The mechanical force to polymeric materials in vacuum at 77 K produces mechano radicals, mechano anions and mechano cations due to homogeneous and heterogeneous scissions of the covalent bonds comprising polymer main chain. The ionic degree of the covalent bond was estimated by calculating the "absolute Delta Mulliken atomic charge," which was defined as the difference between the Mulliken atomic charges of the two adjacent atoms comprising the covalent bond of the polymer main chain. The ionic yield of the covalent bond increased with increasing the absolute Delta Mulliken atomic charge. The empirical formula for the ionic yield was obtained with the absolute Delta Mulliken atomic charge, and indicates that the ionic yield could be estimated from its chemical structure. (c) 2014 Elsevier Ltd. All rights