Chemistry Reference
In-Depth Information
than the starting material, homogeneous limited reactions can be controlled. When the starting
material is incompatible with the product, mutual precipitation or coiling of the chains can take
place. This can also result in limited reactions. In addition, only minor differences in the constitutions
of two polymers can cause incompatibility. For instance, among methacrylate polymers, there are
incompatibilities in benzene solutions that result from differences only in the amount of branching of
the alkyl groups [
29
].
Problems with solvent incompatibility can sometimes be overcome by using mixtures of solvents.
Those that are good for the starting materials can be combined with those that are good for
the products. With careful experimentation, it may be possible to develop a mixture of solvents
that will keep all components in solution [
30
]. In some instances, however, insolubility of the
products might be an advantage. This is the case in alcoholysis reactions of poly(vinyl acetate),
where the polymer precipitates during the reactions and in doing so absorbs the catalyst with it.
The phenomenon permits complete alcoholysis, particularly with the higher molecular weight species
that precipitate first.
Secondary reactions, like cross-linking and gelation, can result in precipitations from solution.
The extent of the reactions, however, is not necessarily limited, because diffusions of low
molecular weight species are still possible. Isolation of useful products, however, often becomes
very difficult.
9.1.5 Effects of Crystallinity
Crystallinity can only affect reactivity when the reactions are carried out on polymers in the solid
state and at heterogeneous conditions. The differences in accessibility to the reactive sites vary with
the amount of crystallinity. Cellulose, for instance, is often reacted in the solid state and the degree of
crystallinity is expressed in terms of reactivity to various reagents [
31
]. The progress of a reaction can
sometimes be monitored by a loss of crystallinity. What is more significant, however, is that greater
accessibility to amorphous regions results in reaction products with special properties. An example is
heterogeneous and homogeneous chlorination of polyethylene. Two different products are obtained
[
32
]. The material from heterogeneous chlorination is much less randomly substituted and remains
crystalline up to a chlorine content of 55%. The products from the homogeneous reactions, on the
other hand, are amorphous after 35% substitution.
9.1.6 Reactions That Favor Large Molecules
Hydrophobic interactions play important roles in many polymeric reactions. They are, for instance,
significant in the hydrolyses of low molecular weight esters when catalyzed by polymeric sulfonic
acid reagents, like poly(styrene sulfonic acid). In these reactions, the hydrogen ions are located close
to the macromolecules [
19
]. The hydrolytic cations are located in the regions of the macromolecules
and not in the bulk of the solution. The rates of the reactions are high. Low molecular weight
catalysts, on the other hand, like HCl, have all the hydrogen ions distributed evenly throughout the
reaction medium. As a result, the rates are lower. Adsorption of the ester groups to the polymeric sites
is accompanied by an increase in the apparent rate constant, as compared to reactions with HCl.
