Biomedical Engineering Reference
In-Depth Information
.
CH(R) NH
2
.
HOTos
n
TosOH
H
2
N
CH(R)
CO
O
D
O
CO
+
or
n
Cl
CO
A
CO
Cl
R
1
O
CO
ACO O
R
1
n
pTSA and HCl acceptor
HN
CH(R)
CO
O
D
O
CO
CH(R) NH
CO
A
CO
n
PEAs
Figure 5.5
Synthesis of PEAs by IP and SP (AP).
epoxy-succinic acids, as well as bis-(succinic acid)-
-
diacyl - bis -glycolic acids, see Scheme 5.2), the preference should be given to SP
using DADs as bis-electrophilic monomers. The SP via active diesters of various
classes - DADs, DBCs, and ACs - is called “ active polycondensation ” ( AP ) [42] to
distinguish it from traditional polycondensation methods. Hereafter we use the
term AP for polycondensation with participating active diester of diacid. The AP
with DADs is normally carried out in polar aprotic solvents DMA, DMSO, etc., or
in common organic solvents like chloroform, THF, etc., at 20 - 80 ° C using mostly
triethylamine (TEA) as TosOH acceptor [14- 16, 20 -22, 24, 25, 27, 28, 32, 33, 42-
47] . It was shown that DADs are stable against both amide-type solvents and terti-
ary amines [48] under the conditions of AP that minimizes undesirable side
reactions and results in the formation of high-molecular-weight polymers.
It has to be noted that PEAs composed of the same three building blocks -
α
,
ω
- alkylene diesters and
O
,
O
′
- AA
(glycine), fatty diols, and dicarboxylic acids - were synthesized recently [5] using
the third method - thermal polycondensation ( TP ) in melt, in the presence of
titanium butoxyde as a catalyst at 160 - 220 ° C.
The advantage of TP is the possibility to process polymers from melt directly
after the polycondensation, that is, without the separation and purifi cation of the
resulting polymers. However, the method is less suitable for thermally sensitive
and unstable monomers including optically active ones since high reaction tem-
perature can cause racemization and destruction. The use of metalorganic catalyst
is one of the drawbacks as well.
The AABBPs type PEURs can be synthesized on the basis of TAADs under the
conditions of either IP or AP similar to Figure 5.5 using as bis-electrophilic mono-
mers BCFs instead of DDCs, and DBCs instead of DADs.
Like for the PEA, the PEUR synthesis by IP is less suitable with short-chain
DBC due to their hydrolytic instability that results in low-molecular-weight poly-
mers. Kohn
et al.
[49 - 51] suggest that more appropriate monomers for poly-
urethane synthesis via IP are DBCs that are hydrolytically more stable. The results
α
