Mechanochromic polymers, which change their colors under mechanical stress, have attracted much attention in materials science. The main strategy for building mechanochromic polymers involves incorporation of mechanochromophores into polymer chains. Recently, we have reported that the central carbon–carbon bond in tetraarylsuccinonitrile (TASN), can be cleaved to afford pink-colored radical by either thermal or mechanical stimuli. The generated carbon-centered radical species are tolerant of oxygen and detectable by electron paramagnetic resonance (EPR) spectroscopy, resulting in quantitative evaluation of the mechanical stress in air. However, the thermo-responsiveness of TASN limits its application as a mechanochromophore under heating conditions. Herein, we report the design and synthesis of a novel mechanochromophore, difluorenylsuccinonitrile (DFSN), whose central carbon–carbon bond is more tolerant of thermal stimulus, and mechanochromic polymers with a DFSN skeleton at their mid-chains. A DFSN derivative with two hydroxyl groups was synthesized from commercially available 2-fluorenecarboxaldehyde in 7 steps. Thermal properties of the obtained DFSN derivative were evaluated by EPR measurements, which revealed that the cleavage ratio of central carbon–carbon bond in DFSN at 373 K was suppressed to less than 1/2000 in comparison with counterpart TASN. In addition, DFSN derivatives bearing two initiator moieties for atom transfer radical polymerization (ATRP) was synthesized and applied to ATRP of methyl methacrylate, affording DFSN-containing poly(methyl methacrylate) (PMMA-DFSN-PMMA). PMMA-DFSN-PMMA exhibited a remarkable color change from white to pink upon grinding. EPR measurement supported that this color change is originated from dissociation of a DFSN moiety at the middle of a PMMA chain. It is concluded that DFSN works as a thermally stable mechanochromophore but sufficiently sensitive to mechanical stress.

Thermally stable mechanochromic polymers having difluorenylsuccinonitrile

256th National Meeting (2018)

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