Chemistry Reference
In-Depth Information
The intra-intermolecular propagations can result in ring structures of various sizes. For instance,
three-membered rings can form from
transanular
polymerizations of bicycloheptadiene [
100
,
101
]:
n
m
o
Four-membered rings form in free-radical polymerization of perfluoro-1,4-pentadiene [
103
]. The
size of the ring that forms depends mainly on the number of atoms between the double bonds:
F
F
gamma
rays
F
F
FF
F
F
F
F
F
F
11,000 atm.
110
o
C
F
F
F
F
F
F
CF
3
F
FF
Formation of many five-membered rings is also known. One example is a polymerization of 2,3-
dicarboxymethyl-1,6 hexadiene [
98
]:
R
R
R
n
n
R
where, R
COOCH
3.
The polymer that forms, shown above, is cross-linked, but spectroscopic analysis shows that 90%
of the monomer placement is through ring formation [
104
]. Formations of six-membered rings are
also well documented. Two examples were shown above in the polymerization of a quaternary
diethyldiallylamine and in the polymerization of 2,6 disubstitued, 1,6-heptadiene. Many other 1,6-
heptadienes yield linear polymers containing six-membered rings [
105
].
This tendency to propagate intra-intermolecularly by the unconjugated dienes is greater than can
be expected from purely statistical predictions [
106
]. Butler suggested that this results from
interactions between the olefinic bonds [
107
-
109
]. Ultraviolet absorption spectra of several unconju-
gated diolefins does show bathochromic shifts in the absorption maxima relative to the values
calculated from Woodward's rule [
102
,
104
]. This supports Butler's explanation [
107
,
109
].
A RAFT type of controlled/living polymerization (see Sect.
3.14.4
) was applied to cyclopoly-
merization of
ΒΌ
-butyl (hydroxyethyl) acrylate ether dimer [
108
]. The polymerizations, carried out in
xylene at 70
C, yielded polymers with six-membered tetrahydropyran repeat units:
t
O
O
n
n
RO
OR
O
COOR
ROOC
O
3.5 The Termination Reaction
The termination process in free-radical polymerization is caused, as was shown early in this chapter,
by one of three types of reactions: (1) a second order radical-radical reaction, (2) a second order
radical-molecule reaction, and (3) a first order loss of radical activity.
