Reversible click chemistry at the service of macromolecular materials. 2. Thermoreversible polymers based on the Diels-Alder reaction of an A-B furan/maleimide monomer

Article abstract of DOI:10.1002/pola.23957

A novel structure was synthesized from renewable resources, bearing a turan and a masked maleimide as terminal moieties. Its thermal deprotection generated in situ the Diels-Alder (DA) A-B monomer which was polymerized (forward DA) and then depolymerized (retro DA) in several cycles, as a function of the medium temperature. The ensuing linear polyadduct represents the first thermally reversible macromolecular material prepared from a furan-maleimide A-B monomer.

Full text of DOI:10.1002/pola.23957

Reversible Click Chemistry at the Service of Macromolecular Materials.
2. Thermoreversible Polymers Based on the Diels-Alder Reaction of an
A-B Furan/Maleimide Monomer
ALESSANDRO GANDINI, ARMANDO J. D. SILVESTRE, DORA COELHO
Department of Chemistry, CICECO, University of Aveiro, Aveiro 3810-193, Portugal
Received 27 November 2009; accepted 24 January 2010
DOI: 10.1002/pola.23957
Published online in Wiley InterScience (www.interscience.wiley.com).
KEYWORDS: A-B monomer; click chemistry; Diels-Alder reversible polymerization; furan; 1H NMR spectroscopy; maleimide; renew-
able resources
INTRODUCTION The dwindling of fossil resources, together
with their unpredictable price fluctuations, raise serious ques-
tions about the medium-term pursuit of their exploitation,
whether as a source of energy and/or as purveyors of organic
commodities and materials, with the additional concern of the
ecological impact related to their exploitation. Hence, the ensu-
ing surge of initiatives devoted not only to the search for alter-
native sources of energy providers, but also to the context of
polymers from renewable resources, namely the vegetable bio-
mass.1,2 Among these, polymers from furan monomers consti-
tute a unique class of materials based on vegetable renewable
resources, whose structures can simulate virtually all their
current fossil-derived counterparts.2,3
The emphasis here is on the role of the temperatures chosen
to shift drastically the equilibrium from predominant adduct
formation (polymerization–DA), viz. around 65 ꢀC for a rea-
sonable reaction rate, to the predominant reversion to iꢀts pre-
cursors (depolymerization—retro-DA), viz. around 110 C. We
have undertaken a systematic study on the application of the
DA and retro-DA reactions, first to model monofunctional sys-
tems, then to linear polymerizations with difurans (A-A) and
bismaleimides (B-B),5 as well as to non-linear systems using
A3 and B-B monomers.6 It is from our preliminary study on
equilibrium position and kinetics related to the model com-
pounds5 that the choice of both forward and backward reac-
tion temperatures was made for the subsequent polymeriza-
tions5,6 and hence also for this investigation.
One of the peculiar chemical features of the furan hetero-
cycle is its pronounced dienic character, which makes it par-
ticularly suited to intervene in the [4 þ 2] Diels-Alder (DA)
cycloadditon as the dienic reagent. The DA reaction is a
splendid example of reversible click chemistry, which has
opened new ways to synthesize polymers with unique prop-
erties, including self-mendability and network recyclability
through reversible cross-linking.2–4 Scheme 1 illustrates the
essence of the mechanism involved in the DA and retro-DA
reactions, between a furan and a maleimide end-group:
The use of A-B monomers bearing both the furan and the mal-
eimide moieties in their structure represents an interesting al-
ternative for linear polymerizations, because it ensures the ideal
initial stoichiometry. The synthesis of a monomer of this type
was first explored by Mikroyannidis,7 who investigated a series
of such structures comprising different bridging moieties. How-
ever, the spectroscopic characterization of the monomers did
not provide convincing evidence in favor of the expected struc-
tures, and no NMR spectra of the final materials could be taken,
because they were not soluble in any tested solvent, despite the
fact that they should all have had a linear macromolecular
structure, given the difunctional character of their precursors. A
few years later, the same issue was briefly investigated by
Gousse´ and Gandini,8 who prepared, characterized, and poly-
merized monomer I, but the single methylene bridge joining the
two complementary DA moieties made the molecule difficult to
handle, and the study was not pursued. More viable AB struc-
tures (II) have now been synthesized in our laboratory using
aminoacids as precursors, providing, on the one hand, stable
SCHEME 1 The DA equilibrium between growing species bear-
ing, respectively, furan and maleimide end groups.
