Polyhydroxyalkanoates: Structure, Properties and Sources - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 35

reaction with sodium azide (DSN3 ¼ 1). Well defined PHBHV oligomers
containing an alkyne end group were prepared in a one-step reaction by
direct alcoholysis from natural polyesters using propargyl alcohol with
dibutyltin dilaurate as a catalyst. The presence of PHBHV on the dextran
backbone led to the formation of stable nanoparticles (160 nm) without
surfactant by an emulsion-solvent evaporation method. 89
Poly(ethylene oxide) (PEO) as a hydrophilic and biocompatible
polyether is widely used in biomedical applications. 90 An amphiphilic
triblock copolymer, an enantiomerically pure telechelic OH-terminated
poly[(R)-3-hydroxyoctanoate] (PHO-diol), poly[(R)-3-hydroxyoctanoate]-co-
poly[(R)-3-hydroxy-7-oxooctanoate] (PHOO-diol), and poly[(R)-3-hydro-
xyoctanoate]-co-poly[(R)-3-hydroxy-7-octenoate] (PHUO-diol) have been
synthesized by catalytic transesterification with ethylene glycol. 91 The
number average molecular weights (Mn) of these telechelic diols reached
(2.0 3.0) 10 3 . For PHOO-diol and PHUO-diol, the side chain functional
groups remained, which provides additional reactive groups for further
polymerization or modification. The glass transition temperatures (T g ) of the
telechelic diols are between 46 and 56 1C and the melting transition
temperatures (T m ) are lower than 40 1C, all determined by DSC. 92,93
Aliphatic polyesters coupled with monomethoxy poly(ethyleneoxide),
mPEO, as PHB-b-PEO diblock copolymers are often used as drug delivery
systems, and many applications in this field involve polylactides (PLA)
coupled to mPEO because of the biodegradability and biocompatibility
of PLA. Three and four-arm star polyethylene oxide-polylactide copolymers
(s-PEO-PLA) can be synthesized by the use of triethanolamine and pentaer-
ythritol and initiating agent, respectively. 94 Similarly, because PHB is also a
chiral aliphatic polyester, PHB-bmPEO diblock copolymer can be syn-
thesized. For this, PHB and poly(ethylene glycol) methyl ether are melted
under vacuum at 190

d n 2 r 4 n g | 8

.

1C in the presence of bis(2-ethylhexanoate) tin

catalyst. 95
New diblock copolymers of selected PHAs (PHB, PHBV, PHO) with
atactic poly[(R,S)-3-hydroxybutyrate] (a-PHB) were obtained by using
low-molecular weight macroinitiators derived from natural poly(3-hydro-
xyalkanoates), which contain olefinic units and activated by 18-crown-6 ether
carboxylic end groups. These new polymers are suitable for use as blend
compatibilizers. 96
A chemical synthesis of functionalized atactic poly(3-hydroxybutanoic
acid) and its copolymers with the aid of activated anionic initiators as well as
enzymatic synthesis of PHB copolymers (using lipase PPL) are described by
Jedli´ski et al. (1999). Using these new synthetic approaches, PHBs with
defined chemical structures of the end groups as well as block, graft and
random copolymers have been obtained. 97 The chemical synthesis of co-
polymers containing structural segments derived from natural origin PHAs
is based on the concept of partial depolymerization of a natural polyester to
oligomers containing end groups suitable for initiation of polymerization of
other monomers. 75,98