Page 27 - Biodegradable Polyesters

1.1 Historical Overview on the Origin of Polymer Science and Synthesis of Polyamides 5

suggested the possible condensation products via intra- or interpolymerization
with bifunctional monomers as limited atomic ring or high-molecular-weight
chains [10]. In 1929, Carothers and Arvin [14] prepared some esters by heating
∘
a variety of acids and 5% excess of glycol for about 3 h at 175–185 and then
∘
at 200–250 and 0.2 mmHg for 3 h. Some solid esters had molecular weights
ranging from 2300 Da to the highest of 5000 Da as polymers and in the final
chemical structure of the polyesters it was assumed that HO groups were
present at each end of the chain because of the presence of one more molecule
of glycol than that of acid. Carothers, who analyzed the hydroxy-acids of the
series, found that HO(CH ) COOH might condense with them, but in most
2 x
cases the bifunctional reaction led to a lower-atomic-number ring lactone.
Higher-molecular-weight condensed substances were only synthesized by the
oxidation of the corresponding cyclic ketones with persulfuric acid [15]. In 1930,
Carothers et al. [11] synthesized powder-like polymeric ethyl oxalates in at least
two forms by heating; these polyesters had higher melting points (soluble form
∘
∘
m.p. 159 C and insoluble form m.p. 172 C) than the monomer ethyl oxalate
∘
∘
(m.p. 144 C). The material with m.p. 172 C was insoluble in all common organic
solvents. Most importantly, they found that the polymeric ethyl oxalateswere all
not stable and purified polymers were partially depolymerized during standing.
It was found that the monomer of ethyl oxalate crystals was also not stable in
ambient condition.
1.1.2
Initial Knowledge about Polyesters

A brief introduction to commonly used polyesters would be in order here. The
most well-known and daily used man-made polyester is polyethylene terephtha-
late (PET)(more often written as poly(ethylene terephthalate), whose chemical
structure is shown in Figure 1.3). PET is a thermoplastic polymer resin of the
polyester family and is used in the form of synthetic fibers. This polyester is gen-
erally nonbiodegradable. Whinfield synthesized PET in 1941 and called it terylene
[16] and Hardy [17] characterized teryleneusing a couple of methods (Figure 1.3).
The general chemical formula for polyesters can be summarized in short form
as –(COOR) – but the R groups are different and bring in varying properties to
x
the final polyesters. PET has an aromatic ring in its main-chain structure, as a
result of which it is not readily biodegradable. In contrast to PET, the aliphatic
polyesters listed in Table 1.1 are readily biodegradable and there are no aromatic
rings in their main chain structures. However, chemical composition is not the
only determining factor for polymer biodegradability.

O Figure 1.3 Poly(ethylene terephthalate).
C C O CH 2 CH 2 O
O
n