Chemistry Reference
In-Depth Information
Fig. 3.1
Illustration
of the effects of inhibitors
and retarders. (A) Normal
polymerization rate,
(B) effect of a retarder,
(C) effect of an ideal
inhibitor, and (D) effect
of a non ideal inhibitor.
The time between A and
C is the induction period
caused by an ideal inhibitor
A
% Conversion
C
B
D
induct. period
Time
where Z is the inhibitor or the retarder in chain-growth termination:
k
Z
M
n
þ
Z
!
M
n
þ
Z
ð
and/or
M
n
Z
Þ
To simplify the kinetics it is assumed that Z and ~
M
n
Z do not initiate new chain growth and do
not regenerate Z upon termination.
3.10 Thermal Polymerization
A few monomers, such as styrene and methyl methacrylate, will, after careful purification and
presumably free from all impurities, polymerize at elevated temperatures. It is supposed that some
ring-substituted styrenes act similarly. The rates of such thermal self-initiated polymerizations are
slower than those carried out with the aid of initiators. Styrene, for instance, polymerizes only at a rate
of 0.1% per hour at 60
C and only 14% at 127
C. The rate of thermal polymerization of methyl
methacrylate is only about 1% of the rate for styrene [
163
,
164
]. Several mechanisms of initiation
were proposed earlier. The subject was reviewed critically [
165
]. More recently, the initiation
mechanism for styrene polymerization has been shown by ultraviolet spectroscopy to consist of an
initial formation of a Diels-Alder dimer. The dimer is believed to subsequently transfer a hydrogen to
a styrene molecule and as in doing so form a free radical [
166
]:
CH
2
H
s
tyren
e
H
+
2
dimer
Thermal polymerization of methyl methacrylate, on the other hand, appears to proceed through an
initial dimerization into a diradical [
167
]. This is followed by a hydrogen abstraction from any
available source in the reaction mixture.
