To understand the mechanism in more detail, polymerization of the simplest monomer (1) was investigated using DFT calculations (Fig. and S52-S78 for additional details of the method and data). The initiation step was studied first (Fig. ). As discussed previously, the calculated reduction potential for monomer 1 (-2.29 V versus Fc/Fc + ) was consistent with the experimentally observed value (-2.22 V versus Fc/Fc + ) (Fig. ). Upon reduction and formation of radical anion 6, the monomer does not spontaneously ring-open, but the Gibbs free energy barrier for it to do so is only 18.9 kJ/mol, which is considerably less than the corresponding 165.0 kJ/mol barrier for ring-opening of the neutral (unreduced) monomer 1. In Figure , the ring-opened, reduced species 8 is drawn with a formal negative charge on the terminal monosulfide and a formal radical on the terminal disulfide; this depiction best reflects the computed spin-and charge-density distribution, though we note that both sulfur end groups of 8 have some anionic and some radical character. Either of these sulfur centers of intermediate 8 may in principle attack the neutral monomer to start the polymerization process, with attack by the terminal monosulfide best represented as an anionic ring-opening process, and attack by the terminal disulfide best represented as a radical process (Fig. ). Of both pathways, attack by the monosulfide anion was found to be considerably more kinetically favorable than radical attack by the disulfide terminus, so the latter can be ruled out. The monosulfide anion can then attack the terminal sulfur in the monomer to form a disulfide linkage and a new terminal disulfide anion (Fig. ). Alternatively, the central sulfur atom can be attacked to form a trisulfide linkage and a terminal monosulfide anion (Fig 8B ). Based on the calculations, both reactions are likely to occur, with an approximate 8:1 preference for the latter. As both products are thus likely to be produced, they were both considered for the next propagation step.