Chemistry Reference
In-Depth Information
3.4 Propagation
The transition state in a propagation reaction can be illustrated as follows:
XXX
XX
X
X
X
+
XXXX
In the above transition state, the macroradical electron is localized on the terminal carbon. Also,
the two
electrons of the double bond are localized at each olefinic carbon. Interaction takes place
between p-orbital of the terminal atom in the active polymer chain with associated carbon of the
monomer. This results in formation of
p
-bonds [
67
].
The rate of the propagation reaction depends upon the reactivity of the monomer and the growing
radical chain. Steric factors, polar effects, and resonance are also important factors in the reaction.
Another factor that can affect the rate of propagation is interaction between propagating radicals.
Siegmann and Beuermann [
60
] studied the rate of propagation of 1
s
-tridecafluorooctyl
methacrylate and compared it to the rate of propagation of methyl methacrylate. They observed that
k
p
for 1
H
,1
H
,2
H
,2
H
k
p
for methyl
methacrylate. They concluded that this higher rate is due to less interactions occurring between the
propagating macroradicals.
Bowman and coworkers studied the impact of intermolecular and intramolecular interactions on
the polymerization kinetics of monoacrylates [
69
]. They carried out polymerization studies in the
presence of extensive amounts of solvent. This was an attempt to elucidate the effects of intermolec-
ular interactions, such as bulk medium polarity,
H
,1
H
,2
H
,2
H
-tridecafluorooctyl methacrylate polymerization is 1.9 times that of
p
p
stacking, and hydrogen bonding and characterize
the contribution of intramolecular conformational effects to monomer reactivity. Solution polymeri-
zation kinetics of various monomers were measured in the presence of 95 wt% 1,4-dioxane. The
results were compared to bulk polymerization kinetics. The studies revealed that aliphatic acrylates
such as hexyl acrylate exhibit approximately two to threefold reduction in reactivity upon dilution.
Monomers characterized by only hydrogen-bonding features such as hydroxyethyl acrylate exhibit an
8- to 12-fold reduction upon dilution. Monomers possessing only aromatic ring stacking interactions
such as phenyl acrylate exhibit approximately a five to tenfold reduction upon dilution under similar
conditions. Even at a concentration of 5 wt% monomer in 1,4-dioxane, there were approximately two
to fivefold differences in reactivity observed between various acrylates. Bowman and coworkers
attributed these reactivity differences between various acrylates, upon extensive dilution, solely to
intramolecular interactions [
69
].
-
3.4.1 Steric, Polar, and Resonance Effects in the Propagation Reaction
The steric effects depend upon the sizes of the substituents. The resonance stabilization of the
substituents has been shown to be in the following order [
70
]:
O
O
Cl
>
>
>
N
>
>
OCH
3
O
