Biomedical Engineering Reference
In-Depth Information
Table 1
Synthesis of hyperbranched polytriazoles
M
w
a
PDI
a
Entry
Monomers
Time (h)
Polymer
Yield (%)
Thermally Initiated Polymerization
b
1
30
(4) +
20
72
hb-r-
P
30
(4)-
20
64.0
5500
2.0
2
30
(6) +
20
72
hb-r-
P
30
(6)-
20
75.7
11 400
2.7
Transition Metal-Catalyzed Polymerization
b
3
62.5
c
30
(4) +
20
0.33
hb-1,5-
P
30
(4)-
20
5350
2.4
hb-1,5-
P
30
(6)-
20
4
30
(6) +
20
0.50
74.9
9370
2.7
a
Estimated in THF by GPC on the basis of a polystyrene calibration
b
[
20
]
0
/[
30
(
m
)]
0
=2:3,[
20
]
0
= 0.12 M;undernitrogenin1,4-dioxaneat101
◦
C (entries
1and2)orinTHFat60
◦
CusingCp
∗
Ru(PPh
3
)
2
Cl as catalyst (entries 3 and 4)
c
Soluble fraction (total yield: 84.5%)
tion resulted only in insoluble polymers. Stereoregular 1,5-substituted
hb
-
PTAs, however, were successfully obtained when Cp
∗
RuCl(PPh
3
)
2
was used
as the catalyst. Compared to their regiorandom congeners, completely sol-
uble polymers
hb
-1,5-P(
30
-
20
) with similar isolation yields and molecular
weights were obtained in much shorter polymerization times (Table 1, en-
tries 3 and 4). One reason for the solubility difference in the hyperbranched
PTAs might be because the triazole rings function as ligands that form com-
plexes with copper but not ruthenium ions, which thus cross-links the
hb
-1,4-
PTA spheres [55]. Another possible cause might be due to the difference in
the architectural structures of 1,4- and 1,5-disubstituted triazole rings. The
1,4-isomer experiences little steric interaction with the neighboring phenyl
groups and can arrange in a relatively planar confirmation (Fig. 1). In con-
trast, its 1,5-substituted congener experiences strong steric repulsion, forcing
the molecule to adopt a more twisted conformation. The structural planarity
of the 1,4-isomer may facilitate
π
-
π
stacking of the aromatic moieties, lead-
ing to a lower solubility.
Fig. 1
Simulated conformations of
A
tris[4-(1-methyl-1
H
-1,2,3-triazol-4-yl)phenyl]amine
and
B
tris[4-(1-methyl-1
H
-1,2,3-triazol-5-yl)phenyl]amine
