synthesis and characterization of polyurethanes from epoxidized methyl oleate based polyether...

Synthesis and Characterization of Polyurethanes fromEpoxidized Methyl Oleate Based Polyether Polyols asRenewable Resources

G. LLIGADAS,1 J. C. RONDA,1 M. GALIA,1 U. BIERMANN,2 J. O. METZGER2

1Departament de Quımica Analıtica i Quımica Organica, Universitat Rovira i Virgili, Campus Sescelades,Marcellı Domingo s/n, 43007 Tarragona, Spain

2Institute of Pure and Applied Chemistry, University of Oldenburg, 26111 Oldenburg, Germany

Received 27 June 2005; accepted 10 October 2005DOI: 10.1002/pola.21201Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Oligomeric polyether polyols were obtained through the acid-catalyzedring-opening polymerization of epoxidized methyl oleate and the subsequent partialreduction of ester groups to give primary alcohols. The oligomers were characterizedwith titration, spectroscopic techniques (Fourier transform infrared and nuclear mag-netic resonance), matrix-assisted laser desorption/ionization time-of-flight mass spec-trometry, size exclusion chromatography, and differential scanning calorimetry.Depending on the degree of reduction, polyols of different hydroxyl content values wereobtained and were reacted with 4,40-methylenebis(phenyl isocyanate) to yield polyur-ethanes. These materials, which were characterized by differential scanning calorime-try, thermogravimetric analysis, and dynamic mechanical thermal analysis, couldbehave as hard rubbers or rigid plastics. VVC 2005 Wiley Periodicals, Inc. J Polym Sci Part A:

Polym Chem 44: 634–645, 2006

Keywords: polyether; polyurethanes; renewable

INTRODUCTION

Sustainable development has become the keyideal of the 21st century. In the search for sus-tainable chemistry, considerable importance isbeing attached to renewable raw materials,which exploit the synthetic capabilities of natureand may eventually substitute for fossil, deplet-ing feedstocks.1 The encouragement of the envi-ronmentally sound and sustainable use of renew-able natural resources is an important aim ofAgenda 21.2 Oils and fats of vegetable and ani-mal origin make up the greatest proportion ofthe current consumption of renewable raw mate-rials in the chemical industry because they offer

to chemistry a large number of possibilities forapplications that can be rarely met by petro-chemistry. Vegetable oils containing unsaturatedfatty acids can be used in polymerizations tomake biobased polymers.3–5 Numerous fattyacids are now available in a purity that makesthem attractive for synthesis and as raw materi-als for the chemical industry.6

There have been many studies on the synthe-sis and characterization of a wide variety of poly-mers based on vegetable oils.7,8 Although theypossess double bonds, which are used as reactivesites in coatings, they cannot be converted easilyto high-molecular-weight products without theintroduction of more reactive functional groups,such as hydroxyl, epoxy, or carboxyl groups. Vari-ous chemical pathways for the functionaliza-tion of triglycerides and fatty acids have beenstudied.9 Epoxidation is one of the most impor-

Correspondence to: M. Galia (E-mail: [email protected])or J. O. Metzger (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 634–645 (2006)VVC 2005 Wiley Periodicals, Inc.

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tant functionalization reactions of the C��C dou-ble bond. The chemistry of the Prileshajev epoxi-dation of unsaturated fatty compounds is wellknown.10 A short-chain peroxy acid, preferablyperacetic acid, is prepared from hydrogen peroxideand the corresponding acid either in a separatestep or in situ.11 For chemical synthesis on the lab-oratory scale, better results can be achieved withpreformed peroxy acids such as m-chloroperben-zoic acid. Other methods for the epoxidationinclude the use of dioxiranes,12 the generation ofperacids from aldehydes and molecular oxygen,13

and the use of alkyl hydroperoxides with transi-tion-metal catalysts.14 Recently, a convenientmethod for the chemoenzymatic self-epoxidation ofunsaturated fatty acids was developed.15 Unsatu-rated fatty compounds are preferably epoxidizedon an industrial scale by the in situ performic acidprocedure.16 Epoxidized fatty acids are monomerssuited for ring-opening polymerizations.17

