Conformations of linear hydrocarbon polymers

This shows the successive rotation hy 120 about the central C-C hond of the adjacent methylene group. Initially all carbon bonds lie in a plane and then after each rotation a hydrogen atom lies in the initial plane. A detailed analysis may be made of the rotational 

The structures considered above have been concerned with the behaviour of the backbone of the polymer. On proceeding from polyethylene to the next member in the series of olefin polymers, polypropylene, [-CH2-CH(CH3)-] , an asymmetric centre has been introduced into the backbone, in this case the carbon bearing the methyl group. An asymmetric centre is one where it is possible to recognize two isomeric forms that are mirror images and not superimposable. These are often described as optical isomers and the terms d and I are introduced for dextro (right-) and laevo (left-) handed forms. For small molecules these isomers may be resolved optically since they will rotate the plane of polarization in opposite directions. 

As will be discussed later, special synthetic techniques are required to achieve isotactic and syndiotactic structures, and polypropylene, the example above, achieved commercial success only through the discovery of stereoregular polymerization to achieve the isotactic structure. The measurement of the degree of tacticity of a polymer is achieved through C NMR studies of the polymer in solution (Koenig, 1999). 

In syndiotactic polypropylene, the methyl groups are well separated and the TT form is favoured, but there are other energy minima among the gauche conformations and TT/G G and TT/G G sequences can generate left- and right-handed helices, respectively, where the repulsions are minimized (Boyd and Phillips, 1993). The chains may crystallize both in the TT and in the TTG G form, so syndiotactic polypropylene is polymorphic. 

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