Communication

A Double Click Strategy Applied to theReversible Polymerization ofFuran/Vegetable Oil Monomers

Carla Vilela, Letizia Cruciani, Armando J. D. Silvestre, Alessandro Gandini*

This investigation describes preliminary results related to the preparation of monomers basedon vegetable oil derivatives bearing furan heterocycles appended through thiol-ene clickchemistry, and their subsequent polymerization via the Diels-Alder (DA) polycondensationbetween furan and maleimide complementary moieties, i.e. a second type of click chemistry.Following the optimization of these interactions with model compounds, two basicapproaches were considered for these DA polymerizations, namely (i) the use of monomerswith two terminal furan rings in conjunction with bis-maleimides (AAþBB systems) and (ii) the use of ABmonomers incorporating both furan and maleimideend-groups. This ongoing study clearly showed that bothapproaches were successful, albeit with different out-comes, in terms of the nature of the products. The appli-cation of the retro-DA reaction confirmed theirthermoreversible character, i.e. the clean-cut return totheir respective starting monomers.

Introduction

Polymers from renewable resources have acquired a

respectable status within the macromolecular community

and their scope is widening incessantly.[1] Among the

numerous sources of novelmaterials, vegetable oils occupy

a prominent position, further enhanced by the growing

research activities of the last several years,[2] focussed in

particular on the application of metathesis, thiol-ene

chemistry and other original mechanistic approaches.

The ensuingmaterials include polyurethanes,[3] polyesters,

and other macromolecular structures[2] in which the long

aliphatic segments play a critical role in imparting

properties likehydrophobicity and lowthermal transitions,

C. Vilela, L. Cruciani, Prof. A. J. D. Silvestre, Prof. A. GandiniCICECO and Chemistry Department, University of Aveiro, 3810-193Aveiro, PortugalFax: (þ351)234370084; E-mail: agandini@ua.pt

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whereas the associated polar moieties can favour biode-

gradability.

Our interest in vegetable oils stems from the idea of

applying previous experience on the use of furan mono-

mers and furan chemistry to these substrates, through the

joint exploitation of two click-chemistry mechanisms, viz.

the thiol-ene and the Diels-Alder (DA) reactions. The

purpose of this ongoing investigation is therefore to

append furan moieties to molecules issued from triglycer-

ides through thiol-ene coupling[4] and then polymerize

these furan-bearing monomers using their Diels-Alder

polycondensation with complementary maleimide func-

tions.[1,5]

The reactivityof furfuryl thiol towardsboth terminaland

internal C¼C unsaturations was first assessed under

various conditions using model compounds (including

various vegetable oil derivatives) bearing a single alkene

moiety, since no published information was available on

this type of system. Subsequently undecenyl compounds

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C. Vilela, L. Cruciani, A. J. D. Silvestre, A. Gandini

were used as substrates to append two terminal furan (A)

heterocycles or both a furan (A) and a maleimide (B) end-

groups, calling upon the thiol-ene reaction in conjunction

with more classical chemical condensations. The ensuing

AAandprotectedABmonomerswere thenpolymerized via

the DA reaction, the former with different bismaleimides

(BB), the latter on its own, after deprotection.

Experimental Section

Materials

2-Furanmethanethiol (FT Aldrich, 98%), furfuryl sulfide (Aldrich,

�98%), furfuryl methyl sulfide (SAFC, �97%), furfuryl alcohol (FA,

Aldrich, 98%), 10-undecenoic acid (UDA,Aldrich, 98%), 10-undecen-

1-ol (UDOL,Aldrich,98%), allyl alcohol (Aldrich,�99%), 3-buten-1-ol

(Aldrich, 96%), vinyl isobutyl ether (Aldrich 99%), 4-maleimidobu-

tyric acid (MBA, Fluka�98%), methyl oleate (Aldrich, 99%), methyl

linoleate (Sigma �99%), vinyl stearate (Aldrich), N,N0-dicyclohex-

ylcarbodiimide (DCC, Aldrich, 99%), 4-dimethylaminopyridine

(DMAP, Sigma), 2,20-azobis(2-methylpropionitrile) (AIBN, Fluka

�98%), 1,6-bismaleimidohexane (BMH, Tyger Scientific Inc.) andall

the solvents were commercial products, used as received. The

photoinitiator Irgacure 651 (2,2-dimethoxy-2-phenylacetophe-

none, DMPA) was generously provided by BASF, Ludvigshafen.

