Chemistry Reference
In-Depth Information
The applied catalytic system consisted of a Ru-Noyori-type racemization catalyst
of acyl donors, and it was expected that it would allow the use of bifunctional acyl
donors for the formation of polycondensates. Before the start of the reaction, the
monomer mixture showed the expected diastereomer ratio of
(
S
,
S
)
:
(
R
,
R
)
:
(
R
,
S
)
of 1:1:2 of the 1,4-diol employed. After 30 h of reaction the
(
S
,
S
)
-enantiomer al-
most completely disappeared, whereas the ratio of
-monomer was
ca. 3:1 (
R
:
S
ca. 7:1). At a hydroxyl group conversion of 92% after 70 h, no fur-
ther conversion was observed and a final ratio of
(
R
,
R
)
-to
(
R
,
S
)
of 16:1 (
R
:
S
ca.
33:1) was obtained. Unfortunately, the molecular weights of the polymer were mod-
erate at best
(
R
,
R
)
to
(
R
,
S
)
and Novozym 435 had to be added every few hours to
compensate for the activity loss of the lipase. This suggests that Ru-catalyst
1
and
Novozym 435 are not fully compatible.
van As et al. improved the catalytic system by changing the racemization catalyst
dimethyl-3-pentanol (DMP) to suppress dehydrogenation reactions [
103
]. DMP is a
sterically hindered alcohol and is not accepted by Novozym 435 as a substrate. Al-
though Shvo's catalyst
2
is a significantly slower racemization catalyst than
1
, it does
not require the addition of K
2
CO
3
. This base, required to activate the precatalyst of
1
, appears to contribute to the deactivation of Novozym 435 in DKR polymerization
conditions. For the 1,4-diol, the improved catalyst combination resulted in polymers
with
ee
of 94% and peak molecular weight (
M
p
)
(
M
w
=
3
.
4kDa
)
α
,
α
-
Dimethyl-1,3-benzenedimethanol (1,3-diol), showed a higher solubility in toluene,
and slightly better results under similar conditions were obtained. After an optimiza-
tion study using 1,3-diol and diisopropyl adipate as the monomers, chiral polymers
were obtained with
M
p
values up to 15 kDa,
ee
values up to 99%, and (at most) 1-3%
ketone functional groups in
of 8.3 kDa within 170 h.
120 h as a result of dehydrogenation. Aliphatic sec-
ondary diols were also employed as the substrate, but DKR polymerization of these
diols did not lead to enantiopure polymers. At most, an
ee
of 46% was obtained with
low molecular weights in the range of 3.3-3.7 kDa. The latter was attributed to the
low selectivity of Novozym 435 for these secondary diols, as revealed by kinetic res-
olution experiments of 2,9-decandiol with vinyl acetate and Novozym 435. Appar-
ently, the
∼
(
)
-alcohol showed significant reactivity, decreasing the
ee
of the polymer.
comprising a secondary hydroxyl group and an ester moiety did show high
The
E
was high for all monomers studied
S
(
E
>
200
)
. An additional advantage
OH
O
p
Scheme 11
Novozym-435-
catalyzed transesterification
reaction of aliphatic AB
monomers containing a
secondary hydroxyl
O
p = 1: Me-6HH
p = 2: Me-7HO
p = 3: Me-8HN
p = 8: Me-13HT