Correspondence to: A. Gandini (E-mail: agandini@ua.pt)
C
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THERMOREVERSIBLE POLYMERS, GANDINI, SILVESTRE, AND COELHO
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
monomers and, on the other hand, the possibility of using both
precursors derived from renewable resources.
M aqueous HCl (2 ꢁ 5 mL) and a saturated sodium hydrogen
carbonate aqueous solution (2 ꢁ 5 mL). Additional DCU was
precipitated and removed by filtration. The organic phase was
dried over anhydrous sodium sulfate and concentrated under
reduced pressure. The expected product was obtained as a yel-
low solid with a yield of 5.66 g (90.0%).10 Compound 5 was
stable and could not undergo DA condensation since the malei-
mide moiety was protected as its furan adduct.
IR (ATR): 3124 (furan ¼¼CH), 3000 (¼¼CH), 2933 (asCH2),
2857 (sCH2), 1775, 1736 and 1693 (C¼¼O), 1503 (furan
C¼¼C), 1274, 1226, and 1150 (CAO), 1016 (ring breathing),
915, 811, and 734 (furan CAH), 877 (CAH), 593 (ring defor-
The present communication deals specifically with monomer
II with n ¼ 2.
1
mation) cmꢂ1. H NMR (CDCl3): d (ppm) ¼ 2.64 (t, J ¼ 7.33
Hz; NCH2CH2CO), 2.82 (s; ¼¼CHCHCHCO), 3.78 (t, J ¼ 7.34 Hz;
NCH2CH2CO), 5.05 (s; OCH2-2-Furan), 5.26 (s; ¼¼CHCHCHCO),
6.35 (d, J ¼ 3.29 Hz; furan H3), 6.40 (dd, J ¼ 3.24, 1.86 Hz;
furan H4), 6.52 (s; ¼¼CHCHCHCO ), 7.42 (d, J ¼ 1.60 Hz; furan
H5). 13C NMR (CDCl3): d (ppm) ¼ 31.8 (NCH2CH2CO), 34.5
(NCH2CH2CO), 47.3 (¼¼CHCHCHC(O)N), 58.3 (OCH2-2-Furan),
80.8 (¼¼CHCHCHC(O)N), 110.6 and 110.9 (furan C3 and C4),
136.5 (¼¼CHCHCHC(O)N), 143.3 (furan C5), 149.0 (furan C2),
170.1 (C¼¼Oester), 175.8 (C¼¼Oimide).
EXPERIMENTAL
Materials and Instrumentation
3-Aminopropanoic acid (b-alanine, 99þ%), exo-3,6-epoxy-
1,2,3,6-tetrahydrophthalic anhydride (furan-maleic anhydride
DA adduct, 90%) and anhydrous sodium carbonate were
purchased from Acros Organics. N,N0-dicyclohexylcarbodii-
mide (DCC, 99%), 4-dimethylaminopyridine (DMAP), and
1,1,2,2-tetrachloroethane-d2 (TCE-d2) were purchased from
Sigma-Aldrich. All reagents and solvents (analytical grade)
were employed as received. The IR measurements were per-
formed using a Mattson 7000 spectrophotometer working in
Polycondensation of the A-B Monomer (Deprotected 5)
A typical procedure follows: 0.6 M solution of 5 (0.19 g, 0.6
mmol) in TCE-d2 (1.0 mL) was prepared and left at 110 ꢀC
for 24 h under N2 atmosphere. This step promoted the
retro-DA elimination of furan from the adduct moiety and
the ensuing maleimide function was hence set free to poly-
merize. The solution was transferred to a NMR tube and the
1
the ATR mode. H NMR and 13C NMR spectra were acquired
with a Brucker advance spectrometer working, respectively,
at 300.13 MHz and 75.47 MHz.
