Page 248 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: GNH/GRI P2: GTV Final pages
Encyclopedia of Physical Science and Technology EN012B-596 July 27, 2001 18:18

Polymers, Synthesis 757

require the in situ acetylation of the aromatic phenols. the largest volume polyamide, or nylon, in the world. This
Condensation of the acetate and aromatic carboxylic is called poly(hexamethylene adipamide), but in addition,
acid readily occurs at 250 C with the liberation of the common nylon terminology is frequently used. Nylon
◦
acetic acid, and the polymerization mechanism is often nomenclature is based simply on the number of carbons
referred to as acidolysis. Although the polymerization in the reactants. Thus, there are six methylenes in hexam-
mechanism appears straightforward, Hall and coworkers ethylenediamine and six carbons in adipic acid, hence, the
have recently elucidated the very complex nature of this term Nylon 6,6.
polymerization process. Industrial attention has focused In order to achieve perfect stoichiometry in the AABB
on the development of suitable manufacturing processes system, one relies on the capacity of these species to form
that can handle the corrosive reaction environment and the a one-to-one “nylon salt,” which essentially is an ammo-
high polymerization temperatures required in the viscous nium carboxylate, as indicated. On carefully heating the
◦
melt phase. Significant attention has also been devoted ammonium carboxylate to ∼200 C initially, then as high
◦
to the preparation of liquid crystalline polyesters derived as 280 C in order to exceed the crystalline melting point
from aliphatic glycols and biphenyl dicarboxylic acids. of the polymer, one can transform the ionic carboxylate
Many excellent reviews are available on the synthesis into amide bonds with the release of water. The termi-
ofnylons.Melt-phasepolycondensationprocessessynthe- nal amine groups of the oligomeric polyamides react with
size most aliphatic and partially aromatic nylons commer- terminal carboxylic acid groups until appropriate high-
cially. In cases where crystalline polyamides are prepared molecular weight materials are obtained. The molecular
and melt-phase viscosities limit desired molecular weight weight is typically 20,000–30,000 for many fiber-forming
formation, sold-state polymerization processes have been polymers.
employed. For nylons, the formation of the “nylon salt” Many variations on polyesters and polyamides are
is very useful in purifying the monomers and obtaining possible, and in the case of wholy aromatic systems it is
the exact stoichiometry to be able to get a high degree common to use either solution or interfacial processes,
of polymerization. Unlike polyesters, the equilibrium in as depicted in Scheme 6. Due to the lower reactivity
a polyamide condensation lies for to the right and thus of aromatic amines and aromatic bisphenols, it is often
the polymerizations can be charged stoichiometrically and necessary in these cases to utilize the more reactive
initially run under pressure to react all the diamine and acid chloride derivative of carboxylic acids. Thus, many
maintain the stoichiometry. The step-growth process for aromatic polyamides and polyesters are known and are
polyamides is illustrated in Scheme 5. The reaction prod- important in both fiber and thermoplastic materials
uct of hexamethylenediamine and adipic acid produces technology. In these systems, either terephthaloyl or
isophthaloyl chlorides or mixtures of the two are used
frequently to disrupt the chain symmetry and produce
amorphous materials. Utilization of a single component
may allow crystalline textures to be produced that are
important, for example, in fiber technology.
A much different solution process involves the use of di-
aryl halides and diamines to prepare aramids via the Heck
carbonylation reaction. This route to aramids was first dis-
closed using dibromo aromatics. Relatively low molecu-
lar weight polymers were formed. Diiodo aromatics were
found to react at a much higher rate and to give polymers
of much higher ultimate molecular weight, presumably
due to fewer side reactions. This process eliminates the
use of corrosive and moisture-sensitive acid chlorides, but
requires the use of expensive palladium catalysts.
Other important polymers are synthesized via step-
growth polymerization, although we shall provide only
four additional examples that have received significant at-
tention. Since the early 1960s polycarbonates have be-
come an extremely important and fast-growing, clear,
amorphous thermoplastic for injection molding and ex-
truded sheet products. The workhorse polycarbonate resin
SCHEME 5 Synthesis of polyamides. is based on bisphenol A (BPA PC) and has a unique