Chemistry Reference
In-Depth Information
7.18.1 Dendrimers and Hyperbranched Polymers
Highly branched polymeric materials with large number of end groups can offer unique physical
behavior. A variety of applications have been forecast for these highly branched globular
macromolecules. One early approach to dedrimer synthesis relied upon the Michael reaction using
repeated sequential additions of an amine to
a
,
b
-unsaturated ester [
248
]:
H
2
NCH
2
CH
2
NH
2
NH
3
+
O
CH
2
CH
2
CONHCH
2
CH
2
NH
2
O
H
2
NCH
2
CH
2
NHCOCH
2
CH
2
N
CH
2
CH
2
CONHCH
2
CH
2
NH
2
repeat
(CH
2
)
2
CN(CH
2
)
2
NH
2
O
H
2
N(CH
2
)
2
NC(CH)
2
N
H
2
N(CH
2
)
2
NC(CH)
2
(CH
2
)
2
CN(CH
2
)
2
NH
2
H
O
H
O
O
N(CH
2
)
2
NC(CH
2
)
2
N
(CH
2
)
2
CN(CH
2
)
2
NH
2
H
H
2
N(CH
2
)
2
NC(CH)
2
H
O
H
2
N(CH
2
)
2
NC(CH)
2
N
O
H
O
H
O
(CH
2
)
2
CN(CH
2
)
2
NH
2
H
O
In the first step, ammonia is added to an excess of methyl acrylate. The product is reacted with an
excess of ethylenediamine to form a star molecule with three arms. The product of the second step is
then reacted with an excess of methyl acrylate. This is followed by a reaction with an excess of
ethylenediamine to form a molecule with six arms. The sequences are repeated to yield a star
polymer. Such star polymers are sometimes referred to as
.
A recent publication described preparation of dendrimers from functional aliphatic polyesters that
are based on 2,2-bis(hydroxymethyl)propionic acid [
249
].
starburst polymers
N
0
-dicyclohexylcarbodiimide was used
as the coupling agent in a double-stage convergent approach that reduced the number of synthetic and
liquid chromatographic steps required in the preparations and purifications. The hydroxyl functional
dendrimers were then subjected to a variety of surface modifications by reactions with different acid
chlorides [
249
].
Another publication describes formation of polyphenylene dendrimers that formed nanoparticles
with 102 benzene rings [
250
]. In this preparation, an A—B type monomer, 2,3,4,5-tetrakis(4-
triisopropylsilylethynylphenyl)-cyclopenta-2,3-dienone,
N
,
is condensed in a [4 + 2]cycloaddition
with 3,3
0
,5,5
0
-tetraethynyl-biphenyl:
