Chemistry Reference
In-Depth Information
0
P
0
P
ln
½
M
¼DH
=RT
c
DS
=R
T
c
is the
ceiling temperature
for the
equilibrium monomer concentration
.
It is a function of the temperature of the reaction. Because the heat content is a negative quantity, the
concentration of the monomer (in equilibrium with polymer) increases with increasing temperatures.
There are a series of ceiling temperatures that correspond to different equilibrium monomer
concentrations. For any given concentration of a monomer in solution, there is also some upper
temperature at which polymerization will not proceed. This, however, is a thermodynamic approach.
When there are no active centers present in the polymer structure, the material will appear stable even
above the ceiling temperature in a state of metastable equilibrium.
The magnitude of the heat of polymerization of vinyl monomers is related to two effects: (1) Steric
strains that form in single bonds from interactions of the substituents. These substituents, located on
the alternate carbon atoms on the polymeric backbones, interfere with the monomers entering the
chains. (2) Differences are in resonance stabilization of monomer double bonds by the conjugated
substituents [
70
].
Most 1,2 disubstituted monomers, as stated earlier, are difficult to polymerize. It is attributed to
steric interactions between one of the two substituents on the vinyl monomer and the
In the above equation,
-substituent on
the ultimate unit of the polymeric chain [
94
]. A strain is also imposed on the bond that is being formed
in the transition state.
The propagation reaction usually requires only an activation energy of about 5 kcal/mol. As a
result, the rate does not vary rapidly with the temperature. On the other hand, the transfer reaction
requires higher activation energies than does the chain-growth reaction. This means that the average
molecular weight will be more affected by the transfer reaction at higher temperature. When
allowances are made for chain transferring, the molecular weight passes through a maximum as the
temperature is raised. At temperatures below the maximum, the product molecular weight is lower
because the kinetic chain length decreases with the temperature. Above the maximum, however, the
product molecular weight is also lower with increases in the temperature. This is due to increase in
the transfer reactions. The above assumes that the rate of initiation is independent of the temperature.
The relationship of the kinetic chain length to the temperature can be expressed as follows [
5
]:
b
2
dln
n=
d
T ¼ðE
P
1
=
2
E
T
1
=
2
E
I
Þ=RT
where,
E
I
means that if the temperature of polymerization is raised, the kinetic chain length decreases. This is
affected further by a greater frequency of chain transferring at higher temperatures. In addition, there
is a possibility that disproportionation may become more significant.
E
P
,
E
T
, and
E
I
are energies of propagation, termination, and initiation, respectively. A large
3.4.4 Autoacceleration
When the concentrations of monomers are high in solution or bulk polymerizations, typical auto-
accelerations of the rates can be observed. This is known as the
gel effect
or as the
Trammsdorff
effect
, or also, as the
Norrish-Smith effect
[
66
]. The effect has been explained as being caused by a
decrease in the rate of termination due to increased viscosity of the medium. Termination is a reaction
that requires two large polymer-radicals to come together and this can be impeded by viscosity. At the
same time, in propagation the small molecules of the monomer can still diffuse for some time to the
radical sites and feed the chain growth.
