Electrochemical manipulation of adhesion strength of polybenzoxazines on metal surfaces: from strong adhesion to dismantling

Abstract

A novel electrochemical redox process for the manipulation of adhesion of polybenzoxazine thermosets on metal surfaces is reported. The method pertains to the electrochemically driven hydroquinone–quinone redox couple. Several antraquinone based bisbenzoxazines possessing phenyl, benzyl and methyl furfuryl substituents were synthesized and characterized. The antraquinone bisbenzoxazines were shown to readily undergo thermally activated ring-opening polymerization in the absence of a catalyst forming cross-linked networks on metal surfaces. The substituent effect on thermal curing behaviour and thermal stability of the cured polymers were investigated. The strong binding affinities of phenolic hydroxyl groups of the anthraquinone moiety in the cured polymers promote adhesion on the metal surface. Electrochemical oxidation converts hydroquinone groups into quinone moieties resulting in the dismantling of the coated films. The generality of this electrochemical method is demonstrated by initial results on platinum electrodes as well as steel plates.

---

Introduction

The history of benzoxazine chemistry dates back to the 1940s with the first synthesis of benzoxazines by Burke et al.¹ The polymerizability of these interesting structures was discovered in 1970s.^2,3 However, interest in this chemistry started in the 1990s after the seminal papers of Ishida et al. showing attractive properties of polybenzoxazines as a candidate for high performance thermosetting materials.^4,5 These polymers possess a number of outstanding characteristics such as, no requirement for harsh catalyst, long shelf lives, no or limited generation of byproducts during curing, low water uptake, almost zero shrinkage in the course of curing reaction.^6,7 Recent synthetic advances provide promising strategies to combine this chemistry with classical polymers.⁸ Many different end chain,^9–11 main chain^12–18 or side chain^19–21 polybenzoxazine prepolymers were synthesized by various research groups overcoming problems associated with the processability of brittle polybenzoxazines. Blending benzoxazines with polymers or fillers and fibers was also proposed as an alternative route to reach desired ultimate performances.^22–35 Independently from the above strategies employed, monomer design approach has been repetitively used to incorporate desired functionalities to the end structure of polybenzoxazine thermosettings, since their molecular structure provides immense design flexibility by means of using various phenolics, primary amines and formaldehyde (Scheme 1).^36–41

Scheme 1 Synthesis of mono functional benzoxazines.

Apart from straightforward synthesis, another striking feature of benzoxazines is their undemanding polymerizability in the absence of a catalyst at temperatures between ca. 160–250 °C.^5,42–44 Depending on the specific groups on the monomer, even much lower polymerization temperatures were reported.^45–47 Polymerization occurs through heterocyclic ring-opening reaction yielding a network possessing phenolic groups and amine bridges in the structure (Scheme 2).^48–50 It is known that polymers involving hydroquinone–quinone or similar redox couples have diverse applications ranging from electrical conductors, batteries, electrode coatings for sensors, catalysts for electrochemical reactions, antioxidants to reaction inhibitors.⁵¹ When incorporated into benzoxazines, the redox cycle between quinone–hydroquinone couples would make these compounds particularly useful as electroactive thermosets. Some properties of the polybenzoxazines can, thus, be altered in a cyclic manner between two distinct points by electrochemical stimulus.

Scheme 2 Thermally induced ring-opening polymerization of a bisbenzoxazine monomer.

The adhesion of polybenzoxazines onto steel surfaces is acquired by the attractions between phenolic hydroxyls and iron atoms.^33,52 Quinone–hydroquinone conversions can affect the hydroxyl content of polybenzoxazines and as a consequence, the adhesive property can dramatically be changed between these two redox points.

Herein, we describe our efforts to synthesize benzoxazine monomers possessing anthraquinone groups in the structure to generate corresponding polybenzoxazine networks by thermally activated ring opening polymerization capable of changing adhesion property by external electrochemical stimulation.

Experimental

Materials

4,4-Isopropylidenediphenol (Acros, 97%), benzylamine (Merck), furfurylamine (Fluka, ≥99%), paraformaldehyde (Acros, 96%), sodium hydroxide (Acros, >97%), 1,4-dioxane (Aldrich, 99%), 1,5 dihyroxyanthraquinone tech. (Alfa Aesar, 90%) and chloroform (Merck, 90–99%), ethanol (Aldrich), toluene (AnalaR Normapur) were used as received. Aniline was vacuum distilled before use.

Methods

¹H NMR spectra of all samples were recorded in chloroform, using an Agilent VNMRS 500 MHz instrument. The FTIR spectra were recorded at Perkin Elmer Spectrum One with ATR accessory (ZnSe, Pike Miracle Accessory) and cadmium telluride (MCT) detector. DSC measurements were performed on Perkin-Elmer Diamond DSC with a heating rate of 15 °C min⁻¹ under nitrogen flow. TGA measurements were performed on Perkin-Elmer Diamond TA/TGA with a heating rate of 10 °C min⁻¹ under nitrogen flow (200 mL min⁻¹). Cyclic voltammetry and electrochemical studies: electrochemical properties of corresponding monomers and polymers were investigated using a Voltalab 50 Potentiostat and in a three electrode cell consisting of platinum wires as the working and counter electrodes, and a Ag wire as the reference electrode.

Single scan cyclic voltammogram of monomers and polymers were recorded in 0.1 M tetrabutylammonium hexafluorophosphate (Bu₄N⁺PF₆⁻)/acetonitrile (ACN) solution. For this experiment, Pt foil electrode was used as the working electrode. Monomers were dissolved in CHCl₃ and drop casted on Pt electrode. Resulting CVs were recorded in Bu₄N⁺PF₆⁻/ACN electrolyte system. Then, to explore the redox behaviour of polymers, freshly prepared Pt electrodes stored in the oven for 3 hours at 180 °C. Single scan CVs of resulting polymers were also investigated with the same conditions used for the monomers. (Note: abbreviations of corresponding polymers are as; PB-Ant-A for aniline, PB-Ant-Bn for benzylamine, PB-Ant-F for furfurylamine based polybenzoxazines).

¹³C CPMAS NMR spectrum was collected using an Inova 500 MHz NMR Varian system fitted with a Jacobsen brand CP-MAS probe. ¹³C CPMAS spectra were acquired at 125.654 MHz using Si₃N₄ rotors set to 6 Hz. Pulses were separated by a 1 s delay.

General procedure for the synthesis of anthraquinone based benzoxazine monomers

1,5-Dihydroxyanthraquinone (8.32 mmol, 2 g), aniline (0.01 mol, 5.5 g), paraformaldehyde (0.03 mol, 1 g) were dissolved in a mixture of ethanol–toluene (1 : 2, v/v, 150 mL) in a 250 mL round bottom flask and refluxed overnight. The reaction mixture was filtered and mixture of solvents was evaporated under vacuum. Resulting product was dissolved in diethylether and washed with 0.1 N NaOH aqueous solutions and distilled water, for many times. Then, the solution was dried with anhydrous magnesium sulfate, and solvents were removed by evaporation under vacuum. Yield (∼50%). (Note: abbreviations are; B-Ant-A for aniline, B-Ant-Bn for benzylamine, B-Ant-F for furfurylamine based benzoxazine monomers.)

Results and discussion

As stated previously, functional benzoxazine monomers with complex structures can easily be prepared by using appropriate phenol or amine in the modified Mannich reaction. Thus, the desired antraquinone–benzoxazine monomers were readily obtained from 1,5-dihydroxyanthraquionone, primary amines and formaldehyde (Scheme 3).

Scheme 3 Synthesis of anthraquinone based bisbenzoxazines.

The structure of the monomers was confirmed by spectral analysis. As can be seen from Fig. S1–S3,† the ¹H-NMR spectra of the monomers exhibit the specific signals of benzoxazine ring. Notably, the two broad signals in the range of 4.9 and 3.7 ppm corresponding to –CH₂ protons of benzoxazine rings are revealed. The FT-IR absorption bands of the benzoxazines were previously reported in detail.^53,54 In Fig. S4,† the important bands of infra–red absorptions of benzoxazines observed at 932 and 1496 cm⁻¹ can be attributed to the benzene ring and the peak at 1229 cm⁻¹ is assigned to the asymmetric stretching of C–O–C group of oxazine ring. A detailed view of the region below 1700 cm⁻¹ of Fig. S4 is included in the ESI as Fig. S5.†

It is well known that 1,3-benzoxazines and related polymeric precursors can be cured by thermally-activated ring-opening polymerization and the exothermic process can be monitored using differential scanning calorimeter (DSC). The curing maximum varies from 160 to 250 °C, depending on the functionalities present on the benzoxazines or related polymeric precursors. In some cases, even curing temperatures below 160 °C was also reported.⁴⁷ Fig. 1 shows the overlaid DSC profiles of antraquinone–benzoxazines. It is clear that the monomers are curable and the curing temperature varies between 180–230 °C. In detail, an exotherm belonging to B-Ant-A is detected having an onset of polymerization reaction at 221 °C with a maximum at 227 °C. On the other hand, the exotherm of B-Ant-Bn has a maximum at 195 °C; also, B-Ant-F shows an exothermic peak starting from 170 °C and reaching its maximum at 181 °C. B-Ant-F and B-Ant-Bn monomers exhibited an endotherm right after the end-set temperatures showing that some volatile compounds are formed or sublimation occurred at that temperature. However, benzoxazine coated Pt electrodes were treated at 180 °C, which is even below the maximum cure temperature. Thus, possible side product formation during thermal treatment can be discarded (Table 1).

Fig. 1 DSC traces of B-Ant-F (a), B-Ant-Bn (b), B-Ant-A (c).

Table 1 DSC characteristics of antraquinone monomers

Benzoxazine	Curing maximum (°C)	On-set (°C)	End-set (°C)	ΔH (J g^−1)
B-Ant-A	229	221	234	−118
B-Ant-Bn	196	170	230	−111
B-Ant-F	181	170	202	−107

---

The polymerization of benzoxazines occurs in response to the benzoxazine ring structure having distorted semi-chair conformation. The ring strain created from this molecular structure assists six-membered ring to undergo ring-opening reaction under thermal conditions with a mechanism through two main pathways. The first step is the heterolytic cleavage of the C–O bond of the oxazine yielding a carbocation. Subsequent attack of the corresponding carbocation occurs to either the ortho or para position of the neighbour aromatic ring, which can be considered as a kind of Friedel–Crafts reaction.

Thus, cross-linked structures can easily be obtained even using mono-functional benzoxazines with free ortho and para positions on the phenolic structure. Due to the nature of the reaction mechanism, if the benzoxazine monomer contains aromatic ketonic moieties in the structure, these groups can survive under curing condition. Indeed, the FT-IR spectra of cured monomers reveal the ketonic carbonyl in the range between 1670–1685 cm⁻¹ indicating the preservation of the quiononic structure (Fig. S6 and S7†). Moreover, the progress of the curing reaction was monitored by IR and the each cure stage, from 120 to 210 °C, is shown in Fig. S7.† Expectedly, the bands corresponding to the oxazine and substituted benzene rings at 1373, 1214, 1068 and 987 cm⁻¹ decrease and finally disappear with increasing temperature. Ultimately, polybenzoxazine resins exhibiting a redox capability through hydroquinone–quinone transformation can readily be obtained by thermally activated curing of the corresponding monomer (Scheme 4).

Scheme 4 Thermally induced ring-opening polymerization of antraquinone–benzoxazines and subsequent redox process.

In order to prove that the ring-opened materials are truly polybenzoxazines and discard possible thermal degradation during the curing process, solid state NMR was conducted (Fig. S8†). According to the NMR results, two types of polybenzoxazine structures are present in the final product. The first structure has free phenolic hydroxyl groups as a result of Friedel–Crafts reaction. The second phenoxy ether type structure is formed by the attack of imine intermediate to the phenolic oxygen. Similar reactions were previously reported by Wang et al.^55,56 Scheme 5 briefly illustrates the mechanism of the formation of the two types of the polymers.

Scheme 5 Plausible mechanism for polymerization of benzoxazine monomer producing two different types of polybenzoxazine.

The electroactivity of the monomers and resulting polymers was investigated by cyclic voltammetry. As illustrated in Fig. 2, while aniline, benzyl and furfuryl based benzoxazines have oxidation peaks at around 1.5 V, all peaks were irreversible and all oxidations disappear in the CVs of polymers. While the reduction behaviors of monomers compared to polymers (Fig. 3), in the negative side there is a significant change in the reduction peaks from −0.5 V to −1 V.

Fig. 2 Single scan cyclic voltammogram of monomers (B-Ant-A (a), B-Ant-Bn (b), and B-Ant-F (c) based benzoxazines) in 0.1 M tetrabutylammonium hexafluorophosphate (NH₄⁺PF₆⁻)/acetonitrile (ACN) solution at a scan rate of 100 mV s⁻¹.

Fig. 3 Single scan cyclic voltammogram of polymers PB-Ant-A (a), PB-Ant-Bn (b), and PB-Ant-F (c) based polybenzoxazines in 0.1 M (NH₄⁺PF₆⁻)/ACN solution at a scan rate of 100 mV s⁻¹.

Quinone–hydroquinone transformation generates a cycle between tetrahydroxy and dihydroxy aromatic structures. It is well known that, phenolic hydroxyl groups promote the adhesion of molecules to metals.⁵⁷ Using quinone–hydroquinone cycle generate two different adhesion strength since the number of phenolic hydroxyls doubles in reduced form and this structure has high adhesion strength on steel surfaces. The oxidized form of the polybenzoxazine resins showed a drastic decrease in adhesion and simply dismantled from the surface of the steel specimen. Moreover, it can also be considered that the increase of the internal stress of the materials caused by the volume change may contribute to the dismantling behaviour. The adhesion values and the photographs of dismantling of the polybenzoxazine resins are presented in Table 2 and Fig. 4, respectively.

Table 2 Adhesion data of thermally cured benzoxazines

Polymer	Reduced form	Oxidized form
PB-Ant-A	5B	0B
PB-Ant-Bn	5B	0B
PB-Ant-F	5B	0B

---

Fig. 4 The photographs of reduced PB-Ant-A coated on steel (a), oxidized and dismantled (b).

Standard tests for measuring adhesion by tape test⁵⁸ were carried out on stainless steel plates (5 cm × 5 cm). Polymers were prepared using the same procedure described for Pt foil electrode. All polymers are well adhered and durable versus adhesion tests.

In order to confirm the proposed redox mechanism, electrochemical studies were combined with FTIR spectral analysis. Thus, monomer films were prepared on stainless steel (5 cm × 5 cm) electrodes by spray processing and then polymerized at 180 °C similar to the Pt electrodes. Resulting polymers were subjected to the constant potential at −0.3 V for 20 hours to reduce the carbonyl bonds in the polymer backbone to –OH units. Although the initial current was 3 μA, it was drastically decreased to 0.5 μA after 20 h reduction. FTIR spectra of the polymer films before and after reduction were recorded. The concurrent appearance of a new peak at 2965 cm⁻¹ and disappearance of the carbonyl peak at 1655 cm⁻¹ clearly confirms the successful redox process (see Fig. S9†). When the reduced polymer films were subjected to oxidation at +1.8 V, dismantling of film was observed in a matter of seconds due to the oxidation of the polymer resulting in the formation of ketone moieties.

Thermogravimetric behaviour of the cured polybenzoxazines was investigated by TGA under nitrogen atmosphere. The TGA and derivative thermal gravimetry (DTG) profiles are shown in Fig. 5 and 6, respectively. Thermal characteristics are also summarized in Table 3. It can be seen that aniline functional anthraquinone based polybenzoxazine shows significantly higher thermal stability than the other two polybenzoxazines. Moreover, methyl furan based polybenzoxazine has higher char yield compared to benzyl based benzoxazine. The differences observed in their thermal stability can be explained by the aromatization of the furan groups. This phenomenon is well known and reported in carbonization studies of furans.^59,60

Fig. 5 TGA curves of PB-Ant-A (a), PB-Ant-F (b), PB-Ant-Bn (c).

Fig. 6 Derivative weight (%) of (a) PB-Ant-F (b) PB-Ant-Bn (c) PB-Ant-A.

Table 3 Thermal properties of the anthraquinone based polybenzoxazines

Polymer^a	T5%^b (°C)	T10%^c (°C)	Tmax^e (°C)	Char yield^d (%) at 800 °C
a Curing was performed in TGA at 220 °C for 15 min. under N2 stream (200 mL min^−1).b T5%: the temperature for which the weight loss is 5%.c T10%: the temperature for which the weight loss is 10%.d Yc: char yields at 800 °C under nitrogen atmosphere.e Tmax: the temperature for maximum weight loss.
PB-Ant-A	211	233	296	19
PB-Ant-Bn	159	171	200	1
PB-Ant-F	156	168	192	11

---

DTG curves of benzyl and methyl furan based polybenzoxazines exhibit peaks at 192 °C, 200 °C and inflection points at 240 °C, 254 °C, respectively. However, aniline based polybenzoxazine showed a peak at relatively higher temperature, 296 °C, indicating a different degradation pathway than that of benzyl and furan derivatives.

Conclusion

It has been demonstrated that adhesion of anthraquinone based polybenzoxazine thermosets on metal surfaces can be manipulated efficiently by electrochemical redox process. The method relies on the electrochemically driven hydroquinone–quinone redox couple. The strong binding affinities of phenolic hydroxyl groups on the metal surface can be increased by increasing the number of phenolic hydroxyls via reduction of anthraquinone moieties in the cured polymers, which promotes adhesion drastically. Furthermore, electrochemical oxidation converts reduced groups into quinone moieties resulting in the dismantling of the coated films. Consequently, this work is very appealing due to its very simple design and manipulation of adhesion property supported by convincing electrochemical and adhesion test data. And it is anticipated that the approach can be applied in recycling processes of polybenzoxazine coated steels.

Acknowledgements

The authors thank to Istanbul Technical University Research Fund for financial support and B.K. thanks the FABED (Fevzi Akkaya Scientific Activity Support Fund) for financial support by means of a Young Investigator Award.

Notes and references

1. W. J. Burke, J. Am. Chem. Soc., 1949, 71, 609–612.
2. H. Schreiber, German Patent 2 255 504, 1973.
3. G. Riess, J. M. Schwob and G. Guth, Abstr. Pap. Am. Chem. Soc., 1984, 188, 85-Poly.
4. X. Ning and H. Ishida, J. Polym. Sci., Part A: Polym. Chem., 1994, 32, 1121–1129.
5. H. Ishida and Y. Rodriguez, J. Appl. Polym. Sci., 1995, 58, 1751–1760.
6. N. N. Ghosh, B. Kiskan and Y. Yagci, Prog. Polym. Sci., 2007, 32, 1344–1391.
7. K. D. Demir, B. Kiskan, B. Aydogan and Y. Yagci, React. Funct. Polym., 2013, 73, 346–359.
8. Y. Yagci, B. Kiskan and N. N. Ghosh, J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 5565–5576.
9. S. Ates, C. Dizman, B. Aydogan, B. Kiskan, L. Torun and Y. Yagci, Polymer, 2011, 52, 1504–1509.
10. B. Kiskan and Y. Yagci, Polymer, 2005, 46, 11690–11697.
11. A. Yildirim, B. Kiskan, A. L. Demirel and Y. Yagci, Eur. Polym. J., 2006, 42, 3006–3014.
12. T. Takeichi, T. Kano and T. Agag, Polymer, 2005, 46, 12172–12180.
13. K. D. Demir, B. Kiskan, S. S. Latthe, A. L. Demirel and Y. Yagci, Polym. Chem., 2013, 4, 2106–2114.
14. B. Aydogan, D. Sureka, B. Kiskan and Y. Yagci, J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 5156–5162.
15. B. Kiskan, Y. Yagci and H. Ishida, J. Polym. Sci., Part A: Polym. Chem., 2008, 46, 414–420.
16. A. Chernykh, J. P. Liu and H. Ishida, Polymer, 2006, 47, 7664–7669.
17. A. Nagai, Y. Kamei, X. S. Wang, M. Omura, A. Sudo, H. Nishida, E. Kawamoto and T. Endo, J. Polym. Sci., Part A: Polym. Chem., 2008, 46, 2316–2325.
18. K. D. Demir, B. Kiskan and Y. Yagci, Macromolecules, 2011, 44, 1801–1807.
19. B. Kiskan and Y. Yagci, Polymer, 2008, 49, 2455–2460.
20. B. Hanbeyoglu, B. Kiskan and Y. Yagci, Macromolecules, 2013, 46, 8434–8440.
21. B. Gacal, L. Cianga, T. Agag, T. Takeichi and Y. Yagci, J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 2774–2786.
22. M. R. Vengatesan, S. Devaraju, K. Dinakaran and M. Alagar, J. Mater. Chem., 2012, 22, 7559–7566.
23. M. Sponton, D. Estenoz, G. Lligadas, J. C. Ronda, M. Galia and V. Cadiz, J. Appl. Polym. Sci., 2012, 126, 1369–1376.
24. Y. Si, T. Ren, B. Ding, J. Yu and G. Sun, J. Mater. Chem., 2012, 22, 4619–4622.
25. C. Yan, X. Fan, J. Li and S. Zhiqi Shen, J. Appl. Polym. Sci., 2011, 120, 1525–1532.
