The copolymerization of tert-butyl glycidyl ether (tBuGE), allyl glycidyl ether (AGE), ethoxyethyl glycidyl ether (EEGE) and 1,2-epoxybutane (BO) with γ-thiobutyrolactone was investigated using benzyl alcohol–phosphazene bases as initiating systems. The prepared copolymers display perfect (poly­(ester-alt-sulfide)) alternating structures for all epoxide monomers as evidenced by 1H, 13C, and 2D NMR and MALDI–TOF mass spectrometry. A marked influence of the reaction temperature on the occurrence of transesterification side reactions has been evidenced. In particular, at elevated temperature, transesterification reactions led to the formation of alternating macrocycles. The choice of the phosphazene base (tBuP4, tBuP2, or tBuP1) has a strong impact on the system reactivity and on the control of the polymerization. The use of tBuP1 led to very slow polymerization rates. The polymerization is much faster in the presence of tBuP2 even at low temperature. The use of tBuP4 shows an intermediate polymerization rate and enabled a good polymerization control at moderate temperatures. Finally, the synthesis of well-defined polyether-block-poly­(ester-alt-sulfide) and the polymerization of a challenging substituted γ-thiolactone were proven to be feasible.