Chemistry Reference
In-Depth Information
polymerizations. Conversely,
tetrahydrofuran is a useful solvent
in some
anionic processes but is polymerized by appropriate cationic initiators.
3.
The growth of vinyl polymer molecules in cationic reactions is limited by
transfer rather than by true termination processes. Transfer to monomer is
often the most important of these processes.
4.
Some monomers isomerize during cationic polymerization. The net result is
that the structure of the polymer repeating unit is not the same as that of the
monomer. This occurs if the carbocation which is formed by initial attack on
the monomer can isomerize to a stabler form. Isomerization is more prevalent
in cationic than in anionic or free-radical polymerizations because carbenium
or oxonium ions rearrange easily.
11.4.2
Initiation of Polymerization of Vinyl Monomers
Initiation is usually by attack of a cation on a monomer to generate a carbocation
which can propagate the kinetic chain by adding further monomers. (We shall
refer to this sp
2
-hybridized trigonal species as a carbenium ion. It is also called a
carbonium ion
.) The initiator is itself usually a proton or a carbenium ion. The
actual initiation process is often preceded by a reversible reaction or series of
reversible reactions between the initiator and other species. An example is
1
(11-30)
1
Al R
2
Cl
+
RCl
R
Al R
2
Cl
2
CH
3
CH
3
1
1
(11-31)
R
Al R
2
Cl
2
+
CH
2
CR
CH
2
C
Al R
2
Cl
2
CH
3
CH
3
The preinitiation reaction sequence may be quite complex and attainment of
the equilibria involved is often slower than subsequent initiation or propagation
reactions. The following paragraphs review a few of the more important initiators
for vinyl monomers.
11.4.2.1
Bronsted (Protonic) Acids
The general reaction between an olefin and a protonic acid HA can be written as
R
R
(11-32)
H
A
A
+
CH
2
C
CH
3
C
R
′
R
′
It is necessary for practical polymerizations that the anion A not be a strong
nucleophile or else it will react with the carbenium ion to form the nonpropagat-
ing covalent compound.
