The photoregulation of a mechanochemical polymer scission

Control over mechanochemical polymer scission by another external stimulus may offer an avenue to further advance the fields of polymer chemistry, mechanochemistry, and materials science. Herein, we demonstrate that light can regulate the mechanochemical behavior of a diarylethene-conjugated Diels–Alder adduct (DAE/DA) that reversibly isomerizes from a weaker open form to a stronger closed form under photoirradiation. Pulsed ultrasonication experiments, spectroscopic analyses, and density functional theory calculations support the successful photoregulation of the reactivity of this DAE/DA mechanophore, which is incorporated at the mid-chain of a polymer, and indicate that higher force and energy are required to cleave the closed form of the DAE/DA mechanophore relative to the open form. The present photoregulation concept provides an attractive approach toward the generation of new mechanofunctional polymers.


General Experimental Details
All solvents and reagents were purchased from Sigma-Aldrich, Wako Pure Chemical Industries, Tokyo Chemical Industry, or Kanto Chemical and used as received, unless otherwise noted. Methyl acrylate was purified by basic aluminum chromatography to remove the polymerization inhibitor. Cu(0) wire (diameter: 1.0 mm, purity >99.9%) was washed with H2SO4 just before use to purify the surface.
Ultrasound experiments were performed using a Branson sonifire model 250D with a 1.1 cm diameter solid probe. The distance between the titanium tip and bottom of the glass cell was 1 cm. The ultrasonic intensity was calibrated using the method outlined by Hickenboth et al. 1 1 H NMR spectra were measured at 25 °C using a 300 MHz Bruker spectrometer or 500 MHz Bruker spectrometer with tetramethylsilane (TMS) as an internal standard in chloroform-d (CDCl3),  or acetonitrile-d3 (CD3CN). IR spectra were obtained with a Perkin-Elmer Spectrum One infrared spectrometer. Size exclusion chromatography (SEC) measurements were carried out at 40 °C on TOSOH HLC-8320 SEC system equipped with a guard column (TOSOH TSK guard column Super H-L), three columns (TOSOH TSK gel SuperH 6000, 4000, and 2500), a differential refractive index detector, and a UV-vis detector. Tetrahydrofuran (THF) was used as the eluent at a flow rate of 0.6 mL/min. Polystyrene (PS) standards (Mn = 4430−3242000; Mw/Mn = 1.03−1.08) were used to calibrate the SEC system. UV/vis absorption spectroscopy was performed on a JASCO V-650 spectrophotometer at 25 °C. Photochemical reactions were carried out employing a USHIO UXL-500SX Xe lamp equipped with either ASAHI SPECTRA optical filter at 313 nm for ring-closing and Hoya colored optical glass Y44 for ring-opening reactions.
The reaction progress was monitored by 1 H NMR measurement ( Supplementary Fig. 1). 1

Wavelength / nm
After UV irradiation at PSS

After visible light irradiation for PSS solution
General procedure for the synthesis of poly(methyl acrylate) (PMA) containing a chain-centerd initiating unit.
A representative procedure for the synthesis of polymer P1 is provided. A 30 mL Schlenk flask equipped with a stir bar was charged with freshly cut copper wire (0.75 cm×2 length, 1.0 mm in diameter) and initiator DAE/DA-dibromide (13.3 mg, 14.8 µmol). The flask was sealed with a stopcock followed by the addition of DMSO (6.2 mL) and methyl acrylate ( Fig. 4). The 1 H NMR spectrum showed the progress of retro-DA reaction and SEC curve shifted to lower molecular weight region, and the estimated molecular weight was half of the original, which indicated that DAE/DA unit is at the center of polymer chain.

CoGEF Analysis
CoGEF calculations were performed using Gaussian 16 according to previously reported methods. [3][4][5] Ground state energies were calculated using DFT at the UB3LYP 6-31G* level of theory. Starting from the equilibrium geometry of the unconstrained molecule (Energy = 0 kJ/mol), the distance between the methyl esters of the truncated structure was increased in increments of approximately 0.1 Å (0.05 Å in the area surrounding the breaking point) and the energy was minimized at each interval.    Fig. 18. Photoregulation of the polymer chain scission upon exposure to pulsed ultrasound.