Biomedical Engineering Reference
In-Depth Information
Table 2
Thermosensitive cyclotriphosphazenes N
3
P
3
(NHCHR
1
COOR
3
)
3
(R
2
)
3
Hydrophobic
substituent
(R
1
)
(R
3
)
Hydrophilic
substituent (R
2
)
T
LCST
(ºC)
References
Glycine
H
Et
O(CH
2
CH
2
O)
2
CH
3
>100
[
59
]
O(CH
2
CH
2
O)
2
CH
3
10.5
[
59
,
105
]
Bz
65.5
[
59
,
105
]
O(CH
2
CH
2
O)
7
CH
3
COOEt
Et
O(CH
2
CH
2
O)
7
CH
3
95.0
[
59
]
Aminomalonic acid
l
l-Aspartic acid
CH
2
COOEt
Et
O(CH
2
CH
2
O)
2
CH
3
47.5
[
59
,
105
]
O(CH
2
CH
2
O)
7
CH
2
CH
3
21.5
[
59
]
83.0
[
59
,
105
]
O(CH
2
CH
2
O)
7
CH
3
O(CH
2
CH
2
O)
7
CH
3
42.5
[
59
,
105
]
Bz
69.0
[
59
,
105
]
O(CH
2
CH
2
O)
11
CH
3
78.5
[
59
,
105
]
O(CH
2
CH
2
O)
16
CH
3
l
l-Glutamic Acid
(CH
2
)
2
COOEt
Et
O(CH
2
CH
2
O)
2
CH
3
30.0
[
59
]
73.0
[
59
]
O(CH
2
CH
2
O)
7
CH
3
Notes
Bz
Benzyl
chloride may still be able to coordinate to the backbone nitrogen atom and decom-
pose the polymer backbone, leading to a decreased molecular weight [
50
,
52
].
Nevertheless, the biodegradable thermogelling poly(organophosphazenes) still
have much higher molecular weight than most of currently used biodegradable
thermogelling block copolymers, which will give poly(organophosphazenes) more
flexibility in molecular structure design over other polymer systems.
Another interesting and unique feature of the synthesis of
poly(organophosphazenes) is that reaction conditions on polymers can be stud-
ied and optimized by using small molecules, cyclotriphosphazenes, as model
compounds, due to its structural similarity with polyphosphazene, but more well-
defined molecular structures [
44
,
49
,
53
,
102
]. This model study will provide bet-
ter understanding of more complicated and challenging reactions carried out later
on polymer side groups. Therefore, a series of thermosensitive cyclotriphospha-
zenes bearing various amino acid and PEG segments were synthesized and their
properties, such as LCST, degradation rate and ability as drug release carriers,
were studied [
59
,
103
-
106
]. However, no thermogelling behaviour was observed
for all of the thermosensitive trimers. Examples of thermosensitive cyclotriphosp-
hazenes are shown in Table
2
.
2.2 Thermosensitivity and Thermogelling Behaviour
The temperature-based responsive behavior of poly(organophosphazenes) were
found by using several different functional side groups as shown in Fig.
3
. The
first thermosensitive poly(organophosphazene) exhibiting LCST in aqueous
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