Chemistry Reference
In-Depth Information
are isolated from each other. Encounters between macroradicals are hindered as a
consequence, and termination reactions are less frequent than in comparable sys-
tems in which the reaction mixture is not subdivided. Emulsion polymerizations
thus often yield high-molecular-weight products at fast rates when suspension or
bulk reactions of the same monomers are inefficient.
Both the emulsion and suspension processes use water as a heat sink.
Polymerization reactions are easier to control in both these processes than in bulk
or solution systems because stirring is easier and removal of the exothermic heat
of polymerization is facilitated.
The free-radical kinetics described in Chapter 8 hold for homogeneous sys-
tems. They will prevail in well-stirred bulk or solution polymerizations or in sus-
pension polymerizations if the polymer is soluble in its monomer. Polystyrene
suspension polymerization is an important commercial example of this reaction
type. Suspension polymerizations of vinyl chloride and of acrylonitrile are
described by somewhat different kinetic schemes because the polymers precipitate
in these cases. Emulsion polymerizations are controlled by still different reaction
parameters because the growing macroradicals are isolated in small volume ele-
ments and because the free radicals which initiate the polymerization process are
generated in the aqueous phase. The emulsion process is now used to make large
tonnages of styrene
butadiene rubber (SBR), latex paints and adhesives, PVC
“paste” polymers, and other products.
Emulsion polymerizations vary greatly, and no single reaction mechanism
accounts for the behavior of all the important systems. The kinetics and mecha-
nism of emulsion polymerizations are reviewed in detail in
[4]
. It is important to
note that the nature of the products made by emulsion reactions is highly depen-
dent on the details of the process whereby they were produced. (Incidentally, this
is an advantage industrially, because, although the component monomers in com-
plicated structures can be identified fairly readily, competitors cannot easily
deduce the polymerization process by examining the final product.) Emulsion
polymerization is described in a qualitative format in this introductory text, partly
in order to emphasize the versatility of the process. Mathematical features of the
reaction kinetics have not been covered because emulsion polymerizations are
controlled most effectively by the rates and modes of reactant addition.
Emulsion polymerization is a particularly attractive route for the production
and control of polymer structures on a size scale from a few hundred nanometers
to several microns. This section is then the first instance in this text in which
polymerization reactions are considered on a scale larger than molecular.
The essential ingredients in an emulsion polymerization are the water, a
monomer which is not miscible with water, an oil-in-water emulsifier, and a com-
pound or compounds that release free radicals in the aqueous phase. Other ingre-
dients which may be used in practical recipes are mentioned briefly later. Typical
proportions (by weight) are monomers 100, water 150, emulsifier 2-5, and initia-
tor 0.5, although these ratios may vary over a wide range.
