Biomedical Engineering Reference
In-Depth Information
a
O
O
O
O
L-lactide
m
O
O
O
O
H
OH
m
O
O
Polyglycidol
O
n
Polyglycidol-
g-
PLA
(BranchedPLA)
b
O
O
O
O
O
O
H
N
H
N
L-lactide
Ac
Ac
O
O
O
O
O
O
O
O
OH
OH
O
O
CH
2
O
O
CH
2
OH
1-x
x
1-x
x
n
n
O
poly[LA-
r
-(Glc-Ser)]
H
O
O
O
PLA-
g-
PLA
(comb-typePLA)
O
m
c
O
O
O
O
LA unit
ML unit
O
+
O
O
O
O
O
O
OH
O
O
O
L-lactide
(LA)
DL-mavalonolactone
(ML)
hyper-branched PLA
Fig. 6 Synthesis of branched PLAs: (a) polyglycidol-
g
-PLA, (b) PLA-
g
-PLA (comb-type PLA),
and (c) hyperbranched PLA
polymer as a kind of branched PLA (Fig.
6a
)[
168
]. The obtained branched PLA
showed modified degradability and physical properties.
Graft copolymers having aliphatic polyester main chains can be synthesized
by “grafting to” methods. For example, PLA-
g
-PEG having hydrophobic PLA-based
main chain and hydrophilic PEG graft chains could be obtained by a coupling reaction
of poly(DP-
co
-LA) having carboxylic acid side chains [poly(Glc-Asp)-
co
-LA] with
one terminal reactive PEG (MeO-PEG-OH) [
111
]. The obtained amphiphilic graft
copolymers showed temperature-responsive sol-gel transition behavior.
2.3.2 Branched Polymers
Introduction of branched structures is an effective method for modifying the
physical properties of polymers by changing crystallinity, hydrodynamic diameter,
and entanglement of polymer chains. Branched PLA, a graft polymer where both
the main chain and graft chains are PLA (i.e., PLA-
g
-PLA or comb-type PLA),
could be synthesized by a “grafting from” method using poly(DP-
co
-LA) having
hydroxyl side chains [poly(Glc-Ser)-
co
-LA] [
107
,
169
] (Fig.
6b
). The obtained
branched PLAs exhibited controllable degradability and physical properties based
on their branched structures. Branched aliphatic polyesters can be synthesized
by polyol compounds such as penta-erythritol [
170
] as multifunctional initiators
to give star-shaped polyesters. Multiarmed PEG (four-arm, eight-arm) [
171
-
173
]
and cyclodextrins (CDs) [
174
] can also be used as multifunctional initiators to give
star-shaped block copolymers of PEG and PLA and star-shaped PLAs having a CD
