Intra-versus Intermolecular CN Oligomerization

The emphasis here is on the word forced . As amply discussed in Section 2.1, the mutual repulsion between adjacent CN groups in the same chain is large, and tends to hold them apart in a helix-like structure. The dipole moment is certainly still quite high, if a free radical attack has transformed a CN group into a R—C=N radical. Neighboring CN groups from different macromolecules, on the other hand, attract themselves. This kind of arguments appears to favor the intermolecular reaction (Formula 5) over the intramolecular reaction (Formula 3). 

Fitzer et al. suggested, based on the shrinkage behavior of PAN fibers during 

The exothermic peak evidently does not account for the full amount of exothermic reaction taking place, since the exotherm is partly masked by the endotherm. Fig. 5 c shows, for comparison, a statistic copolymer containing AN and VCl in an approximately 1 1 ratio (commercial product). Although sequences of AN units can be assumed to be present in this copolymer, the picture is very similar to Fig. 5b. 

We conclude that the intermolecular reaction is certainly feasible, and possibly has an even greater importance than the intramolecular reaction  

In copolymers with larger sequences of AN units, there are probably one or several intramolecular steps in between crosslinks. At any rate, however, extensive cross-linking takes place already in the early stages of heat treatment. 

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