Chemistry Reference
In-Depth Information
The cationic polymerization was initiated at room temperature upon irradiation with light in the
visible region in CH
2
Cl
2
solutions in the presence of diphenyliodonium hexafluorophosphate and
the thiophene derivative. According to the suggested mechanism, Ayodgan et al. [
12
] discuss the
formation of
(see Chap.
10
) by the absorption of light. Subsequent electron transfer from
excited Photosensitizer to iodonium ion yields radical cations of the thiophene derivatives. The
resulting strong Bronsted acid derived from this process catalyzes the cationic polymerization.
exciplex
5.3.2 Anionic Polymerization
Anionic polymerizations of ethylene oxide were originally observed as early as 1894 [
13
]. A step-growth
mechanism for these polymerizations was proposed later [
14
]. This mechanism is now well established
[
14
]. The conversion increases linearly with time and the molecular weight also increases with conver-
sion. Reactions with bases like sodium or potassium hydroxides or alkoxides yield only low molecular
weight polymers. The initiation, an S
N
2 displacement, results in formation of an alkoxide ion:
O
R
O
R
O
+
O
Subsequent propagation may occur by nucleophilic displacement involving a new alkoxide ion
formation in each addition:
O
R
O
R
O
+
O
O
O
Termination takes place by a transfer to a hydroxy group of another molecule. It can also take place to
a terminal hydroxyl group of a formed polymer, which starts new chain-growth from the second unit:
O
R
HO
O
O
+
O
O
R
O
OH
+
O
O
The chain ends remain active and higher molecular weights can be obtained by further additions of
the monomer.
The reaction is often more complex than is shown above, because many such polymerizations,
catalyzed by alkali hydroxides or alkoxides, are carried out in the presence of alcohols. This is done to
achieve a homogeneous system. Such conditions, however, lead to exchange reactions:
R
O
ROH
O
Na
+
O
n
R
O
+
RO
Na
O
OH
n
Of course, the exchanges, as shown above, affect the kinetics of the process. The extent of these
reactions is subject to the acid strength of the alcohols present, including the terminal hydroxyl groups
of formed polymers. If their acidities are approximately equal, the exchange reactions take place
throughout the whole course of the polymerization [
2
].