Thiol-ene Reactions

Test reactionswere carriedout in anNMRtube inCD2Cl2 or C2D2Cl4,using a 5–10 molar excess of FT relative to the moles of C¼C

unsaturations of the substrate. The activationwas induced by heat

with AIBN (10 mol% of the substrate) at 75 8C, or photochemically

by illuminating the tubewith the 365nmoutput of a 9Wmedium-

pressure Hg lamp. 1HNMR spectrawere taken to follow the course

of these reactions, by observing the progressive disappearance of

the alkenyl protons.

Other Functionalizations

All direct esterification reactions were carried out in dichloro-

methaneat 25 8C, using stoichiometric amounts of acid andalcohol

in the presence of DCC and DMAP.

Diels-Alder Polymerizations

AARBB – Stoichiometric quantities of the two monomers were

dissolved in C2D2Cl4 at concentrations adapted to the technique

used to follow the reactions, which were conducted at 65 8C,namely UV and 1HNMR spectroscopy. The retro-DA depolymerisa-

tion reactions were followed at 110 8C.AB – The protected monomer was dissolved in C2D2Cl4 and the

solution brought to 110 8C while a gentle stream of nitrogen was

bubbled through it for several hours to remove the furan generated

by the retro-DA of themaleimide adduct. The ensuing unprotected

AB monomer was then allowed to polymerize at 65 8C and the

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reaction followed by UV and 1H NMR spectroscopy. Again, the

depolymerisation of the DA polymer was carried out at 110 8C.

Characterizations

All FTIR-ATR spectra were taken with a Perkin Elmer FT-IR System

SpectrumBX spectrophotometer equippedwith a single horizontal

Golden Gate ATR cell. 1H and 13C NMR spectra were recorded in

CDCl3 or C2D2Cl4 on a Brucker AMX 300 operating at 300.13 and

75.47MHz for the 1H and 13C spectra, respectively. Gas chromato-

graphy-mass spectrometry (GC-MS) analyseswere carriedoutwith

a Trace GC Ultra 2000 instrument equipped with a mass selective

detector DSQ II and a DB-1 J&W capillary column (30m�0.32mm

i.d., 0.25mm film thickness). Electronic spectra were run on a

temperature-controlled Jasco V-560 spectrophotometer using 1 cm

Hellma suprasil cells equipped with a 9.9mm quartz spacer and a

quartz-to-pyrexgraded seal. DSC thermogramswereobtainedwith

a Perkin Elmer Diamond DSC unit using aluminium pans under

nitrogen with a heating rate of 20 8C �min�1 in the temperature

range of�80 to 80 8C. Themolecularweights andmolecularweight

distributions of the polymers were determined by size-exclusion

chromatography (SEC) with a PL-110 instrument, using N,N-

dimethylacetamide (DMA) as the mobile phase.

Results and Discussion

Monomer Synthesis and Characterization

To the best of our knowledge, the click reaction of FT with

alkenyl functionshadnever been reportedbefore this study

and it was therefore essential to investigate it first with

simple model compounds. Its occurrence was confirmed

with terminal vinylmoieties, usingvinyl isobutyl etherand

vinyl stearate, with which coupling did take place albeit at

moremodest rates thanwith conventional aliphatic thiols.

The use of reagents incorporating internal CH¼CH groups,

likemethyl oleate, resulted in a very sluggish process and it

was thereforedecided toconcentrateonend-unsaturations.

The reasons for the modest reactivity of FT, compared with

that of aliphatic counterparts, are probably related to the

steric hindrance of the furan heterocycle.