The preparation of polyols from fatty acids andoils for general polyurethane use has been thesubject of many studies,18–20 but limited attentionhas been paid to the preparation of polyetherpolyols from this kind of compound. Polyetherpolyols are important building blocks for poly-urethane applications with molecular weightsof 200–10000 g/mol.21 Polyols with molecularweights of about 3000 or more are used to produceflexible polyurethanes, and polyols of about 200 to1200 g/mol are used for rigid polyurethanes. Poly-ether polyols are usually produced by the anionicring-opening polymerization of alkylene oxides,such as ethylene oxide or propylene oxide.

Industrial polyether polyols, mainly from eth-ylene oxide and propylene oxide, are produced

for applications in polyurethane foams. Longerchain oxiranes or functionalized epoxides showlower reactivity because of a higher sterical hin-drance and side reactions, and high-molecular-weight polymers cannot be obtained with anionicor cationic catalysts.22 Coordinative ring-openingpolymerization has been described for functional-ized epoxides as phenyl glycidyl ether deriva-tives23 or x-epoxy alkanoates.24 As has beenmentioned previously, polyether polyols of about200–1200 g/mol are used to obtain rigid polyur-ethanes. These low-molecular-weight polyetherscan be synthesized by cationic ring-opening poly-merization, which requires an acid catalyst.Several classes of cationic initiators have been de-veloped, such as photosensitive onium salt initia-tors, which are inactive under ambient condi-tions and can release the acid initiator by UVradiation or heat17 and have been applied toepoxidized fatty compounds.25–27

In this article, we report the synthesis andcharacterization of polyether polyols from epoxi-dized methyl oleate (EMO; Scheme 1). Polyols(PAn and PCn; Scheme 1) were prepared throughthe combination of the cationic polymerization ofEMO oligomers (A–D, Scheme 1) and the con-trolled reduction of the carboxylate groups tohydroxyl moieties with lithium aluminum hy-dride (LiAlH4) as a reducing agent. Polyols werecharacterized by chemical methods, spectroscopictechniques [Fourier transform infrared (FTIR)and nuclear magnetic resonance (NMR)], matrix-assisted laser desorption/ionization time-of-flightmass spectrometry (MALDI-TOF MS), and ther-mal techniques [differential scanning calorimetry(DSC)]. They were reacted with 4,40-methylene-

Scheme 1

METHYL OLEATE BASED POLYETHER POLYOLS 635

bis(phenyl isocyanate) (MDI) to obtain polyur-ethanes. The properties of the prepared polyur-ethanes were studied with DSC, thermogravi-metric analysis (TGA), and dynamic mechanicalthermal analysis (DMTA).

EXPERIMENTAL

Materials

The methyl oleate used in this study was sup-plied by Cognis (Dusseldorf, Germany). EMOwas synthesized with a literature procedure.28

The following chemicals were obtained from thesources indicated: formic acid (Scharlau), hydro-gen peroxide, 50% (w/v; Aldrich), fluoroantimonicacid (HSbF6; Aldrich), LiAlH4 [1.0 M in tetrahy-drofuran (THF); Aldrich], and MDI. They wereused as received. THF was distilled from sodiumimmediately before use. Other solvents werepurified by standard procedures.

Oligomerization of EMO

HSbF6 (0.05 g; 0.5 wt %) was added to 10 g ofEMO. The mixture was stirred at room tempera-ture for 1 h. The reaction was quenched by theaddition of 1.5 mL of water followed by 100 mL ofdiethyl ether. The organic solution was washedwith a sodium bicarbonate solution, dried overanhydrous magnesium sulfate, and filtered.Finally, the solvent was evaporated off in vacuo,and this yielded oligomer A as a viscous oil. Asimilar procedure was followed to obtain oligo-mers B–D with the addition of 10, 20, and 50 mol %water to the oligomerization mixture

