Chemistry Reference
In-Depth Information
acid and the cocatalyst adds to the monomer to initiate polymerization. The
monomer cation that is formed does not form a covalent link with the complex
anion, because the size and solvation of the latter reduce its nucleophilicity.
The use of cocatalysts is desirable and possibly absolutely necessary in many
Lewis acid systems. The concentration of cocatalyst must be carefully controlled,
however, and optimum Lewis acid/cocatalyst concentration ratios can be established
for particular polymerizations. This is because the cocatalyst must be more basic
than the monomer; otherwise the Lewis acid would react preferentially with the
monomer. If excess co-catalyst BA is present, however, it can compete with the
monomer for reaction with the primary Lewis acid/cocatalyst complex. For example,
H
2
O
(11-36)
BF
3
+
H
2
OH
BF
3
OH
H
3
O
BF
3
OH
where excess water transforms the bare proton of reaction (11-33) into the weaker
acid H
3
O
"
which cannot initiate polymerization of olefins.
In many practical systems the aprotonic acids are difficult to purify and sub-
stances may be present that act as unidentified cocatalysts. The dependence of the
rate of polymerization on the concentrations of Lewis acid and ostensible cocata-
lyst cannot then be elucidated.
Alkyl halides are widely used as cocatalysts in combination with aluminum alkyl
halides or aluminum halide Lewis acids. The reaction scheme in
Fig. 11.2
illustrates
the complicated equilibria that may affect the initiation process. Each carbenium ion
can initiate polymerization or remove an ethyl group from the counterion to produce
a saturated hydrocarbon, REt, and a new more acidic Lewis acid. The propagating
macrocarbenium ions can also terminate by the same process to produce ethyl-
M
M
Polymer
RCl
+
AlEt
2
Cl
R
AlEt
2
Cl
2
RM
AlEt
2
Cl
2
REt
+
AlEt
2
Cl
2
M
M
RM
AlEt
2
Cl
3
R
AlEt
2
Cl
3
Polymer
RCl
+
AlEt
2
Cl
2
REt + AlCl
3
M
M
RM
AlCl
4
Polymer
RCl
+
AlCl
3
R
AlCl
4
R = Alkyl or aryl groups
Et = CH
2
CH
3
—
M = Monomer (styrene)
FIGURE 11.2
Equilibria in initiation by alkyl aluminum halides and alkyl halides
[2]
.
