Chemistry of the Degradation of Vinyl Polymers to give Carbon Fibre Precursors

Abstract

An investigation of the oxidative degradation of polyacrylonitrile,
a precursor for carbon fibre manufacture, has been made by examining the
oxidation of 'model* compounds of the polymer. During the attempted
preparation of one of these 'models', 2,)-dicyanopentane, it became
apparent that this compound was unstable in the presence of sodium
cyanide.

Polyacrylonitrile was found to be unstable under similar conditions;
refluxing a solution of the polymer in dimethylformamide with sodium
cyanide produced a black nonflammable material with very similar
properties to oxidatively degraded polyacrylonitrile, The initial
product from the cyanide, polyacrylonitrile reaction is extremely
sensitive to oxidation, absorbing oxygen at room temperature. This
behaviour can not be explained by the formation of a hydrogenated
naphthyridine structure by cyclisation of the nitrile groups in the
polymer.

An alternative mechanism for the depradetion under these conditions
is proposed. Before cyclisation the propogating ketimine groups
(>C =H) undergo tautomeric rearrangement to the enamine form
( >C-=C—NH2,), Subsequent cyclisation results in a condensed
1 ,4.-dihydropyridine structure which, by analogy with the known chemistry
of 1,4-dihydropyridines, will oxidise on exposure to air to a fully
aromatic condensed pyrinoid system.

This theory has been extended to cover the thermal and oxidative
degradations of polyacrylonitrile in the solid state, It is concluded
that the condensed 1 ,-dihydropyridine is formed first and, under
oxidative conditions, is transformed to a mixture of condensed pyrinoid
and 4«pyridone sequences. Chain scission also occurs but hydrogen
bonding through the 4=pyridone species prevents loss of orientation and
damage to the fibre structure.

Date of Award	1972
Original language	English