A minimalist furan-maleimide AB-type monomer and its thermally reversible Diels-Alder polymerization

Article abstract of DOI:10.1039/c6ra08129k

The present study reports an easy and direct route for the synthesis of highly pure 2-furfurylmaleimide, a minimalist furan-maleimide AB-type monomer, which was used for the preparation of a rigid thermally reversible polymer based on the Diels-Alder reaction. This strategy has high potential for the development of novel polymers and copolymers mainly derived from renewable resources.

Full text of DOI:10.1039/c6ra08129k

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A minimalist furan-maleimide AB-type
monomer and its thermally reversible Diels-
Alder polymerization
Talita M. Lacerda,a,b,c Antonio J. F. Carvalho,c Alessandro Gandinib,c*
a Biotechnology Department, Engineering School of Lorena, University of São Paulo,
Estrada Municipal do Campinho, s/no, 12602-810 Lorena, SP, Brazil.
b São Carlos Institute of Chemistry, University of São Paulo, Avenida Trabalhador
São-carlense 400, 13566-590 São Carlos, SP, Brazil.
c Department of Materials Engineering, São Carlos School of Engineering, University
of São Paulo, Avenida João Dagnone 1100, 13563-120 São Carlos, SP, Brazil.
The present study reports an easy and direct route for the synthesis of highly
pure 2-furfurylmaleimide, a minimalist furan-maleimide AB-type monomer,
which was used for the preparation of a rigid thermally reversible polymer
based on the Diels-Alder reaction. This strategy has a high potential for the
development of novel polymers and copolymers mainly derived from renewable
resources.
The versatile Diels-Alder (DA) click reaction of the furan (diene)-maleimide
(dienophile) pair is a well-defined tool in organic synthesis and has recently gained
much attention in the field of polymer synthesis and modification, offering a green
connotation by combining furan compounds as renewable resources with clean and
straightforward processes.1 Additionally, based on its thermal reversibility (Scheme
1), this reaction opens the way to the preparation of self-mendable and recyclable

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macromolecular materials possessing a variety of architectures, thus enhancing their
potential applications, as recently reviewed,1 with updates2 and extension to
biomaterials.3
Scheme 1. The furan-maleimide DA equilibrium illustrating a thermally reversible
polycondensation step (the endo and exo structures of the adduct are not shown).1
We describe here a peculiar synthesis and polymerization of a highly pure
AB-type monomer containing a single methylene moiety bridging the furan
heterocycle with the maleimide counterpart, viz. 2-furfuryl-N-maleimide (5, Scheme
2), for the preparation of a rigid, thermally reversible linear polymer based on the
DA/retro-DA (rDA) reactions based on Scheme 1. A previous study4 had tackled this
topic, but with unsatisfactory results because of the inadequate approach to the
monomer synthesis, which gave rise to poor polymerization processes. We decided
therefore to tackle the issue through a different approach that involved the in situ
synthesis of the corresponding polymer 4 (Scheme 2) to be used as a precursor to the
monomer 5 by calling upon its rDA depolymerisation. Scheme 2 highlights this
strategy.
The procedure consisted initially in carrying out the 1:1 reaction of 2-
furfurylamine (1) and the previously synthesized classical furan-maleic anhydride DA
adduct (exo-3,6-epoxy-1,2,3,6-tetrahydrophthalic anhydride, 2), conducted in
methanol at 55-60 °C for 3 days, to prepare the protected analogue (ProtAB, 3) of the
1
AB monomer 5, which was characterized by FTIR and H NMR spectroscopy. Its
deprotection, a simple rDA reaction for the elimination of furan, was conducted in

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1,1,2,2-tetrachloroethane (TCE) at 110 °C for 5h. Once the release of the protecting
furan had been completed, leaving a maleimide group susceptible to undergo the DA
reaction with the complementary furan moiety at the other end on this very short AB
structure 5, the temperature of the system was decreased to 60 °C, which shifted the
DA equilibrium towards the formation of the adduct (Scheme 1) and hence promoted
the monomer polycondensation giving both endo and exo stereoisomer units in the
1
backbone.1 The ensuing polymer (PolyAB, 4) was characterized by FTIR and H
NMR spectroscopy and thermal analyses. GPC could not be carried out because the
polymer was not soluble in the organic solvents commonly used with this technique,
which might be a consequence of its inherent rigid structure. In the final key step, the
desired AB monomer 5 was generated by heating polymer 4 at 150 °C for 6 h in a
high-vacuum flask equipped with a liquid nitrogen cold finger. Its rDA
depolymerization reaction released the AB monomer, which was collected on the cold
finger as high-purity white crystals and whose structure was confirmed by FTIR and
1H NMR spectroscopy. The thermal properties of the monomer were also evaluated.
Scheme 2. Schematic route for the isolation of pure 2-furfuryl-N-maleimide and its
DA polymerization.

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Figure 1 shows the FTIR spectra of 3, 4 and 5 in which the most relevant
discriminating features are the differences around 1000 cm-1, related to the presence
(3 and 5) and absence (4) of the typical furan breathing peak, and around 840 cm-1,
related to the presence (5) and absence (3 and 4) of the characteristic maleimide peak.
1
Figure 2 displays their respective H NMR counterparts with the corresponding
assignments. In the case of polymer 4 spectrum, the presence of both endo and exo
adducts gave rise to broader resonance features.
Figure 1. FTIR spectra of ProtAB (3), PolyAB (4) and 2-furfuryl-N-maleimide (5),
highlighting different wavenumber regions.

