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Synthesis and Characterization of a New Thermoreversible Polyurethane Network Cristian-Dragos Varganici,† Oana Ursache,‡ Constantin Gaina,‡ Viorica Gaina,‡ Dan Rosu,*,† and Bogdan C. Simionescu†,§ †

Centre of Advanced Research in Bionanoconjugates and Biopolymers, “Petru Poni” Institute of Macromolecular Chemistry, 41A Gr. Ghica-Voda Alley, 700487 Iasi, Romania ‡ Laboratory of Polyaddition and Photochemistry, “Petru Poni” Institute of Macromolecular Chemistry, 41A Gr. Ghica-Voda Alley, 700487 Iasi, Romania § Department of Natural and Synthetic Polymers, “Gh. Asachi” Technical University of Iasi, 73 Dimitrie Mangeron Boulevard, 700050 Iasi, Romania S Supporting Information *

ABSTRACT: A new polyurethane network was synthesized by the Diels−Alder cross-linking reaction of a polyurethane to bisfuryl monomer. Attenuated total reflectance in conjunction with Fourier transform infrared spectroscopy (ATR-FTIR) spectra of the network showed the disappearance of the absorption bands of maleimide and the appearance of new bands attributed to furan-maleimide cycloadduct. Chemical shifts characteristic to the cycloadduct appeared in the proton nuclear magnetic resonance spectra. ATR-FTIR and differential scanning calorimetry (DSC) demonstrated the thermal reversibility of the material by the reproduction of the retro-Diels−Alder and Diels−Alder processes upon heating and cooling. Global kinetic nonisothermal decomposition parameters in nitrogen were determined by the Flynn-Wall-Ozawa method. A three successive stage thermal decomposition mechanism depicted by n order reaction model for each stage was proposed. The validity of the chosen kinetic model and the values of the kinetic parameters of the individual decomposition stages were determined by the multivariate nonlinear regression method.

1. INTRODUCTION The Diels−Alder (DA) reaction is an important and versatile tool applied in a wide range of organic synthesis because most functional groups do not hinder the reaction. This process generally consists of a [4 + 2] cycloaddition reaction between a dienophile and a diene.1 The DA process can also be applied for cross-linking polymers.2−6 Three advantages of the process make it an environmentally friendly reaction: it proceeds under relatively mild conditions, requires no catalyst, and generates no byproducts. Furthermore, a DA reaction is a thermally reversible process, thus the polymer network synthesized by DA reaction will be thermally amendable. This means that the respective material can be decomposed into the comprising monomers by heating, i.e. retro-Diels−Alder (rDA) reaction.7 This important aspect makes such types of materials suitable for chemical recycling processes which consist of the depolymerization and repolymerization cycles with the reproduction of the initial polymeric material.8,9 The DA reaction and its analogue are thus considered a highly promising route in the quantitative recovery of polymers and their corresponding monomers from polymer-containing waste. Two different approaches in the applications of the DA reaction sequence have been extensively discussed especially during the past decade: the polymerization of multifunctional monomers10−13 for example, a di- or trifunctional furan derivative and a bismaleimide; (ii) the formation of cross-linked polymer networks14−18 from linear thermoplastics bearing pendant furan or maleimide groups. Some advantages of the cross© 2013 American Chemical Society

linked polymer networks consist of their superior mechanical properties and increased chemical and thermal stability in comparison to their linear analogues.19−21 In this paper authors report the synthesis of a new thermoreversible polyurethane network obtained via DA reaction and its characterization using proton nuclear magnetic resonance (1H-NMR), attenuated total reflectance in conjunction with Fourier transform infrared spectroscopy (ATRFTIR), differential scanning calorimetry (DSC), and dynamic thermogravimetric analysis (TGA) methods. The purpose of this paper is the accumulation of new knowledge concerning the synthesis and thermal remendability and stability of polyurethane based networks with thermoreversible properties, since the synthesis pathways of the material are different from the authors previous reported work and the new material presented some advantages concerning characterization which are further described in this paper.

