Thermal coloration and insolubilization in polyacrylonitrile

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Abstract
The thermal coloration reaction in polyacrylonitrile is shown to be similar to that which occurs in polymethacrylonitrile, consisting essentially of the linking up of adjacent nitrile groups to form conjugated carbon‐nitrogen sequences. Thus infrared spectral changes in both polymers consist of a decrease in CN absorption accompanied by the development of CN and conjugated CN absorption. The reactions are accelerated similarly, by nucleophilic reagents such as acids either copolymerized into or external to the polymer chains. There are, however, two major differences between the coloration behaviors of these two materials. Firstly, while polymethacrylonitrile remains color‐stable below 200°C. in the absence of nucleophilic initiating agents, pure polyacrylonitrile colors at a finite rate which is not reduced by further attempts at monomer purification. This is due to the fact that the tertiary CH structures in polyacrylonitrile, absent in polymethacrylonitrile, are capable of initiating coloration. It seems more likely that this occurs intramolecularly than intermolecularly. Secondly, while polymethacrylonitrile remains soluble even when extensively colored, polyacrylonitrile rapidly insolubilizes. This is not due to crosslinking in an intermolecular self‐initiation process; rather, it appears that the successive linking up of nitrile groups can move. from molecule to molecule in polyacrylonitrile owing to the more intimate packing of the chains, so that “propagation crosslinks” are produced. This theory of “propagation crosslinking” is supported by the observed influence of various comonomers in inhibiting crosslinking.