Chemistry Reference
In-Depth Information
counterion form an organic salt that may exist in several forms in the reaction
medium. The degree and nature of the interaction between the cation and anion
of the salt and the solvent (or monomer) can vary considerably.
If we represent the organic ionic species as U
"
V
~
, then at least four forms
must be considered in principle:
-
-
-
+
+
+
V
U /
V
U+
V
UV
U
(11-1)
covalent
bonding
contact
ion pair
solvent
separated
ion pair
free
solvated
ions
The forms that are actually important in a given polymerization will depend
on the natures of the species U and V, the solvating ability of the medium, and
the temperature. It is not unusual to find two of these forms coexisting in signifi-
cant quantities in a given polymerization. In general, more polar media favor
solvent-separated ion pairs or free solvated ions. Free solvated ions will not exist
in hydrocarbon media, where other equilibria may occur between ion pairs and
clusters of ions.
It is well known in organic chemistry that the reactivities of these various
forms of organic ions and counterions may differ significantly
[1]
, and it is thus
not surprising that the same influences are detectable in ionic polymerizations. In
general, propagation rates are higher the more the macroion and its counterion are
separated. Steric control of the polymer microstructure is greater, however, when
the macroion and the counterion are associated.
Rates of ionic polymerizations are by and large much faster than in free-
radical processes. This is mainly because termination by mutual destruction of
active centers occurs only in free-radical systems (Section 8.3.3). Macroions with
the same charge will repel each other and concentrations of active centers can be
much higher in ionic than in free-radical systems. Rate constants for ionic propa-
gation reactions vary but some are higher than those in free-radical systems. This
is particularly true in media where the ionic active center is free of its counterion.
The high reactivity of ionic active centers which yields fast propagation rates also
results in a greater propensity toward side reactions and interference from trace impuri-
ties. Low polymerization temperatures favor propagation over competing reactions,
and ionic polymerizations are often performed at colder temperatures than those used
in free-radical processes, which would be impossibly slow under the same conditions.
In this chapter we first review anionic polymerizations and then discuss cat-
ionic processes. A final section describes coordinated polymerizations.
11.2
Anionic Polymerization
11.2.1
Overview
Anionic polymerizations are chain-growth processes in which the active center to
which successive monomers are added is a negative ion that is associated with a
