Chemistry Reference
In-Depth Information
no necessity for high conversions characteristic of equilibrium step-growth pro-
cesses. Further, since the polymerization is diffusion controlled, one does not
need to start with an exact balance of coreactive equivalents. The molecular
weight distributions obtained are also quite different from the random one which
is found for reversible systems, and the distributions of interfacial polymers may
vary depending on the details of the particular interfacial reaction. As explained
above, however, interfacial polyesters and polyamides will tend to randomize dur-
ing subsequent melt processing.
The choice of organic solvents for interfacial polycondensation has an impor-
tant effect on the molecular weight of the polymer that is made. Liquids that swell
the polymer permit the formation of higher molecular weight products than media
that are effectively nonsolvents for the polymer. If the polymer is insoluble in
either phase, the migration of acid chloride will be hindered enough to limit the
polymer yield and molecular weight.
Interfacial polymerizations are generally characterized by the following fea-
tures
[4]
: (1) Reactive monomers are used; (2) the polymerization is irreversible
at the reaction temperature; (3) the reaction rate is determined by the rates of
encounter of complementary species; (4) the growing polymer is dissolved or
highly swollen during the reaction; and (5) the aqueous phase contains a base to
remove by product acid from the polymerization zone.
Interfacial polycondensations can also be carried out in vapor
liquid systems.
Reaction takes place at the interface between an aqueous solution of a bifunc-
tional active hydrogen compound and the vapor of diacid chloride. Interfacial
condensation is commercially important in the synthesis of polycarbonates (1-52).
Polymerizations based on diacids are always less expensive than those that use
diacid chlorides. In the polycarbonate case, however, the parent reactant, carbonic
acid, is not suitable and the derived acid chloride, phosgene (COCl
2
), must be
used.
Similar reactions to those used in interfacial polymerizations are sometimes
carried out in solution and are employed to prepare some polymers which yield
ultra-high-strength high modulus fibers. These species typically contain para-
linked aromatic rings and amide or ester linkages in the polymer backbone.
Aromatic polyamides are generally made by low-temperature reactions of aro-
matic diamines and aromatic diacid chlorides in special solvents such as a 1:3
molar mixture of hexamethylphosphoramide: N-methylpyrrolidone, as in the
first reaction shown in Section 4.6. Intensive stirring is required to attain high
molecular weights because the polymer precipitates. These macromolecules are
very rigid and rodlike. They form oriented liquid crystalline arrays in solution
and require little post-spinning orientation to produce extremely strong and
stiff fibers. The polymer would not be made in the melt because it is infusible.
It must be synthesized and handled in solution, and this requires the use of
reactive precursors.
