Biomedical Engineering Reference
In-Depth Information
Scheme 6.3
(a) Synthesis of polyurethanes from cyclic carbonates. (b) Enzymatic synthesis of
polyester-based polyurethanes by direct polymerization and cyclization with subsequent
ring - opening polymerization.
(DUD) which was polycondensated with
- diols containing six or more carbon
atoms and rendered samples with
M
n
molecular weights between 1600 and 3500.
The process required the presence of tin catalysts, such as Bu
2
Sn(OCH
3
)
2
or
Bu
2
SnO, to prevent thermal decomposition of the urethane group.
Other authors have prepared PURs with a regular sequence by ring-opening
polymerization of cyclic trimethylene urethane and tetramethylene urethane at
100 ° C [58] . A more recent route was developed by Schmitz
et al.
[59] , who studied
the copolymerization of equimolar amounts of 2,2-dimethyltrimethylene carbon-
ate and tetramethylene urea. The obtained copolymers had microstructures that
were strongly dependent on reaction conditions and the catalyst used.
α
,
ω
6.4.2
Enzymatic Synthesis of Polyurethanes
Enzymes have remarkable characteristics compared with conventional chemist
catalysts, like high catalyst activity, stereoselectivity, and lack of side reactions.
Furthermore, enzymes are interesting as a renewable and naturally occurring
catalyst. Because enzymatic polymerization can be considered the reverse reaction
of degradation, polyester-based PEUs obtained using a biological catalyst are
expected to be biodegraded by the same enzyme.
Many authors have reported the enzymatic synthesis of PEUs by enzymatic
polyesterifi cation [55]. Thus, lipase has been employed to promote the esterifi ca-
tion reaction between a low-molecular-weight DUD and various diacids/diesters.
Two strategies can be applied (Scheme 6.3b): (i) direct polycondensation of the
DUD and the appropriate diester, and (ii) ring-opening polymerization of a cyclic
ester-urethane oligomer which has been previously prepared by a lipase-induced
transesterifi cation reaction between a biodegradable DUD and a diester. The latter
method gives higher molecular weights.
