Chemistry Reference
In-Depth Information
M
R
p
¼
d
½M=
d
t ¼ K
p
ð½
M
Þ½
M
Þ
The expression can be integrated to yield:
In
½
M
0
½
M
C
M
t
¼ k
p
½
½
M
½
M
C
where [M]
0
is the initial monomer concentration
5.3 Polymerization of Oxiranes
Polymerizations of oxiranes or epoxides occur by one of three different mechanisms: (1) cationic, (2)
anionic, and (3) coordination. In this respect the oxiranes differ from the rest of the cyclic ethers that
can only be polymerized with the help of strong cationic initiators. It appears, though, that sometimes
coordination catalysis might also be effective in polymerizations of some oxetanes. The susceptibility
of oxirane compounds to anionic initiation can be explained by the fact that these are strained ring
compounds. Because the rings consist of only three atoms, the electrons on the oxygen are crowded
and are vulnerable to attack [
2
].
5.3.1 Cationic Polymerization
Various Lewis and protonic acids are capable of initiating cationic polymerization of epoxies. Among
them, the following metal salts are effective in polymerizations of ethylene and propylene oxides
[
3
,
4
]: ZnCl
2
, AlCl
3
,SbCl
5
,BF
3
, BCl
3
,BeCl
2
, FeCl
3
, SnCl
4
, and TiCl
4
. Often these polymerizations
can be carried out in bulk without any solvent, particularly in the laboratory. The mechanism of these
reactions can be complex, however, depending upon the particular Lewis acid used. In fact, not all of
these polymerizations can even be treated in general terms as cationic. For instance, ferric chloride
initiated polymerizations of epoxides initially proceed by a mechanism that has all the superficial
features of cationic polymerization. After the initial stages, however, the polymerizations proceed by
a coordination mechanism. This is discussed further in this section.
Stannic chloride yields only low molecular weight poly(ethylene oxide) from ethylene oxide
(molecular weight below 5,000) when the reaction is carried out in ethylene chloride at room
temperature. Some dioxane and dioxolane also form in the process. Following reaction scheme was
proposed [
2
-
6
]:
Initiation
O
O
O
SnCl
4
SnCl
4
+
O
O
