Introduction

Polyurethanes (PUs) are one of the most important polymers due to their outstanding properties such as good chemical resistance, high elasticity, and strong abrasive performance. Thus, PUs have found versatile applications in coatings, paints, adhesives, foams, elastomers, and so on [1,2,3]. Consequently, global PUs output is expected to reach 18 million tons in 2016 [4]. For example, PUs show the best comprehensive properties in organic coatings in comparison with methacrylate resins, epoxy resins, polyesters, etc. But carbamate groups in PUs are conventionally formed by reactions between diols/polyols and diisocyanates/polyisocyanates. The use of highly toxic isocyanates in PUs manufacturing processes may inevitably affect the health of production workers and limit their wider scale applications in high-safety-required medical or bio-materials. To solve this problem, non-isocyanate polyurethanes (NIPU) are desired and have been studied by chemists and chemical engineers [5,6,7]. There are mainly three approaches for preparation of NIPU: ring-opening reactions between five-membered cyclic carbonates and di/multi-amines, leading to hydroxyl-containing non-isocyanate polyurethanes (HNIPU) [8]; the other is self-polymerization of monomers containing hydroxyl and in situ-forming isocyanate groups [9]; the third is the exchange reactions between carbamates [10, 11]. Among them, the first method plays the most important role due to its advantages such as: (1) green-house gas CO2 is utilized to form cyclic carbonate; (2) other materials such as epoxides and amines are commercially available or can be easily prepared; (3) the resulting HNIPU can form H-bonds, which can help to improve mechanical properties of polymers. For example, Leitsch et al. studied the effect of hydroxyl groups on the properties of HNIPU, and found that hydroxyl groups played critical roles in controlling nano-phase separation, possibly leading to novel broad-temperature-range acoustic and vibration damping materials [12]. In addition to HNIPU obtained from cyclic carbonates and amines, similar polymers can also be obtained by reactions between cyclic dithiocarbonates and amines. This polymer belongs to polythiourethanes (PTUs) group [13,14,15]. In addition to the excellent properties of PUs, PTUs also show outstanding optical properties due to the presence of sulfur, and has found applications in sealing materials, coatings, optical lenses, and so on [16,17,18].

In the second method, isocyanates are usually formed in situ by acyl azides under conditions of Curtius rearrangement [19, 20]. This method can only be conducted in laboratories due to harsh reaction conditions and possible danger of explosion of azides. Recently, our research group reported preparation of ricinoleic acid-based NIPU, in which isocyanate can be formed in situ from amide of ricinoleic acid instead of acyl azide [21].

Conventional fossil resources as well as their derived substances are facing serious crisis, which drive scientists to find and develop alternatives [22,23,24]. Resources from plants or animals such as cellulose, lignin, vegetable oils [25], cardanol [26, 27], and proteins, are renewable and have attracted enormous attention both from academia and industry in recent years [28]. Eugenol, a phenol derivative with an allylic group at para-site of the phenyl ring, is a natural extract of cloves [29, 30]. This essential oil has been studied and found applications in dentistry, medicine, perfume, and functional materials [31,32,33]. For example, an interesting work reported in 2015 by Deng and coworkers indicated that eugenyl methacrylate can form microspheres with diameters ranging from 500 to 800 μm by suspension polymerization [34]. The microspheres showed remarkable ability in large oil absorbency and could be re-used at least five times. More recently, our research group has designed terminal diene derivatives of eugenol and subsequently prepared a self-healable polymer by combination of thiol-ene and thiol-oxidation reactions [35]. Herein, we report preparation and characterization of eugenol-based HNIPUs and non-isocyanate PTUs. Significance of our work is as follows: (1) eugenol, a renewable resource, is used to synthesize polyurethanes; (2) toxic isocyanate is avoided in PU synthesis; (3) green-house gas CO2 is utilized to produce HNIPUs; and (4) cardanol, another renewable resource, and the prepared eugenol-based PTUs are combined to afford UV-crosslinked resins.

