Chemistry Reference
In-Depth Information
O
O
a
lipase
H
O
RO
O
R-OH
+
n
x
n
x
b
O
O
lipase
RO
OH
n
H
+
n
+
2n R-OH
n
HO
OH
OR
x
O
O
y
x
y
O
O
O
O
c
lipase
OH
O
n
+
RO
RO
H
n R-OH
x
x
n
Fig. 2
Synthetic routes to procure polyesters using lipase catalysis:
(
a
)
ROP of cyclic esters,
(
b
)
polycondensation of AA-BB monomers, and
(
c
)
polycondensation of AB monomers
monomer sequence during ROP and the endgroup fidelity will be discussed. Then,
we will briefly discuss the application of lipases in polycondensation reactions, con-
centrating on the production of multifunctional polyesters in a one-step approach.
3.1
Lipase-Catalyzed ROP of Cyclic Esters
3.1.1
Monomer Activation During ROP
In lipase-catalyzed ROP, it is generally accepted that the monomer activation pro-
ceeds via the formation of an acyl-enzyme intermediate by reaction of the Ser
residue with the lactone, rendering the carbonyl more prone to nucleophilic attack
acyl-enzyme intermediate by an appropriate nucleophile such as water or an alcohol
to produce the corresponding
-hydroxycarboxylic acid or ester. Propagation, on
the other hand, occurs by deacylation of the acyl-enzyme intermediate by the termi-
nal hydroxyl group of the growing polymer chain to produce a polymer chain that
is elongated by one monomer unit.
Mechanistic investigations of lipase-catalyzed ROP of unsubstituted lactones
revealed that the polymerizations follow Michaelis-Menten kinetics and that the
formation of the acyl-enzyme intermediate is the rate-determining step [
62
]. In
substituted lactones, depending on the size of the substituent, the deacylation step
constant,
K
M
, and the maximal rate of polymerization,
V
max
, of unsubstituted lac-
tones of varying ring size showed that
K
M
is relatively independent of the ring
ingly, lactones possessing a
cisoid
ester bond conformation appeared less reactive in
CALB-catalyzed reactions than did large ring lactones in a
transoid
conformation
where ring strain in the small
cisoid
lactone is the driving force for reaction and
ω
