Page 39 - Biodegradable Polyesters

1.3 Biodegradable Polyesters 17

For instance, a polymerization of ε-caprolactone was carried out in two steps:
(i) pretreatment of raw materials and (ii) polymerization. ε-Caprolactone was
∘
purified by distillation over calcium hydride under reduced pressure, b.p. 117 C
at 20 mbar. Tetrahydrofuran (THF) was distilled twice, over sodium first and
then from a dilute solution of sodium benzophenone in THF. 1,1-Diphenyl-
3-methylpentyllithium is obtained by addition of sec-butyllithium (BuLi) on
∘
1,1-diphenylethylene (DPE). This initiator is prepared at −60 Cinanhydrous
THF. 1,4-Dilithio-1,1,4,4-tetraphenylbutane was synthesized from lithium and
DPE in THF at room temperature. The polymerizations were conducted in tight
∘
reactors, under inert atmosphere. The THF was cooled down to −90 C, a few
drops of initiator 1,1-diphenyl-3-methylpentyllithium were used to neutralize
solvent impurities. Then, more amount of the initiator was added, the concentra-
−1
tions of which ranged from 5 × 10 −4 to 10 −3 mol L . The initial concentration of
−1
the monomer was kept at 0.5 mol L . The monomer (diluted in THF) was added
slowly. Initiation was not instantaneous: the red color vanished after approxi-
∘
mately 30 min. The propagation step was carried out between −20 and −10 C.
The polymerization was stopped with a few drops of acetic acid at rather low
conversion (30%), after a period of 2–10 min, depending upon the desired molec-
ular weight. The monomer conversion was determined from the size exclusion
chromatography (SEC) diagrams obtained on polymerization mixtures [107].

Chemical and Physical Properties The major physical and mechanical properties of
poly(caprolactone) are summarized briefly in Table 1.5. Its physical and mechani-
cal properties depend mainly on its molecular weight and crystallinity. In general,
aromatic and some polar solvents such as benzene, toluene, cyclohexanone,
dichloromethane and 2-nitropropane are good solvents for PCL. Water, alcohols,
petroleum ether, diethylether are poor solvents for PCL. PCL can be slightly sol-
uble in acetonitrile, acetone, 2-butanone, ethyl acetate and dimethylformamide.
PCL has high polymer-polymer miscibility with most of the other polymers such
as poly(vinyl chloride), poly(styrene-acrylonitrile), poly(acrylonitrile butadiene

Table 1.5 Properties of poly(caprolactone) [97].

Property Range

−1
Number average molecular weight (M /g mol ) 530–630 000
n
∘
Melting temperature (T / C) 56–65
m ∘
Glass transition temperature (T / C) (−65) to (−60)
g
−3
Density (g cm ) 1.071–1.200
∘
Decomposition temperature ( C) 350
3 −1
Inherent viscosity ( /cm g ) 100–130
inh
3 −1
Intrinsic viscosity ( /cm g ) 0.9
int
Tensile strength ( /MPa) 4–785
Young modulus (E/GPa) 0.21–0.44
Elongation at break ( /%) 20–1 000