Polyhydroxyalkylation of Urea with Ethylene Carbonate and Application of Obtained Products As Components of Polyurethane Foams
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http://www.e-polymers.org e-Polymers 2008, no. 166 ISSN 1618-7229 Polyhydroxyalkylation of urea with ethylene carbonate and application of obtained products as components of polyurethane foams Iwona Zarzyka-Niemiec* *Rzeszów University of Technology, Department of Organic Chemistry, Al. Powstańców Warszawy 6, 35-959 Rzeszów, Poland; e-mail: [email protected] (Received: 17 March, 2008; published: 27 December, 2008) Abstract: The reaction between urea and ethylene carbonate occur with partial release of CO2 and partial incorporation of carbonate groups into products. The carbonate groups were found to be attached both to nitrogen of urea and to oxyethylene chain. The most effective catalyst of the synthesis was potassium carbonate. The hydroxyethyl and hydroxyethoxy groups of urea derivatives undergo partial dimerization to form carbamate groups in the products. The products of reaction between urea and ethylene carbonate have good thermal stability, they start to decompose at 200 0C. The obtained products can be used as polyol components for polyurethane foams. Polyurethane foams obtained from hydroxyethoxy derivatives of urea (EC8) are rigid products of low water uptake, good stability of dimensions, low mass loss on 30 days heating at 150 C, enhanced thermal stability and good compressive strength. Introduction Hydroxyalkyl derivatives of urea can be obtained by reaction of urea with corresponding aminoalcohols (I, y = 1) [1, 2]: O O 2H N CH O H+H N C NH H O CH HN C NH CH O H+2NH (1) 2 2 n y 2 2 2 n y 2 n y 3 (I) where: n = 2-5, y = 1, 2 or 3. Similarly the hydroxyalcoxy derivatives of urea (I, y = 2 or 3) can be obtained using aminoetherol substrates [3, 4]. The synthesis is accompanied by ammonia release, which must be removed to improve the yield of synthesis. Moreover, the competitive formation of carbamates by the reaction between hydroxyl groups with urea takes place. We have experienced that purification of products, especially tetrakis(hydroxyalkyl)- and alcoxy- derivatives of urea was ineffective because they do not precipitate from the reaction mixture and decompose upon distillation even under reduced pressure. Another method of synthesis of hydroxyalkylurea and hydroxyalcoxyurea derivatives is based on the reaction of urea with oxiranes, like ethylene oxide or 2,3-epoxybutane according to the following scheme [5, 6]. 1 O O CH CH O H H O CH CH z H N C NH + R CH CH R x N C N 2 2 R R R R (2) O H O CH CH CH CH O H y w R R R R where: R = CH3-, H-, x + y +z + w = 2 4. However, oxiranes are toxic and cancerogenic and additionally the synthetic protocol requires high pressure. It seemed reasonable to replace them with alkylene carbonates, which are less ecologically hazardous [7, 8]. They posses higher boiling points and are good solvents for urea. They are expected to react with urea in analogous manner as oxiranes. Here the attempts of such a synthesis are described together with the application of polyol products to obtain polyurethane foams of enhanced thermal stability. Results and discussion The reactions of urea with EC were performed in presence of potassium carbonate as catalyst at 120-160 C temperature range (Table 1). Tab. 1. Reaction conditions of urea with EC. Initial Amount of Time Kind of Temp. Presence of Synth. Molar catalyst of Reaction Catalyst [ C] Carbodiimide Ratio [mole/mole urea] [h] 1. 1 : 1 K2CO3 0.06 120 10 + 2. 1 : 2 K2CO3 0.03 120 18.5 + 3. 1 : 2 K2CO3 0.06 120 11 + 4. 1 : 2 K2CO3 0.03 140 13.5 + 5. 1 : 2 K2CO3 0.06 140 6 + 6. 1 : 2 DABCO 0.06 120 28 + a 7. 1 : 2 ZnCl2 0.06 120 27 + 8. 1 : 3 K2CO3 0.06 140 6.5 + 9. 1 : 4 K2CO3 0.06 120 35 + 10. 1 : 4 K2CO3 0.06 140 8.5 - 11. 1 : 8 K2CO3 0.09 160 10.5 - 12. 1 : 12 K2CO3 0.09 160 16 - 13. 