1H NMR [CDCl3/tetramethylsilane (TMS), d,ppm]: 3.63 (s, ��OCH3), 3.62–3.16 (m, polyetherbackbone), 2.27 (t, ��CH2COO��), 1.70–1.10 (m,��CH2�� aliphatic backbone), 0.85 (t, CH3)

Representative Procedure for the Reductionof Oligomers A and C

Oligomer A (6 g, 3.2 mmol) was dissolved in60 mL of anhydrous THF under nitrogen. A 1 Msolution (6 mL) of LiAlH4 in THF (6 mmol ofLiAlH4) was added slowly with stirring. After theLiAlH4 addition was complete, the mixture wasstirred vigorously at room temperature. After30 min, excess LiAlH4 was decomposed by theaddition of 20 mL of ethyl acetate dropwise, thena saturated 10% H2SO4 solution (aqueous) was

added, the phases were separated, and the aque-ous layer was extracted with ethyl acetate. Thecombined organic phase was washed with a satu-rated aqueous NaCl solution, dried over anhy-drous magnesium sulfate, and filtered, and thesolvent was removed in vacuo at 50 8C.

1H NMR (CDCl3/TMS, d, ppm): 3.63 (s, ��OCH3), 3.55 (t, ��CH2OH), 3.62–3.16 (m, polyetherbackbone), 2.27 (t, ��CH2COO��), 1.70–1.10 (m,��CH2�� aliphatic backbone), 0.85 (t, CH3)

Representative Procedure for the Synthesisof Polyurethanes (PUAn and PUCn)

Polyurethanes were synthesized by the reactionof the polyols with MDI with a 2% molar excessof isocyanate (MDI). The samples were preparedthrough the mixing of a proper amount of polyoland MDI at 60 8C. Curing was carried out at60 8C for 2 h and 110 8C overnight.

Characterization

NMR spectra were recorded on a Bruker AM 300spectrometer. The samples were dissolved in deu-terochloroform, and 1H NMR and 13C NMR spec-tra were obtained at room temperature withTMS as an internal standard. The IR spectrawere recorded on a Bomem Michelson MB 100FTIR spectrophotometer with a resolution of4 cm�1 in the absorbance mode. An attenuatedtotal reflection (ATR) accessory with thermal con-trol and a diamond crystal (Golden Gate heatedsingle-reflection diamond ATR, Specac-Teknok-roma) was used to determine FTIR spectra.

MALDI-TOF MS measurements were per-formed with a Voyager DE-RP mass spectrometer(Applied Biosystems, Framingham, MA) equip-ped with a nitrogen laser delivering 3-ns laserpulses at 337 nm. 2,5-Dihydroxybenzoic acid(DHB) was used as a matrix, and silver triflu-oroacetate was used as a dopant. Size exclusionchromatography (SEC) analysis was carried outwith a Waters 510 pump system equipped with aShimadzu RID-6A refractive-index detector. THFwas used as an eluent at a flow rate of 1.0 mL/min. The calibration curves for SEC analysiswere obtained with polystyrene standards.

Calorimetric studies were carried out with aMettler DSC822e thermal analyzer with N2 asthe purge gas. A heating rate of 20 8C/min wasused. Thermal stability studies were carried outwith a Mettler TGA/SDTA851e/LF/1100 with N2

as the purge gas at a scanning rate of 10 8C/min.

636 LLIGADAS ET AL.

The mechanical properties were measuredwith a TA DMA 2928 dynamic mechanical ther-mal analyzer. Specimens 1.2 mm thick, 5 mmwide, and 10 mm long were tested in a three-point-bending configuration. The various thermaltransitions were studied between �100 and140 8C at a heating rate of 5 8C/min and at afixed frequency of 1 Hz.

General Procedure for the Unilever MethodHydroxyl Value Determination29

For an expected hydroxyl value (see ref. 29), anaccurately measured volume of acetic acid anhy-dride was added to a known amount of a sampleweighed into a round-bottom flask. After mixing,the flask was placed in an oil bath at 95–100 8C.After 1 h, the flask was cooled, and 1 mL of dis-tilled water was added. After shaking, heatingwas continued for 10 min to convert acetic acidanhydride into acetic acid. After the mixture wascooled again, 5 mL of 95% ethanol was added,and the contents were titrated with a 0.5 M etha-nolic potassium hydroxide solution with phe-nolphthalein as an indicator. A blank determina-tion was carried out with a similar procedure.

