Substituent Effects of the DAAN Derivatives on the Radical Generation via Polymer Mechanochemistry. The following two points are considered to be important for the generation of radicals from DAAN derivatives (Figure ). 1) The hydrogen atom at the benzylic position of the DAAN derivatives is easily abstracted by polymeric mechanoradicals. 2) The generated DAAN radicals are more stable than other mechanoradicals due to their aromaticity and electron-withdrawing cyano group. To confirm these hypotheses, the thermodynamic parameters of each DAAN derivative were determined using DFT calculations, and the amount of DAAN radicals generated upon ball-milling mixed samples of polystyrene and DAAN derivatives was estimated based on EPR measurements by comparing the integral area of each observed spectrum with that of a solution of 4-hydroxy-2,2,6,6-tetramethylpiperidin-1oxyl (TEMPOL) as a standard. Unless otherwise noted, DFT calculations were carried out at the UM06-2X/6-311+G(d,p) level given that this level has been reported to be appropriate for fluorescent radicals. The unpaired electron of the mechanoradical generated by the ball-milling of polystyrene is accommodated in the a-SOMO. To abstract the hydrogen atom from the DAAN derivative for the mechanoradical, the a-SOMO of the mechanoradical must interact with the C(sp 3 )-H bonding orbital (sC-H) of the DAAN derivatives (Figure ). Accordingly, the orbital energy of the a-SOMO of the 1-phenylethyl radical and 2-phenylethyl radical, which was used as the model structure for the mechanoradical generated by the polymer-chain scission of polystyrene, and the sC-H energy of the DAAN derivatives were calculated using DFT calculations. The results of the calculations suggested that the sC-H energy level of the DAAN derivatives and the reactivity with the a-SOMO of the mechanoradical increase with increasing electron-donating properties of the substituents on the DAAN derivatives because the energy difference between a-SOMO of mechanoradical and sC-H decreases (Table , Figures ).