2. MATERIALS, SYNTHESIS, AND METHODS 2.1. Materials. Maleic anhydride, 5-aminoisophtalic acid, dibutyltin dilaureate, furfural, and pentaeritritol were purchased from Aldrich, while polycaprolactone diol having M̅ n = 2000 was purchased from Sigma-Aldrich and was used as received. Received: Revised: Accepted: Published: 5287

January 29, 2013 March 12, 2013 March 20, 2013 March 20, 2013 dx.doi.org/10.1021/ie400349b | Ind. Eng. Chem. Res. 2013, 52, 5287−5295

Industrial & Engineering Chemistry Research

Article

Scheme 1. Synthesis of NPU

Dimethylformamide (DMF) and diethyleneglycol (DEG) were purchased from Aldrich and were dehydrated before use. 2.1.2. Synthesis of 5-Maleimidoisophtalic Diisocyanate. 5-Maleimidoisophtalic diisocyanate (DIFM) was prepared in the laboratory using maleic anhydride and 5aminoisophtalic acid as raw materials according to the method in our previous article.22 2.1.3. Synthesis o f 3.9-Di-2-furyl-2,4,8,10tetraoxaspiro[5,5]undecane. 3.9-Di-2-furyl-2,4,8,10tetraoxaspiro[5,5]undecane (DFTOUD) was prepared by acetalization of pentaeritritol with 2-furfural according to the method presented in the literature. The melting point was 163−164.5 °C determined with the Gallenkamp hot-block point apparatus and 165 °C from DSC.23 2.1.4. Synthesis of Poly(ester-urethane) Containing Maleimide Ring (PU). The polyurethane was synthesized by a conventional two step method under inert atmosphere of highly pure nitrogen in a 100 mL three-neck round bottomed flask equipped with a magnetic stirrer and thermometer. To a 10% w/v solution of polycaprolactone diol (PCD-2000) in dry DMF and dibutylthin dilaureate (10−2 mol/L) as catalyst, an excess (2:1) of 5-maleimidoisophtalic-diisocyante was added (NCO:OH ratio = 2). The solution was stirred at 60 °C for 2 h, and an isocyanate terminated prepolymer was obtained. In a second step a stoichiometric amount of dried DEG in DMF (3 mL) was added, and the stirring continued at 60 °C for 2 h. Then, the solution was cooled (25 °C), and the polymer was precipitated in cool distilled water and washed with methanol at room temperature. The sample was then dried at 50 °C for 24 h under reduced pressure. The polyurethane film was casted from a 8% w/v solution of polyurethane in DMF. 2.1.5. Synthesis of Polyurethane Network (NPU). To a solution of polyurethane (1.2 g) in DMF (10 mL) a stoichiometric amount of bisfuryl monomer (0.1367 g) in DMF (3 mL) was added. The mixture was stirred at 60 °C for 24 h, after which the solution was casted on a Teflon Petri dish and the solvent was evaporated at 60 °C for 24 h. The film was

removed from the Teflon Petri dish by soaking in cold water. The obtained film was used for all characterizations. 2.1.6. Synthesis of the Model Compound. The model compound was prepared from stoichiometric amounts of DFTOUD and 4-maleimido-acetophenone (AMI). Thus, 0.0118 g of DFTOUD and 0.0173 g of AMI were dissolved in 1 mL of deuterated DMSO and transferred imediately in the NMR tube. The 1H NMR spectrum of the reaction mixture was recorded right after preparing the sample. 2.7. Methods. 2.7.1. ATR-FTIR. The Fourier transform infrared (FTIR) spectra were recorded on a Bruker Vertex 70 (Austria) Instrument equipped with a Golden Gate single reflection ATR accessory and a temperature controller. The measurements were performed in the 4000−600 cm−1 spectral range, using a heating/cooling rate of 5 °C min−1 for the sample recorded at 160 °C and at room temperature after cooling. 2.7.2. Melting Points Determination. Melting points were determined with a Gallenkamp hot-block point apparatus. 2.7.3. 1H-NMR. The 1H NMR spectra were recorded on a Bruker NMR spectrometer, Avance DRX 400 MHz, using DMSO-d6 as solvent and tetramethylsilane as an internal standard. 2.7.4. DSC. DSC measurements were conducted on a DSC 200 F3Maia (Netzsch, Germany). A mass of 10 mg of each sample was heated in pressed and pierced aluminum crucibles at a heating/cooling rate of 10 °C min−1. Nitrogen was used as inert atmosphere at a flow rate of 50 mL min−1. The temperature against heat flow was recorded. The baseline was obtained by scanning the temperature domain of the experiments with an empty pan. The enthalpy was calibrated with indium according to standard procedures. 2.7.5. TGA. Thermogravimetric measurements were conducted on a STA F1 449 Jupiter device (Netzsch, Germany). Around 10 mg of each sample was heated in alumina crucibles at four different heating rates of 10, 15, 20, and 25 °C min−1. 5288

dx.doi.org/10.1021/ie400349b | Ind. Eng. Chem. Res. 2013, 52, 5287−5295


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Nitrogen was used as inert atmosphere at a flow rate of 50 mL/ min.

new structure allowed the evidentiation of the dienic process of the NPU on the cooling curve in the DSC experiments. Also, the thermoreversibility of NPU studied by ATR-FTIR spectroscopy could be better explained (comparing with our other previous work), the urethane CO absorption band behavior being another proof of the thermoreversibility of the network. The ATR-FTIR spectra of PU film (Figure 1a) presented the absorption bands of ester, urethane, and imide free carbonyls which appear overlapped between 1735 and 1710 cm−1. The absorption peaks at 2858 and 2950 cm−1 correspond to symmetric and asymmetric CH2 groups, respectively. The bands at 1459, 1434, 1417, and 1369 cm−1 appear due to various modes of CH2 vibration. The peak at 1552 cm−1 is characteristic to amide II (a combination peak of N−H bending and C−N stretching). The absorbance at 1090 cm−1 was attributed to the ether group from the center of the polycaprolactone segment.24 The absorption band at 3334 cm−1 is caused by the hydrogen bonded N−H urethane stretching. The bands at 3103, 1612, 1394, 829, and 689 cm−1 are characteristic to maleimide groups. The 1H NMR spectrum of PU is presented in Figure 2. The attributions of different signals are also given in the figure. All expected signals of protons are clearly noted. Furthermore, the integration of all proton peaks shows that the structure of PU corresponds to the one illustrated in Scheme 1. NPU was prepared by the Diels−Alder cross-linking reaction of PU to bisfuryl monomer, as represented in Scheme 1. ATRFTIR spectra of the network (Figure 1b) showed the disappearance of the absorption bands of maleimide groups at 3103 and 829 cm−1 and the appearance of new bands at 1776 cm−1 and 935 cm−1 attributed to the furan-maleimide cycloadduct. Also, it is observed the change in the absorption of the carbonyl group. In order to study the DA and rDA reactions by 1H NMR spectroscopy a model compound was synthesized according to Scheme 2. When mixing the monomers in deuterated DMSO and recording the 1H NMR spectrum we can see the presence of the chemical shifts specific to the protons of both monomers (Figure 3). After maintaining the NMR tube with the sample at 60 °C for 60 h and recording the spectrum one can observe that the reaction occurs in a proportion of 55% (Figure 3). New chemical shifts characteristic to the cycloadduct appeared at 3.05−3.35 and 3.9 (CH succinimide exo and endo from cycloadduct), 4.98−5.05 (CH bridgehead exo), 5.11−5.16 (CH bridgehead endo), and 6.57−6.72 (−CHCH− endo and exo from cycloadduct). After the Diels−Alder reaction takes place both endo and exo isomers are formed. According to the 1H NMR spectrum at 60 °C the exo isomer is majority formed. By treating the sample at 140 °C for ten minutes the retrodienic reaction takes place almost completely (Figure 3). 3.2. Thermal Remendability by DSC. The DSC method offers useful insights in characterizing materials with thermally remendable properties.25,26 Figure 4 indicates two heating cycles and a cooling curve of the NPU. A glass transition temperature (Tg) was observed at a value around 110 °C attributed to the Tg of the cross-linked network which underwent a broad endothermic peak at a temperature value of 150 °C. The endothermic process is attributed to the rDA reaction which consists of the chemical debonding between maleimide and furfuryl moieties.27,28 On the cooling segments, the

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization by 1H-NMR and ATR-FTIR. PU was prepared by a two step addition method in

Figure 1. ATR-FTIR spectra of (a) PU and (b) NPU.