1 : 16 K2CO3 0.09 160 21 - a: EC did not react completely. The resin-like product was obtained at 1:1 molar ratio of substrates (Table 1, synthesis 1), which has shown the same ratio of oxyethylene to urea. The initial stoichiometry was chosen at the assumption that total elimination of CO2 occurs (reaction 3). However the spectral analysis of product evidenced the presence of carbonate groups in the product by the presence of the primary amide proton resonance at 6.4 ppm in the 1H-NMR spectrum (Figure 1) as well as the resonance at 2 7.5 ppm attributed to imide group proton (H2N-CO-NH-COO-) [9]. Thus the carbonate groups are preserved at product, which can be represented by the formula (III). O O C NH C NH CH CH OH CO O O O 2 2 2 + 2 (II) NH C NH + CH CH 2 2 2 2 O O NH2 C NH C O CH2 CH2OH (3) (III) In the spectrum of product (III) the resonance of methylene group proton at -CH2-O-CO was found at 4.15 ppm [10] while those at 5.4 and 6.0 ppm were attributed to primary and secondary amide protons of N-(2-hydroxyethyl)urea (II) [9]. From the integral intensity of the NH2 group resonances in (II) and (III), respectively, it can be concluded that N-(2-hydroxyethyl)urea (II) is the major component in obtained mixture (ca 60%). Fig. 1. 1H-NMR spectrum of the product of reaction between urea and EC at 1:1 molar ratio in the presence of 0.06 mole K2CO3/mole urea at 120 C. The IR spectrum (Figure 2) of the product corroborates well with the NMR data. The valence C=O stretching ester and imide group band are present at 1730 cm-1 while I amide bands of primary and secondary amides are observed at 1663 and 1607 cm-1, respectively. The asymmetric and symmetric valence bands of ester -C-O- group occur at 1250 and 1150 cm-1, respectively. The C-OH valence band of primary alcohol is observed at 1059 cm-1. Some minor products were also found in the reaction mixture obtained from 1:1 urea: EC system. Derivative (IV) was tentatively identified by the strong IR band at 2157 cm-1, which was assigned to valence band of carbodiimide fragment N C N. Carbodiimides were found to be formed upon dehydration of N,N’-disubstituted ureas [11]. 3 1150 2157 1730 1250 1059 1663 1607 Fig. 2. IR spectrum of the product of reaction between urea and EC at 1 : 1 molar ratio in the presence of 0.06 mole K2CO3/mole urea at 120 C. The product (V) was identified based on 7.0 ppm resonance at 1H-NMR spectrum of the product which was assigned to secondary amide protons in N-substituted urea with accepting groups [10]. O O C HO CH CH NH C NH CH CH OH + 2CO 2 2 2 2 2 O O O (IV) H N C NH +2CH CH 2 2 2 2 O O O C NH C +CO HO CH2 CH2 NH CH2 CH2 OH 2 (4) (V) In the reaction between urea and EC at 1:2 molar ratio a mixture of products is also found, some with preserved carbonate group (V) and others without it (IV). The 1H- NMR of this product is similar to that obtained from 1:1 system. The presence of the amine proton resonances centered at 5.4 and 6.4 ppm from (II) and (III) indicates the partial decomposition of EC and formation of mono-substituted derivatives and/or N,N’-bis(hydroxyalcoxy) derivatives of urea. The synthesis at 120 C in presence of 0.03 mole K2CO3/mole of urea lasts too long (18.5 hours), therefore the process was performed at higher concentration of catalyst (0.06 mol; Table 1, syntheses 2 and 3). During this process the ammonium carbamate sublimed into reflux condenser (the product was identified by IR spectrum). It forms in the reaction between urea and water (present in the reaction mixture due to dehydration of N,N’-disubstituted derivatives of urea with formation of carbodimide). This by-product was also formed at higher temperature irrespective of the catalyst concentration (Table 1, syntheses 4 and 5), while it was not observed under less basic conditions (in presence of 0.03 mole K2CO3/mole of urea (Table 1, synthesis 2) at lower temperatures. The percentage of ammonium carbamate did not exceed 10 wt.-%. 4 The reaction is slower in presence of DABCO catalyst instead of K2CO3; the time of reaction was 28 hours at 120 °C (Table 1, synthesis 6). The amount of ammonium carbamate and carbodiimide of by-products remains unaltered. Also the major products are the same as previous, namely (II)‚(V). Therefore the acid catalyst; zinc chloride was also tested (Table 1, synthesis 7), which is known to catalyze the reaction between amines and alkylene carbonates [12]. In the presence of this catalyst the reaction was much slower and although ammonium carbamate was not formed, the carbodiimide by-product was still present. Further studies were performed with potassium carbonate as catalyst. When 3-molar excess of EC related to urea was applied (Table 1, synthesis 8) the percentage of by-products was considerably reduced. When four-fold excess of EC was applied at 120 C (Table 1, synthesis 9) ammonium carbamate was absent. At higher temperature (140 C; Table 1, synthesis 10) also carbodiimide did not form.