Page 31 - Biodegradable Polyesters

1.3 Biodegradable Polyesters 9

by microorganisms in nature [33]. The first report on the isolation of lactic acid
was in 1780 [34]. The dimerization of lactic acid monomers into a form of lactide
followed by ring-opening polymerization was reported by Carothers et al. [18],
who found that the ability of lactic acid to undergo reversible polymerization is
generally characteristic of six-membered cyclic esters. The ester rings of five atoms
or more than six atoms do not polymerize under the action of heat. The polymers
formed from six-membered cyclic esters are linear polyesters and, at least in cer-
tain instances, the chains are open and terminated by HO and COOH groups.
Both the polymerization and the depolymerization take place with a process of
ester interchange [18]. It was known [14, 18] that the polymer based on lactyl
units was instable in humid atmosphere and the application of this kind of poly-
mers was not considered as meaningful earlier than the 1960s. In the 1960s, their
biodegradability and nontoxicity for use in medical applications became apparent
[35]. Research on lactic acid-based polymers intended for medical applications has
markedly increased since then and boomed in the last two decades in many other
areas of interest [36–43]. Polylactides have been of significant research interest
due to their biocompatibility and biodegradability, leading to applications in med-
ical science and biotechnology.

Synthesis PLA is a thermoplastic aliphatic polyester which is formed by con-
densation polymerization of lactic acid, as mentioned in the preceding. Lactic
acid is isolated from tapioca, corn and other plant root starches, sugarcanes , or
other resources. Bacterial fermentation is normally used to produce lactic acid
from starch or sugar. However, lactic acid cannot be straight away be polymerized
into a useful material as one condensation reaction by two lactic acids gener-
ates one molecule of water. The generated water degrades the oligomer chain to
result in low-molecular-weight lactide. Two lactic acid molecules then undergo a
single esterification and get catalytically cyclized to form a cyclic dilactate ester.
Although dimerization also generates water, it can be separated before polymer-
ization owing to a significant drop in polarity (Figure 1.4).
The polymerization of lactic acid to lactide or high-molecular-weight lactic
acid-based polymers can be conducted in several ways:
1) Lactic acid through condensation polymerization to produce lower-
molecular-weight PLA (degree of polymerization (DP) is normally less than
100).

O
O
CH 3 CH 3
H 3 C
O Ring-opening O OH
HO O
O Polymerization
O O
CH 3 CH 3 n
O
Lactide Poly(lactic acid)
Figure 1.4 Synthesis of poly(lactic acid) via ring opening.