Breaking Strong Alkynyl-Phenyl Bonds: Poly(<i>para</i>-phenylene ethynylene)s under Mechanical Stress

Stronger chemical bonds withstand higher mechanical forces; thus, the rupture of single bonds is preferred over the rupture of double or triple bonds or aromatic rings. We investigated bond scission in poly(dialkyl-<i>p</i>-phenylene ethynylene)s (PPEs), a fully conjugated polymer. In a scale-bridging approach using electron-paramagnetic resonance spectroscopy and gel permeation chromatography of cryomilled samples, in combination with density functional theory calculations and coarse-grained simulations, we conclude that mechanical force cleaves the sp-sp² bond of PPEs (bond dissociation energy as high as 600 kJ mol^-1). Bond scission primarily occurs in shear bands with locally increased shear stresses. The scission occurs in the middle of the PPE chains. Breaking sp-sp² bonds into free radicals thus is feasible but requires significant mechanical force and an efficient stress concentration.