Thermal degradation of polymethacrylonitrile. Part V. The mechanism of the initiation step in coloration reactions

The coloration reaction of polymethacrylonitrile, which involves linking up of adjacent nitrile units, has previously been shown to be initiated by traces of methacrylic acid present in the polymer chains. It also occurs when pure polymer is heated with small quantities of a wide variety of substances including carboxylic acids, phenols, imides, and aliphatic amines. The carboxylic acid initiated reaction exhibits first order kinetics. The rates of initiation by a number of aromatic acids have been measured and compared with the pK a values. The initiating power is strongly influenced by the nature of substituents in the acid molecule, weaker acids giving a higher rate of initiation. A semiquantitative treatment has been applied to phenols with similar results. A mechanism for the initiation step in the color reaction is presented involving nucleophilic attack on a nitrile group. This is consistent with the results obtained for acids and phenols, and the observations of other workers on the alkali‐induced coloration in solution are explained similarly. It is noted that all the substances capable of acting as initiators are Lewis bases.