Chemistry Reference
In-Depth Information
There are different techniques for carrying out solution polymerization reactions. Some can be as
simple as combining the monomer and the initiator in a solvent and then applying agitation, heat and
an inert atmosphere [
292
]. Others may consist of feeding into a stirred and heated solvent the
monomer or the initiator, or both continuously, or at given intervals. It can be done throughout the
course of the reaction or through part of it [
293
]. Such a set up can be applied to laboratory
preparations or to large-scale commercial preparations. It allows a somewhat better control of the
exotherm during the reaction.
In both techniques the initiator concentration changes only a few percent during the early stages of
the reaction, if the reaction temperature is not too high. The polymerization may, therefore, approach
a steady state character during these early stages. After the initial stages, however, and at higher
temperatures, the square root dependence of rates upon the initiator concentration no longer holds.
This is a result of the initiator being depleted rapidly. The second technique, where the initiator, or the
monomer and the initiator are added continuously was investigated at various temperatures and rates
of addition [
294
-
299
]. If the initiator and monomer are replenished at such a rates that their ratios
remains constant, steady state conditions might be extended beyond the early stages of the reactions.
How long they can be maintained, however, is uncertain.
Suspension
polymerization [
298
] can be considered as a form of mass polymerization. It is carried
out in small droplets of liquid monomer dispersed in water or some other media and caused to
polymerize to solid spherical particles. The process generally involves dispersing the monomer in a
non solvent liquid into small droplets. The agitated stabilized medium usually consists of nonsolvent
(often water) containing small amounts of some suspending or dispersing agent. The initiator
is dissolved in the monomer if it is a liquid or it is included in the reaction medium, if the monomer
is a gas.
To form a dispersion, the monomer must be quite insoluble in the suspension system. To decrease
the solubility and to sometimes also increase the particle size of the resultant polymer bead, partially
polymerized monomers or prepolymers may be used. Optimum results are obtained with initiators
that are soluble in the monomer. Often, no differences in rates are observed between polymerization
in bulk and suspension. Kinetic studies of styrene suspension polymerization have shown that all the
reaction steps, initiation, propagation, and termination, occur inside the particles [
299
].
The main difficulty in suspension polymerization is in the forming and in the maintaining uniform
suspensions. This is because the monomer droplets are slowly converted from thin immiscible liquids
to sticky viscous materials that subsequently become rigid granules. The tendency is for the sticky
particles to attach to each other and to form one big mass. The suspending agent's sole function is to
prevent coalescing of the sticky particles. Such agents are used in small quantities (0.01-0.5% by
weight of the monomer). There are many different suspending agents, both organic and inorganic.
The organic ones include methylcellulose, ethyl cellulose, poly(acrylic acid), poly(methacrylic acid),
salts of these acids, poly(vinyl alcohol), gelatins, starches, gums, alginates, and some proteins, such as
casein or zein. Among the inorganic suspending agents can be listed talc, magnesium carbonate,
calcium carbonate, calcium phosphate, titanium and aluminum oxides, silicates, clays, such as
bentonite, and others. The diameter of the resultant beads varies from 0.1 to 5 mm and often depends
upon the rate of agitation. It is usually inversely proportional to the particle size. Suspension
polymerization is used in many commercial preparations of polymers.
Zhang, Fu, and Jiang, reported a study of factors influencing the size of polystyrene microspheres
in dispersion polymerization [
300
]. The found that that the size of polystyrene microspheres
decreased with an increasing amount of stabilizer and also increased with increasing the amount of
monomer and initiator. The amount of stabilizer and monomer concentration were the major factors
influencing the size distribution of polystyrene microspheres. The size of the microspheres decreased
with an increase of the solvency of reaction media. The size distribution, however, hardly changed.
The size of polystyrene microspheres increased with an increase in the reaction temperature. but the
size distribution hardly changed.
