Pseudo block and triblock copolymers were synthesized by the cationic ring-opening copolymerization of 1,5,7,11-tetraoxaspiro[5.5]-undecane (SOC1) with trimethylene oxide (OX) via one-shot and two-shot procedures, respectively. When SOC1 and OX were copolymerized cationically with boron trifluoride etherate (BF3OEt2) as an initiator in CH2Cl2 at 25 degrees C, OX was consumed faster than SOCI. SOCI was polymerized from the OX-rich gradient copolymer produced in the initial stage of the copolymerization to afford the corresponding pseudo block copolymer, poly [(OX-grad-SOC1)-b-SOC1]. We also succeeded in the synthesis of a pseudo triblock copolymer by the addition of OX during the course of the polymerization of SOCI before its complete consumption, which provided the corresponding pseudo triblock copolymer, poly[SOC1-b-(OX-grad-SOC1)-b-SOC1].