Figure 2. 1H NMR spectrum of PU.

DMF solution by the reaction of 5-maleimidoisophtalic diisocyanate with PCD (having number-average molecular weights of 2000) using DEG as chain extender, in a 2:1:1 molar ratio, in the presence of dibutylthin dilaureate as catalyst, at 60 °C and under a stream of dry nitrogen (Scheme 1). The reaction was considered completed when the absorption band from 2270 cm−1 attributed to NCO disappeared. The inherent viscosity of PU was 0.43 dL/g in 0.5% w/v DMF solution. The 5289



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Scheme 2. Synthesis of the Model Compound

Figure 3. 1H NMR spectrum of the model compound.

Figure 4. DSC thermograms of NPU.

130 °C. Process enthalpies yielded almost identical values for the rDA reaction, as it can be observed. Thermal kinetic

network reconnected via DA reaction. A wider and less intense exothermic peak was observed at a temperature value around 5290



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approaches to the rDA process for similar structures was discussed in previous papers.29,30 3.3. Thermal Remendability by ATR-FTIR. Another method used for demonstrating the thermoreversibility of the polyurethane was the ATR-FTIR spectroscopy. The spectra were recorded at room temperature, at 160 °C, after cooling and after treating the sample at 100 °C for 2 h in an oven (Figure 5). So, in the spectrum of the compound, recorded at 160 °C, one can observe the disappearance of the band characteristic to the cycloadduct, namely the one at 1778 cm−1. By cooling the sample, the band specific to the cycloadduct reappears, but the band characteristic to the maleimidic double bond does not disappear. After maintaining the sample for 2 h at 100 °C in an oven and recording again the spectrum one can observe that it is identical to the initial one. The debonding of the cycloadduct at 160 °C is also evidenced by the disappearance of the band at 1195 cm−1 which is due to the C−N−C from succinimide ring. After treating the sample in the oven, due to the reformation of the cycloadduct the succinimide C−N−C absorption band at 1195 cm−1 appears again. Some changes occur also in the region of the CO band. This region includes the ester CO band, the urethane CO band, and the cycloadduct CO band. The first band that appears represents the overlapping of the ester CO band and the free urethane CO band. The second one is the result of the overlapping of the three bands, namely the disordered hydrogen bonded and ordered hydrogen bonded urethane CO stretching and cycloadduct CO band.18,31 The free urethane CO groups are the ones which are not implied in hydrogen bonds, while the disordered and ordered hydrogen bonded ones are the hydrogen bonded groups associated with amorphous phase and crystalline phase, respectively. When heating the sample, the hydrogen bonds decompose and the hydrogen bonded urethane CO stretching band disappears. Also, the CO band which was characteristic to the cycloadduct shifts to higher wavenumbers and overlaps with the ester and free CO band, due to the disconnection of the cycloadduct in maleimide and furyl groups. When cooling the sample and recording the spectrum, one can observe that the hydrogen bonds between the urethane CO and NH started forming. After treating the sample in the oven and the complete reformation of the cycloadduct, the C O band characteristic to it completely shifts to lower wavenumber values to its initial position. 3.4. Thermal Stability Studies. Figure 6 depicts the TG and DTG curves of the structure recorded at four heating rates. The DTG curves indicate three stages of thermal decomposition of which the last two are partially overlapping and corresponding to one single mass loss stage on the TG curves. The first stage of thermal decomposition showed a larger amount of weight loss. The rDA reaction occurred in the heating process to generate some free maleimide and furan groups in the sample, which contributed to the weight loss. On the other hand, self-addition and cross-linking reaction occurred between some free maleimide groups to enhance its thermal stability and to reduce its amount of weight loss in the second and third stages of thermal decomposition.32 Also, all the above-mentioned processes may overlap with other complex decomposition phenomena generated by random chain scissions. These processes consist of the depolymerization of the urethane bonds and soft segment decomposition on the first stage of thermal decomposition, whereas on the second and third stage hard segment cleavage occurs.22

Figure 5. ATR-FTIR spectra of NPU.

Figure 6. TG and DTG thermograms of NPU recorded at four different heating rates.

