The important results from the substantial body of experimental data on the formation of poly(1,4-phenylene sulfide) by polymerization of copper(I) 4-bromobenzenethiolate (CBT) have been documented. Single-electron transfer (SET) mechanisms are proposed that are capable of explaining all the experimental observations in a self-consistent manner. An S(RN)1-type mechanism is thought to be dominant, with a possible contribution from a free-radical mechanism as the polymerization progresses. Both mechanisms have the characteristics of chain polymerization in the initial stages, but as the monomer concentration reduces due to polymerization the same chemistry leads to a change in the nature of the polymerization to step-wise growth, in accord with the observed two-stage variation of molar mass with reaction time. The published experimental data in the more general area of polymerization of metal(I) 4-halobenzenethiolates (MHTs) also are most satisfactorily explained in terms of the proposed SET mechanisms. However, possible contributions from an S(N)2Ar mechanism cannot be eliminated, especially for polymerizations of MHTs of alkali-metal ions with Cl or F as the halogen. It is clear, however, that SET mechanisms are dominant in polymerization of CBT.