Lipase-Catalyzed Synthesis of Furan-Based Oligoesters and their Self-Assembly-Assisted Polymerization

Article abstract of DOI:10.1002/cssc.201800446

We investigate the synthesis of bio-based hydrophilic and hydrophobic oligoesters, which in turn are derived from easily accessible monomers from natural resources. In addition to the selection of renewable monomers, Novozyme 435, an immobilized lipase B from Candida antarctica was used for the oligomerization of monomers. The reaction conditions for oligomerization using Novozyme 435 were established to obtain a moderate-to-good yield. The average number of repeating units and the molecular weight distribution of hydrophilic and hydrophobic oligoester were identified by using NMR spectroscopy, gel-permeation chromatography, and MS. The oligoester derived from a hydrophilic monomer self-assembled to form a viscous solution, which upon further heating resulted in the formation of a polymer by the intermolecular Diels–Alder reaction. The viscosity of the solution and the assembly of oligoester to form a fibrous structure were investigated by using rheological studies, XRD, and SEM. The molecular weight of the cross-linked polymer was identified by using matrix-assisted laser desorption/ionization–MS. The thermal properties of the bio-based polymers were investigated by using thermogravimetric analysis and differential scanning calorimetry. For the first time, the self-assembly-assisted polymerization of an oligoester is reported using the intermolecular Diels–Alder reaction, which opens a new avenue in the field of polymer science.

Full text of DOI:10.1002/cssc.201800446

Accepted Article
Title: Lipase Catalysed Synthesis of Furan Based Oligoesters and their
Self-Assembly Assisted Polymerization
Authors: Nagarajan Subbiah, Muthusamy K, Lalitha Krishnamoorthy,
Siva Prasad Yadavali, Thamizhanban Ayyapillai, Sridharan
Vellaisamy, and Maheswari C. Uma
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10.1002/cssc.201800446
ChemSusChem
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Lipase Catalysed Synthesis of Furan Based Oligoesters and their
Self-Assembly Assisted Polymerization
Kumarasamy Muthusamy,a Krishnamoorthy Lalitha,a Yadavali Siva Prasad,a Ayyapillai
Thamizhanban,a Vellaisamy Sridharan,a,b C. Uma Maheswaria and Subbiah Nagarajana,c*
Dedication ((optional))
Abstract: Bio-based polyesters are well-known biodegradable
materials that are frequently used in our daily life, which include food
industries and biomedical fields. The journey towards the
development of sustainable polymer materials and technology
postulate the replacement of traditionally using petrochemical-based
monomers, transition metal catalyst, and more intensive purification
techniques, which do not agree with the green chemistry principles.
This contribution investigates the synthesis of bio-based hydrophilic
and hydrophobic oligoesters, which in turn derived from easily
accessible monomers of natural resources. In addition to the selection
of renewable monomers, Novozyme 435, an immobilized lipase B
from Candida antarctica was used for the oligomerization of
monomers. The reaction condition for oligomerization using
Novozyme 435 was established to get moderate to good yield. The
average number of repeating unit and molecular weight distribution of
hydrophilic and hydrophobic oligoester was identified using NMR,
GPC and Mass spectral analysis. To our delight, oligoester derived
from hydrophilic monomer was found to self-assemble to form a
viscous solution, which on further heating resulted in the formation of
polymer via the intermolecular Diels-Alder reaction. The viscosity of
solution and assembly of oligoester to form fibrous structure was
investigated by rheological studies, XRD and SEM analysis.
Molecular weight of cross linked polymer was identified using MALDI
mass spectral analysis. Thermal properties of bio-based polymers
were investigated using TGA and DSC analysis. For the first time we
are reporting the assembly assisted polymerization of oligoester using
intermolecular Diels-Alder reaction, which would initiate a new avenue
in polymer science field.
renewable raw materials and CO2 emission contributing
significantly to global warming have emphasized the researchers
to focus on renewable feedstock for the fuel, value-added
chemicals and monomers for polymer synthesis.1-4 In the field of
polymer chemistry, implementation of strict environmental
regulations to reduce the carbon content,5 pollution and other
concerns influenced the demand of biopolymers and plastics, and
projected to perceive a CAGR of 12% from 2016 to reach a
market size of USD 5.08 billion by 2021.6 Consequently, there has
been an increasing interest in utilization of bio-based monomers
derived from renewable resources for developing environmental
friendly and renewable polymeric materials.7-9 Generally, natural
resources such as vegetable oils, carbohydrates, lignin, cardanol,
catechins, terpenes, rosins and organic acids are used in the
production of high performance bio-based polymers.10 Thus far,
polyesters are frequently used commodity polymers in our daily
life which include biomedicine for bone fixation, ligament
attachment, surgical sutures, food containers, packing, fibers,
biodegradable films, surface coatings, composites, plasticizers,
membrane filters, wound healing accelerators, controlled drug
delivery system and scaffold for cell and tissue growth.11 In
practice, polyesters were derived from petroleum-based diacids
and diols with metallic alkoxides or transition metals as catalyst.12
However, in recent years, as a concern on environmental pollution
and energy storage, functional monomers derived from
renewable feedstock are promising candidates for bio-based
polyester synthesis in both academic and industrial sectors.13,14
Among the various bio-derived functional monomers available for
the synthesis of green polyester synthesis, 5-hydroxymethyl
furfural is considered as one of the most promising platform
chemical derived from carbohydrates such as glucose, fructose,
sucrose and cellulose.15-17 Continuous effort has been devoted for
the synthesis of bifunctional monomers such as 2,5-
furandicarboxylic acid (FDCA), 2,5-bis(hydroxymethyl)-furan
(BHMF) and 5-oxy(bismethylene)-2-furaldehyde from HMF.18-20
Recently, US Department of Energy (DOE), DuPont Industrial
Bioscience and Archer Daniels Midland Co. has declared FDCA
as one of the most important value added chemicals, which is
having the potential to generate exciting high-performance
renewable materials in the 21st century.21,22 By employing FDCA
as a platform chemical, a wide variety of renewable polymers
were generated23,24 with biocompatibility, good thermal and
electrical conductivities, for example, poly(ethylene-2,5-
furandicarboxylate) (PEF), structural analogue of poly(ethylene
terephthalate) (PET) have excellent thermomechanical and
barrier properties than PET.25,26 Despite the potential application
of FDCA based polymers such as polyesters,27 polyamides,28
epoxy resins29 in producing bioplastics,30 3D printing,31 coatings
Introduction
From the environmental perspective, particularly the finite
availability of fossil resources, increased demand of non-
aMr. K. Muthusamy, Dr. K. Lalitha, Mr. Y. Siva Prasad, Mr. A.Thamizhanban,
Dr. V. Sridharan, Dr. C. Uma Maheswari and Dr. S. Nagarajan*
Organic Synthesis Group, Department of Chemistry & The Centre for
Nanotechnology and Advanced Biomaterials, School of Chemical and
Biotechnology, SASTRA Deemed University, Thanjavur
E-mail: snagarajan@nitw.ac.in, snrajannpt@yahoo.co.in,
Fax: 04362264120; Tel: 04362304270
-
613401
b Department of Chemistry and Chemical Sciences, Central University of
Jammu, Rahya-Suchani (Bagla), District-Samba, Jammu-181143, Jammu
and Kashmir, India.
c Department of Chemistry, National Institute of Technology Warangal,
Warangal -506004, Telangana, India.
KM and KL have contributed equally to the work
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cssc.201800446
ChemSusChem
FULL PAPER
in automotive and marine industries,32 fabricating biomedical
devices,33 etc., an intensive investigation on environmental
friendly synthesis for bio-based polymer and advanced method
for generating high molecular weight polymers have not been
completely explored. Most of the protocols reported for the
synthesis of bio-based polymers displaying various physical and
chemical properties involves harsh reaction conditions, metal
catalysts and intense purification steps.24 In order to maintain
sustainable society, a widely accepted concept “green polymer
chemistry” has to be introduced in each and every step.35-38 Thus,
the enzymatic polymerization is desirable, which allows precise
synthesis of bio-based polyesters under mild reaction condition
with control in structure, substrate, functional group, stereo- and
regioselectivities.39 Moreover, polymerization using enzymatic
reactions show high catalytic efficiency without forming any side
products, and after the completion of reaction enzymes can be
separated easily either by filtration or centrifugation.40-42
Eventhough a wide range of enzyme catalysts available for the
synthesis of macromolecules, Candida antarctica lipase B (CALB)
possesses many desirable advantages including compactable
synthesize bio-based polymer involves the polycondensation of
monomers derived from renewable resources by enzyme catalytic
pathway. Precursors for furan-based monomers 3a-d, such as
HMF 1 and -C-glycosidic ketone 2a were synthesized by
following the literature procedure44,52,53 involving the dehydration
of fructose under amberlyst 15 condition and condensation of D-
glucose with pentane-2,4-dione in aqueous basic condition
respectively. Furan-based monomers with hydrophilic saccharide
moiety 3a and hydrophobic alkyl moiety 3b and 3c were
synthesized by aldol condensation of 1 with 2a-c in presence of
pyrrolidine as catalyst and DCM as a solvent in good yields
(Scheme 1).
with
a magnitude of substrate and solvent, regio- and
enantioselectivities.43,44 The immobilized form of CALB,
Novozyme 435® displayed high potential in polyester synthesis
because of its reusability, enhanced thermal stability, propensity
toward bulky substrates and stable catalytic performance in
various chemical environment.45,46 Recently Kobayashi and
coworkers have briefly discussed the production of value-added
functional macromolecules via an environmental friendly enzyme
catalysis method.47 He emphasized the significance on migration
of petroleum-based processes to enzymatic renewable-resource
based processes, that contribute to the sustainable society for the
future. Overall, perceiving the practical application of bio-based
polymers in biomedical, drug-delivery, pharmaceutical and other
areas, there is a demand to develop functional bio-based
polymers using green polymer chemistry.48 In this study, we
present a sustainable approach toward furan-based polyester
synthesis via the formation of assembled structure. A new class
of bifunctional monomers with different hydrophilic and
hydrophobic character were generated from HMF using aldol
condensation step. Further, furan-based bifunctional monomers
were enzymatically polymerized in the presence of Novozyme
435® under optimized reaction condition. The resultant hydrophilic
oligoesters undergoes polymerization via Diels-Alder reaction.
Microstructure, thermal and mechanical properties of furan-based
polymers were studied with respect to its molecular structure.
Scheme 1. Synthesis of bifunctional monomers 3a-c.