Table 1. Global Kinetic Parameters of Thermal Decomposition Process of NPU Resulted from the Flynn− Wall−Ozawa Method kinetic parameters Flynn-Wall-Ozawa α

log A (s−1)

E (kJ mol−1)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.95

5.66 4.86 4.70 4.79 5.03 5.20 6.18 11.81 27.45 24.21

91 83 82 83 86 89 101 176 400 467

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Figure 7. Variation plot of E with α.

The conversion rate is described by eq 3, where t is time (min), A is the pre-exponential factor (s−1), E is the activation energy of thermal decomposition (kJ mol−1), R is the gas constant (8.314 J mol−1K−1), T is temperature (K), and f(α) is the conversion function. E dα = k(T )f (α) = Ae− RT f (α) dt

(3)

A new equation of the thermal degradation rate is obtained by inserting the rate of heating (eq 4), β = dT/dt. dα A E = e− RT f (α) β dT

(4)

Global kinetic parameters were evaluated using an isoconversional method which uses shifts in thermograms with heating rate increase, due to temperature delay as a function of heating rate.34−36 Only one isoconversional method is sufficient in order to obtain the global kinetic parameters values.37 After integration of eq 4 between the limits T0 and Tp the integral function of conversion was obtained which was noted G(α) in eq 5. G (α ) =

Figure 8. Plot of logβ as a function of 1000/T according to the Flynn−Wall−Ozawa method for NPU.

∫T

Tp

0

E

e− RT dT =

∫0

αp

dα f (α )

(5)

T0 is the initial temperature corresponding to α = 0, and Tp is the temperature corresponding to the peak from DTG curve, where α = αp. The integral function of conversion depicts the mechanism of thermal degradation.38 The applied isoconversional method is the integral one of the Flynn-Wall-Ozawa method39−41 which uses the Doyle approximation42 of the temperature integral in eq 5. The relationship between kinetic parameters and the heating rate is given by eq 6.

3.5. Nonisothermal Kinetics of Thermal Decomposition. The parameter which is most affected by temperature is the rate constant (or rate coefficient) (k).33 It was assumed that the thermal decomposition process of the studied films is described by the reaction model given in eq 1, where the solid material M(s) decomposes into solid residue R(s) and gases G(g).

M(s) → R(s) + G(g)

A β

⎛ AE ⎞ E ln β = ln⎜ ⎟ − ln G(α) − 5.3305 − 1.052 ⎝ R ⎠ RT

(1)

The kinetic parameters were evaluated from nonisothermal experiments. The conversion degree (α) was calculated using eq 2, where mi, mt, and mf represent the weights of the sample before degradation, at a time t, and after complete degradation. m − mt α= i mi − mf (2)

(6)

To calculate the kinetic parameters with eq 6, it was considered that the process was described by a first order reaction for G(α) (i.e., 1 − α). For the same value of α, the plot of lnβ as a function of 1/T is a straight line with the slope proportional with the activation energy. The different expressions of G(α) which are specific to the thermal 5292



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Table 2. Nonisothermal Kinetic and Statistic Parameters Obtained after the Nonlinear Regression Method through the Thermal Degradation Mechanism of NPU kinetic parameters values code

parameter

stage I

stage II

stage III

correlation coefficient

Fcritical (0.95)

Fexp

Fn

logA (s−1) E (kJ mol−1) n logA (s−1) E (kJ mol−1) logA (s−1) E (kJ mol−1) logA (s−1) E (kJ mol−1) logA (s−1) E (kJ mol−1) logA (s−1) E (kJ mol−1) n a1 a2 a3 logA (s−1) E (kJ mol−1) logA (s−1) E (kJ mol−1) logA (s−1) E (kJ mol−1) logA (s−1) E (kJ mol−1) logA (s−1) E (kJ mol−1) logA (s−1) E (kJ mol−1) logA (s−1) E (kJ mol−1)

4.54 77.95 0.75 5.18 85.56 4.71 80.45 5.45 85.28 3.68 74.5 5.26 83.2 2.15 0.13

14.1 198.84 1.4 22.68 335.6 9.28 156.81 11.51 150.79 9.88 162.34 11.52 152.75 2.05

