Chemistry Reference
In-Depth Information
Table 3.7
Affinity of methyl radical for olefins
a
[
56
,
57
,
67
]
Monomer
Structure
Methyl affinity
Styrene
792
a
-methyl styrene
92.6
cis
-
b
-methyl styrene
40
Trans
-
b
-methyl styrene
92.5
a
,
b
-dimethyl styrene
66
a
,
b
,
b
0
-trimethyl styrene
20
a
From Carrick, Szwarc, Leavitt, Levy, and Stannett, by permission of
American Chemical Society
The reactivities of the propagating polymer-radicals, however, exert greater influence on the rates
of propagation than do the reactivities of the monomers. Resonance stabilization of the polymer-
radicals is a predominant factor. This fairly common view comes from observations that a methyl
radical reacts at a temperature such as 60
C approximately 25 times faster with styrene than it does
with vinyl acetate [
72
]. In homopolymerizations of the two monomers, however, the rates of
propagation fall in an opposite order. Also, poly(vinyl acetate)-radicals react 46 times faster with
n
-butyl mercaptan in hydrogen abstraction reactions than do the polystyrene-radicals [
71
]. The
conclusion is that the polystyrene radicals are much more resonance stabilized than are the poly
(vinyl acetate)-radicals. Several structures of the polystyrene-radicals are possible due to the conju-
gation of the unpaired electrons on the terminal carbons with the adjacent unsaturated groups. These
are resonance hybrids that can be illustrated as follows:
There is not such opportunity, however, for resonance stabilization of the poly(vinyl acetate)
radicals because oxygen can accommodate only eight electrons. The effect of steric hindrance on the
affinity of a methyl radical is illustrated in Table
3.7
[
56
,
57
].