RESULTS AND DISCUSSION

Oligomerization of EMO

EMO was prepared by the reaction of methyl ole-ate with formic acid and hydrogen peroxide. Pre-cipitation with acetone was applied to purify theproduct. Gas chromatography analysis showedthat EMO was obtained in a purity of 96%.

EMO was cationically oligomerized. We testedseveral acids as catalysts: HSbF6, hexafluorophos-phoric acid (HPF6), and trifluoromethanesulfonicacid (HSO3CF3). Among these, only HSbF6 pre-

dominantly produced oligoether species, as couldbe observed from MALDI-TOF MS analysis. Theoligomerization was carried out with differentamounts of HSbF6 at room temperature (0.1, 0.5,1.0, and 1.5 wt %). NMR analysis of the oligomerobtained with 0.1 wt % initiator revealed the pres-ence of nonreacted epoxide, whereas higheramounts of the initiator gave complete oligomeri-zation. Therefore, the cationic polymerization ofEMO was carried out in the presence of 0.5 wt %HSbF6 at room temperature for 1 h. The catalystwas completely soluble in EMO at room tempera-ture, and the oligomerization was performedhomogeneously in the absence of a solvent, thusbeing an advantageous process from an environ-mental viewpoint.1 The oligomerization reactionwas monitored by FTIR spectroscopy, whichshowed a decrease in the absorption centered at836 cm�1, corresponding to an oxirane ring, and aslight increase in the absorption at 1075 cm�1

(C��O��C ether) and in the broad band around3500 cm�1 (O��H hydroxyl group). This indicatedthat oxirane ring opening took place to form etherlinkages and hydroxyl-terminated polymer.

The cationic oligomerization of EMO resultedin a clear, yellow, viscous oil, and the results aresummarized in Table 1 (oligomer A). The 1HNMR spectrum of oligomer A is given in Figure 1.1H NMR showed that no epoxy groups (2.8 ppm)remained after the polymerization, in accord-ance with FTIR results, thus showing that theepoxy group of the methyl ester is reactiveenough to polymerize under the set conditions. Anew broad peak due to the oligoether backbonewas observed between 3.1 and 3.6 ppm. Signalsin the 1H NMR spectrum at 3.6 (��OCH3), 2.3(��CH2��COO��), 1.1–1.7 (��CH2�� aliphaticbackbone), and 0.8 ppm (��CH3) confirmed theoligomer structure. Traces of ketones, formed bythe acid-catalyzed isomerization of the oxiranering, were observed in the product of oligomeri-

Table 1. General Properties of the Oligomers Obtained by the Cationic Ring-Opening Polymerization of EMOa

OligomerWater(mol %)

HV(mg of KOH/g)

EW(g/equiv)b

Mn

(SEC; g/mol)Mw/Mn

(GPC) Functionalityc

A — 52 1079 1235 1.59 1.1B 10 74 758 1076 1.48 1.4C 20 89 630 1025 1.47 1.6D 50 108 519 961 1.45 1.8

a Polymerization conditions: 0.5 wt % HSbF6, 1 h, and room temperature.b Equivalent weight calculated from the HV.c Obtained by the division of the experimental molecular weight (Mn) by the EW.


zation initiated by HSbF6 by1H NMR at 2.4 ppm.

In contrast, we observed the formation of higheramounts of corresponding ketones when usingHSO3CF3 as an initiator.

MALDI-TOF MS was used to analyze theoligomer distribution. Figure 2 shows the MALDI-

TOF MS spectrum of oligomer A, which is domi-nated by a series of peaks ranging from a mass of750 Da to a mass of 2630 Da, corresponding to lin-ear hydroxyl-terminated oligomers doped with Agþ

ions of type H��[O�� C19H36O2]n��OH��Agþ (mass¼ 312n þ 18 þ Agþ); n values varying from 2 to 8

Figure 1. 1H NMR spectrum (CDCl3, 300 MHz) of oligomer A.

