Chemistry Reference
In-Depth Information
Cyclic allylic sulfides were shown to polymerize by a free-radical ring-opening mechanism [
231
].
The key structural unit that appears to be responsible for the facile ring-opening is the allylic sulfide
fragment. In it the carbon-sulfur bond is cleaved [
231
]:
S
S
S
R
etc.
n
R
S
n
S
R
n
S
S
S
S
or
S
It was also reported recently that a controlled free-radical ring-opening polymerization and chain
extension of the “living” polymer was achieved in a polymerization of 2-methylene-1,3-dioxepane in
the presence of 2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO) [
232
] The reaction was
initiated with di-
tert
-butyl peroxide at 125
C
At high concentrations of the piperidinyloxy radical, the polydispersity of the product was
1.2 [
232
]
5.17 Thermodynamics of Ring-Opening Polymerization
The stability of the ring structure as well as the stability of the resultant linear polymer determines the
polymerizability of cyclic monomers. Thermodynamic factors, therefore, are of paramount impor-
tance in ring-opening polymerizations [
233
]. Actually, the polymerization of many bond-strained
ring monomers is favored thermodynamically. Thus,
for instance,
DH
, for three-membered
69.1 J/mole
C. It was shown (Sawada) that in three-
and four-membered ring structures, the change in enthalpy is a major factor in determining
cycloalkanes is
113.0 kJ/mole and
DS
is
DF
, the
change in free energy. For three-membered cycloalkanes
DF
is
92.0 kJ/mole, while for four-
membered ones it is
is a major factor in polymerization of
five-membered cyclic monomers. The six-membered ring monomers that are relatively strain free are
very hard to polymerize. An exception is trioxane, whose
90.0 kJ/mole. The entropy change,
DS
is close to zero. On the other hand, the
enthalpy and entropy factors contribute about equally to the free energy change of larger rings. This
means that with increases in temperature
DH
becomes less and less negative and above certain
temperatures some large cyclic monomers will not polymerize. The transanular strain in seven- and
eight-membered rings contributes to their polymerizability. Presence of substituents in cyclic
monomers has a negative effect on the thermodynamic feasibility to polymerize. On the other
hand, thermodynamic feasibility alone does not determine whether a cyclic monomer will
polymerize.
The entropy changes do not show much dependence of on angle strain. They are susceptible,
however, to configurational influences. Sawaada [
233
] writes the entropy change of polymerization as
a function of the probability of ring closure:
DF
DS
p
¼b
ln
P a
where
P
is the probability of ring closure and
a
and
b
are constants. The probability of ring closure for
a chain with
repeating units can be taken as a function of the probability that the chain ends will
come together. This probability is usually expressed as a radius of gyrations, (
n
2
), the root square
r
