Chemistry Reference
In-Depth Information
it may be possible for the monomers to lie directly on top of each other, resembling a stack of coins.
Such stacking greatly facilitates reactions between guest molecules [
210
,
211
].
Polymerizations in thiourea canal complexes yields high melting crystalline
-1,4 polybutadi-
ene, 2,3-dimethylbutadiene, 2,3-dichlorobutadiene, and 1,3-cyclohexadiene. Cyclohexadiene mon-
oxide, vinyl chloride, and acrylonitrile also form stereoregular polymers. On the other hand,
polymerizations of isobutylene and of vinylidene chloride fail to yield stereospecific polymers.
Sodium montmorillonite can also be used to polymerize polar monomers between the lamellae.
Here too, the organization of monomer molecules within the monolayers influences the structure of
the resultant polymers [
212
,
213
]. Poly(methyl methacrylate) formed in sodium montmorillonite is
composed of short, predominantly isotactic stereosequences [
211
]. The percentage of isotactic
component increases with an increase in the ion exchanging population on the surface of the mineral
and is independent of the temperature between 20 and 160
C. In this way, it is possible to vary the
population of isotactic triads at will up to 50% composition [
205
].
Perhydrotriphenylene also forms channel-like inclusions with conjugated dienes. Polymerization
of these dienes yields some steric control [
216
,
218
].
Uemura and coworkers [
217
] carried out radical polymerizations of vinyl monomers (styrene,
methyl methacrylate, and vinyl acetate) within various nanochannels of porous coordination
polymers. They studied the relationships between the channel size and polymerization behaviors,
such as monomer reactivity, molecular weight, and stereostructures. They reported that in these
polymerization systems, the polymer-growing radicals were remarkably stabilized by efficient
suppression of the termination reactions within the channels, resulting in relatively narrow molecular
weight distributions. A significant nanochannel effect on the polymer stereoregularity was also seen,
leading to a clear increase of isotactic placement in the resulting polymers.
There were attempts at controlling steric placement by a technique called
template polymeriza-
tion
. An example is methyl methacrylate polymerization in the presence of isotactic poly(methyl
methacrylate) [
208
,
209
]. Thus template polymerization is a process of polymerizing a monomer in
the presence of a polymer, usually from a different monomer. The presence of template polymers,
however, only results in accelerating the rates of polymerizations [
219
].
trans
3.14 Controlled/“Living” Free-Radical Polymerization
Living polymerizations
are chain-growth reactions where the propagating centers on the growing
chains do not terminate and do not undergo chain transfer. Such polymerizations are noted for
preparations of polymers with controlled molecular weights, desired end groups and low
polydispersities. In addition, the preparations of polymers with predetermined molecular weights
and narrow molecular weight distributions require fast initiations and fast exchanges between sites of
variable activities and variable lifetimes. Such chain-growth reactions, ionic in nature, are discussed
in Chap.
4
. In typical homogeneous free radical polymerizations, however, bimolecular terminations
between two growing radicals cannot be avoided and, therefore, typical living free radical polymeri-
zation cannot be fully realized. Also, in conventional free radical polymerizations, the initiations are
slow, while high-molecular-weight polymers form shortly after the start of the reactions. As the
reactions progress, polydispersities increase, while the molecular weights actually decrease. It is
possible, however, to adjust conditions of some radical polymerizations in such a way that polymers
with controlled molecular weights and relatively low polydispersities form [
220
,
221
]. These are not
true living polymerization as such because termination reactions do occur. They possess, however,
some characteristics that are similar to living polymerizations and are referred to by many as
controlled/“living”
polymerizations. Such reactions yield polymers with controlled molecular
weights, exhibit increase in molecular weight with conversion, yield narrow molecular weight
