Stereochemistry of a-Olefin Enchainment

If consecutive olefin insertions all occur with the same stereochemical orientation, i.e. on the same olefin enantioface, all alkyl substituents at the polymer backbone will have the same orientation. In such an isotactic polymer chain, all the tertiary C atoms inside its backbone are of the same configuration. If all consecutive olefin insertions occur with opposite enantiofacial orientation, the resulting syndiotactic polymer contains in its backbone tertiary C atoms of strictly alternating configuration. An atactic polymer will finally result when olefin insertions occur randomly, without enantiofacial preference. 

In other cases, for example in syndiotactic propylene polymerization with certain unbridged 0,N-chelated Ti complexes, chain-end chirality can control the stereochemistry of 2,1-olefin insertions by adjusting the chirality of a ligand framework so as to minimize mutual repulsions [P. Corradini, G. Guerra, L. Cavallo, Acc. Chem. Res. 2004, 37, 231]. 

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