Specially designed block copolymers have played a role as compatibilizing agents in the system of immiscible polymer blends. We applied lauryl lactam (LA)caprolactone (CL) block copolymer [P(LA-b-CL)] as a compatibilizing agent for immiscible poly(vinyl chloride) (PVC) blends with various polymers. These blends possess high thermal performance and toughness. We investigated the effect of P(LA-b-CL) as a compatibilizing agent for immiscible PVC blends with poly(omega-lauryl lactam) [polyamide 12 (PA12)]. We also described the invention of a new compatibilizing agent system involving P(LA-b-CL) for PVC/polypropylene (PP) blends. The mechanical and thermal properties of (1) PVC/PA12 blend compatibilized with P(LA-b-CL) and (2) PVC/PP blend compatibilized with P(LA-b-CL)/PA12/maleic anhydride-modified PP were both enhanced. (C) 2003 Wiley Periodicals, Inc.