SYNTHESIS AND PROPERTIES OF CROSS-LINKED POLYMERS CONTAINING DIARYLBIBENZOFURANONE BY ADMET POLYMERIZATION T. Ohishi, 1 K. Imato, 2 T. Kanehara, 2 A. Takahara, 1,2 and H. Otsuka 1,2 1 Institute for Materials Chemistry and Engineering, Kyushu University, 2 Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan - e-mail: t-ohishi@cstf.kyushu-u.ac.jp Keywords: dynamic covalent bonds, cross-linked polymers, acyclic diene metathesis polymerization ABSTRACT Diarylbibenzofuranone (DABBF) derivatives can be reversibly cleaved to the corresponding arylbenzofuranone (ABF) radicals under mild conditions. We recently reported the synthesis and physicochemical properties of the cross-linked polyurethane containing DABBF unit. The autonomous structural transformation and the macroscopic self-healing of separated gel pieces under air at room temperature without any stimuli were accomplished by a dynamic covalent approach. Since the self-healing property was derived from DABBF units, one can expect the self-healing of DABBF-containing cross-linked polymers with various structures. We here report the acyclic diene metathesis (ADMET) polymerization of multifunctional olefin monomers containing DABBF unit. This is because a low-polarity olefin polymer can be obtained by ADMET polymerization. The DABBF-diolefin monomer was first polymerized at room temperature in the presence of Grubbs catalyst (2nd generation) in CH 2 Cl 2. However, the yield of the obtained polymer was low (29%). Under this condition, it is considered that the ethylene molecules produced during polymerization can not be removed. The polymerization was also performed at 40 C to give polymers in good yield (83%). Furthermore, cross-linked polymers were synthesized by ADMET polymerization of DABBF-tetraolefin monomer and copolymerization of DABBF-bifunctional olefin monomer and triolefin monomer. All cross-linked polymers were obtained in good yields. These polymers showed high swelling properties in organic solvents with relatively low polarity. 1. INTRODUCTION A reversible system can repeat the self-healing process for numerous cycles when noncovalent bonds or dynamic covalent bonds are used. 1 Such noncovalent and dynamic covalent bonding 2 can be reversed under equilibrium controls. Modified by external stimuli or through continuous component exchange, noncovalent and dynamic covalent bonds can be reorganized or reshuffled to cure the system. Diarylbibenzofuranone (DABBF), a dimer of arylbenzofuranone, exists as a state of equilibrium between dissociation and association and its central C-C bond can act as a dynamic covalent bond at room temperature. 3 We recently reported the synthesis and physicochemical properties of the cross-linked polyurethane containing DABBF 355
unit. 4 The autonomous structural transformation and the macroscopic self-healing of separated gel pieces under air at room temperature without any stimuli were accomplished by a dynamic covalent approach. Since the self-healing property was derived from DABBF units, one can expect the self-healing of DABBF-containing cross-linked polymers with various structures. We here report the acyclic diene metathesis (ADMET) polymerization of multifunctional olefin monomers containing DABBF unit. This is because a low-polarity olefin polymer can be obtained by ADMET polymerization. 2. MATERIALS Grubbs second-generation catalyst was purchased from Aldrich. Substrates and reagents were purchased form Tokyo chemical industry or Wako pure chemical industries; 4-hydroxymanderic acid monohydrate, 2,4-di-tert-butylphenol, methane sulfonic acid, 3-chloro-1-propanol, 3-chloro-1,2-propanediol, di-tert-butyl peroxide, 10-undecenoyl chloride, and 1,1,1-tris(4-hydroxyphenyl)ethane were used as supplied. Organic solvents were purchased from Kanto chemical or Wako pure chemical industries at the highest purity and used as supplied. DABBF-diol and DABBF-tetraol were prepared according to the literature. 4 The olefin groups were then introduced by condensation of DABBF-diol and/or DABBF-tetraol 10- undecenoyl chloride at room temperature. This reaction gave the monomers 1 or 2 in high yield (Scheme 1). 3.METHODS Polymerization: An example of the procedure for the synthesis of polymer containing DABBF unit is as follows. In a nitrogen-filled Schlenk flask, Grubbs secondgeneration catalyst was added to the solution of monomer 1 in CH 2 Cl 2, which was degassed with three freeze-vacuum-thaw cycles. Then the mixture was heated to 40 C and stirred for 24 h. The reaction was quenched by adding ethyl vinyl ether using syringe and stirred for 12 h at room temperature. CH 2 Cl 2 was added to the reaction mixture and treated with tris(hydroxymethyl)phosphine (THP) in order to remove the residual catalyst. The polymer was purified by reprecipitation from 356
