Chemistry Reference
In-Depth Information
The propagating anion and its counterion exist in relatively nonpolar solvents
mainly in the form of associated ion pairs. Different kinds of ion pairs can be
envisaged, depending on the extent of solvation of the ions. As a minimum, an
equilibrium can be conceived between intimate (contact) ion pairs, solvent-
separated ion pairs, and solvated unassociated ions. The nature of the reaction
medium and counterion strongly influences the intimacy of ion association and
the course of the polymerization. In some cases the microstructure of the polymer
that is produced from a given monomer is also influenced by these variables. In
hydrocarbon solvents, ion pairs are not solvated but they may exist as aggregates.
Such intermolecular association is not important in more polar media where the
ion pairs can be solvated and perhaps even dissociated to some extent.
Anionic chain growth polymerizations are particularly distinguished from
free-radical polymerizations in the following respects.
1.
Initiation is the slow reaction in the initiation
termination
sequence in free-radical reactions whereas selected initiation reactions in
anionic systems can be very rapid compared to the subsequent propagation
reaction. This facilitates the preparation of anionic polymers with narrow
molecular weight distributions, if the polymerization is conducted carefully.
2.
Spontaneous termination reactions are effectively absent in a number of anionic
polymerizations. Some block copolymers can therefore be produced by adding
a second monomer to the active anionic end of a polymer made from a
different, first monomer. (Macromolecular species in which the propagation
reaction can be interrupted for lack of monomer and then resumed with the
same or a selected different monomer are called “living polymers.” Living
polymerizations are reactions in which only the initiation and propagation steps
are significant.) Alternatively, polymers with specific end groups can be
formed by deliberately adding suitable terminating agents to the system.
3.
The choice of initiator has no effect on the propagation reactions in free-
radical polymerizations but it can influence ionic propagations because
the reactivity of the active center is partly determined by the nature of the
counterion that is derived from the initiator.
4.
The choice of reaction medium is much more significant in anionic and
cationic reactions than in free-radical polymerizations because the character of
the growing chain end is altered if the ion pair is more or less solvated.
5.
Geometrical isomerism of polymers made from conjugated diolefins can be
regulated in some anionic polymerizations. The tacticity of vinyl polymers is,
however, not always controlled in anionic reactions. The products of anionic
vinyl polymerizations are usually atactic, as in free-radical syntheses.
propagation
11.2.2
Anionic Initiation
Anionic initiators are all electron donors of varying base strengths. The initiator
type required for a particular polymerization depends on the ease with which an
