Photo-triggered solvent-free metamorphosis of polymeric materials

Liquefaction and solidification of materials are the most fundamental changes observed during thermal phase transitions, yet the design of organic and polymeric soft materials showing isothermal reversible liquid–nonliquid conversion remains challenging. Here, we demonstrate that solvent-free repeatable molecular architectural transformation between liquid-star and nonliquid-network polymers that relies on cleavage and reformation of a covalent bond in hexaarylbiimidazole. Liquid four-armed star-shaped poly(n-butyl acrylate) and poly(dimethyl siloxane) with 2,4,5-triphenylimidazole end groups were first synthesized. Subsequent oxidation of the 2,4,5-triphenylimidazoles into 2,4,5-triphenylimidazoryl radicals and their coupling with these liquid star polymers to form hexaarylbiimidazoles afforded the corresponding nonliquid network polymers. The resulting nonliquid network polymers liquefied upon UV irradiation and produced liquid star-shaped polymers with 2,4,5-triphenylimidazoryl radical end groups that reverted to nonliquid network polymers again by recoupling of the generated 2,4,5-triphenylimidazoryl radicals immediately after terminating UV irradiation.


Synthesis of star-PBA Br .
Into a flask equipped with three-way cock were placed CuBr (390 mg, 2.73 mmol), dNbpy (2.2 g, 5.6 mmol), and 1 (5.0 g 6.82 mmol), BA (12.3 mL, 137 mmol), and toluene (12 mL), and the mixture was degassed by three "freeze-pump-thaw" cycles. The test tube was placed in an oil bath to proceed the reaction with stirring at 80 °C for 13 h under vacuum. The reaction was then quenched by cooling with liquid nitrogen. After allowed to warm to room temperature, acetone was added to the mixture and the resulting solution

Synthesis of star-PBA Lophine .
Into a flask, star-PBA Azide (3.8 g, 1.5 mmol), 4 (2.2 g, 6.2 mmol), CuBr (890 mg, 6.2 mmol), PMDETA (1.3 mL, 6.2 mmol), and DMF (20 mL) were added and the resulting mixture was stirred at room temperature for 52 h. The reaction mixture was diluted with acetone and passed through a plug of alumina. The eluent was concentrated to dryness and the residue was further purified by silica gel column chromatography using EtOAc/hexane (1/3, v/v) as an eluent to afford star-PBA Lophine . The yield was 3.0 g (57%).

NMR Measurements.
1 H NMR spectra were recorded on a Bruker AVANCE III spectrometer operating at 500 MHz. CDCl3, DMSO-d6 ,or acetone-d6 was used as the solvent and chemical shifts were reported relative to tetramethylsilane (TMS) (δ = 0.00 ppm) or solvent residual signals.

GPC Measurements.
GPC measurements were performed using a Waters e-2695 high-speed liquid chromatograph equipped with RI and UV detectors. Two series-connected TSKgel SuperMultipore HZ-H columns were employed with tetrahydrofuran (THF) as the eluent at a flow rate of 0.35 mL/min at 40  C. For polymers containing lophine or HABI groups, THF/TEA (99.5/0.5, v/v) was used as the eluent to avoid interaction between the polymers and column packing materials.

MALDI-TOF Mass Measurement.
MALDI-TOF mass spectra were recorded on a Bruker ultrafleXtreme mass spectrometer equipped with a nitrogen laser ( = 337 nm). The spectrometer was operated at an accelerating potential of 20 kV with a linear-positive ion mode. THF solutions of a polymer sample (1 mg/mL), dithranol (10 mg/mL) and sodium trifluoroacetate (1 mg/mL) were mixed (1/1/1, v/v/v), and a portion of the mixture was deposited onto a sample target plate.

ESR Measurements.
ESR spectra were recorded on a JEOL JES-TE3000 spectrometer at room temperature.
For UV irradiation experiments, UV was directly irradiated to the sample from a window equipped with sample cavity by using a light guide connected to the Hamamatsu LIGHTNING CURE LC8 L9588.

UV-Vis Absorbance Measurements.
UV-vis absorption spectra were recorded on a JASCO V-560 spectrometer using a quartz cell of 10 mm optical path length. For UV-vis absorbance measurements under the UV irradiation (365 nm, 150 mW/cm 2 ), the light guide was set inside the sample chamber and UV was directly irradiated to the quartz cells containing THF solutions of network-PBA HABI (1, 5, 10, 50, and 100 mg/mL).