1
The H NMR spectrum of furan based monomer 3a-c displayed
the resonance of the alkenic proton around δ 7.2 and 6.2 ppm
with the coupling constant ranging between 15.2-16.4 Hz,
confirms the trans isomeric form of alkene. Bifunctional monomer
3a possesses two primary hydroxyl groups along with three
secondary hydroxy groups, whereas 3b and 3c has each one
primary hydroxy and carboxylic acid groups. Thus, the diol in 3a
undergoes polycondensation with diacids to generate
heteropolymer and hydroxyacids self-condense themselves
produce homopolymer in the presence of either chemical catalyst
or Novozyme 435®. However, to address the global issues on
production of polymers and motivation by the general concern
about environmental issues, past two decades profound research
on the development of bio-based polyester using chemical and
biocatalysts has been performed by many research groups.54,55
Specifically, only few reports were available on lipase catalyzed
polycondensation of furan-based monomers. Loos and coworkers
have reported the synthesis of bio-based furan polyesters using
biocatalyst.56-59 To ensure the sustainability and regioselectivity in
the generation of bio-based polymers, we have chosen the
enzymatic polycondensation method.
Results and Discussion
The rationale behind the growing interest in bio-based polymers
originates from sustainability and exploring the possibility of
chemical features associated with the nature of substituents
present in the polymers.49,50 To achieve the target, two main
strategies been widely used. First strategy is polycondensation of
bio-based monomers in the presence of suitable catalyst and the
later one involves Diels-Alder reaction of diene-dienophile
combination under thermal condition.51 However, our approach to
To explore the possibility of our planned bio-based polymer
synthesis using Novozyme 435®,60 we consecrate our effort to the
identification of a suitable reaction condition. It has been reported
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10.1002/cssc.201800446
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that Novozyme 435 displayed prominent catalytic activity in a wide
range of solvents such as toluene, acetone, dodecane, isooctane,
t-butanol, DMF, hexane and diphenyl ether, and the selection of
solvent is based on the nature of substrate.61-63 For the
optimization step, we have selected diol 3a and a non-activated
diacid, azelaic acid as a model substrate and studied the
polycondensation process in the presence of a variety of solvent
under diverse conditions. Reaction was carried out in a wide
range of solvents and solvent mixtures. Use of acetone,
acetonitrile and dioxane at 50 °C did not afford the product,
instead, the reactants remain insoluble even at elevated
temperature (Table 1, entries 1-3). Polar protic solvents such as
isopropanol, t-butanol yielded 10-15% of oligomer, whereas
DMSO, water and DMF yielded traces of product (entries 4-8). To
our delight, the use of toluene as a solvent at 50 °C furnished 24%
of oligomer (entry 9).
mixture in the ratio of 1:3 did not improve the yield (entry 15,16).
Gratifyingly, the use of toluene-tBuOH (1:3) mixture at 70 °C and
48 h furnished 68% of oligomer (entry 17). The change of solvent
ratio substantially reduce the yield of oligomer (entry 18,19). The
enhanced catalytic activity in optimal mixture of solvent is due to
the retention of structural integrity of Novozyme 435 and
increased solubility of monomers. With the optimized reaction
condition in hand, next we turned our attention to study the
substrate scope of the established methodology and the results
are summarized in the Scheme 2.
First, the utilization of various diacids combining with diol 3a for
the regioselective esterification process to obtain the
corresponding furan-based polyester with hydrophilic sugar and
hydrophobic flexible alkyl unit has been investigated. The use of
glutaric acid as an acid source for regioselective esterification of
3a did not afford the corresponding oligoester, whereas adipic
acid and azelaic acid furnished the respective oligomers 5a and
5b in 68 and 76% respectively. Self-condensation of monomers
3b and 3c in the presence of novozyme 435 under optimized
reaction condition provided the hydrophobic oligoesters 5c and
5d respectively in good yield (Scheme 2). Nonetheless, the use
of optimized condition for the polymerization of 3b and 3c
furnished the corresponding low molecular weight oligomer in
good yield, whereas the prolonged reaction time yielded the
respective furan-based polyester in good yield. Altogether, we
have synthesized a series of furan-based oligoesters with a range
of hydrophilic-lipophilic character and flexible alkyl units (Scheme
2, Figure 1), which could be potentially used for the synthesis of
high molecular weight polymers and self-assembly studies.
Formation of oligomeric ester was confirmed by 1H NMR spectral
analysis. Before oligomerization, methylene group attached to the
furan ring resonate at δ 4.5 ppm, whereas after oligomerization
with diacid, methylene protons experienced deshielding and
resonate at δ 5.1 ppm. A careful analysis of 1H NMR spectra and
integral value of methylene group attached to the furan moiety of
oligomeric esters 5a and 5b revealed that the oligomer was
formed by involving three units of monomer 3a and two units of
4a or 4b. Oligomeric ester formed by the self-condensation of 3b
and 3c at 70 oC, 48 h displayed a signal of equal intensity at δ 4.5
ppm and δ 5.1 ppm respectively for furan-CH2 group. Further the
prolonged reaction time furnished the high molecular weight
oligomeric ester with the signal ratio of 3:1 (Figure S18, S19). The
number average molecular weight (Mn), weight average molecular
weight (Mw) and dispersity (Ð) of the oligoesters were determined
by gel permeation chromatography (GPC) using DMF as a solvent.
