Chemistry Reference
In-Depth Information
The degree of polymerization decreases with increasing reaction temperature
with every mode of initiation except photochemical, where
E
d
is close to zero.
In general, then, when the temperature of a free-radical polymerization is
increased, the rate of polymerization is strongly enhanced and the molecular
weight of the polymer is reduced. These effects are a consequence primarily of
the strong temperature dependence of the rate at which chemical
initiators
decompose.
There is little quantitative information about the effects of temperature on
chain transfer reactions. Activation energies for transfer reactions of polystyryl
radical with its own monomer and with transfer agents like isopropylbenzene are
in the range of 40
60 kJ/mol at 60
C. The activation energy for the transfer con-
stant
C
(
Eq. 8-76
) of polyethylene radical and chain transfer agents like propane
or isobutane are about 16 kJ/mol at 1360 atm pressure and 130
200
C
[22]
.In
general, transfer agents with low transfer constants have higher activation ener-
gies for chain transfer.
As a general rule, higher reaction temperatures result in lower polymer molec-
ular weights because of higher initiation rates and enhanced rates of chain
transfer.
Chain transfer to polymer increases with reaction temperatures. The backbiting
reaction (8-89) results in the production of polyethylene with more short branches
at higher polymerization temperatures. This reaction changes the polymer consti-
tution but not its molecular weight.
8.16.2
PolymerizationDepolymerization Equilibrium
Most polymerizations are characterized by negative enthalpy (
ΔH
p
) changes since
the reactions are exothermic and by negative entropy (
Δ
S
p
) values because the
total disorder of the monomer
polymer system is decreasing. Since the free
energy of polymerization is
Δ
G
p
5
Δ
H
p
2
T
Δ
S
p
(8-120)
Δ
S
p
term
increases at higher temperatures. If side reactions due to chemical degradation do
not intervene, a temperature
T
c
may be reached at which
G
p
will become less negative as the positive contribution of the
T
Δ
0. It will not be
possible to produce high-molecular-weight polymer at temperatures
Δ
G
p
5
.
T
c
, just as
liquids do not aggregate into crystals at temperatures above their melting point.
In a dynamic equilibrium situation, forward and reverse reactions proceed at
equal rates. Thus, reaction (8-12) should be written more generally as
k
p
!
k
dp
M
i
1
M
!
M
i
1
1
(8-121)
where
k
dp
is the reaction rate constant for depolymerization.
Equation (8-13)
is
then replaced by