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Figure 2. 1H NMR spectra of ProtAB (3), PolyAB (4) and 2-furfuryl-N-maleimide
(5).
The thermogravimetric analysis of polymer 4, performed under a nitrogen
atmosphere, showed an initial degradation at 110 °C with a more prominent weight
loss from 150 °C, with only 10% of residue left at 230 °C (Figure 3A). This
behaviour was in tune with a depolymerisation process induced by the rDA reaction
of both the endo and exo motifs.5 Differential Scanning Calorimetry (DSC) showed a
glass transition around 90 °C, before the onset of depolymerisation (Figure 3B).

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Figure 3. (a) TGA and (b) DSC thermograms of polymer 4.
The rigid regular structure of polymer 4 suggested the possible occurrence of partial
crystallization, which was however not confirmed by X-ray diffraction analysis
(Figure 4). This could be explained by the stereochemistry of the furan/maleimide
adduct, present as endo and exo isomers, which would interfere with the formation of
highly ordered macromolecular structures.

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Figure 4. X-ray diffractogram of polymer 4.
DSC was also used to measure the melting temperature of 5, which appeared
as a rather sharp endothermic peak centred at 94 °C, immediately followed by its DA
exothermic polymerization in the molten state (Figure 5). These results are in
accordance with monitoring glass capillary heating features. The AB monomer 5 was
also characterized by mass spectrometry, which indicated the formation of dimers by
the presence of a prominent [M-H]- (negative ion mode) peak at 353 mu.
It is important to emphasize that monomer 5 was found to be thoroughly stable
at room temperature, i.e. no evidence of its spontaneous DA self-polymerization was
detected within more than one-year storage in its crystalline form.
Figure 5. DSC thermogram of monomer 5.

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Polymerizations of the isolated 5 were conducted with a 0.3M solution in
1
TCE-d2, kept at 60 °C. A typical H NMR spectrum acquired during this process is
depicted in Figure 6.
Figure 6. 1H NMR spectrum of the initial phase of the AB-Mon 5 polymerization.
The evolution of the spectral features strongly indicated the progressive
formation of the DA polymer, by the appearance of the broader peaks already
characterized in the initial PolyAB spectrum (Figure 2) and the corresponding
decrease in the intensity of the peaks associated with both furan and maleimide
moieties. It was possible to estimate an approximate value of degree of
polymerization (DPn) of PolyAB by calculating the ratio between the integration of
the peaks arising from the protons of the monomer units positioned within the
polymer chain (adducts) and those from terminal moieties (unreacted furan and
maleimide rings). Following this approach, the ratio between the integration of the
peaks ranging from 5.09 to 5.36 ppm, which correspond to the hydrogen atoms of the
furan-maleimide adducts and the peaks ranging from 7.27 and 7.41 ppm, attributed to
terminal furan motifs, led to a DPn of approximately 10 (Mn ~1800 Da) for
homogeneous polycondensations, i.e. before polymer precipitation took place. Indeed,
at the advanced stage of all these experiments a precipitate appeared. Its

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characterization confirmed that it bore the same structure as the polymer being
generated, indicating that saturation had been reached and thus suggesting that higher
DPs could not be attained under those conditions. Other solvents and initial monomer
concentrations did not give rise to improved solubility. These observations were in
tune with the difficulty mentioned above in preparing adequate solutions for GPC
analysis and reiterated the polymer stiffness interpretation.
Although the synthesis of this compact AB-type DA monomer did not provide
the opportunity to synthesize high-DP homopolymers, its utilization as a comonomer
in systems involving other more flexible AB structures, as in the case of plant oil-
based monomers,6 opens the way to random and block copolymers with tuneable
properties, such as glass transition temperature and mechanical performances. This
possibility is being pursued.
Acknowledgements
We thank FAPESP for T.M.L.’s postdoctoral fellowship (Grant 2012/00124-9) and
CNPq for A.G.’s visiting professorship (Science Without Borders program, Grant
PVE 401656/2013-6) and financial support to A.J.F.C. (Grants 305864/2012-2 and
476709/2013-0). We also thank FAPESP (2010/11391-2) for the X-ray diffraction
analysis.
Notes and References
*The authors declare no competing financial interest.
1 A. Gandini, Prog. Polym. Sci., 2013, 38, 1-29.
2 A. Gandini, T. M. Lacerda, Prog. Polym. Sci., 2015, 48, 1-39. A. Gandini, T. M.
Lacerda, A. J. F. Carvalho, E. Trovatti, Chem. Rev., 2016, 116, 1637-1669.

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3 M. Gregoritza, F. P. Brandl, Europ. J. Pharm. Biopharm., 2015, 97, 438-453.
4 C. Goussé, A. Gandini, Polym. Bullet., 1998, 40, 389-395.
5 J. Canadell , H. Fischer , G. de With , R. A. T. M. van Benthem, J. Polym. Sci. Part
A, Polym. Chem., 2010, 48, 3456-3467.
6 C. Vilela, L. Cruciani, A. J. D. Silvestre, A. Gandini, RSC Adv., 2012, 2, 2966-
2974.

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GRAPHICAL AND TEXTUAL ABSTRACT
An original approach to prepare and polymerize the shortest Diels-Alder AB monomer.