The basic structure selected as a typical and viable

vegetable oil derivative was the undecene moiety, i.e. the

well-known fragment arising from the pyrolysis of castor

oil, in the form of its acid derivative UDA. Two approaches

were adopted to append furan end-groups onto it, viz. (i)

esterification with FA at the carboxylic terminal and ene

reactionwith FTat theunsaturated counterpart (Scheme1),

or (ii) esterification with allyl alcohol, followed by a double

ene reactionwith FT at both alkenyl end-groups (Scheme2).

In both instances, a, v-difuran macromonomers (AA and

AA’) were obtained and characterized.

The corresponding alcohol UDOLwas used to prepare the

protected AB macromonomer through esterification of its

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OH

O

OOH

DMAP, DCCCH2Cl2, 25ºC

OO

O

OSH hν , 25ºC

O

O

OS

O

AA

Scheme 1. Synthesis of the AA macromonomer.

OH

O DMAP, DCCCH2Cl2, 25ºC

O

O

OSH hν , 25ºC

O

O

SO

AA'

OH

SO

Scheme 2. Synthesis of the AA’ macromonomer.

A Double Click Strategy Applied to the Reversible Polymerization of Furan/Vegetable Oil Monomers

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primary OH end-group with the masked MBA (readily

prepared from the DA reaction of MBA with furan) and

again the ene reaction of its terminal unsaturationwith FT

(Scheme 3).

The FTIR spectrum of the AAmacromonomer (Scheme 1)

clearly showed the furan heterocycle bands at 3111, 1503,

1346, 1009, 748 cm�1, the ester C¼OandC�Obands at 1732

and1150 cm�1, respectively, and theC�Sbandat 666 cm�1.

The 1H NMR analysis confirmed the expected structure

through the appearance of resonances typical of the

methylene protons of the ester moiety at 5.1 ppm

(OCH2-2-furan), the methylene protons between the S

atom and the furan heterocycle at 3.7 ppm (SCH2-2-furan),

DMAP, DCCCH2Cl2, 25ºC

OSH hν , 25ºC

AB

O

OO

O

N

O

OO

O

N S O

OHO

O

O

OHO

O

NO

Scheme 3. Synthesis of the protected AB macromonomer.

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and the furan ring protons at about d 6.3

(H3), 6.4 (H4) and 7.4 (H5) ppm, respec-

tively, apart from the typical resonances

of the aliphatic chain protons of

10-undecenoic acid.

The 13C NMR spectrum of AA provided

further corroboration of the validity of

its structure with, in addition to the

unchanged resonances related to the

aliphatic chain carbons, resonances at

57.8 ppm(OCH2-2-Furan), 107.2and110.4

(furan C3 and C4), 142.0 (furan C5), 151.9

(furan C2) and the C¼Oester peak at

173.5 ppm. Finally, its EI-MS spectrum,

obtained by GC-MS, showed a molecular

ion at m/z 378, which was consistent

with the AA C21H30O4S molecular for-

mula.

Similarly, the changes in the FTIR,1H and 13C NMR spectra associated with

the functionalization of 10-undecenoic

acid to give macromonomer AA’

(Scheme 2) reflected the same pattern,

confirming the success of both the

esterification and the thiol-ene reactions.

Its EI-MS spectrum, obtained by GC-MS,

showed a molecular ion at m/z 452,

which was consistent with the AA’ C24H36O4S2 molecular

formula.

The FTIR spectrum of the masked AB macromonomer

was consistent with the structure shown in Scheme 3, by

the presence of all the relevant peaks and, at the same time,

the absence of (i) the OH band of primary alcohols, (ii) the

OH and C¼O bands of the carboxylic group and (iii) the SH

band of the thiol moiety. The 1H NMR analysis clearly

confirmed this structure through the presence of the furan

ring protons at d 7.4, 6.4 and 6.2 ppm, as well as of the

protons of the protected maleimide moiety at about 6.5

(¼CHCHCH), 5.3 (¼CHCHCH) and 2.83 ppm (¼CHCHCH).