Experimental

Material

Commercial epichlorohydrin (99%), 4,4′-diaminodiphenyl methane, 1,6-hexanediamine, p-xylene diamine, meta-chloroperbenzoic acid (m-CPBA, 70–75%), acetic acid, dichloromethane, ethyl acetate, petroleum ether, methanol, tetrahydrofuran, triethyl amine, N-methyl-2-pyrrolidone (NMP), 1,6-hexanediol diacrylate, benzophenone, aqueous dimethyl amine (33%), LiBr, CO2, CS2 were all AR grade purity, and were used straight as received without any treatment. Commercial eugenol and cardanol are industrial grades. Silica gel is CP grade. The above reagents were used directly as received from Shanghai Aladdin Corporation Ltd.

Characterization

Structures of the intermediates were characterized by 1H NMR and 13C NMR spectroscopy techniques on a Bruker AV 400 MHz spectrometer, with tetramethylsilane (TMS) as the internal standard. CDCl3 was used as the solvent in NMR measurement. The Bruker FTIR (V70) spectra were recorded through KBr pellet method. Melting point was measured by an XT-4 melting point meter. Gel permeation chromatography (GPC) was performed on an HP 1100 HPLC, equipped with a Waters 2414 refractive index detector and three Styragel HR 2, HR 4, HR 5 of 300 × 7.5 mm columns (packed with 5 mm particles of different pore sizes). The column packing allowed the separation of polymers over a wide molecular weight range of 500–1000,000. THF was used as the eluent at a flow rate of 1 mL/min at 40 °C. PMMA standards were used as the reference. Elemental analysis was conducted on a Thermo Scientific FLASH 2000 CHNS/O Elemental Analyzer. Hardness of polymer films was measured by standard pencils.

Synthesis of eugenyl acetate

Eugenol (16.4 g, 0.1 mol) was dissolved in acetic acid (15 mL). The solution was heated to 80 °C and kept at this temperature for about 2 h. When TLC indicated complete consumption of eugenol, the reaction mixture was cooled to room temperature, and then poured into an excess of ice-water. The resulting mixture was extracted with 3 × 25 mL of dichloromethane, and the organic phase was combined, washed twice with aqueous sodium bicarbonate, once with brine, and dried on anhydrous sodium sulfate. After filtration, the filtrate was condensed to remove solvent, and the residue was further purified on a silica gel column (petroleum ether/ethyl acetate = 10:1, v/v) to obtain the ester product as a light yellow liquid [17.8 g; 86%; boiling point 162–165 °C (30 mmHg); 1H NMR (400 MHz, CDCl3): δ 6.88–6.86 (m, 1H), 6.74–6.71 (m, 2H), 5.91–5.84 (m, 1H), 5.09–5.01 (m, 2H), 3.75 (s, 3H), 3.34–3.30 (m, 2H), 2.24 (s, 3H)].

Synthesis of epoxide of eugenyl acetate (1)

Eugenyl acetate (10.3 g, 50 mmol) was dissolved in dichloromethane (20 mL) with stirring, followed by adding dropwise m-CPBA (17.2 g, 75 mmol) in dichloromethane (180 mL). After addition, the reaction mixture was stirred at room temperature overnight. The reaction mixture was washed three times with 10% aqueous NaHSO3, then washed with saturated NaHCO3 and brine, respectively, and dried on anhydrous sodium sulfate. After filtration, the filtrate was distilled under reduced pressure to remove dichloromethane, and the residual crude product was purified by silica gel chromatography (eluent: petroleum ether/ethyl acetate = 5:1) to afford 1 as a white crystal [9.0 g; 81%; melting point 58–59 °C; 1H NMR (400 MHz, CDCl3): δ 6.99–6.95 (m, 1H), 6.87–6.83 (m, 2H), 3.84 (s, 3H), 3.17–3.14 (m, 1H), 2.86–2.84 (m, 2H), 2.83–2.81 (m, 1H), 2.58–2.53 (m, 1H), 2.32 (s, 3H)].

