Chemistry Reference
In-Depth Information
Fig. 12
Lipase-catalyzed polycondensation of 1,6-hexanedithiol with dicarboxylic acid diesters
diesters of varying spacer length with hexane-1,6-dithiol. Low molecular weight
polythioesters were obtained in 75-90% yield (
M
n
3.7-6.0 kDa, PDI 1.7-2.0).
A higher molecular weight polythioester (
M
n
50 kDa, PDI 2.3) was obtained by
preparation of cylic, oligomeric polythioesters under dilute conditions, followed by
Imbalance in the stoichiometry of polycondensation reactions of AA-BB-type
monomers can be overcome by changing to heterofunctional AB-type monomers.
Indeed, 11MU has been subjected to bulk polycondensation using lipases as cata-
lyst in the presence of 4 Å molecular sieves. At 70
◦
C, CALB showed 84% monomer
conversion and a low molecular weight polymer (
M
n
1.1 kDa, PDI 1.9). No signifi-
cant polymerization was observed with other lipases (except
P. cepacia
lipase, 47%
conversion, oligomers only) and in reference reactions with thermally deactivated
CALB or in the absence of enzyme. Further optimization of the reaction conditions
(60 wt% CALB, 110
◦
C, 3 days, 4 Å molecular sieves) gave a polymer with
M
n
of
7
Conclusions
Lipase-catalyzed polymerizations have found a place in the field of synthetic poly-
mer chemistry. Detailed research conducted in the past decade has brought the
understanding of this novel technology to a level that is known in traditional poly-
merization techniques. This led to a situation where lipase-catalyzed ROP and
polycondensation can be compared directly to its chemical counterpart, and its spe-
cific advantages identified. The recent breakthroughs in enzymatic ROP are to a
large extent due to Novozym 435. The commercial availability of this stable, robust
immobilized lipase formulation has been very important in bridging the worlds of
biocatalysis and polymer chemistry. It has also allowed polymer chemists to rou-
tinely apply biocatalysis without the necessity of a deep knowledge of enzymology
or biotechnology.
Nevertheless, the limitations of Novozym 435 have also been clearly identified.
Water is the preferred nucleophile, making 100% end-functionalization very chal-
lenging, while the preference of CALB for
transoid
ester bonds limits the potential
to reach low polydispersities in a ROP. Multidisciplinary projects focusing both on
the enzymology side of the biocatalyst (e.g., by improving the lipase by mutations)
and on the polymers required for specific applications can in the near future lead
