Introduction

Biomass-based renewable materials have attracted considerable attention as a source of green plastics.1, 2, 3, 4, 5 Lignin is a waste biomass that has three-dimensional network structures containing aromatic rings, and its conversion into versatile soluble monomers has been requested. In a previous study, we succeeded in the metabolic conversion of lignin into 2-pyrone-4,6-dicarboxylic acid (PDC) on a large scale via protocatechuate by a transformed bacterium.6 PDC consists of a polar pseudo-aromatic ring and two carboxylic acids, suggesting the potential use as a bifunctional monomer for polycondensation and polyaddition. As PDC was not accessible from petrochemicals, its chemical reactivity and physical properties were comprehensively investigated.7, 8, 9 During the course of the synthetic studies of PDC polymers, it was found that PDC is unstable under basic conditions. Thus, all the PDC polymers reported so far were synthesized by base-free reactions. In the past few years, a series of PDC polyesters was synthesized by the direct dehydrated polycondensation with various diol co-monomers, and their excellent biodegradable, mechanical and adhesive properties were demonstrated.10, 11, 12, 13, 14, 15, 16 However, the major problem of the PDC polyesters was their relatively low molecular weights. In order to improve the polymer properties, other efficient base-free polymerization methods were desired.

A CuI-catalyzed azide-alkyne cycloaddition (CuAAC), known as a representative click chemistry reaction, became an important polymerization method for achieving high-molecular-weight polymers.17, 18, 19, 20, 21, 22 Owing to the chemical orthogonality, a wide variety of functional substances was polymerized under mild conditions. Although the reaction usually requires basic amines as a ligand of the Cu catalysts, there are also some reports of a ligand-free CuAAC.23, 24 In addition, the powerful metal-adhesive features of the poly(triazole) products were previously demonstrated.25, 26, 27 As the biomass-based PDC polymers essentially show strong adhesive properties, it is reasonable to pursue the novel PDC polymers by the ligand-free CuAAC. In other words, the purpose of this work is to investigate the adhesive properties of the PDC polymers prepared by the ligand-free CuAAC. In this paper, we report for the first time the click polymerization of a PDC monomer, which was analyzed by 1H NMR and IR spectroscopies, as well as its gel permeation chromatography (GPC) profiles. (For a preliminary communication of this work, see Michinobu et al.28) The adhesive properties of the in-situ prepared PDC polymers with various metal surfaces are also reported.

Experimental procedure

Materials

Chemicals and dehydrated solvents were purchased from Kanto (Tokyo, Japan), Tokyo Kasei (Tokyo, Japan) and Wako (Osaka, Japan), and used as received. 1,2-Bis(2-azidoethoxy)ethane,29 and 2,2-bis(azidomethyl)propane-1,3-diol,30 diprop-2-ynyl isophthalate 231 were prepared according to the literature method.

Measurements

1H NMR and 13C NMR spectra were measured on a JEOL (Tokyo, Japan) model AL300 spectrometer at 20 °C. Chemical shifts are reported in p.p.m. downfield from SiMe4, using the solvent's residual signal as an internal reference. The resonance multiplicity is described as s (singlet), br s (broad singlet), d (doublet) and m (multiplet). Infrared spectra were recorded on a JASCO (Tokyo, Japan) FT/IR-4100 spectrometer. Gel permeation chromatography was measured on a JASCO system equipped with a HITACHI L-4000 UV detector (275 nm), a Spectra-Physics SP8800 ternary HPLC pump, and polystyrene gel columns (Shodex Asahipak GF-310 HQ, GF-410 HQ, and GF-710 HQ with the exclusion limit molecular weight of 40 000, 300 000 and 10 000 000, respectively) using dimethylformamide (DMF) as an eluent at a flow rate of 0.5 ml min−1 after calibration with standard polystyrenes (TOSOH (Tokyo, Japan) TSK standard polystyrene). Thermogravimetric analyses were carried out on a Rigaku (Tokyo, Japan) Thermo plus TG8120 under nitrogen flow at the heating rate of 10 °C min−1 from 20 to 500 °C. X-ray photoelectron spectroscopy measurement was performed using a JASCO JPS-9010TR.

Tensile strength measurements were performed according to JIS K 6849-1994 testing method. The size and length of metal plate edges (brass, Fe, Cu) for adhesion measurements were 5.0 × 28.0 mm2 and 50 mm, respectively. The edge plane surfaces were smoothened by a rotating disk diamond cutter or a horizontal milling machine, followed by polishing with diamond sands/oil or alumina powders/oil combinations. The surface coarseness of <0.6 μm was confirmed by surface profilometry using a Toyo Technica (Tokyo, Japan) Dek-Tak IIA. Then, the plates were washed under sonication with a series of solvents, such as 1,2-dichloroethane, methanol and water. A solution of 1/2 and 2,2-bis(azidomethyl)propane-1,3-diol was cast on the metal plate surfaces. A set of two plates was in contact with each other and cured at 70 °C for 4 h or at 100 °C for 1 h. The adhesion strengths of the cured films were measured at 21±2 °C under dry nitrogen using a Tensilon testing machine (Auto COM/AC-50A, TS Engineering (Kenosha, WI, USA)).

Synthesis

Diprop-2-ynyl 2-oxo-2H-pyran-4,6-dicarboxylate (1)

To a solution of propargyl alcohol (0.706 ml, 12.0 mmol) and N,N-dimethylaminopyridine (133 mg, 1.09 mmol) in 1,2-dimethoxyethane (100 ml), PDC (1.00 g, 5.43 mmol) was added under nitrogen. After cooling to 0 °C, dicyclohexylcarbodiimide (2.69 g, 13.0 mmol) was added, and then warmed to 20 °C and stirred for 5 h. The precipitates were filtered off and the filtrate was washed with water and dried over Na2SO4. Evaporation of the solvents and column chromatography (SiO2, acetone/hexane 1:1) afforded 1 (1.02 g, 72%) as a white solid.

1H NMR (300 MHz, DMSO-d6, 293 K) δ 3.72 (s, 2H), 4.99–5.01 (m, 4 H), 7.14 (d, J=3 Hz, 1 H), 7.28 p.p.m. (d, J=3 Hz, 1 H); 13C NMR (75 MHz, DMSO-d6, 293 K) δ 53.90, 54.09, 77.40, 78.94, 108.24, 122.82, 141.94, 148.12, 157.91, 159.24, 161.61 p.p.m.; IR (neat): ν 3272, 3087, 2122, 1752, 1733, 1716, 1559, 1440, 1418, 1378, 1340, 1275, 1249, 1178, 1129, 1092, 1019, 981, 962, 931, 891, 778, 763, 721, 690, 668 cm−1.

(P1)

To a solution of 1,2-bis(2-azidoethoxy)ethane (387 mg, 1.92 mmol) and copper(I) bromide (50 mol%, 276 mg, 1.92 mmol) in dehydrated DMF (10 ml), 1 (500 mg, 1.92 mmol) was added at 20 °C. After stirring for 70 h, the reaction mixture was filtered off and the filtrate was evaporated to be approximately 2–3 ml. Precipitation into CH3OH (300 ml) afforded the target polymer P1, which was filtered off, thoroughly washed with CH3OH and then dried in vacuo (621 mg, 70%).

1H NMR (300 MHz, DMSO-d6, 293 K) δ 3.74 (s, 4n H), 4.48 (br s, 8n H), 5.38 (br s, 4n H), 7.03 (br s, n H), 7.17 (br s, n H), 8.20 p.p.m. (br s, 2n H); IR (neat): ν 2924, 2876, 1734, 1642, 1559, 1436, 1395, 1351, 1314, 1253, 1124, 959, 826, 761, 710 cm−1.

(P2)

To a solution of 2,2-bis(azidomethyl)propane-1,3-diol (180 mg, 0.967 mmol) and copper(I) iodide (13 mg, 0.068 mmol) in dehydrated DMF (3 ml), 1 (252 mg, 0.967 mmol) was added at 20 °C. After stirring for 43 h, the reaction mixture was filtered off and the filtrate was evaporated to be approximately 2–3 ml. Precipitation into CH3CN (60 ml) afforded the target polymer P2, which was filtered off, thoroughly washed with acetone, THF and CHCl3 and then dried in vacuo (314 mg, 73%).

1H NMR (300 MHz, DMSO-d6, 293 K) δ 3.08–3.40 (m, 8n H), 4.21–4.38 (m, 2n H), 5.01–5.40 (m, 4n H), 6.97–7.23 (m, 2n H), 8.19 p.p.m. (s, 2n H); IR (neat): ν 3420, 2934, 2883, 2108 (w), 1736, 1654, 1560, 1440, 1390, 1328, 1247, 1171, 1115, 1050, 959, 876, 761, 665 cm−1.

Results and Discussion

Polymer synthesis and characterization

The transformation of the carboxylic-acid moieties of PDC into either terminal alkynes or azide functions was attempted. It was found that the common condensing agents, such as dicyclohexylcarbodiimide and N,N-dimethylaminopyridine, were effective for the esterification of the carboxylic-acid moieties of PDC. Thus, 2 equivalent of propargyl alcohol was reacted with PDC, providing 1 in 72% yield (Scheme 1). The dipropargyl PDC 1 was stable in neutral pH, but the ethynyl moieties and the pyrone ring were unstable in the acidic and basic pH regions, respectively. We also tried to prepare a diazide-functionalized PDC monomer as a complementary monomer for the click polymerization with 1. However, the esterification of the PDC carboxylates with 2-bromoethanol followed by the reaction with sodium azide gave an unidentified product, probably due to the intramolecular side reaction between the pyrone ring and azide moieties.

The click polymerization of 1 was carried out at 20 °C in DMF with an equimolar amount of the diazide co-monomers, 1,2-bis(2-azidoethoxy)ethane or 2,2-bis(azidomethyl)propane-1,3-diol, in the presence of the CuI catalyst. It is generally known that the conventional click polymerization using CuAAC proceeds very smoothly. In contrast, a partial polymerization progress could be monitored by the GPC profiles in this study, because the reaction rate became slower because of the absence of basic amino ligands. As shown in Figure 1, there were mainly three fractions after the polymerization for 15 h, that is, a small-molecular-weight fraction with the molecular weight of <104, a moderate-molecular-weight fraction with very sharp peaks at approximately 105, and a very large-molecular-weight fraction with the molecular weight of approximately 106. The presence of the large-molecular-weight fraction suggested that the polymerization rate was basically fast. When the polymerization time was prolonged to 27 h, the small-molecular-weight fractions almost disappeared, whereas the moderate-molecular-weight fractions still existed. The polymerization time was further prolonged to 70 h, resulting in the saturation of the molecular weight increase in the large-molecular-weight fractions. This saturation was most likely caused by a significant increase in the solution viscosity or deposition from the polymerization solution. Therefore, the polymerization was terminated and the products were purified by completely washing with methanol.

Figure 1
figure 1

Gel permeation chromatography (GPC) profiles (eluent: dimethylformamide (DMF)) of P1 for the polymerization time of (a) 15, (b) 27 and (c) 70 h.

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The polymerization was also monitored by the IR spectra of the powder samples. In the IR spectrum of the 15-h polymerization product, the azide peak at 2109 cm−1 still remained, whereas the peaks at 3272 and 2122 cm−1, ascribed to the C-H and C≡C vibrations of the terminal alkyne moieties, respectively, were almost negligible (Figure 2). In accordance with the GPC profiles, when the polymerization time was prolonged, the azide peak completely disappeared, indicating the efficient conversion into very large-molecular-weight products.

Figure 2
figure 2

Infra red spectra (KBr pellet) of P1 for the polymerization time of (a) 15, (b) 27 and (c) 70 h.

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On the basis of these results, the high-molecular-weight polymers P1 and P2 were prepared at 20 °C under base-free conditions. The molecular weights of the resulting linear polymers, determined by GPC using DMF as an eluent, exceeded 1 000 000 (Table 1). Longer polymerization time tended to produce the higher-molecular-weight polymers, whereas an increase in the catalyst amount from 7 to 50 mol% did not improve the molecular weights, suggesting that a catalytic amount of CuI was sufficient to achieve high-molecular-weight polymers. The problem with increasing the CuI amount was the difficulty in perfectly removing the Cu ions because of the presumed interaction of CuII with the formed triazole products.32 The residual Cu was indeed detected when the X-ray photoelectron spectroscopy measurement of P1, prepared in the presence of 50 mol% CuI, was performed (Supplementary Figure S1). Although these remaining Cu ions were thought to interact with the triazole moieties, they did not significantly affect the GPC profiles. In contrast, the chemical structure of the di-azide co-monomers affected the polymerization results. Taking into account the molecular weights of the high-molecular-weight fractions obtained under the same conditions (7 mol% of CuI catalyst, 20 °C, 43 h), 1,2-bis(2-azidoethoxy)ethane was a better co-monomer compared with 2,2,-bis(azidomethyl)propane-1,3-diol. Despite the high molecular weight exceeding 1 000 000, these polymers were soluble in DMF and DMSO. In addition, P1 was sparsely soluble in THF. Thus, the 1H NMR spectra of P1 and P2 were measured in DMSO-d6 at 20 °C. The PDC protons and triazole proton were detected in the desired aromatic region (Figure 3). Owing to the asymmetric chemical structure of PDC, multiple peaks were observed for some of these protons. The aliphatic protons were also clearly detected, suggesting the occurrence of the copolymerization.

Table 1 Polymerization results of dipropargyl-PDCa
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Figure 3
figure 3

1H NMR spectra of (a) P1 and (b) P2 in DMSO-d6 at 20 °C.

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Thermal properties

The thermal properties of the PDC polymers were investigated by a thermogravimetric analysis at the heating rate of 10 °C min−1 under a nitrogen atmosphere. Although most of the reported PDC polyesters showed a single-step decomposition with the onset decomposition temperature of >200 °C,10, 11, 12, 13 the decomposition of P1 and P2 proceeded in two or three steps with the onset decomposition temperature of 190 and 160 °C, respectively, (Figure 4). The decreased thermal stabilities of P1 and P2 were probably due to the co-monomer structures. However, the residual soot amounts of P1 and P2 at 500 °C were 55 and 35%, respectively, which were much greater than the previous PDC polyesters. Thus, it is suggested that an increase in the nitrogen content most likely leads to a higher soot amount from the thermal decomposition process. Note that the possibility of the residual Cu contamination was not ruled out. Differential scanning calorimetry measurements of P1 and P2 did not show any transitions in the temperature range from −50 to 150 °C.

Figure 4
figure 4

Thermogravimetric analyses curves of P1 and P2 at the heating rate of 10 °C min−1 under flowing nitrogen.

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Adhesive properties

It was reported that the PDC polyesters show strong adhesive properties toward metal and glass surfaces.11, 15 In addition, the first noticeable application of CuAAC in materials science was the specific adhesion of poly(triazole) derivatives to a Cu surface, which was achieved by the partial crosslinking of the polymer chains with a small amount of the leached CuI.25

As P1 and P2 contain both PDC and triazole moieties, they are promising materials for adhesive applications. We focused on the adhesive properties of P2 to various metal surfaces, because the PDC and triazole contents of P2 are greater than those of P1. After a solution of an equimolar amount of 1 and 2,2-bis(azidomethyl)propane-1,3-diol was cast on the metal surface, they were in-situ polymerized at 70 °C for 4 h or at 100 °C for 1 h. The polymerization conditions were determined so that no thermal decomposition occurred. When Cu was used as a metal, a small amount of the leached CuI was expected to serve as a catalyst to promote the polymerization and regioregularity control. Note that the products are basically a mixture of 1,4-triazole and 1,5-triazole derivatives on other metal surfaces.24 Tensile strength measurements were performed against brass, Fe and Cu plates, according to the JIS K 6849-1994 testing method. For all the three metal plates, P2 prepared at 70 °C for 4 h showed higher tensile strengths than that prepared at 100 °C for 1 h (Figure 5 and Table 2). In addition, a slightly better elongation occurred under the former conditions. Among the three metals, P2 displayed the strongest adhesion to Cu, probably due to the catalytic effect and/or the crosslinking through the multivalent coordination to the triazole rings. Thus, the maximum tensile strengths to brass, Fe and Cu plates were 2.89, 3.16 and 3.70 MPa, respectively. The previously reported PDC polyesters displayed a stronger adhesion to brass and Fe plates than to Cu plates.11 This interesting inversion is apparently caused by the introduction of the click-formed triazole rings. Moreover, the effect of the PDC ring was also elucidated using diprop-2-ynyl isophthalate 2, in place of 1 (Scheme 2). The tensile strength measurements of the cured films prepared from 2 and 2,2-bis(azidomethyl)propane-1,3-diol were attempted. However, these films were too fragile to be measured, although numerous combinations of curing conditions and various metals were tested. These results clearly suggested the important role of the pyrone ring to generate the specific adhesion. A plausible adhesion mechanism is based on the carbonyl group coordination to the metal atoms, accompanying the pyrone ring opening.11

Figure 5
figure 5

Adhesion properties of the in-situ prepared P2 with brass, Fe and copper (Cu) under different conditions (at 70 °C for 4 h or at 100 °C for 1 h).

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Table 2 Adhesion conditions and tensile strength (MPa) of P2 by JIS K 6849-1994 testing
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Conclusion

Very high-molecular-weight PDC polymers with well-defined chemical structures were for the first time synthesized by the ligand-free CuAAC. As the PDC polymers were soluble in DMF and DMSO and sparsely soluble in THF, they were fully characterized. Investigations of the adhesive properties to various metals revealed that the pyrone ring is essential for better adhesion, and the triazole rings show specific interactions to Cu. Thus, the maximum tensile strength to Cu plates was 3.70 MPa. Further optimization of the chemical structures, for example, multi-functionalized monomers for efficient crosslinking, is expected to improve the adhesive properties.

scheme 1

Click polymerization of dipropargyl-PDC. (a) Propargyl alcohol, dicyclohexylcarbodiimide, N,N-dimethylaminopyridine (DMAP), 0–20 °C, 72%; (b) CuBr, dimethylformamide (DMF), 20 °C.

scheme 2

Chemical structure of 2.