Oligomerization of Nitrile Groups

3 The Chemistry of Stabilization and Carbonization 3.1 Oligomerization of Nitrile Groups 

One of the most significant steps in the preparation of carbon fibers from acrylic precursor fibers is the oligomerization of the nitrile groups. This reaction has originally been studied in context with the problem of thermal discoloration of PAN (e.g. McCarthney Grassie and McNeill Grassie and Hay It was supposed to lead to a so-called ladder structure  

Intermolecular reaction of CN groups also takes place. The dipole-dipole interactions between CN groups of neighboring macromolecules (cf. Sect. 2.1) brings these groups into an ideal position for reaction. 

As all free radical polymerizations of double and triple bonds, this reaction is strongly exothermic. The heat of polymerization is of the order of 6 to 7 kcal per mol of AN monomer unit (Grassie and McGuchan Grassie The resulting structures 

It should, however, be noted that a number of subsequent reactions (oxidation, dehydrogenation, condensation) take place in the later phases of stabilization they will be discussed in detail in Section 3.2. 

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On the other hand, a slow controlled thermal oligomerization of nitrile groups, in air and at moderate temperatures, leads to highly conjugated, crosslinked, oxidized structures which make the fiber non-flammable and infusible. Such fibers are said to be stabilized. They can be further heat-treated in an inert atmosphere to form the carbon fibers (>99% carbon). 

Note that such cis-dienic structures can be present only if intermolecular oligomerization of nitrile groups has taken place.) The Diels-Alder product interrupts conjugation, thus reducing color formation. In the particular case of maleic anhydride, and under the relatively rough conditions of stabilization, this type of reaction should lead to crosslinking, by cross-anhydridization ... 

Olive GH, Olive S, Inter- versus intramolecular oligomerization of nitrile groups in polyacrylonitrile, Polym Bull, 5, 457-461, 1981. 

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