for most polymers. Therefore in the subsequent sections dealing with the stereochemistry of propagation only head-to-tail placements will be considered. 

For polymers prepared from monomers of the general structure CH2 = CXY, where X and Y are two different substituent groups, there are two distinct configurational arrangements of the repeat unit. 

Polypropylene (X=H, Y=CH3) provides a good example of the importance of tacticity. The commercial material is essentially isotactic and 

Polymers with more complex tacticities are formed from monomers of the general structure XCH=CHY since each backbone carbon atom 

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Given the vastness of the subject matter I have limited myself to dealing with the structural (or static) aspects of macromolecular stereochemistry. An adequate treatment of the stereochemistry of polymerization, with specific regard to the polymerization of olefins and conjugated diolefins, would have occupied so much space and called for such a variety of additional information as to make this article excessively long and complex. I trust that others will successfully dedicate themselves to this task. However, the connection between polymer structure and polymerization mechanism is so important that the fundamentals of dyruunic macromolecular stereochemistry cannot be completely ignored in this chapter. 

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