Novel sequence-ordered poly(S-alkyl thioester)s were synthesized on the basis of transformation of a backbone of the poly(S-aryl thioester) (P-1) using the regioselective insertion of thiiranes. The insertion reaction of 3-butoxypropylene sulfide (BPS) into P-1 using catalysts such as tetrabutylammonium chloride (TBAC), tetrabutylphosphonium chloride, or 18-crown-6-ether/potassium chloride in DMAc at 70 degrees C for 6 h proceeded completely and regioselectively to provide an ordered poly(S-alkyl thioester) (P-2a) with a new sequence. The molecular weight of P-2a was reasonably higher than that of the starting polymer P-1. The structure of P-2a including a head-to-tail sequence of BPS unit was well-characterized by IR, H-1 NMR, C-13 NMR spectroscopies, GPC, and elemental analysis. Various sequence-ordered poly(S-alkyl thioester)s were successfully synthesized by the insertion reaction of 3-phenoxypropylene sulfide, BPS, or cyclohexene sulfide using the TBAC catalyst under the same conditions. It was demonstrated that the transformation of the polymer backbone because of the regioselective insertion of thiiranes using quaternary onium salts was a useful method for achieving the synthesis of sequence-ordered polymers.