Chemistry Reference
In-Depth Information
Most successful dispersants are based on block or graft polymers that are
physically adsorbed on the polymer particle surfaces. For a copolymer segment to
be sufficiently insoluble to function as an anchor group the molecular weight is
usually
1000. The soluble portion of the dispersant is also at least about the
same size. The chemical nature of the soluble portion is not as important as the
requirement that it be freely soluble in the diluent. The same soluble segment has
been used to stabilize dispersions of widely different polymers. A case in point is
the use of poly(12-hydroxy stearic acid) in aliphatic hydrocarbon continuous
phases. The anchor component of the dispersant is, of course, specific to the par-
ticular polymer that is being synthesized, and can be provided by graft polymeri-
zation of the disperse polymer during its synthesis. Such graft copolymerizations
in free-radical systems are initiated by the creation of radicals on the backbone of
the soluble dispersant portion, by atom abstraction. The mechanism is exactly as
in chain transfer to polymer (Section 8.8.4). The precursor soluble component
may be modified, if necessary, to facilitate free-radical chain transfer.
Covalent links or acid
.
base interactions are alternatives to physical adsorp-
tion of the anchor segments of the dispersants. Such procedures are valuable in
special cases, where other factors compensate for the added complication of the
polymerization process.
Mention of several possible operating variations may help the reader's under-
standing of the dispersion polymerization process. It is important to note, first,
that dispersion polymerizations are usually conducted so that the monomers and
other appropriate ingredients are metered into the vessel during the course of the
reaction. (That is, these are so-called semibatch operations, which are considered
in more detail in Chapter 12.) In polymerizations in a nonpolar diluent like an ali-
phatic hydrocarbon, the introduction of a small amount of a highly polar comono-
mer at the start of the reaction reduces the solubility of both the disperse polymer
and the anchor group. As a result, anchoring is stronger and the precipitated poly-
mer particles are finer. Conversely, to obtain a coarser particle size product a
small quantity of strong solvent for the polymer may be added at the beginning of
the polymerization. Alternatively, the process may be started with an increased
amount of monomer in the reaction vessel, since monomers are usually solvents
for their own polymers.
10.2
Emulsion Polymerization
An emulsion consists of a discontinuous liquid phase dispersed throughout a dif-
ferent, continuous liquid phase. Milk and the sap of the rubber tree are examples
of naturally occurring emulsions. The term
latex
is used also to denote aqueous
dispersions of polymers.
Most emulsion polymerizations are free-radical reactions. The main difference
from alternative free-radical polymerizations, such as those in bulk, solution, and
suspension systems, is that the propagating macroradicals in emulsion reactions
