Chemistry Reference
In-Depth Information
be hard to control, particularly if it is done on a large scale. Also there is a chance that local hot spots
might develop. Once bulk polymerization of vinyl monomers is initiated, there can be two types of
results, depending upon the solubility of the polymer. If it is soluble in the monomer, the reaction may
go to completions with the polymer remaining soluble throughout all stages of conversion. As the
polymerization progresses, the viscosity of the reaction mixture increases markedly. The propagation
proceeds in a medium of associated polymeric chains dissolved in or swollen by the monomer until all
the monomer is consumed.
If the polymer is insoluble, it precipitates out without any noticeable increase in solution viscosity.
Examples of this type of a reaction can be polymerizations of acrylonitrile or vinylidene chloride. The
activation energy is still similar to most of the polymerizations of soluble polymers and the initial
rates are proportional to the square root of initiator concentration. Also, the molecular weights of the
polymerization products are inversely proportional to the polymerization temperatures and to initiator
concentrations. Furthermore, the molecular weights of the resultant polymers far exceed the solubility
limits of the polymers in the monomers. The limit of acrylonitrile solubility in the monomer is at a
molecular weight of 10,000. Yet, polymers with molecular weights as high as 1,000,000 are obtained
by this process. This means that the polymerizations must proceed in the precipitated polymer
particles, swollen and surrounded by monomer molecules.
The kinetic picture of free-radical polymerization applies best to bulk polymerizations at low
points of conversion. As the conversion progresses, however, the reaction becomes complicated by
chain transferring to the polymer and by gel effect. The amount of chain transferring varies, of course,
with the reactivity of the polymer radical.
Bulk polymerization is employed when some special properties are required, such as high
molecular weight or maximum clarity, or convenience in handling. Industrially, bulk polymerization
in special equipment can have economic advantages, as with bulk polymerization of styrene. This is
discussed in Chap.
6
.
Solution
polymerization differs from bulk polymerization because a solvent is present in the
reaction mixture. The monomer may be fully or only partially soluble in the solvent. This, the
polymer may be (1) completely soluble in the solvent, (2) only partially soluble in the solvent, and
(3) insoluble in the solvent.
When the monomer and the polymer are both soluble in the solvent, initiation and propagation
occur in a homogeneous environment of the solvent. The rate of the polymerization is lower,
however, than in bulk. In addition, the higher the dilution of the reactants the lower is the rate and
the lower is the molecular weight of the product. This is due to chain transferring to the solvent. In
addition, any solvent that can react to form telomers will also combine with the growing chains.
If the monomer is soluble in the solvent, but the polymer is only partially soluble or insoluble, the
initiation still takes place in a homogeneous medium. As the chains grow, there is some increase in
viscosity that is followed by precipitation. The polymer precipitates in a swollen state and remains swollen
by the diffused and adsorbed monomer. Further propagation takes place in these swollen particles.
Because propagation continues in the precipitated swollen polymer, the precipitation does not
exert a strong effect on the molecular weight of the product. This was demonstrated on polymeriza-
tion of styrene in benzene (where the polymer is soluble) and in ethyl alcohol (where the polymer is
insoluble). The average molecular weight obtained in benzene at 100
C was 53,000 while in ethyl
alcohol at the same temperature it was 51,000 [
280
]. When the monomers are only partially soluble
and the polymers are insoluble in the solvents the products might still be close in molecular weights to
those obtained with soluble monomers and polymers. Polymerization of acrylonitrile in water can
serve as an example. The monomer is only soluble to the extent of 5-7% and the polymer is
effectively insoluble. When aqueous saturated solutions of acrylonitrile are polymerized with
water-soluble initiators, the systems behave initially as typical solution polymerizations. The
polymers, however, precipitate out rather quickly as they form. Yet, molecular weights over 50,000
are readily obtainable under these conditions.
