Thermal degradation of polymethacrylonitrile. III. Structural abnormalities and degradation processes

The importance of abnormal linkages in polymer degradation processes is well illustrated by the thermal reactions which occur in polymethacrylonitrile. Three quite distinct reactions may be observed in the temperature ranges 20–120°C., 120–220°C., and above 220°C., respectively, and each of these is associated with a different structural tural irregularity. In the low temperature range, in solution, ketene‐imine structures disappear with an accompanying limited decrease in molecular weight. These are presumably created during polymerization by reaction of long chain radicals through the cyanide centers They are formed principally in the chain termination process and disappear in a second‐order reaction whose rate is strongly dependent upon the viscosity of the medium. Above the softening point of the polymer (120°C.) rapid discoloration through yellow and orange to red occurs. This has been attributed to the progressive linking up of sequences of adjacent nitrile groups to form conjugated structures. The reaction is initiated at methacrylic acid units which are present in small concentration in polymethacrylonitrile as it is usually prepared. The reaction can also be initiated by a variety of other substances containing reactive hydrogen atoms and not necessarily incorporated into the polymer chains. A mechanism has been proposed in terms of well‐established chemical processes. Above 220°C. depolymerization to monomer occurs and yields approaching 100% can be attained provided the coloration reaction is suppressed by careful purification of the monomer from which the polymer is prepared. This depolymerization process is superficially similar to that which occurs in polymethyl methacrylate, being initiated most probably by radical formation at the chain terminal structures.