Chemistry Reference
In-Depth Information
The reluctance of olefins like propylene or isobutene to form high polymers in
free radical reactions is a result of degradative chain transfer of allylic hydrogens.
Despite these problems useful polymers are made from allyl monomers. High
initiator concentrations are needed because the kinetic chains terminate at low
degrees of polymerization and multifunctional monomers are used to produce
cross-linked structures even at low conversions.
Methyl methacrylate (Fig. 1.4) and methacrylonitrile (8-4) are allylic-type
monomers that do yield high-molecular-weight polymers in free-radical reactions.
This is probably because the propagating radicals are conjugated with and stabi-
lized to some extent by the ester and nitrile substituents. The macroradicals are
less reactive than those in conventional allyl polymerizations and therefore have
lower tendencies for transfer to monomer.
CH
3
C
CH
2
CN
6-5
8.9
Inhibition and Retardation
Some substances retard or suppress free-radical polymerization by reacting with
primary radicals or macroradicals to yield nonradical products or radicals that are
too stable to add further monomer. Some inhibitors and retarders are chain trans-
fer agents; others act by addition processes.
Inhibitors and retarders provide an alternative reaction path to propagating
macroradicals:
Monomer
M
n+1
(8-91a)
Addition
Chain transfer
With TH
M
n
(8-91b)
M
n
H + T
Addition to
M
n
Q
(8-91c)
Q
If the new radicals T
or M
n
Q
do not react readily with more monomer, there
will be a decrease in the concentration of reactive radicals and a concurrent
reduction in the rate of polymerization. When the rate of reaction (8-91b) or
(8-91c) is very much greater than that of reaction (8-91a) and the new radicals T
or M
n
Q
do not add monomer, then high-molecular-weight polymer will not be
formed and
R
p
will be effectively zero. This is a case of inhibition. In retardation