GPC analysis of oligoesters 5a-5d displayed Mn of 674, 561, 649
and 495 g/mol, and Mw of 1145, 904, 899 and 638 respectively.
The higher polydispersity value ranging from 1.3-1.7 revealed the
existence of uneven molecular distribution due to the
spontaneous intermolecular Diels Alder reaction of oligoesters
(Table S1). Mass spectral analysis of compound 5a-d shows the
formation of oligoesters having repeating units n>3 via the
effective condensation of monomers, which is in good agreement
with 1H-NMR spectral studies (Figure S20-S22). However, all the
synthesized oligomers possess alkenic and furan moiety with a
range of hydrophilic to hydrophobic character.
Table 1. Optimization of reaction condition for Novozyme 435 catalyzed
polymerization.
S.
No
1
2
3
4
5
6
7
Solvent a
Temperature (oC)
Time
(h)
24
24
24
24
24
24
24
24
24
24
24
24
48
72
48
Yield %b,c
Dioxane
Acetone
Acetonitrile
Isopropanol
t-Butanol
DMSO
H2O
DMF
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene +
Acetone (3:1)
Toluene +
Isopropanol
(3:1)
50
50
50
50
50
50
50
50
50
60
70
80
70
70
70
NR
NR
NR
10
15
Tracesd
Tracesd
Tracsed
24
8
9
10
11
12
13
14
15
30
32
30
42
39
30
16
70
48
35
17
18
19
Toluene + t-
Butanol (3:1)
Toluene + t-
Butanol (1:1)
Toluene + t-
Butanol (1:3)
70
70
70
48
48
48
68
53
36
NR- no reaction. a dry solvents were used. b oligomerization was observed.
c isolated yield. d product was not isolated, formation of traces of product was
noticed in TLC.…
The increase in temperature and reaction time up to 70 °C and 48
h improved the yield of oligomer (48%), further elevation in
temperature and prolonged reaction did not improve the yield
substantially, which can be attributed to the loss of activity of
Novozyme 435 at 80 °C (entries 9-14). Encouraged by this result,
in order to improve the yield, we further moved on to the mixture
of solvents. The use of toluene-isopropanol and toluene-acetone
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(d = 0.45 nm), 20.9 (d = 0.42 nm), 23.5 (d = 0.38 nm), 25.80 (d =
0.35 nm), 27.5 (d = 0.32 nm) corresponds to the (010), (010),
(103), (100), (100) and (100) planes and similar XRD diffraction
pattern was reported for furanic polyesters (Figure 2).56-59
However, hydrophobic oligoester 5d lacking with flexible
methylene unit displayed broad peak at 2θ = 10 and 22 evoked
the amorphous behavior. Though SAXD result suggest the
aggregation behavior of oligoester, it is mandatory to analyze the
self-curing by well-established method. Oligoesters 5a-d were not
freely soluble in polar and nonpolar solvents. In order to record
NMR spectra, compound 5a-d were dispersed in DMSO-d6
followed by heating the mixture to 120 oC for 5 min. Upon heating,
oligoesters 5a and 5b slowly get dissolved in DMSO-d6 and the
solution becomes more viscous, whereas such phenomenon was
not observed in case of 5c and 5d. This result clearly indicates
the existence of hydrogen-bonding assisted cross-linking
between the furan ring and unsaturated double bond in 5a and 5b.
In this context, oligoesters 5a-5d after heating at 120 oC in DMSO
for about 15 min, the contents were poured in methanol and the
precipitated solid were investigated using FT-IR.
Scheme 2. Synthesis of furan-based oligoesters 5a-d.
It is worth mentioning that recently Chen and coworkers have
reported the difuranic polyesters with alkenic unit and their self-
curing ability via Diels-Alder reaction.64 It has been observed that
oligomeric ester 5a and 5b displayed self-curing ability and
formed crosslinker polymer, whereas 5c and 5d exhibited
tolerance. The presence of hydrophilic sugar moiety in 5a and 5b
facilitates the molecular self-assembly via the intermolecular
hydrogen bonding, allow the alkene to approach furan ring in a
close proximity, resulted in the formation of cross-linked polymer
via Diels-Alder reaction.
Figure 2. SAXD of self-assembled oligoesters 5a-d.
Figure 1. Furan-based oligoesters 5a-d and their structural features.
The existence of intermolecular hydrogen bonding in oligoethers
has also been identified using FT-IR spectroscopy. The bands
appear around 2930, 2840, 1400 and 1150 cm-1 were assigned to
asymmetric and symmetric stretching vibrations of the CH2
groups, stretching vibrations of C=C and alkenyl ether (C-O)
linkage of furan ring respectively (Figure 3).56-59 In 5a and 5b,
stretching band appeared for furan ring around 1150 cm-1 got
shifted to 1085 further confirms the formation of cross-linked
polymer via intermolecular Diels-Alder reaction (Figure 3,6d).67
Even though monomer of 5b and 5c contains furan ring and
alkene, it failed to form Diels-Alder product because of the
absence of molecular self-assembly.
