Vinyl polymers and tacticity

The three possible ways for adjacent monomer units of a vinyl polymer to join are 

The head-to-tail arrangement predominates for most vinyl polymers prepared by the normal methods, so it is assumed in the following sections that only head-to-tail structures are present unless stated otherwise. This leads to chains that are chemically regular in the sense discussed in section 4.1.1. 

More generally, isotactic means that all repeat units have the same handedness and syndiotactic means that the handedness alternates along the chain. Alternate chemical repeat units along the syndiotactic chain are mirror images of each other. In the translationally symmetric conformations shown in fig. 4.2 the translational repeat unit for the syndiotactic chain is two chemical repeat units, whereas it is only one for the isotactic chain. 

A particular state of tacticity is a particular configuration of the molecule and cannot be changed without breaking and reforming bonds and, at ordinary temperatures, there is not enough thermal energy for this to happen. Rotations around bonds produce only different conformations. A vinyl polymer is therefore unlikely to be appreciably crystalline unless it is substantially either isotactic or syndiotactic the atactic chain cannot get into a state in which it has translational symmetry. 

The following techniques (and others) can give information about tac-ticity. 

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