Chemistry Reference
In-Depth Information
The products of these reactions are linear. Actually, this is common to polymerizations of many
heterocyclics. The propagation reactions proceed by stepwise additions of monomer by S
N
2 mecha-
nism to the growing ends of the propagating chains. The NMR spectra of the growing chains only
shows a presence of the oxonium ions [
55
,
56
]:
CH
2
O
The oxonium ions could, in principle, be in equilibrium with minute quantities of carbon cations, -
CH
2
+
that are more active. All evidence, to date, however, shows that in tetrahydrofuran
polymerizations the presence of carbon cations is negligible in the propagation process [
57
]. Also,
the rate constant for propagation of free macroions with the counterions is equal, within experimental
error, to the rate constant for macroions-counterion pairs. This does not appear to depend upon the
stricture of the anion studied. The above information, however, was obtained on large anions. With
smaller anions, differences in the rates of propagation of macrocations and those of macroion-
counterion pairs has not be ruled out.
An S
N
2 attack requires that the reaction occur at the oxygen carbon bond. In such an attack steric
requirements are less restricted than they are in an anionic polymerization. In addition, positive and
negative charges in the macroion-pairs that contain the oxonium ions are dispersed and the anions are
large. This means that the electrostatic interactions are less important in cationic polymerizations of
this type than they are in anionic ones.
When the polymerization of tetrahydrofuran is carried out with the aid of CF
3
SO
3
H, both covalent
and ionic species are present They can be detected during propagation by means of NMR spectros-
copy. Both species exist in a mobile equilibrium. Solvent polarity, apparently, influences the position
of such equilibria. In nitromethane, 95% of the growing chains are macroions. In carbon tetrachloride
95% of them are macroesters. In methylene chloride both species are present in the reaction mixture,
approximately in equal amounts [
58
-
62
]. The propagation rate of macroions, however, is 10
2
times
faster than that of the macroesters. As a result; chain growth even in carbon tetrachloride is still by
way of the ions. The macroesters, therefore, can be considered as dormant species [
59
], or, as some
suggest, even cases of temporary termination [
59
]. The much higher reactivity of the macroions is
attributed to the contribution of the partially released strain in ionic species [
49
]. Macroions and
macroesters can be illustrated as follows:
H
δ
H
CF
6
SO
2
O
O
O
δ
O
C
H
H
covalent
ionic
5.5.3 The Termination Reaction
The termination reactions in tetrahydrofuran polymerizations can depend upon the choice of the
counterion, particularly if the reaction is conducted at room temperature [
60
]. In many reactions, the
chains continue to grow without any considerable termination or transfer [
63
,
64
]. Some refer to this
process as “
living
” polymerization. thus in polymerizations of tetrahydmfuran [
65
] with PF
6
or
