Relative Mechanical Strengths of Weak Bonds in Sonochemical Polymer Mechanochemistry

The mechanical strength of scissile chemical bonds plays a role in material failure and in the mechanical activation of latent reactivity, but quantitative measures of mechanical strength are rare. Here, we report the relative mechanical strength of polymers bearing three putatively "weak" scissile bonds: the carbon-nitrogen bond of an azobisdialkylnitrile (<30 kcal mol(-1)), the carbon-sulfur bond of a thioether (71-74 kcal mol(-1)), and the carbon-oxygen bond of a benzylphenyl ether (52-54 kcal mol(-1)). The mechanical strengths are assessed in the context of chain scission triggered by pulsed sonication of polymer solutions, by using two complementary techniques: (i) the competition within a single polymer chain between the bond scission of interest and the nonscissile mechanochemical ring opening of gem-dichlorocyclopropane mechanophores and (ii) the molecular weights at long (4 h) sonication times of multimechanophore polymers. The two methods produce a consistent story: in contrast to their thermodynamic strengths, the relative mechanical strengths of the three weak bonds are azobisdialkylnitrile (weakest) < thioether < benzylphenyl ether. The greater mechanical strength of the benzylphenyl ether relative to the thermodynamically stronger carbon-sulfur bond is ascribed to poor mechanochemical coupling, at least in part as a result of the rehybridization that accompanies carbon-oxygen bond scission.