Thermal degradation of polymethacrylonitrile. IV. Formation and decomposition of ketene‐imine structures

The reaction of ketene‐imine structures in polymethacrylonitrile is most conveniently followed by observing the disappearance of the 2012 cm.−1 peak in the infrared spectrum. These structures are stable to 120°C. in the bulk material but disappear rapidly in solution at lower temperatures. Like simple ketene‐imines they are destroyed by addition of water, chlorine, methanol, and hydrogen sulfide. Thermal decomposition in cyclohexanone solution in the temperature range 65–90°C. is a second‐order process. The second‐order rate constant decreases with increasing polymer concentration although, within experimental error, it is independent of the ketene‐imine concentration. The rate constant can also be depressed by addition of polymethyl methacrylate. These observations have been interpreted in terms of diffusion (viscosity) control of the reaction rate. Ketene‐imine structures are formed during polymerization by reaction of the growing polymer radical in the form magnified image By preparing a series of polymers at the same temperature (30°C.) but with different initiator concentrations and determining their ketene‐imine contents, it has been possible to show that the ketene‐imine structures are formed predominantly in the termination step. The ketene‐imine form of the radical appears not to be reactive enough to add monomer readily in a propagation process. The mechanism of the thermal disappearance of ketene‐imine structures is not clear. In particular it is difficult to correlate the second‐order kinetics with the chain‐scission process which is associated with the reaction. There is some evidence, however, that the extent of this chain scission is a measure of ketene‐imine concentration.