The 13C NMR spectrum of AB was also in tune with its

proposed structure. Additionally, the GC-

MSdata corroborated these results,with a

molecular ion at m/z 518, corresponding

to the AB C28H39NO6S formula.

The commonstructural featureof these

three monomers, aside from their term-

inal DA-reactive furan or maleimide

functions, was the long methylene

sequence, i.e. the flexible bridge joining

them. This implies that all the ensuingDA

polymers, namely those derived from AA

and AA’ with the equally flexible bridge

joining the aliphatic bismaleimide BMH,

and that formed by the AB monomer,

im1321

O

O

N+

O

O

NOO

Scheme 4. The DA equilibrium between growing species bearing,respectively, furan and maleimide end groups.

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C. Vilela, L. Cruciani, A. J. D. Silvestre, A. Gandini

were expected to have relatively low glass transition

temperatures.

Diels-Alder Polymerizations

The relevant feature of the DA reaction in the present

context is its reversible character (Scheme 4), where the

temperature is a key factor in determining the position of

the equilibrium, which can be drastically shifted from

predominant adduct formation (DA reaction), up to ca.

65 8C, to the predominant reversion to its precursors (retro-

DA reaction), above ca. 100 8C.[5,6] The kinetic features

associated with the course of the forward and backward

reactions depend on the specific structure of the moieties

attached to both heterocycles, and, of course, the reactant

concentration, the medium and the temperature. The fact

that the forward reaction gives rise to both endo and exo

stereoisomer adducts is not relevant here, since both

participate in the chain growth.

Given the lack of published information about the

reactivity of FT as a diene in its DA interaction with

maleimide dienophiles, preliminary experiments were

carried out on model compounds. Furfuryl sulfide was

found to react with methymaleimide to form the expected

adduct and the same occurred with the larger sulphide

arising from the ene reaction of FT with vinyl stearate.

Additionally, theDApolymerizationof furfuryl sulfidewith

BMH took place, as expected, to give the corresponding

linear polyadduct. These experiments were conducted in

both NMR tubes and UV cells and the course of the

corresponding DA reactions followed, respectively, by

the progressive decrease in (i) the resonance intensity of

the furan and maleimide protons and (ii) the maleimide

peakat300nm(lossof theO¼C�C¼C�C¼Oconjugation, as

shown in Scheme 4), associated with the formation of the

DA adducts. Compared with other systems previously

studied in our laboratory,[6] all these reactions, carried out

in the same conditions of concentration, medium and

temperature, were found to proceed more slowly, suggest-

ing that thepresence of the sulphur atom in the2-Fu-CH2-S-

group played a (modest) retarding role.

Notwithstanding thisminor quantitative difference, the

qualitatively positive outcome of these tests opened the

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way to the study of the programmed polymerization

systems. The complementary bis-dienophile chosen as DA-

polymerization partner for both the bis-dienes AA and AA’

was the bismaleimide BMH, i.e. a monomer bearing, as

mentioned above, an equally flexible aliphatic bridging

structure. As with themodel reactions, all polymerizations

were followed by both 1H NMR and UV spectroscopy using

monomer concentrations of about 0.1M.