Molecular Self-assembly assisted crosslinking behavior of
oligoester was confirmed by various techniques. Small angle X-
ray diffraction (SAXD) is considered as a valuable technique to
study the molecular assembly phenomenon involving H-bonding,
- interaction and van der Waals interaction.66,67 Recrystallized
oligoesters 5a and 5b displayed sharp peaks at 2θ = 7.2 (d = 1.2
nm), 10.1 (d = 0.87 nm), 17.6 (d = 0.53 nm), 21.6 (d = 0.41 nm),
23.9 (d = 0.37 nm), 27.1 (d = 0.33 nm), 29.9 (d = 0.30 nm) and
34.17 (d = 0.26 nm) suggested the formation of well-organized
structure (Figure 2). A careful analysis of diffraction pattern of 5a
and 5b in the lower angle region propose the existence of various
degree of hydrophobic interaction. Hydrophobic oligoester 5c with
flexible alkyl unit displayed peaks at 2θ = 15.6 (d = 0.57 nm), 19.8
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(Hf) was observed at δ 5.7 ppm (Figure 4b, 4e). Similar trend was
observed in case of oligoester 5b and its corresponding cross-
linked polymer (Figure 4c, 4d). However, it is worth mentioning
that oligoester 5b is insoluble in DMSO-d6 at room temperature
and upon heating the mixture to 120 oC for about 3-5 min attained
a viscous character, which displayed signal corresponding to Hf
proton along with the expected Ha-Hd protons (Figure 4c). In figure
4d, the appearance of signal for Hf proton in cross-linked polymer
derived from oligoester 5b further confirms the self-assembly
assisted polymerization. Eventhough oligoesters 5c and 5d
possesses alkene and furan in its core structure, it fails to undergo
crosslinking owing to the lack of molecular self-assembly.
Morphological analysis of self-cured polymer derived from
oligoester 5a were examined by using optical microscopy and
FESEM (Figure 5). Optical microscopy image of viscous solution
obtained from 5a in DMSO-water (1:1 ratio) displayed the
formation of fibrous structure. In order to have the further insight
of cross linked polymer, we have performed FESEM analysis
(Figure 5c-f). The formation of uniformly entangled fibrillar
structure with the dimension ranging from 100-200 nm has been
observed. This result clearly depicts that self-assembled
oligoester 5a and 5b undergoes cross linking via intermolecular
Diels-Alder reaction to form supramolecular architecture.
Figure 3. FT-IR spectra of crosslinked hydrophilic oligoesters 5a, 5b and
hydrophobic oligoesters 5c and 5d.
Molecular self-assembly assisted crosslinking of alkene to furan
moiety via the intermolecular Diels-Alder reaction was further
confirmed by NMR spectral technique. In 1H NMR spectra of
oligoester 5a, alkenic protons (Ha and Hb) resonate at δ 7.4 and
6.6 ppm (J=15.9 Hz), furan protons (Hc and Hd) resonate at δ 6.9
and 6.5 ppm (J=3.3 Hz) and methylene protons (He) attached to
the furan moiety resonate at δ 5.1 ppm respectively (Figure 4a,
4e).
Figure 4. 1H NMR spectra of (a,c) oligoester 5a and 5b in DMSO-d6; (b,d)
crosslinked polymer obtained from oligoesters 5a and 5b via the self-assembly
assisted intermolecular Diels-Alder reaction respectively. Broadening of signal
is due to the increased viscosity of oligoesters and poor solubility of polymers in
DMSO-d6.
Figure 5. Optical microscopy image of self-cured oligoesters (a) 5a and (b) 5b.
FESEM images of self-cured oligoesters (c and d) 5a and (e and f) 5b.
Nano fibrous polymer solution68 are versatile materials play an
important role in industrial and biomedical applications including
cell adhesion, proliferation, tissue engineering and regeneration,
etc. Rheological studies correlate the formulation phenomenon
After self-assembly assisted crosslinking of oligoester 5a, signals
corresponding to alkenic and furan protons (Ha-Hd) disappeared
and new signal representing the formation of Diels-Alder adduct
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with the product performance, thereby act as a bridge that link
laboratory scale preparation and process to the product
development.68 Generally, rheology of nano fibrous polymer
solution is governed by the volume of fibers dispersed in the bulk
solution, which increase the viscosity by flow line distortion and
friction of the particles. The response of viscosity as a function of
applied shear rate for 5a and 5b is shown in Figure 6. When
compared to the solvent, the effective viscosity of 5a and 5b
displayed higher value because of the formation of fibrous
network. At a higher shear rate, however, 5a and 5b behave like
a Newtonian fluid.
adsorbed moisture and self-assembly assisted intermolecular
Diels-Alder reaction between the alkene and furan moiety (Figure
7d). In oligoester 5a and 5b, apart from broad exothermic peak,
clear sharp peak indicating the melting point and crystallization
was not observed. The schematic representation of the existence
of molecular assembly in 5a and 5b via H-bonding and van der
Waals interaction to facilitate the cross linking through Diels-Alder
reaction is shown in Figure 7d. However, 5c displayed sharp
melting peak at 250°C and 5d displayed a broad exothermic peak
along with the melting peak at 240°C respectively. DSC analysis
of oligoesters also confirms the proposed mechanism of
crosslinking without any disputation.
Figure 6. Viscosity vs shear rate characteristics of polymer solution (1% wt/v)
derived from 5a and 5b.
