Chemistry Reference
In-Depth Information
Fig. 3.3
Early stages of emulsion polymerization (from ref. [
306
])
Emulsion
polymerization is used widely in commercial processes [
300
,
301
]. The success of this
technique is due in part to the fact that this method yields high-molecular-weight polymers. In
addition, the polymerization rates are usually high. Water is the continuous phase and it allows
efficient removal of the heat of polymerization. Also, the product from the reaction, the latex, is
relatively low in viscosity, in spite of the high molecular weight of the polymer. A disadvantage of the
process is that water-soluble emulsifiers are used. These are hard to remove completely from the
polymers and may leave some degree of water sensitivity.
The reaction is commonly carried out in water containing the monomer, an emulsifier or a surface-
active agent, and a water-soluble initiator. Initiation may be accomplished through thermal decom-
position of the initiator or through a redox reaction. The polymer forms as a colloidal dispersion of
fine particles and polymer recovery requires breaking up the emulsion.
The full mechanism of emulsion polymerization is still not completely worked out. It is still not
clear why a simultaneous increase in the polymerization rate and in the molecular weight of the
product is often observed. Also, in emulsion polymerization, at the outset of the reaction the monomer
is in a form of finely dispersed droplets. These droplets are about 1
in diameter. Yet, during the
process of a typical polymerization, they are converted into polymer particles that are submicro-
scopic, e.g., 1,000
˚
in diameter.
At the start of the reaction the emulsifier exists simultaneously in three loci: (a) as a solute in water;
(b) as micelles; (c) and as a stabilizing emulsifier at the interface between the monomer droplets and
the water. The bulk of the emulsifier, however, is in the micelles. The monomer is also present in three
loci: (a) in the monomer droplets that are emulsified and perhaps 1-10
m
in diameter; (b) it is
solubilized in the micelles, perhaps 50-100
˚
in diameter; (c) and it is present as individual molecules
dissolved in the water. The bulk of the monomer is in the droplets. There are on the average 10
18
/mL
of monomer-swollen micelles in the reaction mixture at the outset of the reaction [
302
]. At the start of
the reaction there are also on the average 10
12
/mL monomer droplets that act as reservoirs. The
monomer is supplied from the droplets to radical-containing micelles when the reaction progresses by
a process of diffusion through the aqueous phase (Fig.
3.3
).
The first hypothesis of the mechanism of emulsion polymerization was formulated by Harkins
[
305
]. According to this hypothesis, the water-soluble initiator decomposes in the aqueous phase.
m
