Chemistry Reference
In-Depth Information
other one occurs above 240
C. At the low temperature, the polymers develop dark color. There is,
however, very little evolution of volatiles. At the high temperature, on the other hand, there are
evolutions of volatiles and thermally stable residues develop.
The low temperature darkening process of polyacrylonitrile was shown to be intramolecular
cyclization and polymerization of the cyanide groups [
475
,
476
]. The overall reaction can be
illustrated as follows:
CCCC
N
N
N
NNNN
N
The above shown ladder structure is actually a vary idealized picture. Nevertheless, the formation
of fused tetrahydropyridine rings was demonstrated by IR data [
475
,
476
]. At the same time, there are
many irregularities in the above shown structure. Also, it was demonstrated that the longer are the
isotactic sequences in the polymeric structures, the longer are the sequences of ladder structures that
form [
479
].
High temperature degradation of polyacrylonitrile leads to formation of oligomers. The general
form of the oligomerized material can be shown as follows [
480
]:
NC
CN
N
H
n
CN
x
where,
0-5.
The above structure occurs as a result of an initiation and termination process of cyclization at
frequent intervals along the chain. The growth of the ladder structure, however, terminates due to
hydrogen transfer. This process is associated with the atactic sequences in the polymer chain:
x ¼
0-2 and
n ¼
NC
NC
CN
CN
CN
CN
N
H
CN
NN
N
H
N
H
develops color upon heating as a result of linking up of adjacent carbons
and nitrogen atoms in intramolecular cyclization reactions similar to acrylonitrile.
This reaction is initiated primarily by impurities that are often present in the polymer both at the
end of the chains and at various locations at the backbones. Due to this ring formation, the amount of
monomer that can be obtained from the polymer at 200
C is approximately 50%. If, however, the
polymer is prepared from highly purified monomer, the yield of monomer upon thermal degradation
at 300
C is 100%. Also, a yellow color does not develop from such a polymer at temperatures of
120-220
C[
457
].
Poly(vinylidine cyanide)
Polymethacrylonitirle
has the structure that would suggest formation of a double ladder polymer
upon pyrolysis. It was shown, however, in an early study that this does not happen. Instead, the
polymer mostly depolymerizes [
481
,
482
].