methanol. 1 H NMR (400 MHz) spectroscopic measurements were carried out at 25 C with a 400 MHz Bruker spectrometer using tetramethylsilane (TMS) as internal standard in chloroform-d (CDCl 3 ). Gel permeation chromatographic (GPC) measurements were carried out at 40 C on TOSOH HLC-8220 GPC system equipped with a guard column (TOSOH TSK guard column Super H-L), three columns (TOSOH TSK gel SuperH 6000, 4000, and 2500) and a UV-Vis detector. Tetrahydrofuran (THF) was used as the eluent at a flow rate of 0.6 ml min -1. Polystyrene (PS) standards (M n = 4920-3000000; M w /M n = 1.02-1.03) were used to calibrate the GPC system. 4. RESULTS AND DISCUSSION ADMET polymerization of multifunctional olefin monomers containing DABBF The DABBF-diolefin monomer was first polymerized at room temperature in the presence of Grubbs catalyst (2nd generation) in CH 2 Cl 2. However, the yield of the obtained polymer was low (29%). Under this condition, it is considered that the ethylene molecules produced during polymerization can not be removed. The polymerization was also performed at 40 C to give the corresponding polymers in good yield (83%) (Scheme 2). Synthesis and properties of cross-linked polymers containing DABBF Diverse types of cross-linked polymers were synthesized by ADMET polymerization of DABBF-tetraolefin monomer and copolymerization of DABBF-bifunctional olefin monomer and triolefin monomer (Scheme 3). The swelling behavior of the crosslinked polymers containing DABBF was investigated in various organic solvents (Table 1). The cross-linked copolymers containing monomer 1 unit, such as poly1-copoly3 and poly1-co-poly3-co-poly4, did not form a gel in benzene, THF, CHCl 3, and anisole. These results indicates that cross-linked copolymers containing monomer 1 357
have low crosslink density. In contrast, the cross-linked copolymers containing monomer 2 unit, such as poly2-co-poly3, poly2-co-poly4, and poly2-co-poly3-copoly4, swelled and absorbed benzene, THF, CHCl 3, and anisole. Accordingly, the swelling properties of these cross-linked copolymer containing DABBF unit are probably dependent on the cross-linked density. Furthermore, poly2-co-poly4 and poly2-co-poly3-co-poly4 were formed high viscosity solution in low-polar organic solvent such as CH 2 Cl 2. These results imply that swelling behavior of cross-linked copolymers containing DABBF unit depends on polymer main chain polarity and dynamic covalent bond property of DABBF unit. 5. CONCLUSIONS We have demonstrated the synthesis and properties of cross-linked polymers having DABBF prepared by ADMET polymerization. All cross-linked polymers were synthesized by ADMET polymerization of DABBF-tetraolefin monomer and copolymerization of DABBF-bifunctional olefin monomer and triolefin monomer. These polymers showed high swelling properties in organic solvents with relatively low polarity. Furthermore, poly2-co-poly4 and poly2-co-poly3-co-poly4 were formed high viscosity solution in low-polar organic solvent. These results imply that swelling behavior of cross-linked copolymers containing DABBF unit depends on polymer main chain polarity and dynamic covalent bond property of DABBF unit. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support of the Funding Program (Green Innovation GR077) for Next Generation World-Leading Researchers from the Cabinet Office, Government of Japan. REFERENCES [1] Syrett, J. A.; Becer, C. R.; Haddleton, D. M. Polym. Chem. 2010, 1, 978 987. [2] (a) Rowan, S. J.; Cantrill, S. J.; Cousins, G. R. L.; Sanders, J. K. M.; Stoddart, J. F. Angew. Chem., Int. Ed. 2002, 41, 898 952. (b) Corbett, P. T.; Leclaire, J.; Vial, L.; West, K. R.; Wietor, J. L.; Sanders, J. K. M.; Otto, S. Chem. Rev. 2006, 106, 3652 3711. (c) Lehn, J. M. Chem. Soc. Rev. 2007, 36, 151 160. 358
[3] Frenette, M.; Maclean, P. D.; Barclay, L. R. C.; Scaiano, J. C. J. Am. Chem. Soc., 2006, 128, 16432-16433. [4] Imato, K.; Nishihara, M.; Kanehara, T.; Amamoto, Y.; Takahara, A.; Otsuka, H. Angew. Chem. Int. Ed., 2012, 51, 1138-1142. 359