Polyesters - Biodegradable Polymers - page 14

Table 1.4 Comparative degradation times of different polyesters in physiological
conditions [5] .

Polymer Degradation time (months)

Poly( l - lactide) 18 - 24

Poly( rac - lactide) 12 - 16

Polyglycolide 2 - 4

Poly(3 - hydroxybutyrate) >> 36

Polydioxanone 6 - 12

Poly( ε - caprolactone) > 24

Poly([ l - lactide] - co - glycolide) 50:50 2

Poly([ rac - lactide] - co - glycolide) 85:15 5

Poly([ rac - lactide] - co - [ ε - caprolactone]) 90:10 2

proceeding degradation, a loss of weight and increasing porosity lead to a bulk
degradation. Poly([3- R - hydroxybutyrate] - co - [3 - R - hydroxyvalerate]) is hydrolyzed
more rapidly due to lowered crystallinity. The degradation of the poly(3- R - hydroxy-
butyrate) can be accelerated by microorganisms and enzymes [60]. Bacterially
produced poly(3 - R - hydroxybutyrate) is degraded faster than synthetic poly(3- R,S -
hydroxybutyrate), while poly(3 - S -hydroxybutyrate) is not hydrolyzed at all [61].
Enzymatic digestion proceeds at the surface of the sample as proved by scanning
electron microscopy. The enzyme, because of high molecular weight, is too large
to diffuse into the bulk of the material. Another important factor to take into
account when considering the processing of these polymers is that above 200 °C
thermal degradation can occur [62] .

1.5
Beyond Classical Poly(Hydroxycarboxylic Acids)

1.5.1
Alternate Systems

In addition to the broadly studied groups described previously, there are some
notable examples to include when discussing biodegradable polyesters. Scheme
1.6 shows a selection of such alternate polyester structures. Various amino acids
were converted to the corresponding

α -hydroxycarboxylic acid and then polymer-

ized to give stereogenic - substituted polyglycolide analogs [63, 64]. This approach

could give access to polymers with more tailored properties and sophisticated
material properties. Polydepsipeptides are an interesting class of poly(ester amide)
which are copolymers of

α

- amino acids [65] . They
are synthesized from morpholine diones (analogs of diglycolide, in which one
lactone is replaced by a lactam group) by many of the same procedures outlined
for polyglycolide synthesis. Polydepsipeptides degrade through the ester bonds,
whereas the amide linkages remain intact under physiological conditions [66].

α - hydroxycarboxylic acids and α