16.43 250.88 2.99 4.06 101.23 13.88 251.57 16.24 253.63 7.09 139.22 8.49 101.62 0.5

0.999934

1.11

1.00

0.999929

1.11

1.07

0.999879

1.11

1.82

0.999830

1.11

2.55

0.999824

1.11

2.65

0.999811

1.11

2.85

0.999756

1.11

3.65

0.999707

1.11

4.39

0.999688

1.11

4.69

0.999519

1.11

7.21

0.999222

1.11

11.68

0.998414

1.11

23.78

0.994901

1.11

76.30

An F1 F2 R3 Bna

R2 D3 D2 D1 A2 A3 B1

0.25 3.54 71.2 5.65 102 5.8 98.63 7.41 113.04 2.08 49.2 1.57 43.5 3 46.5

9.45 151.52 10.75 150.4 11.3 150.45 9.9 150.71 10.31 150.32 10.28 150.29 11.76 150.3

1.27 6.77 131.55 11.62 205.82 7.67 143.56 8.97 160.04 4.98 100.72 4.91 100.59 6.33 100.7

The nonisothermal data extracted from the thermograms were processed with the software Netzsch Thermokinetics 3. The values of the kinetic parameters obtained through the applied isoconversional method are given in Table 1. It can be observed from Table 1 and Figure 7 that E values vary with α, suggesting a complex decomposition mechanism.43 The straight lines in the Flynn−Wall−Ozawa plot (Figure 8) do not follow the same parallelism, thus indicating that a first order reaction model is not the best option to describe the thermal decomposition process. A kinetic model of thermal decomposition in three successive stages (eq 7) was proposed. In eq 7 A is the initial structure, D represents the thermostabile residue, and B and C are solid intermediates together with the corresponding gaseous products. 1

2

3

A→B→C→D

(7)

The multivariate nonlinear regression method43 was performed to determine the reaction model for the four heating rates and to find the real form of the conversion function describing individual decomposition stages of the studied sample. After testing of 13 reaction types,33 the best results were obtained with n order reaction model as described by eq 8:

Figure 9. Comparison of experimental TG thermograms with those calculated from kinetic data.

dα A E = e− RT (1 − α)n β dT

degradation mechanisms of polymers can be found in the literature.38 5293

(8)



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In eq 8 (1 − α) is derived from the general function form f(α) = f(e, p) where e = (1 − α) which represents the concentration of the educt, while p is the concentration of the solid products during the thermal decomposition process and n is the reaction order which ranges from values 1 to 3. The kinetic data obtained by the multivariate nonlinear regression method are presented in Table 2. It can be observed that thermal decomposition of the studied structure is characterized by activation energy values higher than 90 kJ mol−1. The pre-exponential factor and activation energy values increase very slowly during transitions between decomposition stages. The subunit value of n, in the first stage of decomposition, indicates scission of small molecules from the polymer structure and simultaneous cyclization reactions. The value of n around 2, in the second stage of decomposition, would reflect that the weight loss is related to random scission of the main chain with generation of shorter polymer fragments and intramolecular transfer,44,45 while an order reaction value of 3 may indicate the occurrence of simultaneous scission and cross-linking phenomena. 46 The first stage of thermal decomposition was attributed to isocyanate and alcohol formation by urethane bond dissociation, while the next stages were described by partial polyols dehydration, with the elimination of carbonyl structures and water, and polyester decomposition with carbon dioxide release.36,47 The kinetic data thus confirm the thermal stability studies observations. The experimental TGA curves were compared with those simulated by the software, using data presented in Table 2. Figure 9 shows the comparison of experimental data with the calculated data. The correlation coefficient yielded a value of 0.999934, thus proving the validity of the chosen thermal decomposition model in three successive stages. Moreover, it can be observed that the kinetic parameters corresponding to each individual thermal decomposition stage determined by the multivariate nonlinear regression method are in the range of global ones determined by the FWO method.

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ASSOCIATED CONTENT

S Supporting Information *

Table listing thermal decomposition characteristics of NPU. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +40 232 217 454. Fax: +40 232 211 299. E-mail: [email protected], [email protected]. Corresponding author address: “Petru Poni” Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda, No. 41A, Iasi 700487, Romania. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Two of the authors (C.-D.V. and D.R.) acknowledge the financial support of a grant of the Romanian National Authority for Scientific Research, CNCS−UEFISCDI, project number PN-II-ID-PCE-2011-3-0187.

