Chemistry Reference
In-Depth Information
Termination reactions are unlikely in very pure cationic systems but slight
traces of impurities can exert significant effects because of the great reactivity of
the propagating species.
The following termination reaction is important in the polymerization of iso-
butene at room temperature:
CH
3
CH
3
(11-40)
CH
2
C
+
AlCl
4
CH
2
C
+
H
AlCl
4
CH
3
CH
2
The kinetic chain is interrupted here, but the catalyst-cocatalyst complex is
regenerated and can initiate new kinetic chains. In the production of butyl rubber
(as distinguished from polyisobutene) isobutene is copolymerized with about 3%
(w/w) isoprene, to facilitate sulfur vulcanization (Section 1.3.3). The major chain
transfer reaction to monomer occurs at the isopropenyl end of the macrocation.
Thus the molecular weight of the copolymer is inversely proportional to the con-
centration of isoprene in the monomer feed mixture.
The most important processes that limit molecular weights in cationic vinyl poly-
merizations are transfer reactions to monomer (11-41) or residual water (11-42):
H
H
H
H
(11-41)
CH
2
C
X+
CH
2
C
CH
C
+
CH
3
C
X
R
R
R
R
H
H
H
monome
r
CH
3
C
+ H
X
C
X
CH
2
CX+
H
2
O
CH
2
OH
R
R
R
(11-42)
Facile chain transfer to polymer is thought to account for the failure of cat-
ionic reactions of
-olefins like propylene to yield high-molecular-weight pro-
ducts. Reaction (11-43) produces a relatively stable tertiary carbenium ion from a
more reactive, propagating secondary ion.
α
H
H
CH
2
C
X
+
CH
2
C
CH
2
CH
2
CH
2
CH
3
+
X
CH
3
CH
3
CH
2
C
CH
2
CH
3
(11-43)