Synthesis of 3-(exo-3,6-epoxy-1,2,3,6-
tetrahydrophthalimido)propanoic Acid (3)
1
corresponding H NMR spectrum taken immediately at room
b-alanine (1) (2.41 g, 27.1 mmol) was slowly added to a solu-
tion of furan-maleic anhydride DA adduct (2) (4.5 g, 27.1
mmol) and Na2CO3 (2.87 g, 27.1 mmol) in MeOH (200 mL).
temperature. The tube was then left in a bath at 65 ꢀC for
96 h, while spectra were taken at regular intervals.
ꢀ
The solution was stirred at 56 C for 6 days. Then the solvent
RESULTS AND DISCUSSION
was removed under reduced pressure, and the white residue
dissolved in 100 mL of CH2Cl2 and washed with 3 ꢁ 100 mL
of 0.6 M aqueous HCl. The organic layer was dried over anhy-
drous sodium sulphate and concentrated under reduced pres-
sure. The expected product was obtained as white crystals
with a yield of 3.77 g (59.0%).9 IR (ATR): 3050 (OAH), 3014
(¼¼CH), 2926 (asCH2), 2852 (sCH2), 1779, 1702 and 1689
Scheme 2 depicts the steps leading to the protected A-B
monomer 5. Its FTIR spectrum was consistent with its
expected structure, through the presence of all the relevant
peaks and the absence of the OH and C¼¼O bands of the
1
(C¼¼O), 1258 and 1161 (CAO), 877 (CAH) cmꢂ1. H NMR (ac-
etone-d6): d (ppm) ¼ 2.55 (t, J ¼ 7.75 Hz; NCH2CH2CO), 2.92
(s; ¼¼CHCHCHC(O)N), 3.67 (t, J ¼ 7.76 Hz; NCH2CH2CO), 5.13
(s; ¼¼CHCHCHC(O)N), 6.58 (s; ¼¼CHCHCHC(O)N). 13C NMR (ac-
etone-d6): d (ppm) ¼ 32.0 (NCH2CH2CO), 34.8 (NCH2CH2CO),
48.2 (¼¼CHCHCHC(O)N), 81.6 (¼¼CHCHCHC(O)N), 137.3
(¼¼CHCHCHC(O)N), 171.8 (C¼¼Oacid), 176.8 (C¼¼Oimide).
Synthesis of (2-furfuryl)-3-(exo-3,6-epoxy-1,2,3,6-
tetrahydrophthalimido)propanoate (5)
A solution of 3 (4.74 g, 20 mmol), furfuryl alcohol (4) (5.89
g, 60 mmol) and DMAP (1.95 g, 16 mmol) in dry dichloro-
methane (20 mL) was prepared and after 5 min. of stirring,
DCC (4.54 g, 22 mmol) was added to it, and the reaction
mixture left overnight at room temperature under N2. The
precipitated dicyclohexylurea (DCU) formed during the reac-
tion was removed by filtration and the filtrate washed with 0.5
SCHEME 2 Synthesis of the protected A-B monomer 5.
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RAPID COMMUNICATION
SCHEME 3 Deprotection of 5 and A-B
polymerization.
carboxylic moiety. This was further corroborated by both its
After the in situ deprotection of the maleimide moiety at
high temperature, followed by cooling to 65 ꢀC to promote
polymerization of the ensuing A-B monomer, the polyconden-
sation (Scheme 3) was followed by 1H NMR spectroscopy
(Fig. 1).
1
1H NMR and 13C NMR spectra. In the H NMR spectrum, the
most relevant features included the methylene protons of
the ester function at 5.05 ppm (OCH2-2-Furan) and the furan
protons at 6.4 (H4), 6.3 (H3), and 7.4 (H5) ppm. The 13C
NMR spectrum displayed, among others, resonances at 58.3
ppm (OCH2-2-Furan), 110.6 and 110.9 (furan C3 and C4),
143.3 (furan C5), 149.0 (furan C2) and the C¼¼Oester peak at
170 ppm.
Figure 1 shows the 1H NMR spectra of the initial A-B solu-
tion taken after the deprotection process as well as the pro-
gression of its DA polycondensation. Apart from traces of
FIGURE 1 1H NMR spectra taken during the AB-polimerization at 65 ꢀC (*TCE-d2; **traces of DCU), where t ¼ 0 corresponds to the
initial spectrum of the solution containing the deprotected A-B monomer 6 run at room temperature. The top spectrum was taken
after 3 days at 65 ꢀC, followed by 11 days at room temperature.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
The ensuing polymers, isolated by precipitation in hexane,
filtration and drying to constant weight, had a DPn of 30–35,
as measured from their 1H NMR spectra, and a Tg of 45–50
ꢀC, pending the optimization of the system.