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REFERENCES

(1) Fringuelli, F.; Taticchi, A. Dienes in the Diels−Alder Reaction; John Wiley & Sons: New York, 1990. (2) Miura, M.; Akutsu, F.; Usui, T.; Ikebukuro, Y.; Nagakubo, K. Soluble Cyclopentadienylated Polymers. Makromol. Chem. 1985, 186, 473. (3) Chujo, Y.; Sada, K.; Saegusa, T. Reversible Gelation of Polyoxazoline by Means of Diels−Alder Reaction. Macromolecules 1990, 23, 2636. (4) Jones, J. R.; Liotta, C. L.; Collard, D. M.; Schiraldi, D. A. CrossLinking and Modification of Poly(ethylene terephthalate-co-2,6anthracenedicarboxylate) by Diels Alder Reactions with Maleimides. Macromolecules 1999, 32, 5786. (5) Canary, S. A.; Stevens, M. P. Thermally Reversible Crosslinking of Polystyrene via the Furan−Maleimide Diels−Alder Reaction. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 1755. (6) Gheneim, R.; Perez-Berumen, C.; Gandini, A. Diels−Alder Reactions with Novel Polymeric Dienes and Dienophiles: Synthesis of Reversibly Crosslinked Elastomers. Macromolecules 2002, 35, 7246. (7) Engle, L. P.; Wagener, K. B. A Review of Thermally Controlled Covalent Bond Formation in Polymer Chemistry. J. Macromol. Sci. Rev. Part C: Macromol. Chem. Phys. 1993, 1, 239. (8) Endo, T.; Nagai, D. A Novel Construction of Ring-Opening Polymerization and Chemical Recycling System. Macromol. Symp. 2005, 226, 79. (9) Sassw, F.; Emig, G. Chemical Recycling of Polymers. Chem. Eng. Technol. 1998, 21, 777. (10) Goussé, C.; Gandini, A. Diels−Alder Polymerization of Difurans with Bismaleimides. Polym. Int. 1999, 48, 723. (11) Kamahori, K.; Tada, S.; Ito, K.; Itsuno, S. Optically Active Polymer Synthesis by Diels− Alder Polymerization with Chirally Modified Lewis Acid Catalyst. Macromolecules 1999, 32, 541. (12) Mcelhanon, J. R.; Russick, E. M.; Wheeler, D. R.; Loy, D. A.; Aubert, J. H. Removable Foams Based on an Epoxy Resin Incorporating Reversible Diels−Alder Adducts. J. Appl. Polym. Sci. 2002, 85, 1496. (13) Liu, Y.; Hsieh, C. Crosslinked Epoxy Materials Exhibiting Thermal Remendability and Removability from Multifunctional Maleimide and Furan Compounds. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 905. (14) Goussé, C.; Gandini, A.; Hodge, P. Application of the Diels− Alder Reaction to Polymers Bearing Furan Moieties. 2. Diels−Alder and Retro-Diels−Alder Reactions Involving Furan Ring In Some Styrene Copolymers. Macromolecules 1998, 31, 314.

4. CONCLUSIONS A new polyurethane network was prepared by the Diels−Alder cross-linking reaction of PU to bisfuryl monomer. Structural characterization of the product was done by ATR-FTIR, and a model compound was synthesized in order to study the DA and rDA reactions by 1H NMR spectroscopy. The thermally remendable character of the network was also evidenced by ATR-FTIR and DSC techniques. Both methods showed the reproducibility of the rDA reaction upon heating and that of the DA process upon cooling the structure. The material was thermally stable up to 250 °C. Thermal decomposition studies showed that the network decomposed in three stages of which the last two overlapped due to complex simultaneous decomposition processes. Global kinetic parameters were determined by the FWO isoconversional method. The kinetic parameters and the conversion function of each stage of decomposition were determined by the multivariate nonlinear regression method. A conversion function of n order was found. A good correlation between experimental data and simulated data was found proving the accuracy of the chosen kinetic model. 5294



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