Chemistry Reference
In-Depth Information
M
i
OSO
3
1
HSO
4
H
2
O
M
i
OH
1
H
1
(10-8)
-
Since the product here contains ionic and nonionic groups, it will be an anionic
surfactant. Such materials, which are always formed in persulfate-initiated emul-
sion polymerizations, have been termed
. Their nature has not
been studied extensively. In one study of the polymerization of a 64:36 (w/w)
methyl methacrylate:butyl acrylate copolymer in the presence of a chain transfer
agent, more than 50% of the oligomeric radicals containing sulfate end groups pro-
duced surfactants. The surfactant-free polymerization of methyl methacrylate, with
ammonium persulfate initiation, produced
in situ surfactants
surfactants with a mean degree of
polymerization of 8 to 9
[7]
. Oligomers from styrene would be expected to be
shorter and those from vinyl acetate would be longer, in line with the relative water
solubilities of the monomers
[8]
.
We turn now to the other possible destinies of the oligomeric radicals produced
by aqueous phase initiation. The mechanism of “surfactant-free” emulsion poly-
merizations is believed to include at least four stages: initiation (reaction 10-7),
nucleation, coagulation, and particle growth. These intervals may overlap in prac-
tice, but it is convenient to consider them separately. As the oligomeric radicals
grow they become progressively less soluble in water and eventually collapse upon
themselves to form so-called primary particles. Such particles can increase their
surface charge density and colloidal stability through adsorption of
in situ
surfac-
tant and by coagulation, assuming plausibly that all the charged groups remain on
the surface of the growing particle. Incorporation of an ionogenic monomer will
also contribute to the particle stability.
Another reaction path involves the micellization of
in situ
surfactant. Both the
micelles and primary particles can absorb monomer. Particle growth occurs by
polymerization fed by entry into these loci of oligomers generated in the water.
Figure 10.3
is a schematic representation of these mechanisms, the details of
which are still debatable
[9
in situ
13]
.
10.2.4
Mechanism of Emulsion Polymerization
Emulsion polymerizations are normally performed in the presence of micellar sur-
factants at concentrations higher than their cmcs. The most important nucleation
mechanisms are then likely to be as sketched in
Fig. 10.3
, with the important
qualification that the micelles mainly comprise the added soaps. One or other
reaction path may be favored in different emulsion systems; experimental data are
sometimes consistent with either the micellar entry or the homogeneous nucle-
ation route. Stated generally, an aqueous phase monomer-ended radical may
either undergo termination in the water phase, participate in the formation of a
new particle, or enter a polymer particle. Only the first two fates are involved in
particle nucleation. When more hydrophobic monomers are polymerized oligo-
meric growth in the aqueous phase will tend to stop earlier, thus favoring particle
nucleation by polymerization in micelles. Monomers which are more water