Synthesis of eugenol-derived diepoxide compound (2)

Epoxide of eugenyl acetate (1) (11.1 g, 50 mmol) was dissolved in epichlorohydrin (19.5 g, 0.2 mol). NaOH (4.0 g, 0.1 mol) dissolved in EtOH (20 mL) was added dropwise to the previous solution. The mixture was refluxed for 4 h. When the reaction was completed, excess EtOH and epichlorohydrin were distilled off under reduced pressure. To the residue were added dichloromethane and water, and the organic phase was separated, washed with brine, and dried on anhydrous sodium sulfate. After silica gel chromatography (eluent: petroleum ether/ethyl acetate = 5:1) a white crystal was obtained [7.4 g; 63%; melting point 55–57 °C (Ref. [32] 57.5 °C); 1H NMR (400 MHz, CDCl3): δ 6.90-6.85 (m, 1H), 6.81–6.76 (m, 2H), 4.27–4.20 (m, 1H), 4.07–3.99 (m, 1H), 3.88 (s, 3H), 3.41–3.37 (m, 1H), 3.17–3.11 (m, 1H), 2.92–2.88 (m, 1H), 2.83–2.78 (m, 3H), 2.76–2.73 (m, 1H), 2.57–2.52 (m, 1H)].

Synthesis of eugenol-derived dicyclic carbonate (3)

The diepoxide 2 (2.36 g, 10.0 mmol) and LiBr (43 mg, 0.5 mmol) were dissolved in NMP (10 mL), then CO2 (1 atm) was bubbled into the solution. The reaction mixutre was heated to 100 °C and kept at this temperature for 5 h. After completion of the reaction, the mixture was poured into ice-water, followed by extraction with ethyl acetate. The organic phase was washed again with water, and dried with anhydrous Na2SO4. The crude product was purified on a silica gel column (eluent: petroleum ether/ethyl acetate = 1:1) to give 3 as a white solid [2.26 g; 70%; melting point 71–73 °C; IR (KBr): ν 1802 cm−1. 1H NMR (400 MHz, CDCl3): δ 6.91–6.87 (m, 1H), 6.78–6.74 (m, 2H), 5.04–5.01 (m, 1H), 4.95–4.90 (m, 1H), 4.63–4.60 (m, 2H), 4.49–4.44 (m, 1H), 4.25–4.11 (m, 3H), 3.85 (s, 3H), 3.12–2.91 (m, 2H); 13C NMR (100 MHz, CDCl3): δ 154.5, 150.6, 147.1, 129.5, 122.1, 116.8, 114.0, 73.5, 68.5, 67.9, 66.1, 56.0, 39.3. Calcd for C15H16O8: C, 55.56; H, 4.97; Found: C, 55.58; H, 4.96].

Synthesis of eugenol-derived dicyclic dithiocarbonate (4)

The diepoxide 2 (2.36 g, 10.0 mmol) and LiBr (43 mg, 0.5 mmol) were dispersed in tetrahydrofuran (10 mL) and cooled in an ice bath. Carbon disulfide (2.4 mL, 29.8 mmol) was added dropwise into the reaction mixture within 2 h. After addition, the resulting mixture was stirred at room temperature for 24 h. Excess water was poured into the mixture and then extracted with ethyl acetate. After similar workup, product 4 was obtained as a light yellow solid [3.20 g; 79%; melting point 79–81 °C; IR (KBr, cm−1): ν 1031, 1181; 1H NMR (400 MHz, CDCl3): δ 6.91–6.88 (m, 1H), 6.78–6.73 (m, 2H), 5.49–5.44 (m, 1H), 5.30–5.26 (m, 1H), 4.34–4.32 (m, 2H), 3.87–3.94 (m, 4H), 3.78–3.73 (m, 1H), 3.58–3.51 (m, 1H), 3.45–3.40 (m, 1H), 3.17 (d, J = 44.2 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 212.0, 149.5, 147.16, 129.1, 121.6, 116.8, 114.0, 91.1, 88.3, 76.7, 67.8, 56.2, 38.9, 36.5; Calcd for C15H16O4S4: C, 46.37; H, 4.15; Found: C, 46.38; H, 4.16].

Synthesis of 2-(dimethylamino methyl) eugenol (5)

Eugenol (1.64 g, 10 mmol), aqueous dimethyl amine (4.1 mL), and ethanol (10 mL) were mixed in a flask. Paraformaldehyde (0.60 g, 20 mmol) in ethanol (10 mL) was added dropwise into the flask. The mixture was refluxed for 6 h, and then cooled to room temperature. Excess ethanol was removed by distillation, and the residue was dissolved in dichloromethane (20 mL). The solution was washed with water and brine, and dried on anhydrous Na2SO4. The crude product was purified on a silica gel column (eluent: petroleum ether/ethyl acetate = 5:1) to give 5 as a light brown oil [2.93 g; 82%; boiling point 151–154 °C (30 mmHg); 1H NMR (400 MHz, CDCl3): δ 6.62–6.60 (m, 1H), 6.41–6.37 (m, 1H), 5.92–5.86 (m, 1H), 5.28 (s, 1H), 5.06–4.98 (m, 2H), 3.84 (s, 3H), 3.59 (s, 2H), 3.28–3.24 (m, 2H), 2.30 (s, 6H); Calcd for C13H19NO2: C, 70.56; H, 8.65; N, 6.33; Found: C, 70.55; H, 8.63; N, 6.32].

Preparation of eugenol-based HNIPU-D

Dicyclic carbonate (4) (162 mg, 0.5 mmol) and 4,4′-diaminodiphenylmethane (0.1 g, 0.5 mmol) were dissolved in dimethyl sulfoxide (2 mL). Under nitrogen atmosphere, the solution was stirred at 100 °C for 48 h. The reaction mixture was cooled to room temperature, followed by adding excess cold methanol to form a precipitate. After filtration, the filtrate cake was collected and dried to give the product [0.2 g; 80%; IR (KBr, cm−1): ν 3594, 3286, 1802, 1594, 1118, 1049; and Mn 3700 Da (GPC relative to PMMA standards)].

Preparation of eugenol-based HNIPU-H

In a similar manner, polymerization of dicyclic carbonate (4) (162 mg, 0.5 mmol) and 1,6-hexanediamine (58 mg, 0.5 mmol) gave the polymer HNIPU-H [0.18 g; 83%; IR (KBr, cm−1): ν 3350 (br), 2943, 1796, 1693; and Mn 15,970 Da (GPC relative to PMMA standards)].

Preparation of eugenol-based HNIPU-X

In a similar manner, polymerization of dicyclic carbonate (4) (162 mg, 0.5 mmol) and p-xylene diamine (68 mg, 0.5 mmol) produced the polymer HNIPU-X [0.20 g; 87%; IR (KBr, cm−1): ν 3389 (br), 1769, 1632; and Mn 84,930 Da (GPC relative to PMMA standards)].

Preparation of eugenol-based PTU

The eugenol-derived dithiocarbonate (4) (0.202 g, 0.5 mmol) and 4,4′-diaminodiphenylmethane (0.1 g, 0.5 mmol) were dissolved in tetrahydrofuran (2 mL). The solution was stirred at ambient temperature for 48 h followed by addition of excess cold methanol. The precipitate was collected by suction filtration and then dried. A light brown solid (0.24 g; 80%) was obtained [IR (KBr, cm−1): ν 1035, 1100, 1164, 1295, 1404, 1455, 1519, 1606; and Mn 2800 g/mol (GPC relative to PMMA standards)].

Preparation of UV-crosslinked resin of eugenol-based PTU

The eugenol-based PTU (1.0 g), benzophenone (0.02 g), 2-(dimethylamino methyl) eugenol (5) (0.05 g), and 1,6-hexanediol diacrylate (0.05 g) were mixed and dissolved in tetrahydrofuran (5 mL). The solution was coated uniformly on a glass substrate, followed by UV (365 nm) irradiation for a certain period of time.

Preparation of UV-crosslinked resin of eugenol-based PTU and cardanol

In a similar manner as previous procedure, eugenol-based PTU and cardanol, together with other additives, were irradiated under UV (365 nm) for a certain period of time. UV-crosslinked PTU and cardanol were prepared similarly.

Chemical resistance of UV-crosslinked resins

A sheet of UV-crosslinked resin (ca. 0.1 g) was immersed in 2 mL of THF and kept for one week. The dried sample, and the mass changes were determined before and after immersion.

Results and discussion

Synthesis of eugenol-derived diepoxide 2

There is an allylic group in the molecule of eugenol, which allows the synthesis of diepoxide compound through epoxidation reactions (Scheme 1). We have tried to design a two-step route to compound 2, i.e., substitution with epichlorohydrin followed by epoxidation using m-CPBA as the oxidant. However, the complex brown mixtures obtained were difficult to purify in the epoxidation reaction step. Thus, a three-step route was used to synthesize 2 with a 43.9% total yield. 1H NMR spectra of compounds from eugenol to compound 2 are indicated in Fig. 1. Disappearance of peaks at 5–6 ppm indicates that the carbon–carbon double bond in the side chain of eugenol has been converted to epoxide group. Disappearance of the single peak at about 2.3 ppm and appearance of peaks at 4.0–4.3 ppm demonstrate the removal of acetyl protecting group and formation of an ether between eugenol and epichlorohydrin.

Scheme 1
scheme 1

Three-step synthesis of eugenol-derived diepoxide 2

Full size image
Fig. 1
figure 1

1H NMR spectra of compounds from eugenol to 2

Full size image

Synthesis of eugenol-derived dicyclic carbonate 3 and dicyclic thiocarbonate 4

There are several routes to the synthesis of cyclic carbonates, but the most promising one is the way from selective addition between epoxides and CO2 [5, 36,37,38,39]. Various catalysts from inorganic, organic to complex structures are designed and studied for preparation of these compounds [40,41,42]. According to literature, we think that the most convenient method for preparation of cyclic carbonates is to use metallic halides such as LiBr under atmospheric pressure of CO2 [43]. Thereafter, the diepoxide 2 under CO2 atmospheric pressure together with the catalyst LiBr were heated to 100 °C for hours to afford the corresponding dicyclic carbonate 3 in moderate yield (Scheme 2). 1H NMR spectrum of the dicyclic carbonate 3 is shown in Fig. 2. The singlet peak (a) at 3.8 ppm is the signal of methoxy group. Two multiplet peaks (b) at 6.5–7.0 ppm represent three protons of aryl ring. Two multiplet peaks (c and g) at 4.8 and 5.0 ppm mean two protons (2CH-O) of cyclic carbonates. A doublet-like peak (d) at 4.65 ppm represents two protons of methylene of the ether. The multiplet peaks (f) at 4.0–4.5 ppm may correspond to methylene protons (2CH 2O) of cyclic carbonates. The multiplet peak (e) at 3.0 ppm is methylene protons (ArCH 2-) of benzyl group. Note that weak peaks at 0.5–2 ppm may be solvent and water residues which have no negative effects on the subsequent polymerization.

Scheme 2
scheme 2

Synthesis of eugenol-derived dicyclic carbonate 3 and thiocarbonate 4

Full size image
Fig. 2
figure 2

1H NMR spectrum of eugenol-derived dicyclic carbonate 3

Full size image

In addition to CO2, the molecule CS2 can also undergo addition reactions with epoxide groups under certain catalytic conditions, leading to cyclic dithiocarbonates. The mixture of substrate 2, CS2 and LiBr in THF was stirred at room temperature for 24 h, the desired cyclic dithiocarbonate 4 was easily obtained as a viscous yellow solid (Scheme 2). 1H NMR spectrum of 4 is shown in Fig. 3. Main peaks of this spectrum are similar to those of carbonate 3. The chemical shift of peaks (f) is at 3.3–4.0 ppm, shifting to higher field compared to 4.0–4.5 ppm for carbonate 3. This high-field shift for thiocarbonate 4 can be explained as follows: the methylene group in carbonate 3 is bonded to oxygen (i.e., CH 2-O), while that in 4 it is bonded to sulfur (i.e., CH 2-S). It is known that oxygen is more negative than sulfur. Thus, the weak inductive effect of sulfur causes a higher electron density around protons. A peak at 212 ppm in the 13C NMR spectrum of 4 is a powerful proof of the existence of carbon sulfur double bond (C=S) (refers to supporting information).

Fig. 3
figure 3

1H NMR spectrum of eugenol-derived dicyclic thiocarbonate 4

Full size image

Preparation of eugenol-based HNIPUs and PTU

Generally, esters can be aminolyzed to form the corresponding amides due to strong nucleophilicity of amino groups. Thus, carbamate groups may be obtained by aminolysis of cyclic carbonates. As a matter of fact, this strategy has been widely studied and used to prepare polyurethanes in recent years. In our reaction system, dicyclic carbonate 3 and 4,4′-diamino diphenylmethane were polymerized to form eugenol-based HNIPU (labeled as HNIPU-D to differentiate it from those made by aliphatic amines) (Scheme 3). Number-average molecular weight of the HNIPU-D is 3700 Da (measured by GPC relative to a PMMA standard). Structures of the HNIPU-D as well as dicyclic carbonate 3 were characterized by IR spectra (Fig. 4). In spectrum (a), a peak at 1802 cm−1 corresponds to the stretching vibration of carbonyl group of carbonate, while this peak almost disappears in spectrum (b), indicating that cyclic carbonate groups are transformed into other groups. The fact that a new peak at 1594 cm−1 appears in spectrum (b) may imply the formation of carbamate groups. Furthermore, a broad peak at 3200–3500 cm−1 in spectrum (b) suggests the existence of hydroxyl (-OH) and/or amino groups in the HNIPU-D.

Scheme 3
scheme 3

Synthesis of eugenol-based HNIPU-D

Full size image
Fig. 4
figure 4

IR spectra of (a) dicyclic carbonate (3); and (b) eugenol-based HNIPU

Full size image

In addition, aliphatic diamines 1,6-hexamethylene diamine and p-xylene diamine were reacted with dicyclic carbonate 3, and the corresponding polymers HNIPU-H and HNIPU-X were characterized by FTIR (Figs. 5 and 6). The appearance of broad peaks at 3200–3600 cm−1 indicated that hydroxyl (-OH) group was formed by the corresponding reactions. The number-average molecular weights (Mn) of HNIPU-H and HNIPU-X were found as 15,970 and 84,930 Da, respectively (measured by GPC relative to a PMMA standard). By contrast, the order of Mn for three HNIPUs is Mn, HNIPU-X > Mn, HNIPU-H > Mn, HNIPU-D. These results may suggest that aliphatic amines are more reactive than aryl amines in the ring-opening polymerization reactions.

Fig. 5
figure 5

IR spectra of (a) mixture of dicyclic carbonate (3) and hexamethylene diamine; and (b) eugenol-based HNIPU-H

Full size image
Fig. 6
figure 6

IR spectra of (a) mixture of dicyclic carbonate (3) and p-xylene diamine; and (b) eugenol-based HNIPU-X

Full size image

Similarly, cyclic dithiocarbonates can undergo ring-opening reactions under nucleophilic attack of amines (Scheme 4). When the cyclic dithiocarbonate 4 and 4,4′-diamino diphenylmethane were mixed in THF, the reaction proceeded under very mild conditions, and nearly 85% of monomer conversion was obtained after 72 h. Number-average molecular weight of the prepared PTU was Mn 2800 Da (measured by GPC relative to a PMMA standard). According to the reaction conditions, ring-opening of the cyclic thiocarbonate 4 was much easier than that of cyclic carbonate 3. This may be attributed to that carbon oxygen single bond (C–O, bond energy 351 kJ.mol−1) which was much stronger than carbon sulfur single bond (C–S, bond energy 272 kJ.mol−1). Note that when we try to measure T g of the prepared PTU, no inflexion point was found.

