Polymers with automatic regularity

Several classes of polymer are regular, in the sense that they potentially have translational symmetry, as a simple consequence of the particular polymerisation process used in their preparation. Important examples are the condensation polymers. For example, there is only one possible kind of chemical repeat unit in poly(ethylene terephthalate) (PET), viz. 

The important consequence of the polycondensation reaction is that the chain can only consist of DCH2CH20 units joined to -(-CO)— —(CO units in an alternating sequence. The single bonds along the backbone of the molecule are not collinear as shown in the chemical formula above, but it is nevertheless possible for the chain to acquire translational symmetry by suitable rotations aroxmd them and, for this molecule, the chemical and translational repeat units are the same. Another example is a polyamide such as nylon-6,6 (Bri-nylon) 

the important thing is that the condensation process ensures that this unit repeats itself always the same way round as the chain is traversed from one end to the other. 

---