Chemistry Reference
In-Depth Information
(
C
N,
C
R,
C
OH or
C
OR )
O
promote attack of an anionic species on the double bond and may also stabilize
the anion formed by delocalization of the charge:
O
O
H
H
H
-
-
(8-4)
A + CH
2
C
A
CH
2
C
A
CH
2
C
CN
CN
:
C
N
-
CH
2
) substituents are electron releasing but they
stabilize the product anions by resonance, and so styrene and butadiene can
undergo both cationic and anionic polymerizations. Anionic mechanisms are
more important, however, since they provide better control over the polymer
structure (Chapter 11).
Radical reactions with the
Phenyl and alkenyl (
a
CH
Q
bond of vinyl monomers are not nearly as selec-
tive as ionic attack, and free-radical initiators cause the polymerization of nearly
all vinyl and vinylidene monomers. (Some of these polymerizations are not effi-
cient because of side reactions. Propylene is a case in point as described in
Section 8.8.5
.) Resonance stabilization occurs to some extent with most vinyl
monomers but it is important in radical polymerizations only when the monomers
contain conjugated C
π
a
C double bonds as in styrene, 1,3-butadiene, and similar
molecules:
H
H
H
H
H
A
+ CH
2
C
C
A
CH
2
A
CH
2
C
A
CH
2
C
A
CH
2
C
(8-5)
Steric hindrances prevent the polymerization of most 1,2-disubstituted ethy-
lenes by any mechanism. However, 1,1-disubstituted monomers and vinylidene
monomers usually polymerize more readily than the corresponding vinyl analogs.
8.3
Overall Kinetics of Radical Polymerization
Chain-growth polymerizations are so called because their mechanisms comprise
chains of kinetic events. For successful polymerization, the sequence of reactions
must first be initiated by some agent, and monomers must be added consecutively
to a growing macromolecule. This chain of events may then be terminated by a
reaction that is inherent in the system or by the action of impurities. In any case,
we can usefully distinguish between at least three different reaction types in a
kinetic polymerization chain. These are initiation, propagation, and termination
reactions.
(Recall
that
there is only one reaction involved in step-growth