26. J.-M. Huang, S.-W. Kuo, H.-J. Huang, Y.-X. Wang and Y.-T. Chen, J. Appl. Polym. Sci., 2009, 111, 628–634.
27. H.-K. Fu, C.-F. Huang, S.-W. Kuo, H.-C. Lin, D.-R. Yei and F.-C. Chang, Macromol. Rapid Commun., 2008, 29, 1216–1220.
28. T. Agag, V. Taepaisitphongse and T. Takeichi, Polym. Compos., 2007, 28, 680–687.
29. Y. H. Liu, W. Zhang, Y. Chen and S. X. Zheng, J. Appl. Polym. Sci., 2006, 99, 927–936.
30. C. H. Lin, K. Z. Yang, T. S. Leu and J. W. Sie, J. Polym. Sci., Part A: Polym. Chem., 2006, 44, 3487–3502.
31. B. S. Rao, K. R. Reddy, S. K. Pathak and A. Pasala, Polym. Int., 2005, 54, 1371–1376.
32. S. X. Zheng, H. Lu and Q. P. Guo, Macromol. Chem. Phys., 2004, 205, 1547–1558.
33. B. Kiskan, A. L. Demirel, O. Kamer and Y. Yagci, J. Polym. Sci., Part A: Polym. Chem., 2008, 46, 6780–6788.
34. B. Kiskan, N. N. Ghosh and Y. Yagci, Polym. Int., 2011, 60, 167–177.
35. O. S. Taskin, B. Kiskan and Y. Yagci, Macromolecules, 2013, 46, 8773–8778.
36. B. Kiskan, B. Koz and Y. Yagci, J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 6955–6961.
37. B. Kiskan and Y. Yagci, J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 1670–1676.
38. M. Imran, B. Kiskan and Y. Yagci, Tetrahedron Lett., 2013, 54, 4966–4969.
39. R. Kudoh, A. Sudo and T. Endo, Macromolecules, 2010, 43, 1185–1187.
40. C. Zuniga, G. Lligadas, J. Carlos Ronda, M. Galia and V. Cadiz, Polymer, 2012, 53, 3089–3095.
41. H. Xu, Z. Lu and G. Zhang, RSC Adv., 2012, 2, 2768–2772.
42. T. Hayakawa, Y. Osanai, K. Niizeki, O. Haba and M. Ueda, High Perform. Polym., 2000, 12, 237–246.
43. J. A. Cid, Y. X. Wang and H. Ishida, Polym. Polym. Compos., 1999, 7, 409–420.
44. S. Wirasate, S. Dhumrongvaraporn, D. J. Allen and H. Ishida, J. Appl. Polym. Sci., 1998, 70, 1299–1306.
45. C. Wang, C. Zhao, J. Sun, S. Huang, X. Liu and T. Endo, J. Polym. Sci., Part A: Polym. Chem., 2013, 51, 2016–2023.
46. C. Liu, D. Shen, R. M. Sebastián, J. Marquet and R. Schönfeld, Polymer, 2013, 54, 2873–2878.
47. A. Sudo, S. Hirayama and T. Endo, J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 479–484.
48. F. Kasapoglu, I. Cianga, Y. Yagci and T. Takeichi, J. Polym. Sci., Part A: Polym. Chem., 2003, 41, 3320–3328.
49. M. Baqar, T. Agag, R. Huang, J. Maia, S. Qutubuddin and H. Ishida, Macromolecules, 2012, 45, 8119–8125.
50. A. Sudo, R. Kudoh, H. Nakayama, K. Arima and T. Endo, Macromolecules, 2008, 41, 9030–9034.
51. P. Wang, B. D. Martin, S. Parida, D. G. Rethwisch and J. S. Dordick, J. Am. Chem. Soc., 1995, 117, 12885–12886.
52. A. Tuzun, B. Kiskan, N. Alemdar, A. T. Erciyes and Y. Yagci, J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 4279–4284.
53. J. Dunkers and H. Ishida, Spectrochim. Acta, Part A, 1995, 51, 1061–1074.
54. A. B. Rajput and N. N. Ghosh, Int. J. Polym. Mater., 2010, 60, 27–39.
55. C. Wang, J. Sun, X. Liu, A. Sudo and T. Endo, Green Chem., 2012, 14, 2799–2806.
56. Y. X. Wang and H. Ishida, Macromolecules, 2000, 33, 2839–2847.
57. L. A. Varghese and E. T. Thachil, J. Adhes. Sci. Technol., 2004, 18, 181–193.
58. ASTM Spec. Tech. Publ., D3359 − 09e2.
59. M.-M. Titirici, M. Antonietti and N. Baccile, Green Chem., 2008, 10, 1204–1212.
60. M.-M. Titirici and M. Antonietti, Chem. Soc. Rev., 2010, 39, 103–116.

---

Footnote

† Electronic supplementary information (ESI) available: NMR, FT-IR spectra of monomers and polymers. See DOI: 10.1039/c4ra03763d

---