Chemistry Reference
In-Depth Information
11.4
Cationic Polymerization
11.4.1
Overview
The active center in these polymerizations is a cation, and the monomer must
therefore behave as a nucleophile (electron donor) in the propagation reaction.
The most active vinyl monomers are those with electron-releasing substituents or
conjugated double bonds in which the positive charge can be delocalized.
Figure 11.1
illustrates inductive influences in cationic polymerizations. The
electron-releasing inductive influence of alkyl groups causes isobutene to poly-
merize very quickly at low temperatures where propylene reacts inefficiently and
ethylene is practically inert. For similar reasons,
-methylstyrene (11-12) is more
reactive than styrene, and substitution of an electron-withdrawing halogen for an
ortho
-or
para
-hydrogen decreases the monomer reactivity still further. As a cor-
ollary,
ortho
- and
para
-electron-releasing substituents
α
(RO—, RS—, aryl)
increase cation stability and monomer reactivity.
Conjugated olefins, like styrene, butadiene, and isoprene, can be caused to poly-
merize by cationic and anionic as well as by free-radical processes because the active
site is delocalized in all cases. The most practical ionic polymerizations for these spe-
cies are anionic, because such reactions involve fewer side reactions and better control
of the diene polymer microstructure than in cationic systems. Free-radical polymeriza-
tion of styrene is preferred over ionic processes, however, for cost reasons.
For efficient cationic polymerization of vinyl monomers, it is necessary that
the carbon
carbon double bond be the strongest nucleophile in the molecule.
CH
3
CH
3
H
C
CH
2
C
CH
2
C
CH
2
CH
3
H
H
isobutene forms
high polymer
very rapidly
propylene yields
low-molecular-
weight polymer
ethylene is
practically inert in
cationic polymerizations
CH
3
H
H
C
C
C
CH
2
CH
2
CH
2
Cl
Rates of cationic polymerization
FIGURE 11.1
Inductive effects in cationic polymerization of vinyl monomers.
