Biomedical Engineering Reference
In-Depth Information
2.3 Graft Copolymers and Branched Polymers
2.3.1 Graft Copolymers
Graft copolymers have three different parameters: length of main chain, length of
graft chain, and number of graft chains. So, precise control of properties such as
hydrophilic/hydrophobic balance and crystallinity may be achieved by changing
the molecular structure. Graft copolymers having a aliphatic polyester as either
main chain or graft chain have been reported.
To synthesize graft copolymers having aliphatic polyesters as graft chains, any of
three general methods, “grafting through,” “grafting from” and “grafting to,” can be
used. “Grafting through” can be achieved by copolymerization of a macro-monomer
(an aliphatic polyester having a terminal vinyl group) with other monomers to give
graft copolymers having vinyl-type main chains. “Grafting from” can be applied by
ROP of cyclic esters using polymers having hydroxyl side-chain groups as multifunc-
tional macro-initiators. “Grafting to” is a coupling reaction of aliphatic polyesters
having reactive termini onto main chain polymers having reactive side chains.
Vinyl-type graft copolymers having aliphatic polyester graft chains were com-
monly synthesized by the “grafting through” method using aliphatic polyesters having
terminal acryloyl or mathacryloyl groups. PNIPAAm-
g-
PLA and PMPC-
g
-PLA can
be synthesized by this method [
154
,
155
]. Many other examples of such graft
copolymers were introduced in recent reviews [
13
,
16
].
Aliphatic polyester-grafted polysaccharides can be synthesized by both “grafting
from” and “grafting to” methods. We reported preparation of PLA-grafted
polysaccharides, pullulan, amylose, and Dex, by “grafting from” using protected
polysaccharides as macro-initiators (Fig.
5
)[
156
-
161
]. Polysaccharides are highly
hydrophilic polymers and are not soluble in either common organic solvents or
melted cyclic ester monomers. So, the grafting polymerization must be heteroge-
neous using polysaccharide as a macro-initiator. In addition, since polysaccharides
have too many hydroxyl groups, the control of grafting numbers and sites on the
polysaccharide is very difficult. We employed trimethylsilyl (TMS) as a protecting
group, which can be easily removed after graft polymerization. Most hydroxyl
groups were protected by TMS groups, and the obtained TMS-polysaccharide was
soluble in common organic solvent such as THF. Graft ROP of LA in THF solution
proceeded using alkali metal as a catalyst, and the residual (nonreacted) hydroxyl
groups of the polysaccharides were used as initiating groups. After removal of the
TMS groups under mild conditions, PLA-grafted polysaccharides were obtained.
The obtained amphiphilic PLA-grafted polysaccharides showed interesting
properties such as tenacious mechanical properties, microphase separated
structures, accelerated degradation behavior, and anti-cell-adhesive properties,
depending on the polysaccharide content [
156
-
161
]. PLA-grafted polysaccharides
having relatively short graft chains can also be synthesized by coupling using a
“grafting to” method [
162
-
167
]. The obtained oligolactide-grafted polysaccharides
