Transition Metal-Catalyzed Polymerization in Aqueous Systems - Late Transition Metal Polymerization Catalysis - page 229

Scheme 7.1

Possible mechanisms of particle formation in free radical emulsion polymerization.

aqueous phase (also for “water-immiscible” monomers a small equilibrium con-
centration is present in the aqueous phase). Repeated monomer addition results
in the formation of an oligomeric radical, which is initially water-soluble due to
the incorporated sulfate moiety (Scheme 7.1). Upon reaching a certain critical chain
length the growing radical chain becomes insoluble and collapses upon itself, thus
nucleating a particle (“homogeneous nucleation”). This primary particle is stabilized
by adsorption of surfactant, and further polymerization within the particle results in
particle growth. Alternatively, a particle can be nucleated by the water-soluble oligo-
mer entering a surfactant micelle prior to reaching its critical chain length (“het-
erogeneous micellar nucleation”).
During emulsion polymerization the emulsified monomer droplets serve as a
reservoir for monomer; however, polymerization usually does not occur in these
droplets but in the formed polymer particles to which the monomers diffuses
through the aqueous phase (Scheme 7.2).
Particular features of emulsion polymerization and polymer latexes are:

After the polymerization, the polymer can be isolated by precipitation, but for
many applications latexes are used as such. Film formation upon evaporation of
the dispersing medium (Scheme 7.3) is a key step in many applications, and
the fact that colloids are composed of submicron particles is advantageous in
terms of forming continuous films.

Water is non-flammable and non-toxic . This is advantageous not only in the poly-
merization process, but particularly in applications. By film formation from la-
texes, several million tons of water are evaporated into the atmosphere annually.

Particle sizes can be controlled precisely in some cases (albeit usually involving
trial and error). For example, monodisperse polystyrene latexes suitable as cali-
bration standards can be prepared conveniently by emulsion polymerization.