Page 259 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN012B-596 July 27, 2001 18:18

768 Polymers, Synthesis

TABLE XI Emulsion Polymerization Compared with and one may observe pseudo-zeroth-order kinetics for
Other Radical Techniques such polymerizations. The radical micelle then initiates
the polymerization, which essentially continues until all
Advantages Disadvantages
of the local monomer in the micelle is consumed or un-
Faster rates Recovery of solid polymer til a second radical diffuses in to terminate the grow-
more difficult
ing chain. The termination rate is low since a second
High molecular weights Difficult to free completely radical would also have to pass through the interfacial
from emulsifier
boundary. On the average, then, the micelle is believed to
Good heat transfer
contain only one growing chain for most of its lifetime.
May be directly usable (latex)
The rate of polymerization is often approximated by the
500–5000 ˚ A
expression
R p = k p [M]N /2,
which are anionic or nonionic (or both), are used at the
level of several percent. If they are not removed from the where N is the number of colloidally dispersed parti-
final product, they will contribute to the degradation of cles. The rate, then, is proportional to the number of
at least certain polymer properties. In particular, optical colloidally dispersed particles, which means that, if one
properties and electrical properties can be influenced neg- increases the surfactant concentration or the soap concen-
atively by residual of a surfactant. From Table XI we see tration, it is possible to increase the number of particles
that, compared with other processes, emulsion polymer- and hence also increase the polymerization rate. This is
izations have the distinct advantage of providing a fast rate a very fundamental way in which one can decouple the
at the same time as allowing for high molecular weight. effects of initiator concentration on molecular weight and
Good heat transfer is achieved because of the use of water polymerization rate. Thus, the rate can be increased more
as the heat transfer medium. They may also be directly or less independently of the concentration of the initiator.
useful. For example, one can imagine deriving a paint by Emulsion processes are the only free-radical chain poly-
taking an emulsion polymer and simply adding pigments merizations that present this opportunity. Therefore, they
such as TiO 2 . The kinetics of emulsion polymerization are somewhat unique in producing very high molecular
is quite different from that of the other types of poly- weight polymers at very fast rates. It is not difficult, for ex-
merization already described. Bulk, solution and suspen- ample, to make one million-molecular weight polystyrene
sion reactions are often referred to as following homoge- or polybutadiene.
neous free-radical chain kinetics. It may be surprising that As the polymerization proceeds, the locus of the reac-
suspension processes are considered to follow homoge- tion is believed to change from the micelles to a monomer-
neous reactions, but in a sense they are like a microbulk swollen polymer particle at about 10–20% conversion.
reaction. Emulsion processes are quite complex, and we The swollen polymer particle is stabilized at its interface
shall now discuss some of the essential features of these by residual surfactant. Therefore, when all of the monomer
systems. is consumed, the polymer dispersion, or latex, as it is fre-
In an emulsion polymerization, there are typically sev- quently termed, can be quite stable, at least for a small
eral components: the monomer, water, the emulsifier or the range of temperatures and pH. Quantitative studies of the
surface-active agent, a water-soluble initiator, and, option- emulsion kinetics are available in the literature and are
ally, a chain transfer agent. The rate will suddenly begin discussed in greater depth in the references listed in the
to increase at a fairly rapid rate at some critical micelle Bibliography.
concentration (CMC). It is considered that at this CMC,
about 50–100 soap molecules will aggregate into tiny mi- B. Copolymers
celles. The small soap micelles have a very large surface
area. An important result of this is that they will cap- In addition to the synthesis of homopolymers, in which
ture nearly all of the radicals generated in the aqueous there is only one chemical structure in the repeating
phase. It has been calculated that there are ∼10 18 mi- unit, it is possible to prepare copolymers or even ter-
celles per cubic centimeter versus only ∼10 11 monomer polymers by chain reaction processes. There are vari-
droplets per cubic centimeter. Therefore, most of the radi- ous types of copolymers, as indicated earlier in Table I.
calsandsomeofthemonomerbecomepartofthemicelles. Among the most common are the statistical copolymers,
It is believed that the polymerization begins in the soap in which two monomers are randomly distributed through-
micelles. As the monomer polymerizes, new monomer out the chain molecule. A second type is a perfectly al-
may diffuse in from the aqueous phase. Essentially, the ternating copolymer. Two additional types are block and
macromonomer droplets can saturate the aqueous phase, graft copolymers. We will not discuss block and graft