Biomedical Engineering Reference
In-Depth Information
R
1
R
1
O
R
1
O
N
O
R
2
O
HN
O
HN
H
N
O
O
O
R
3
O
R
3
P
N
P
P
N
N
n
n
n
R
3
O
O
R
3
O
O
HN
HN
O
HN
OR
2
H
O
O
R
1
R
1
O
R
1
(1)
(2)
(3)
N
O
H
O
COOH
O
N
O
H
N
N
P
N
P
P
n
n
n
OH
O
COOH
O
OH
N
(4)
(5)
N
(6)
Fig. 2
Examples of biodegradable poly(organophosphazenes)
hydrophilic
ʱ
-amino-
ˉ
-methoxy-poly(ethylene glycol) (AMPEG) segment with a
molecular weight range of 350-750 Da and amino acid esters (such as Isoleucine
ethyl ester [IleOEt] and leucine ethyl ester [LeuOEt]) [
55
-
62
], dipeptides (glycyl-
glycine and glycylglycine ally ester) [
63
-
67
], depsipeptide (ethyl-2-(
O
-glycyl)gly-
colate and ethyl-2-(
O
-glycyl)lactate) [
57
,
60
,
66
-
69
] and oligopeptides (such as
GlyPheLeuEt and GlyPheIleEt) [
70
] as both hydrophobic and hydrolysis-sensitive
moieties. Examples of the molecular structures of biodegradable thermogelling
poly(organophosphazenes) are shown in Table
1
.
2.1 Synthetic Procedures
Several different synthetic routes are available for the synthesis of
poly(organophosphazenes) [
91
-
96
], but the major method for the biodegradable
thermogelling poly(organophosphzenes) involves a ring-opening polymerization
followed by macromolecular substitution reactions as shown in Scheme
1
[
55
-
63
,
65
,
66
,
97
,
98
]. The thermal ring-opening polymerization of a commercially avail-
able cyclic trimer, hexachlorophosphazene (
7
), gives a high molecular weight
poly(dichlorophosphazene) (
8
), which is an organic-soluble, reactive macromo-
lecular intermediate with chlorine atoms that can be replaced with various organic
nucleophiles (
9-11
).
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