The thermal stability of the furan-based oligoester 5a-d was
investigated using thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC).69 The thermal
degradation profile of the furan based oligoesters 5a-d, measured
between 30 to 900 °C at a heating rate of 10 °C/min under a
nitrogen atmosphere is shown in figure 6. The onset of thermal
decomposition (Tstart) between 100 and 200 °C displayed weight
loss of ~5 to 30% of the initial weight. However, furan-based
oligoester displayed two stages decomposition mechanism with
respect to the temperature and molecular structure.69 A plot of
derivative weight (%/°C) vs temperature was used to study the
decomposition behavior of oligoesters. Self-assembled
hydrophilic oligoester 5a and 5b displayed single-stage
decomposition, whereas the hydrophobic oligoesters 5c and 5d
showed the two-stage decomposition. It is worth mentioning that
oligoesters 5a and 5b displayed higher Tstart value than 5c and 5d,
which attributes the perfect molecular assembly of oligoesters via
H-bonding and van der Waals interaction (Figure 7a, 7b). A
complete decomposition of oligoesters was not seen even after
heating to 900 °C and the weight percentage of stable residue
around 19-55% of initial weight was still remained. The amount of
stable carbonaceous materials generated by the polyesters was
15%, 54%, 18% and 38% for the oligoesters 5a, 5b, 5c and 5d
respectively. DSC curves of the polymers 5a-d recorded during
the first heat cycle is shown in Figure 7c. In DSC thermogram of
oligoesters 5a and 5b, the existence of a broad exothermic peak
in the range of about 45-180°C is ascribed to the evaporation of
Figure 7. Thermal analysis of 5a-d. (a) TGA traces of oligoesters 5a-d; (b) a
plot of derivative weight (%/°C) vs temperature; (c) DSC curves of oligoesters
5a-d and (d) schematic representation of self-assembly assisted polymerization
of 5a via intermolecular Diels-Alder reaction.
Conclusions
In this study, we have developed a simple and green protocol for
the synthesis of bio-based hydrophilic and hydrophobic
oligoesters directly from the easily accessible natural monomers
using a biocatalyst, Novozyme 435, an immobilized lipase B from
Candida Antarctica in moderate to high yields. The average
number of repeating unit and molecular weight of hydrophilic and
hydrophobic oligoester was identified using NMR, GPC and mass
spectral analysis. SAXD analysis revealed that hydrophilic
oligoesters were prone to self-assemble via H-bonding and van
der Waals interactions, whereas hydrophobic oligoesters
displayed amorphous structure. The self-assembled oligoesters
in DMSO at 120 oC undergoes intermolecular crosslinking via the
Diels-Alder reaction to form viscous polymer solution. The
viscosity of solution and the morphology of self-cured polymer
was investigated by rheological studies and SEM analysis.
Morphological analysis of polymer showed the formation of
fibrous structure. The robust thermal properties of bio-based
oligoesters were investigated using DSC and TGA analysis. For
the first time, we present the self-assembly aided polymerization
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10.1002/cssc.201800446
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Compound 3a
of bio-based oligoesters, derived via “green polymer chemistry”
using intermolecular Diels-Alder reaction. The reported method
may initiate a new avenue in the development of sustainable
polymer materials and technology of commercial interest.
Yellow viscous liquid; Yield: 87 %; 1H NMR (300 MHz, D2O) δ = 7.37 (d, J
= 15.9 Hz, 1H, Alk-H), 6.76 (d, J = 3.3 Hz, 1H, Fur-H), 6.60 (d, J = 16.2 Hz,
1H, Alk-H), 6.42 (d, J = 3.6 Hz 1H, Fur-H), 4.50 (s, 2H, Fur-CH2), 3.74-3.35
(m, 5H, Sac-H), 3.31-3.23 (m, 2H, Sac-H), 3.17 (t, J = 9.3 Hz, 1H, Sac-H),
3.06 (dd, J = 14.4, 3.0 Hz, 1H, Sac-H), 2.80 (dd, J = 15.3, 9.0 Hz, 1H, Sac-
H). 13C NMR (75 MHz, D2O) δ = 201.9, 157.3, 150.40, 131.6, 122.4, 118.9,
111.2, 79.5, 77.3, 75.77, 73.22, 69.68, 60.75, 55.97.
Experimental Section
Materials and General Methods
Compound 3b
All reagents and solvents needed for the synthesis of furan-based
monomers 3a-c and polyesters 5a-d were purchased from Sigma Aldrich,
Merck, Alfa aesar, TCI chemicals and Avra chemicals and were used as
such without further purification. For purification of compounds, LR grade
and distilled solvents were used, when necessary. The reaction progress
was monitored by thin-layer chromatography using TLC silica gel 60 F254
purchased from Merck and visualized by UV detection, molecular iodine,
p-anisaldehyde stain or sulphuric acid spray. Column chromatography was
performed on silica gel (60-120 mesh) or neutral alumina purchased from
Avra chemicals and Merck respectively.
Yellow solid; Yield: 92%; 1H NMR (300 MHz, CDCl3) δ = 7.35 (d, J = 15.9
Hz, 1H, Alk-H), 6.69 (d, J = 15.6 Hz, 1H, Alk-H), 6.66 (d, J = 3.3 Hz, 1H,
Fur-H), 4.68 (s, 2H, Fur-CH2), 2.97 (t, J = 6.6 Hz, 2H, CH2), 2.74 (t, J = 6.6
Hz, 2H, CH2). 13C NMR (75 MHz, CDCl3) δ = 202.5, 179.3, 163.1, 154.9,
133.6, 127.3, 121.8, 114.6, 61.6, 40.3, 32.9.
Synthesis of bio-based monomers 3c.
