Chemistry Reference
In-Depth Information
[aqueous phase initiation]
xM+SO
-
M
x
SO
-
[aqueous phase polymerization]
[micelles formed by in situ surfactant)
-
[precipitation]
[polymerization in micelle]
primary particle (unstable)
[coagulation]
stable mature particle
[polymerization in particle]
final latex particle
FIGURE 10.3
Schematic of the general mechanism of surfactant-free emulsion polymerization.
soluble will produce oligomers that continue to grow in the continuous medium
until the polymer chain collapses out of solution, and homogeneous nucleation is
then more likely
[14]
.
Macroradicals are isolated from each other in emulsion polymerizations
because they grow in particles, which can accommodate either one or zero radi-
cals at any instant. The distinguishing feature of the kinetics of such reactions is
that the polymerization rate and polymer molecular weight are proportional to the
number of particles, as distinct from free-radical polymerizations in bulk, solu-
tion, or suspension. An interesting consequence is that the rate of polymerization
will be inversely proportional to the particle size. This holds at fixed final poly-
mer content, which is the way such reactions are usually performed. Polymer
molecular weight may also be affected by particle size under the same conditions.
When a mixed monomer feed is used, the course of the copolymerization may
be sensitive to the volume ratio between the monomer and water phases, if the
monomers differ in water solubility. This is a factor especially at high monomer:
