The coupled nature of internal rotations in polymers

Since small molecule studies show that steric interactions prevent the occurrence of two adjacent gg states, the chain can be represented as a zigzag projected onto the planes of a square and would have the conformational structure in which state (1) changes into state (2)  

The process of translation of state (1) to state (2) has efiectively moved two gg arrangements along the chain. For this cooperative motion to occur, there is also a requirement that sufficient intermolecular space should be available. Note that the end-to-end length of the chain has not been changed and the number of conformations with and tt sections is likewise unaltered. 

Using the same conformational analysis as for the previous sequence, we can describe this process as  

This causes not only a change in the direction of the chain, but also a change in the number of tt and conformations. This corresponds to a straightening of the chain and would therefore be observed when a mechanical stress is applied. In a real chain, a mixture of both the above processes is possible, as well as simple rotational isomerism at the chain ends. As the chain length increases, so the motion of the chains will become more constrained and the relative importance of these different processes will change. 

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