CONCLUSIONS
The furan-maleimide A-B monomer described here is the
first example of such structures for which clean-cut DA poly-
merization-depolymerization cycles were shown to apply.
Work is in progress to extend the scope of this investigation
to other A-B monomers, as well as to AB2 and A2B struc-
tures capable of generating thermally-reversible hyper-
branched macromolecules.
D. Coelho thanks FCT for her PhD Grant (SFRH/BD/28271/
2006).
REFERENCES AND NOTES
1 Monomers, Polymers and Composites from Renewable
Resources; Belgacem, M. N.; Gandini, A., Eds.; Elsevier: Am-
sterdam, 2008.
2 Gandini, A. Macromolecules 2008, 41, 9491–9504.
FIGURE 2 1H NMR spectra taken during the A-B retro-DA de-
polymerization (24 h at 110 ꢀC). The spectrum run after 6 h
shown here was essentially identical to that taken after 24 h.
The top spectrum represents the second DA polycondensation
and was taken after 3 days at 65 ꢀC (*TCE-d2; **traces of DCU).
3 (a) Gandini, A. In Encyclopedia of Polymer Science and
Engineering; Mark, H. F.; Bikales, N. M.; Overberger, C. G.;
Menges, G., Eds.; John Wiley & Sons: New York, 1986; Vol. 7, pp
454–473; (b) Gandini, A. ACS Symp Ser 1990, 433, 195–208; (c)
Belgacem, M. N.; Gandini, A. Prog Polym Sci 1997, 22,
1203–1379; (d) Moreau, C.; Gandini, A.; Belgacem, M. N. Top
Catal 2004, 27, 9–28; (e) Gandini, A.; Belgacem, M. N. Furan
Derivatives and Furan Chemistry at the Service of Macromolecu-
lar Materials, ref. 1, Chapter 6.
residual DCU (d ¼ 1.10–1.74 ppm) and evidence of the in-
cipient polymerization (see discussion below), the initial
spectrum clearly confirmed the presence of the unprotected
A-B monomer through the maleimide CH¼¼CH protons at d
6.8 ppm. The expected pattern of the A-B polycondensation
was corroborated by the observation of the progressive
decrease in the peak intensity of the monomer’s relevant
protons and the corresponding increase in that of the peaks
related to the corresponding formation of the adducts. Con-
currently, as the reaction proceeded, the viscosity of the me-
dium also increased.
4 Belgacem, M. N.; Gandini, A. ACS Symp Ser 2007, 954,
280–295.
5 Gandini, A.; Coelho, D.; Silvestre, A. J. D. Eur Polym J 2008,
44, 4029–4036.
6 Gandini, A.; Coelho, D.; Gomes, M.; Reis, B.; Silvestre, A. J.
D. J Mater Chem 2009, 19, 8656–8664.
7 (a) Diakoumakos, C. D.; Mikroyannidis, J. A. J Polym Sci Part
A: Polym Chem 1992, 30, 2559–2563; (b) Mikroyannidis, J. A. J
Polym Sci Part A: Polym Chem 1992, 30, 125, 2017–2025; (c)
Diakoumakos, C. D.; Mikroyannidis, J. A. Eur Polym J 1994, 30,
465–472.
ꢀ
The retro-DA reaction was then followed at 110 C for 24 h
(Fig. 2) and within 6 h the solution already displayed the
resonances of the starting monomer. After 24 h, the system
was allowed to cool to 65 ꢀC to favor a second polymeriza-
tion, as shown in Figure 2. The corresponding 1H NMR was
similar to that taken at the end of the first polymerization,
thus confirming the reversible nature of the system.
8 Gousse´, C.; Gandini, A. Polym Bull 1998, 40, 389–394.
9 Neubert, B. J.; Snider, B. B. Org Lett 2003, 5, 765–769.
10 Berry, D. J.; Digiovanna, C. V.; Metrick, S. S.; Murugan, R.
ARKIVOC 2001, i, 2–13.
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