To a solution of HMF 1 (1 mmol) in 3 mL of pyridine was added malonic
acid (1.2 mmol) and the mixture was refluxed with stirring for 16 h. The
progress of the reaction was monitored periodically using TLC. After the
completion of the reaction as identified by TLC, the reaction mixture was
cooled to room temperature and acidified with 5N HCl to precipitate the
product. The yellow solid thus precipitated was filtered, washed well with
water and dried under vacuum.
Characterization Methods
1H- and 13C NMR spectra were recorded on a Bruker Avance 300 MHz
instrument in CDCl3 or DMSO-d6 or D2O at room temperature. Chemical
shifts (δ) are reported in parts per million (ppm) with respect to internal
standard TMS and coupling constants (J) are given in Hz. Proton
multiplicity is assigned using the following abbreviations: singlet (s),
doublet (d), triplet (t), quartet (q), multiplet (m). Infrared spectrum was
obtained using FTIR Shimadzu 8000 Spectrometer in the spectral range
of 4000 cm-1 to 400 cm-1. Matrix-assisted laser desorption/ionization-time
of flight mass spectrometry (MALDI-TOF MS) measurement was recorded
on Voyager-DE PRO Biospectrometry with dithranol matix. The spectrum
was recorded using DMF solvent in positive and linear mode. Thermal
stability and the thermal transitions of the polyester were characterized by
SDT Q600V20.9 Build 20 thermogravimetric analyzer under nitrogen
atmosphere with a heating rate of 10 °C/min. Small angle X-ray diffraction
(SAXD) spectra were obtained at room temperature using a BRUKER-
binary V3 diffractometer in the angular range of 2-70° (2θ).
Yellow solid; yield: 88%; 1H NMR (300 MHz, D2O) δ = 7.35 (d, J = 15.3 Hz,
1H, Alk-H), 6.66 (d, J = 3.0 Hz, 1H, Fur-H), 6.39 (d, J = 3.0 Hz, 1H, Fur-H),
6.18 (d, J = 16.5 Hz, 1H, Alk-H), 4.50 (s, 2H, Fur-CH2). 13C NMR (75 MHz,
D2O) δ = 170.7, 156.5, 150.2, 132.5, 117.1, 113.9, 110.6, 55.8.
General procedure for the synthesis of bio-based oligomers 5a and
5b.
To a solution of compound 3a (1 mmol) in 5 mL toluene:t-BuOH (3:1
ratio) was added fatty acid (2 mmol) and Novozyme 435® (100 mg). The
reaction mixture was kept in an orbital incubator shaker at the temperature
of 70 °C and 300 rpm for the given period of time. Progress of the reaction
was monitored using TLC by following the consumption of compound 3.
After completion of the reaction as identified by TLC, the reaction mixture
was cooled to room temperature and filtered to remove Novozyme 435®
and washed with acetone. The filtrate was concentrated by rotary
evaporator at reduced pressure and the product was precipitated by
adding methanol. Bio-based polymers 5a and 5b were stored in
refrigerator for further studies.
Synthesis
Synthesis of HMF 1 and -C-Glycosidic ketone 2a.
HMF was synthesized in batch process reported by Simeonov and
Afonso,42 which include the dehydration of fructose using
tetraethylammonium bromide and amberlyst 15 as catalyst. In this method
reaction media and catalyst can be effectively recycled. β-C-Glycosidic
ketone 2a was synthesized by the condensation of unprotected sugar, D-
glucose with pentane-2,4-dione in alkaline aqueous media.41
Compound 5a:
o
1
Pale brown solid; mp: >250 C; H NMR (300 MHz, DMSO-d6)* δ = 7.38
(d, J = 15.9 Hz, 1H, Alk-H), 6.93 (d, J = 3.3 Hz, 1H, Fur-H), 6.58 (d, J =
15.9 Hz, 1H, Alk-H), 6.47 (d, J = 3.3 Hz, 1H, Fur-H), 5.09 (s, 2H, Fur-CH2),
4.44 (s, 2H, Fur-CH2), 4.03 (q, J = 7.1 Hz, 2H, Sac-H), 3.59–3.43 (m, 1H,
Sac-H), 3.41-3.35 (m, 1H, Sac-H), 3.21-3.16 (m, 2H, Sac-H) 3.11–3.04 (m,
2H, Sac-H), 2.93 (m, 1H, Sac-H), 2.72 (dd, J = 16.2, 8.4 Hz, 1H, Sac-H),
2.34-2.28 (m, 1H, Sac-H), 2.15–2.06 (m, 4H, COCH2), 1.49-1.44 (m, 4H,
CH2), 1.26–1.23 (m, 6H, CH2). 13C NMR (75 MHz, DMSO-d6) δ = 197.5,
175.1, 158.6, 149.8, 128.9, 122.9, 117.6, 109.9, 80.6, 77.9, 75.9, 73.5,
70.2, 61.0, 55.7, 43.4, 39.8, 39.2, 38.9, 34.4, 33.2, 28.5, 28.5, 28.4, 28.2,
24.7, 24.3, 21.9. *owing to the formation of oligomer, broadening as well
as shift in signals were observed.
General Procedure for the synthesis of bio-based oligoesters 3a and
3b.
