Chemistry Reference
In-Depth Information
Oxygen is a prime example of such inhibitors. Polymerization can be initiated
ultimately by the breakdown of hydroperoxides or peroxides that were formed by
the oxidation of monomers or other species in the reaction mixture, but the initial
effect of oxygen is to inhibit polymerization by forming unreactive peroxy
radicals:
M
1
O
2
-
M
a
O
a
O
U
(8-92)
Oxygen is a general inhibitor for vinyl polymerizations and good practice
requires the removal of air from the polymerization system before the reaction is
started.
A monomer that forms a stable radical can be used to inhibit the polymeriza-
tion of another monomer that yields a more reactive radical. Styrene inhibits the
polymerization of vinyl acetate, for example.
Monomers are sold with added inhibitors (usually quite highly substituted phe-
nols) to prevent their premature polymerization during storage and transportation.
The inhibitors are either removed by washing with alkaline water before polymer-
ization or else extra initiator is used to destroy the inhibitor. In that case the poly-
merization will exhibit an induction period as shown in
Fig. 8.2
.
Note that only a very small concentration of inhibitor is needed to suppress
polymerization. Consider a reaction initiated by 10
2
3
M
AIBN, for example [reac-
tion (8-41)]. At 60
C,
k
d
is 1.2
10
2
5
sec
2
1
and
f
is close to 0.6. From
Eq.
3
10
2
8
mol/liter/sec and the total number
of radicals produced in 1 h in 1 liter of reaction mixture will be 5
(8-10)
the rate of initiation will be 1.4
3
10
2
5
mol. If
every inhibitor accounts for one primary or monomer-ended radical, the inhibitor
concentration need only be 5
3
10
2
5
M
to suppress polymerization for an hour.
Retarders and inhibitors differ only in the frequency with which propagating
radicals react with them rather than with monomer and possibly also in the ability
of the radicals resulting from such reactions to reinitiate. It is to be expected,
then, that a compound may not exert the same effect in the polymerization of dif-
ferent monomers. For example, aromatic nitro compounds that are inhibitors in
vinyl acetate polymerizations are classified as retarders in polystyrene syntheses.
3
8.10
Readily Observable Features of Free-Radical
Polymerizations
The rate of polymerization
d
[M]
/dt
and the molecular weight of the polymer pro-
duced are easily measured experimental parameters. The rate of polymerization can
be determined by a number of methods. A commonly used procedure involves
quenching the polymerization with an inhibitor like hydroquinone or ferric chloride,
precipitating the polymer with a nonsolvent, washing the polymer, and weighing it.
The polymerization of vinyl monomers is accompanied by considerable
shrinkage (about 20% in the case of styrene), and the rate of the reaction can be
2