Scheme 4
scheme 4

Synthesis of eugenol-based PTU

Full size image

Preparation and characterization of UV-crosslinked eugenol-based PTU

Number-average molecular weights (Mn) of our eugenol-based HNIPU-D and PTU are 2500–4000 Da, which are more like oligomers rather than polymers. These oligomers cannot be used straight as polymer materials due to their small Mn values. Photo-curing technology is extensively researched and applied in producing photo-cured thermoset resins [44,45,46]. Thiol-ene click reaction has become a powerful tool to prepare various polymers as well as small molecules in recent years [47,48,49]. In eugenol-based PTU, there is a mercapto group in each unit. Thus, if a proper diene or polyene compound is added, the two components may be crosslinked under thiol-ene reaction conditions. Thus, as a preliminary experiment, the eugenol-based PTU, dilution promoter 1,6-hexanediol diacrylate, photoinitiator benzophenone, and co-photoinitiator 2-(dimethylamino methyl)eugenol (5) are dissolved in THF. The solution is then coated on a glass substrate to form a liquid-like membrane that is subsequently irradiated by UV light (365 nm). After 10 min irradiation, the resulting yellow film becomes finger-touch dry and has the pencile hardness of H (Entry 3, Table 1). Furthermore, it is difficult for the film sample to dissolve in THF. These changes prove that thiol-ene crosslinking reactions do occur under UV irradiation. Note that the film of sole eugenol-based PTU without any additive is bright yellow, fully soluble in THF, and has hardness of 2B (Entry 1, Table 1). Even if no dilution promoter is used, the properties of eugenol-based PTU also change after UV irradiation (Entry 2, Table 1). These changes may be ascribed to possible mercapto group oxidation and/or other radical reactions under UV light. Cardanol, a main component of natural cashew nut shell liquid (CNSL), is a liquid phenol derivative with a C15 linear carbon chain at meta-site [50]. This renewable compound can also be used as a dilution promoter in preparing UV-cured resins [26]. When cardanol is used as the dilution promoter instead of 1,6-hexanediol diacrylate, the UV-induced crosslinking reactions can occur, but reaction rate is much slower than with 1,6-hexanediol diacrylate. This phenomenon can be explained as follows: 1,6-hexanediol diacrylate is a good commercial dilution promoter, whose carbon–carbon double bonds are very reactive towards thiol-ene reactions; while in cardanol, carbon–carbon double bonds in side chains are common and not so reactive. Therefore, cardanol and 1,6-hexanediol diacrylate are combined together as dilution promoters, and 10 min of irradiation affords a film with 2H of hardness (Entry 5, Table 1). The UV-crosslinking reactions are illustrated in Scheme 5. When the irradiation time is extended to 30 min, hardness of the film rises to 4H. Further extending irradiation time to 60 min cannot improve hardness, which means that 30 min is enough for full crosslinking. Unfavorably, long irradiation time such as 60 min will make film color become deep brown or even black.

Table 1 Properties of eugenol-derived PTU and UV-crosslinked resins a
Full size table
Scheme 5
scheme 5

UV-induced crosslinking reactions of eugenol-based PTU

Full size image

Conclusion

Hydroxyl non-isocyanate polyurethanes (HNIPUs) and mercapto non-isocyanate polythiourethane (PTU) are successfully prepared by using renewable eugenol as the starting material. When the eugenol-derived diepoxide intermediate reacts with atmospheric pressure CO2, the corresponding dicyclic carbonate is obtained, which is subsequently converted to HNIPUs by ring-opening reactions with aryl and aliphatic diamines. Number-average molecular weights (Mn) of the HNIPUs are Mn HNIPU-D 3700 Da, Mn,HNIPU-H 15,970 Da, and MnHNIPU-X 849,300 Da. If CO2 is replaced by CS2 in the cyclic carbonate-forming reactions, the corresponding dicyclic dithiocarbonate is obtained. The dithiocarbonate intermediate further reacts with diamine to afford mercapto-containing non-isocyanate polythiourethane (PTU). Mn value of the PTU is 2800 Da. To improve its potential application as a material, the oligomer-like PTU further undergoes UV light-promoted thiol-ene crosslinking reactions with cardanol. The optimized conditions for UV-crosslinking are mass ratio of PTU/cardanol/1,6-hexanediol acrylate = 1:1:0.05, and UV irradiation time 30 min. The brown crosslinked polymer prepared under optimized conditions shows certain resistance to THF and has pencil hardness of 4H.