Figure 2. MALDI-TOF MS spectrum of oligomer A doped with silver trifluoroace-tate, showing the ion series H��[O��C19H36O2]n��OH��Agþ (mass ¼ 312n þ 18 þ Agþ),[O��C19H36O2]nAg

þ (mass ¼ 312n þ Agþ), and CH3��[O��C19H36O2]n ��OH��Agþ

(mass ¼ 312n þ 32 þ Agþ; see Schemes 1 and 2).

638 LLIGADAS ET AL.

were detected (Scheme 1). The three most intensepeaks belonging to this series correspond to oligo-mers with n values of 3–5. The spectrum also dis-plays other peaks of lower intensity with masses of312n þ Agþ, corresponding to cyclic structuresformed by back-biting (Scheme 2), and peaks withthe general formula CH3��[O��C19H36O2]n��OH��Agþ (mass ¼ 312n þ 32 þ Agþ), correspondingto oligomers terminated by OH on one end andOCH3 on the other (Scheme 2). These last peakscan appear in the spectrum because of the presenceof traces of methanol, generated by the transesteri-fication of ester groups or as impurities in themonomer, which reacted during polymerization andformed chain ends. Therefore, MALDI-TOF MSconfirmed the presence of a mixture of linear andcyclic oligomers.

As is well known, the cationic polymerizationof epoxides produces macrocycles and hydroxy-terminated open chain oligomers. For disubsti-tuted epoxides under photoinitiated cationic poly-merization, Warwel27 reported obtaining macro-cycles, linear oligomers, and ketones formed bythe rearrangement of the monomer. By the addi-tion of water, macrocyclization was completelysuppressed, and linear oligomers of different mole-cular weights were obtained; this showed that thenucleophilic attack of water competes with theintramolecular attack, and the proportion of linearspecies increases.

Polyols of industrial importance are usuallyrequired to have a low viscosity and a highhydroxyl content value (HV). To increase the pro-portion of linear oligomeric structures, to reducetheir molecular weight, and to increase theirhydroxyl content, water was added to the oligo-

merization system. The oligomerization of EMOwas carried out in the presence of 10, 20, and50 mol % water to give oligomers B, C, and D(Table 1). MALDI-TOF spectra show that thehighest intensity signals correspond to a lowermolecular weight (n ¼ 2 or 3). Moreover, fromMALDI-TOF analysis, a decrease in the signalintensity corresponding to the cyclic oligomerswas observed, and for the product obtained with50%, this signal was almost negligible. Theseresults are in accordance with the ones previ-ously described.27

1H NMR spectra of oligomers B–D did notshow significant differences between them andsample A obtained in the absence of water. FTIRspectra showed a significant increase in thehydroxyl absorption at 3500 cm�1, which indi-cates that the presence of water causes anincrease in the HV.

The HV of the oligomers was determinedaccording to the Unilever method.29 This methoduses a solution of acetic anhydride in pyridine,and the HV is defined as the number of potassiumhydroxides required to neutralize the amount ofacetic acid capable of combining by acetylationwith 1 g of the sample. The results are sum-marized in Table 1; as can be seen, the HV in-creases as the water content in the polymeriza-tion system increases, and SEC analysis hasshown that the molecular weight decreases asthe HV increases, according to the MALDI-TOFresults. The compounds obtained so far are poly-ether diols with secondary alcohol function-ality and side products containing only one or nohydroxyl group. These products are not suitedfor synthesizing polyurethanes. In addition, pri-

Scheme 2


mary hydroxyl groups are more reactive andmuch better suited for the synthesis of polyur-ethanes. Therefore, to obtain a broad range ofpolyol structures that may have different prop-erties and may impart different properties tothe final product when converted to polyur-ethanes, we carried out a partial reduction ofthe carboxylate groups to yield primaryhydroxyl moieties.

