Chemistry Reference
In-Depth Information
monomer is distributed nonuniformly in the polymer particles. The outer shell is rich in monomer
molecules, while the inside is rich in polymer molecules [
319
]. The other explanation is that the
radical ions that form from the water-soluble initiator are too hydrophilic to be able to penetrate the
polymer particles [
320
].
Surfactant-free emulsion polymerization
are carried out in the absence of a surfactant [
321
]. The
technique requires the use of initiators that yields initiating species with surface-active properties and
imparts them to the polymer particles. Examples of such initiators are persulfates. The lattices that form
are stabilized by chemically bound sulfate groups that are derived from persulfate ions. Because the
surface-active groups are chemically bound, the lattices are easier to purify and free the product from
unreacted monomer and initiator. Generally, the particle number per milliliter from a surfactant-free
emulsion polymerization is smaller than the particle number from typical emulsion polymerization.
In an
inverse emulsion polymerization
an aqueous solution of a hydrophilic monomer is
emulsified in an organic solvent and the polymerization is initiated with a solvent soluble initiator.
This type of emulsion polymerizations is referred to as
water in oil
polymerization. Inverse emulsion
polymerization is used in various commercial polymerizations and copolymerization of water-soluble
monomers. Often nonionic emulsifiers are utilized. The product emulsions are often less stable than
the oil in water emulsions.
A special approach to emulsion polymerization is called
miniemulsion polymerization
[
322
].
These reactions contain both micelles and monomer droplets, but the monomer droplets are smaller
than in macrosystems. Usually, a water-soluble surfactant is used for emulsification. An example of
such a surfactant can be sodium dodecyl sulfate. In addition, a highly water-insoluble costabilizer is
added, such as hexadecanol. Thus, miniemulsions are dispersions of critically stabilized oil droplets
with a size between 50 and 500 nm prepared by shearing a system containing oil, water, a surfactant
and a hydrophobic material. Polymerizations in such miniemulsions, when carefully prepared, result
in latex particles which have about the same size as the initial droplets. An appropriate formulation of
a miniemulsion suppresses coalescence of droplets. The polymerization of miniemulsions extends the
possibilities of the widely applied emulsion polymerization and provides advantages with respect to
copolymerization reactions of monomers with different polarity, incorporation of hydrophobic
materials or with respect to the stability of the formed latexes. Although labeled “emulsion,” it
appears that some may involve a combination of emulsion and suspension polymerizations. It was
reported [
323
] that by using a difunctional alkoxyamine as an initiator for the homopolymerization of
butyl acrylate in miniemulsion, to increase the achievable molar mass and to use the polymer as a
difunctional macroinitiator for the synthesis of triblock copolymers in aqueous dispersed systems.
Well-defined polymers with one alkoxyamine functionality at each end were obtained, providing that
monomer conversion was kept below 70%. Beyond this conversion, extensive broadening of the
molar mass distribution was evidenced, as the consequence of termination and transfer to polymer.
Tsavalas et al. [
324
] reported that a phenomenon seemingly unique to hybrid miniemulsion
polymerization was observed by them, where monomer conversion would either plateau at a limiting
value or quickly switch to a dramatically lesser rate. They attributed this phenomenon to a combina-
tion of three factors. The first one is the degree to which the monomer and resinous component are
compatible. The second is the resultant particle morphology after approximately 80% monomer
conversion, which roughly corresponds to the portion of reaction where this morphology is
established. The third factor is the degree of interaction between the growing polymer and the resin
(a grafting reaction). Of these three, the first two factors were found by them to be much more
significant in contributing to the limiting conversion.
RAFT emulsion polymerization
is a new development that has attracted considerable attention.
It be carried out in a regular emulsion polymerization [
325
] and in a reverse emulsion polymeriza-
tion [
326
].
Also, recently, several reports in the literature have described miniemulsion RAFT
polymerizations. In some instances, use is made of water-soluble RAFT agents to control polymer
