Figure shows the barriers and Gibbs free energies for the first and second propagation steps. Four reactions were considered, corresponding to the two most likely products of the first propagation step attacking either the terminal or central sulfur of the trisulfide monomer. As in the case of the first propagation step in which the reaction of the disulfide radical was not favored, the reactions of the disulfide anion are also strongly kinetically and thermodynamically disfavored. In other words, the reactions of intermediates 9 or 11 with monomer 1 have such a high barrier that these reactions are not expected to compete. The monosulfide anion in 10, on the other hand, can attack either the central sulfur atom on the trisulfide monomer to form a new terminal monosulfide anion (12), or the terminal sulfur of the trisulfide monomer to form a new disulfide linkage (11). Barriers for both reactions are similar, with the rate of attack at the central sulfur atom having a 3:1 kinetic preference. While not kinetically preferred, attack at the terminal sulfur, forming a terminal disulfide anion, has a significant (28 kJ/mol) thermodynamic preference. However, based on the calculations in Figure , the disulfide products (such as 9 and 11) of this latter process are predicted to be unreactive to further propagation. Therefore, only addition of a monosulfide anion end group to the central sulfur atom of the trisulfide monomer would lead to further chain growth. As a result, the predominant repeat unit is predicted to be a trisulfide linkage. It should be noted that the propagation reactions are reversible at room temperature, which may limit the attainable molecular weight of the polymer. With that said, the reversibility allows for a "self-correction" mechanism that can convert disulfide products back to the pathway that provides a polymer with trisulfides in the repeating unit. The termination mechanism was not studied computationally, but based on our experimental observations we propose that the termination event is aided by precipitation of the polymer, which occurs during the polymerization process at the measured molecular weights.