Chemistry Reference
In-Depth Information
R
p
/
[M]
2
is kept constant by adjusting [I] during the course of the reaction. DP
n
is
measured in a series of reactions, each with a different [Ta]
/
[M] ratio and the
data are plotted as shown in
Fig. 8.1
, where styrene polymerization is taken as an
example. Each plot yields a straight line with slope
C
.
Table 8.2
lists some transfer constants for various compounds in the polymeriza-
tion of styrene, methyl methacrylate, and vinyl acetate. All transfer reactions involve
a transfer agent and a radical and the reaction rates depend on the characteristics of
both reactants. Propagation reactions in free-radical kinetic chains must be fast in
order to exert a significant effect before termination events intervene. This means
that they must have relatively low activation energies. They cannot be strongly endo-
thermic, as a consequence, because the energy of activation for a reaction cannot be
less than the difference in enthalpies of the reactants and products. The enthalpy
change in a transfer reaction will depend on the nature of the bonds which are broken
and formed and the relative stabilities of the radicals M
n
and T
in reaction (8-67). In
general, a given transfer agent will be more reactive (
C
is greater) for a reactive radi-
cal like those in ethylene or vinyl chloride polymerizations than for a resonance-
stabilized radical like that of styrene. Similarly, when a given monomer is being
polymerized, aliphatic compounds that yield tertiary radicals will be more effective
transfer agents than those that produce secondary radicals. Chain transfer activity is
also enhanced by the possibility for resonance stabilization, and isopropyl benzene is
a more active transfer agent than propane, for example. The slopes of the plots in
Fig. 8.1
and the data in
Table 8.2
conform to these general rules.
200
4
3
2
160
120
10
5
D P
n
80
1
40
0
5
10
15
20
25
[Transfer agent]
[Styrene]
FIGURE 8.1
Plots of
against [Ta]/[M] (
Eq. 8-77b
) for various chain transfer agents in the
polymerization of styrene at 100
C
[7]
. Line 1, benzene; 2, toluene; 3, ethylbenzene; 4,
isopropylbenzene. (The data points have been omitted for clarity.)
ð
1
=
DP
n
Þ
