Effects of chain length on the rates of C-C bond dissociation in linear alkanes and polyethylene.

Molecular dynamics modeling of C-C bond dissociation is performed for a series of linear alkanes and polyethylene macromolecules with the chain lengths ranging from one to a thousand constituent ethylene monomers (PE-1-PE-1000). The rate constants obtained in molecular dynamics calculations are compared with those determined using variational transition state theory with the same potential energy surface. The results of simulations demonstrate a significant accelerating effect of chain length on the rates of C-C bond scission. Per-bond rate constant values increase with the increasing chain length, up to an order of magnitude, in the sequence of linear alkanes from PE-1 (ethane) to PE-5 (decane); this dependence becomes saturated for longer chain lengths. Stiffening the potentials of bending and especially the torsional degrees of freedom diminishes the accelerating effect of chain length, while constraining the bond distances for all C-C bonds except the one undergoing dissociation has no effect. The results of the calculations are compared with existing experimental data on the dependences of the rates of thermal decomposition of linear alkanes on the alkane chain length.