Partial Reduction of Ester Groupsof Oligomers A and C

Oligomers A and C were used as starting materi-als for the partial reduction of the ester groups to

synthesize polyols having primary hydroxyl moi-eties. The reduction of these oligomers was car-ried out with different amounts of LiAlH4 as areducing agent, giving polyols PA1, PA2, and PA3(Table 2). Figure 3 shows the 1H NMR spectra ofoligomer A and the corresponding polyols. A newsignal appears at d ¼ 3.6 ppm due to the protonsof the CH2��OH moiety. As the reduction degreeincreases, the intensity of this signal increases,whereas the intensity of the peak at d ¼ 2.3 ppmdecreases. The same trend can be observed forpolyol C.

The FTIR spectra of the polyols are presentedin Figure 4. In comparison with the spectrum ofA, there was an increase in the hydroxyl group

Table 2. General Properties of the Obtained Polyols

PolyolStartingOligomer

mmol of LiAlH4/gof Oligomer

HV(mg of KOH/g)

EW(g/equiv)a

Mn

(SEC) Functionalityb

PA1 A 0.5 94 597 1220 2.0PA2 A 1.0 184 305 1187 3.8PA3 A 1.5 260 216 1149 5.3PC1 C 0.5 127 442 1010 2.3PC2 C 1.0 216 260 980 3.7PC3 C 1.5 298 188 935 4.9

a Equivalent weight calculated from the HV.b Obtained by the division of the experimental molecular weight (Mn) by the EW.

Figure 3. 1H NMR spectra (CDCl3, 300 MHz) of (a) oligomer A, (b) polyol PA1, (c)polyol PA2, and (d) polyol PA3.

640 LLIGADAS ET AL.

Figure 4. FTIR spectra of (a) oligomerA, (b) polyol PA1, (c) polyol PA2, and (d) polyol PA3.

Figure 5. DSC curves of (a) oligomer A, (b) polyol PA1, (c) polyol PA2, and (d) polyol PA3.


peak (3500 cm�1), whereas a decrease in the ali-phatic ester carbonyl peak (1750 cm�1) wasobserved. We determined the HV of the polyolsaccording to the Unilever method (Table 2). Asthe amount of LiAlH4 increases, the hydroxylvalue increases, as expected.

In this way, polyols with a broad range of func-tionalities have been obtained, and they rangefrom clear liquids to white, waxy solids at roomtemperature. DSC traces for A and its polyolderivatives are collected in Figure 5. As can beseen, A exhibits a broad melting peak centered at

Figure 6. FTIR spectra of the crosslinking of polyether polyol PA2 with MDI to givepolyurethane PUA2 at different curing times (0, 30, and 60 min at 60 and 110 8C over-night).

Table 3. Thermal Properties of the Polyurethanes

PolyurethaneStartingPolyol

Tg (8C) TGA (N2 Atmosphere)

1/2DCpa tan dmax

b T5% lossc Yield800 8C (%)d

PUA1 PA1 �15 — 307 2PUA2 PA2 39 63 310 3PUA3 PA3 80 93 316 5PUC1 PC1 0 — 309 2PUC2 PC2 37 62 303 4PUC3 PC3 52 76 313 4

a Glass-transition temperature obtained by DSC.b Glass-transition temperature obtained by DMTA.c Temperature of 5% of weight loss.d Yield obtained at 800 8C.

642 LLIGADAS ET AL.

�7 8C, which shifts to higher temperatures whenthe HV increases. Moreover, a second meltingendotherm can be observed at 42 8C for PA2 andPA3, with the highest functionality. Multiplepeaks in these polyols should be ascribed to dif-ferent crystalline forms.

Synthesis of Polyurethanes

The six polyols PA1–PA3 and PC1–PC3 were

reacted with MDI at 60 8C to give polyurethanes

PUA1, PUA2, PUA3, PUC1, PUC2, and PUC3.

