Page 26 - Biodegradable Polyesters

4 1 Biodegradable Polyesters: Synthesis, Properties, Applications

Nylon. Nylon is a generic term for a family of synthetic polymers known generi-
cally as aliphatic polyamides, first produced in 1935 at DuPont’s research facility
at the DuPont Experimental Station led by Carothers. As one of the most popular
thermoplastics, Nylon has different chemical structures and trade names such as
nylon-6,6, nylon-6, nylon-6,9, and nylon-11.
There are two typical approaches for the synthesis of nylon. The first approach
includes combining molecules with an acid (COOH) group on each end are
reacted with two chemicals that contain amine (NH ) groups at each end. This
2
process creates nylon-6,6, made of hexamethylene diamine with six carbon
atoms and adipic acid. The second approach consists of a compound having an
acid at one end and an amine at the other and this compound is polymerized
to form a chain with repeating units of (–NH–[CH ] –CO–) . For instance,
2 n x
nylon-6 is made from a single six-carbon substance called caprolactam.In
this equation, if n = 5, then nylon-6 is the assigned name (this may also be
referred to as polymer). The numerical suffix specifies the number of carbons
donated by the monomers: the diamine first and the diacid next. The most
common nylon polymer is nylon-6,6 which refers to the one in which the diamine
(hexamethylene diamine) and the diacid (adipic acid) each donate six carbons to
the polymer chain. It is difficult to get equimolar proportions between the two
involved monomers exactly correct, and deviations can lead to chain termination
at molecular weights less than a desirable 10 000 Da. To overcome this problem,
a crystalline, solid “nylon salt” can be formed at room temperature, using an
exact 1 : 1 equimolar ratio of the acid and the base to neutralize each other.
∘
Heated to 285 C, the salt reacts to form nylon polymer. However, it is impossible
to spin the chains into yarn at above 20 000 Da. To overcome this difficulty,
some acetic acid must be added to react with a free amine end group during
polymer propagation to limit the molecular weight. The general reaction is

O O O O
n C R C + nH N R′ NH 2 C R C N R′ N + 2H 2 O
2
HO OH H H n

Two molecules of water are given off and the nylon is formed. Its properties are
′
determined by the R and R groups in the monomers. In nylon-6,6, R = 4C and
′
R = 6C alkanes, but one also has to include the two carboxyl carbons in the diacid
′
to get the number it donates to the chain. In Kevlar, both R and R are benzene
rings.
In early discovery of polymers, of course there were a lot of arguments on the
basics of polymer science and unclear concepts and terms. Carothers criticized
Berzelius [10, 13] about his misleading concept on polymers as the term polymers
indicated the presence of the same atoms in the same proportions. Carothers
described a “condensation” that requires as starting materials compounds in
which at least two functional groups are present in the same molecule (e.g.,
hydroxy acids, HO–R–COOH, might lead to polyesters, HO[RCOO] RCO–;
x
and amino acids to polyamides, NH [RCONH] RCO–.) [10]. In addition, he
2 y