Chemistry Reference
In-Depth Information
In vinyl monomers, both olefinic carbons are potentially subject to free-radical attack. Each would
give rise to a different terminal unit:
R
X
X
R
+
R
X
The newly formed radicals can again potentially react with the next monomer in two ways. This
means that four propagation reactions can occur:
X
X
1.
+
XX
X
X
X
2.
+
X
X
X
X
3;
+
X
XX
X
X
4.
+
Contrary to the above shown four propagation modes, a “head to tail” placement shown in (3.1),
strongly predominates. This is true of most free radical vinyl polymerizations. It is consistent with the
localized energy at the
-carbon of the monomer. Also, calculations of resonance stabilization tend to
predict head to tail additions [
68
].
The free-radical propagation reactions that correspond to conversions of double bonds into single
bonds are strongly exothermic. In addition, the rates increase with the temperature. It is often assumed
that the viscosity of the medium, or change in viscosity during the polymerization reaction does not
affect the propagation rate or the polymer growth reaction. This is because it involves diffusion of
small monomer molecules to the reactive sites. Small molecules, however, can also be impeded in
their process of diffusion. This can impede the growth rate [
50
].
During chain growth, the radical has a great deal of freedom with little steric control over the
manner of monomer placement. Decrease in the reaction temperature, however, lowers mobility of
the species and increases steric control over placement. This is accompanied by an increase in
stereoregularity of the product [
70
,
71
]. The preferred placement is
trans-trans
, because of lower
energy required for such placement. As a result, a certain amount of syndiotactic arrangement is
observed in polymerizations at lower temperatures [
72
].
a
configurations (with respect to
the carbon atoms in the chains) yield zigzag backbones. This was predicted from observations of
steric effects on small molecules [
74
,
75
]. It was confirmed experimentally for many polymers, such
as, for instance, in the formation of poly(1,2-polybutadiene) [
74
] and poly(vinyl chloride) [
72
]. Also,
in the free-radical polymerizations of methyl methacrylate, syndiotactic placement becomes increas-
ingly dominant at lower temperatures. Conversely, the randomness increases at higher temperatures
[
74
]. The same is true in the free-radical polymerization of halogenated vinyl acetate [
75
].
One proposed mechanism for the above is as follows. The least amounts of steric compression
within macromolecules occur during the growth reactions if the ultimate and the penultimate units are
Trans
-
trans
