Chemistry Reference
In-Depth Information
stabilization was described earlier in connection with nonaqueous dispersion
polymerization; the same mechanism applies in aqueous emulsion systems.
Electrostatic stabilizers are usually anionic surfactants, i.e., salts of organic acids,
which provide colloidal stability by electrostatic repulsion of charges on the parti-
cle surfaces and their associated double layers. (Cationic surfactants are not com-
monly used in emulsion polymerizations.)
The presence of free surfactant is not preferred in some applications of
polymer emulsions. It may, for instance, result in water sensitivity of dried
latex coatings or affect the properties of particles used as substrates in solid-
phase peptide syntheses. Colloidal stability can be conferred by so-called copo-
lymerizable surfactants in cases when it is not feasible to add surfactants.
These surfactants are actually comonomers that are intended to provide surface
activity. It is desirable that they be located preferentially on the particle sur-
faces and this can be accomplished by methods outlined below in the section
on core-shell polymerization. For example, incorporation of methacrylic acid
(in copolymers with styrene or acrylics) or vinyl sulfonic acid (in copolymers
with vinyl acetate) provides electrostatic stabilization under the alkaline condi-
tions used to store most emulsions (acid storage favors corrosion of metal con-
tainers). Copolymerizable steric stabilizers include esters of methacrylic acid
and poly(ethylene glycol), which can be made by transesterification of methyl
methacrylate.
10.2.3
Surfactant-Free Emulsion Polymerizations
In view of the foregoing discussion, it is not surprising that many emulsion
polymerizations can be carried out without the addition of free surfactants.
This is accomplished by use of ionic initiators, like the widely used persul-
fates, with the additional assistance of copolymerizable surfactants, if these are
needed. It should be realized, however, that emulsion polymerization itself
generates surfactants, and the process cannot
therefore be entirely surfactant
free.
The persulfate initiator resides in the aqueous phase and it is there that it first
encounters monomers, which are added to growing oligomeric radicals:
OSO
3
1
MOSO
3
M
-
(10-6)
OSO
3
1
MOSO
3
---
M
y
OSO
3
M
-
(10-7)
Here, M stands for monomer, as usual. The oligomers produced by reaction
(10-7) could meet several fates. If such a radical encounters another radical in the
aqueous phase it may undergo termination by combination or disproportionation
(Section 8.3.3) to yield a molecule with one or two sulfate ion ends, respectively.
The end groups may also reflect whether the second reactant in the termination
reaction is a hydroxyl radical (reaction 10-2) or sulfate ion radical (reaction
10-10), or results from hydrolysis, as in:
