Stereochemistry of Polymers

The units of polypropylene and many other polymers are arranged in head to tail or head to head configuration as shown in fig below. In head to tail configuration the pendant groups are on every next or other carbon atoms . While in head to head conformation the pendant groups are not on every next carbon atoms.

The figure above shows the head to tail and Head to Head conformation of polypropylene.

Natta used the term tacticity to describe the stereospecific isomers of polypropylene.

Isotactic, Syndiotactic and Atactic isomers

The isomer having configuration RRRR is called isotactic, the isomer having configuration RSRS is called Syndiotactic and the polymer having random arrangement of the building units is called Atactic. The figure below shows the schematic structure of Isotactic, Syndiotactic and Atactic PVC.

Isotactic PVC.

Syndiotactic PVC.

Atactic PVC.

Threo and Erythro isomers

While most of the polymers have one chiral / assymetric center, some polymers also show ditacticity when two different groups (R&R') are attached to the chiral center. Here erythro and threo diisotactic and erythro and threo disyndiotactic isomers are used as shown in fig below.

Conformers obtained from rotation

The conformers obtained from rotation of carbon- carbon bond of n butane is given by Newman projection as shown in fig below.

Conformers of n-butane.

The most stable form is anti/ trans in which the methyl groups are as far as possible.The energy difference between anti and eclipsed form is about 3KJ and there are numerous conformers between these two extreme forms( the gauche form is shown above).

In polymer like HDPE the methyl groups in n butane is replaced by methylene groups.

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