The DA polycondensation of AA at 65 8C, followed by UV

spectroscopy, revealed a progressive decrease in the optical

densityof themaleimidepeakat�300nm,accompaniedby

a corresponding increase in the viscosity of themedium. As

in the case of previous studies,[6] the spectral pattern gave

rise to an isosbestic point (progressive replacement of the

maleimide peak at �300nm by the absorption of the

unconjugated carbonyl groups of the adduct at �260nm),

which suggested the occurrence of a single reaction

pathway, viz. the DA condensation in Scheme 4. The

concomitant changes in the 1H NMR spectra displayed a

gradual decrease in the intensity of the maleimide (singlet

at 6.7 ppm) and furan (H5 at 7.4 ppm, H3 and H4 at 6.2–

6.4 ppm) resonances at a rate similar to that of the UV

spectra evolution and the parallel surge of the peaks

associated with the three sets of protons attributed to the

adducts, at about 3.0, 5.3 and 6.5 ppm. Typically, several

days were necessary to reach high conversions. The AA

polymer, isolated by precipitation in petroleum ether

and vacuum drying, gave a Tg of ��40 8C and an Mw of

6500 (DPw� 17),with PDI� 1.4. The features accompany-

ing the polycondensation of AA’were, as expected, entirely

similar to those described above for the AA counterpart,

both qualitatively and in terms of its kinetics. The

corresponding polymer had a Tg of �28 8C and an Mw of

9050 (DPw �20), with PDI �1.5. The SEC tracings of both

these polymers showed evidence of the presence of cyclic

oligomers, including thecorrespondingdimers, throughthe

appearance of individual peaks within the distribution

curves. Given the relatively low monomer concentrations

used in these DA polycondensations, the occurrence of

cyclization is reasonable, as indeed already observed in

a recent study on the DA polymerization of other AB

monomers.[7]

Bothpolymerswerethensubmitted tothecorresponding

retro-DA depolymerisation at 110 8C, followed by 1H NMR

spectroscopy. These processes were characterized by the

reverse pattern with respect to the polycondensations,

consisting in the gradual decrease in the adduct resonance

intensities and the corresponding growth of the furan and

maleimide counterparts, together with the decrease in

viscosity of the solutions. Within a few days, the spectra

revealed the presence of the starting monomers, thus

confirming the thermo-reversible character of these

systems. These regenerated monomers could in turn be

re-polymerized by heating at 65 8C, emphasizing the

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A Double Click Strategy Applied to the Reversible Polymerization of Furan/Vegetable Oil Monomers

www.mrc-journal.de

reproducibility of these cyclic events, as previously

reported.[6] The deprotected AB monomer exhibited a very

similar overall behaviour regarding its polymerization and

depolymerisation features.

Clearly, these systems require further work aimed at

their optimization and at amore thorough characterization

of the ensuing original polymers.

Conclusion

The implementation of this novel double-click approach

using two substrates derived from renewable resources,

and the positive outcome of the first systems to which it

was applied, constitute a stimulating encouragement to

widen its scope in terms of both the depth of the associated

studies and the extension to other substrates. Work is in

progress to conduct both such investigations.

Acknowledgements: AG wishes to thank Professor Mats Johans-son of KTH, Stockholm, for a helpful discussion on the basic idea ofthis study. We thank the Portuguese Foundation for Science andTechnology (FCT) for analytical instrumentation support (POCI2010 and REEQ/515/CTM/2005) and for a doctorate grant to CV(SFRH/BD/44884/2008). LC thanks the Department of CivilEnvironmental and Materials Engineering – DICAM of theUniversity of Bologna. We are also grateful to ProfessorD. Evtuguin for carrying out the SEC experiments.

Received: April 14, 2011; Revised: May 20, 2011; Published online:July 7, 2011; DOI: 10.1002/marc.201100246

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� 2011 WILEY-VCH Verlag Gmb

Keywords: furan-maleimide Diels-Alder polymers; renewableresources; thermoreversible polymerization; thiol-ene click chem-istry; vegetable oil derivatives

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[5] [5a] A. Gandini, M. N. Belgacem, ACS Symp. Ser. 2007, 954, 280;[5b] A. Gandini, Polym. Chem. 2010, 1, 245; [5c] A. Gandini,Furans as Offspring of Sugars and Polysaccharides and Pro-genitors of an Emblematic Family of Polymer Siblings, in R. T.Mathers, M. A. R. Meier, Eds., Green Polymerisation Methods:Renewable Starting Materials, Catalysis and Waste Reduction,Wiley-VCH, Weinheim 2011, p. 29; [5d] A. Gandini, FuranMonomers and Their Polymers: Synthesis, Properties andApplications, in D. Plackett, Ed., Biopolymers: New Materialsfor Sustainable Films and Coatings, John Wiley & Sons, Wein-heim 2011, p. 180.

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