Page 38 - Biodegradable Polyesters

16 1 Biodegradable Polyesters: Synthesis, Properties, Applications

as the –NCO/–OH or –NCO/–NH group ratio increased, the deformation of the
2
polymer decreased, and the tensile strength and modulus increased. In 1981, stan-
nous octoate was used as catalyst in making polyfunctional adhesives, tackifiers,
fillers by ring-opening reaction of ε-caprolactone with poly(vinyl alcohol) [105].
In 1982, Busfield [106] systematically studied the mechanical properties of some
PCL-based, cross-linked, crystallizable polyurethanes. It was disclosed that crys-
tallization in these polymers is facilitated by (i) decreasing cross-link density, (ii)
increasing length of PCL segment, and (iii) using a flexible aliphatic diisocyanate
as linking unit. The crystallization can be easily prevented by quenching with
liquid nitrogen from the melt. Therefore, the mechanical properties as Young’s
moduli were enhanced markedly by increasing the PCL segment length and less
markedly by increasing the cross-link density. Replacing the aromatic linking
units by aliphatic crosslinker decreases Young’s moduli in the glassy region. In
the rubbery region, Young’s modulus is enhanced by having shorter PCL segment
lengths or by the presence of the more rigid linking unit diphenylmethane
diisocyanate and is decreased very significantly at low cross-link densities.
Young’s modulus is not enhanced by heavier cross-linking in the rubbery region.
The above study is a good sample for structure-property of polyurethanes.

Synthesis In general, the anionic ring-opening polymerization of lactones
involves a number of specific features that have to be taken into consideration
for the optimization of the synthesis: for example, the nature of the propagating
site, the slow initiation rate, the ability of the active sites to react with functional
links [97, 98]. An alkoxide (or a carbanionic species) may attack lactones and
lead to ring opening in two different ways: (i) by scission of the O-alkyl bond or
(ii) by cleavage of the O-acyl linkage. In the former case, the propagating site is a
carboxylate, whereas in the latter case, it is an alkoxide [107] (Figure 1.7).
Carboxylates are much weaker nucleophiles than alkoxides, and they are unable
to give rise to an O-acyl scission upon attack of another lactone molecule. The
consequence is that once the propagation site is a carboxylate, it stays as such.
Only if the probability of O-acyl scission is equal to unity can one be sure that all
propagation sites are alcoholates, even at high conversions. At various degrees of
conversion, all experimental data showed that the propagating sites are alkoxides
until the end of the reaction.
Normally, the polymerization is conducted at low temperature because the
activation energy for chain growth is generally rather low, which means that
the variation of the rate of propagation with temperature is not very large.

O O
C − R − C O R − C
O R
(1) (2)
O −
R O
Figure 1.7 Two different ways of ring opening: (1) by scission of the O-alkyl bond or (2) by
cleavage of the O-acyl linkage.