To a stirred solution of suitable methyl ketone (1 mmol) in DCM were
added pyrrolidine (1-2 drops) and HMF (1 mmol) and stirring was
continued for 10-12 h at room temperature. The progress of the reaction
was monitored using TLC. After completion of the reaction as identified by
TLC, the reaction mixture was diluted with water and extracted with ethyl
acetate (3 x 20 mL). The organic layer was dried over Na2SO4 and the
solvent was removed under reduced pressure. The crude residue was
purified by silica gel column chromatography.
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10.1002/cssc.201800446
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Compound 5b:
A dispersed solution of self-assembled oligoesters (10 mg) in DMSO (5
mL) was heated to 120 oC for about 15-25 min. Upon heating, dispersed
oligoesters slowly get dissolved and the solution became viscous in nature.
The clear viscous solution thus formed was cooled to room temperature
and 10 mL of methanol was added to generate solid cross-linked polymers.
Cross linked polymer formed by Diels-Alder reaction was not soluble in
any of the solvent. Cross linking was confirmed by FTIR analysis.
Morphology was identified using optical microscopy and FESEM analysis.
Viscous nature of the polymer solution was identified by rheological
measurements
Pale brown solid; mp: >250 oC; 1H NMR (300 MHz, DMSO-d6)* δ = 7.35-
5.85 (m, 4H, Alk-H, Fur-H), 5.07-4.91 (m, 5H, Sac-H, Fur-H), 4.44-4.32 (m,
4H, Sac-H, Fur-H), 3.56-2.73 (m, 3H, Sac-H), 2.37-2.19 (m, 4H, CH2), 1.49
(s, 4H, CH2). 13C NMR (75 MHz, D2O) δ = 180.9, 176.5, 149.6, 148.6, 130.7,
118.0, 110.3, 108.2, 78.7, 78.6, 76.4, 75.0, 74.5, 72.4, 72.2, 68.8, 68.6,
59.9, 59.7, 55.2, 54.9, 51.6, 48.9, 48.2, 44.6, 35.1, 29.5, 24.9, 24.2, 23.9,
23.2, 22.8, 22.7, 21.7. *owing to the formation of oligomer, broadening as
well as shift in signals were observed. Saccharide peaks were merged with
solvent peak.
Rheological studies
General procedure for the synthesis of bio-based oligomers 5c and
5d.
Flow behavior of polymer solution was identified using a stress controlled
rheometer (Anton Paar 302 rheometer) implemented with a steel-coated
25 mm diameter parallel-plate geometry. The gap between two plates for
rheological testing of polymer solution was fixed as 0.5 mm and
experiments were carried out at 23 °C.
To a solution of compound 3b/3c (1 mmol) in 5 mL toluene:tBuOH (1:3
ratio) was added Novozyme 435® (100 mg). The reaction mixture was kept
in an orbital incubator shaker at the temperature of 70 °C and 300 rpm for
the given period of time. Progress of the reaction was monitored using TLC
by following the consumption of compound 3. After completion of the
reaction as identified by TLC, the reaction mixture was cooled to room
temperature and filtered to remove Novozyme 435® and washed with
acetone. The filtrate was concentrated by rotary evaporator at reduced
pressure and the product was precipitated by adding methanol. Bio-based
polymers 5c and 5d were stored in refrigerator for further studies.
Acknowledgements
This work was financially supported by the Department of Science
and Technology (IFA11-CH-04 and #SB/FT/CS-024/2013), India
and Board of Research in Nuclear Science (#37(1)/20/47/2014),
Department of Atomic Energy. S.N. sincerely thanks SASTRA
University for financial support under DESH-VIDESH scheme.
Compound 5c
Polymer obtained from compound 3b. Pale brown solid; mp: >250 oC; 1H
NMR (300 MHz, DMSO-d6)* δ = 7.41 (d, J = 15.9 Hz, 1H, Alk-H), 6.95 (d,
J = 3.3 Hz, 1H, Fur-H), 6.67 (d, J = 3.3 Hz, 1H, Fur-H), 5.90 (d, J = 16.2
Hz, 1H, Alk-H), 5.09 (s, 2H, Fur-CH2), 4.44 (s, 0.5H, Fur-CH2-OH), 2.96 (s,
2H, CH2), 2.62 (t, J = 5.4 Hz, 2H, CH2) 13C NMR (75 MHz, DMSO-d6) δ =
197.5, 171.9, 152.2, 150.8, 128.6, 123.2, 117.2, 113.3, 57.6, 55.7, 34.7,
27.5.
Keywords: Renewable Resources • Enzyme catalysis •
Biopolymers • Self-Assembly • Diels-Alder Reaction
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Entry for the Table of Contents (Please choose one layout)
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This contribution investigates the
synthesis of bio-based hydrophilic and
hydrophobic furan-based oligoesters,
which in turn derived from easily
accessible monomers of natural
resources by biocatalysis. For the first
time we are reporting the self-
Kumarasamy Muthusamy,
Krishnamoorthy Lalitha, Yadavali Siva
Prasad, Ayyapillai Thamizhanban,
Rajendhiran Saritha, Vellaisamy
Sridharan, C. Uma Maheswari and
Subbiah Nagarajan*
Page No. – Page No.
assembly assisted polymerization of
C-glycosyl furan-based oligoester
using intermolecular Diels-Alder
reaction, which would initiate a new
avenue in the field of polymer science.
Lipase Catalysed Synthesis of Furan
Based Oligoesters and their Self-
Assembly Assisted Polymerization
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