Page 37 - Biodegradable Polyesters

1.3 Biodegradable Polyesters 15

are mainly produced by emulsion or double-emulsion technique followed by
solvent evaporation or spray-drying. Drug encapsulation, particle size, molecular
weight (MW), ratio of lactide to glycolide in PLGA, and surface morpholo-
gies could influence the release characteristics. Encapsulation efficiency and
release rates through nano/microparticle-mediated drug delivery devices can be
optimized to improve their therapeutic efficacy [94].
Owing to their absorbability, poly(glycolide) devices were used in trauma
and bone surgery [95]. Ultra-high-strength implants are manufactured from
PGA and/or PLA polymers using self-reinforcing techniques. The implants are
available for stabilization of fractures, osteotomies, bone grafts, and fusions, as
well as for reattachment of ligaments, tendons, meniscal tears, and other soft
tissue structures. As these implants are completely absorbed, the need for a
removal operation is overcome and long-term interference with tendons, nerves,
and the growing skeleton is avoided. This kind of implants does not interfere with
clinical imaging [96].
1.3.1.3 Poly(caprolactone)
PCL is an aliphatic polyester composed of hexanoate repeat units. It is a semicrys-
talline polymer with a degree of crystallinity which can reach 70%. The physical,
thermal, and mechanical properties of PCL depend on its molecular weight and
its degree of crystallinity [97]. As early as in 1921, Windaus et al. [98] found that
the degradation of certain cholesterol derivatives led to obtain hydroaromatic
acids of the glutaric acid series which are very difficult to oxidize and do not
undergo a smooth thermal degradation. In the presence of silver salt, a good
yield of lactone such as γ-caprolactone is obtained. In 1934, Van Natta et al. [99]
published the first paper on synthesis of ε-caprolactone and its polymer where
ε-caprolactone on heating can be converted to a polymer of high molecular
weight. The process is not easily reversible. Berens [100] introduced a way to
make a PCL polyester copolymer with haloethylene. The copolymers consist of a
mixture of 50–98 wt% haloethylene and 2–50 wt% of a PCL polyester. Polyester
polyols with OH end groups and MW 300–3000 are prepared by ring-opening
polymerization of ε-caprolactone in the presence of water, alkylene oxides,
transesterification, or alkoxylation catalysts Bu SnO, and possibly diepoxides and
2
tertiary amines under 30 atm pressure [101]. PCL is a biodegradable polyester
∘
with a low melting point of around 60 C and a glass transition temperature of
∘
about −60 C. It is prepared by ring-opening polymerization of ε-caprolactone
using a catalyst such as stannous octoate [102].
PCL is commonly used in the manufacture of polyurethanes because of its
imparting good water, oil, solvent, and chlorine resistance to the polyurethane
produced. In 1934, Carothers et al. [103] prepared epsilon-caprolactone for the
first time. Under the condition of heat epsilon-caprolactone is converted to a
polyester of high molecular weight. The 1965, Magnus [104] published details
of his study of effects of components and varying –NCO/–OH or –NCO/–NH 2
group ratio on the low-temperature properties, hydrolytic and heat stability, and
solvent and chemical resistance of the polyurethane elastomers. It was found that