All the polyols had primary OH groups and

Figure 7. (a) Storage modulus (E0)/temperature curves of (a) PUA2, (b) PUA3, (c)PUC2, and (d) PUC3 and (b) tan d/temperature curves of (a) PUA2, (b) PUA3, (c)PUC2, and (d) PUC3.


gelled quickly at the mixing temperature. Thecrosslinking reaction was monitored by FTIRspectroscopy. Figure 6 shows FTIR spectra beforeand after the curing of polyurethane PUA2. Thestarting mixture showed a characteristic peak at2240 cm�1 ascribed to ��N¼¼C¼¼O stretching ofthe isocyanate moiety. After the curing, this peaksignificantly decreased. The crosslinking reactionwas also monitored by the appearance of thecharacteristic absorbances of the urethane link.The band due to the carbonyl stretching vi-bration of polyurethane occurred at 1723 cm�1,and a combination of N��H deformation andC��N stretching vibrations occurred at 1533 and1233 cm�1, respectively. The peak appearing at1309 cm�1 was due to ��OCONH asymmetricstretching vibrations. Moreover, the broad bandcentered at 3500 cm�1, corresponding to theO��H stretching, shifted to lower frequenciesand showed a maximum at 3350 cm�1, character-istic of N��H stretching.

The thermal behavior of the polyurethaneswas investigated with DSC. The glass-transitiontemperature (Tg) could be observed by this tech-nique. The Tg values determined by DSC areshown in Table 3. The data given show clearlyfor both series of polyurethanes the expectedtrend, that Tg was higher when the functionalityof the polyol used was higher, which indicates ahigher degree of crosslinking.

TGA is the most favored technique for the eval-uation of the thermal stability of polymers. Poly-urethanes have relatively low thermal stability,mainly because of the presence of urethane bonds.The thermal stability of the obtained polyur-ethanes was studied with TGA at a heating rateof 10 8C/min in a nitrogen atmosphere, and theobtained data are shown in Table 3. The shapes ofthe weight-loss curves of all the polyurethanesare almost identical, and the differences in thethermal stability appear to be small. The decom-position of the polyurethanes in a nitrogen atmos-phere does not take place below 300 8C.

The dynamic mechanical properties of the poly-urethanes were obtained as a function of tempera-ture beginning in the glassy state, through theTg, and well into the rubbery plateau of eachmaterial. Figure 7(a,b) shows the temperaturedependence of the storage modulus and loss factor(tan d) of the polyurethanes. From the DMTAcurves, the plateau of the elastic modulus in therubbery state can be used to make qualitativecomparisons of the level of crosslinking amongthe various polymers. Figure 7(a) shows that the

value of the storage modulus in the rubber pla-teau decreased as the HV of the starting polyoldecreased. DMTA also made it possible to deter-mine the Tg of the crosslinked materials. It wasdetected as the maximum of tan d. The Tg valuesdetermined by DMTA are shown in Table 3. Asexpected, the Tg values, like tan d, are higherthan the ½DCp values from DSC and increase asthe HV of the starting polyol increases [Fig. 7(b)].This is caused by the higher crosslinking degree,which increases the stiffness of the network struc-ture. A weak b transition at about 0 8C can beobserved. The origin of this peak is not known,but it may be related to the rotational motions ofshort units in the fatty acid chains.30

CONCLUSIONS

Polyether polyols were synthesized by the cati-onic oxirane ring-opening oligomerization ofEMO followed by partial reduction of the estergroups to primary alcohols. The correspondingpolyurethane networks were prepared by thereaction of the polyols with MDI. This showedthat plant oils as renewable resources can beused to make hard rubbers or rigid plastics.

The authors gratefully acknowledge the Comision Inter-ministerial de Ciencia y Tecnologıa (MAT2002-00223)and the Comissio Interdepartamental de Recerca iInnovacio Tecnologica (2001SGR00318) for their finan-cial support of this work and for G. Lligadas’s pre-doctoral (2003FI00765) and mobility (2004BV200032)grants. The authors thank F. Fabbretti for matrix-assisted laser desorption/ionization time-of-flight massspectrometry analysis and D. Topken for the synthesisof epoxidized methyl oleate.

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