The preparation and compatibilizing ability of heteroarm star-shaped block copolymers (6 and 8) composed of polystyrene and nylon 6 units diverged from the phosphazene core were investigated. The heteroarm star-shaped copolymers were prepared by a partial coupling reaction of the acid chloride of hexakis(4-carboxyphenoxy)cyclotriphosphazene [4, (4-ClCOC6H4O)(6)N3P3] with polystyrene having a terminal amino group (PSt-NH2) or poly(styryllithium), followed by the ring-opening polymerization of epsilon-caprolactam (epsilon-CL) with the residual carboxyl group in the phosphazene core. The coupling reaction of telechelic PSt-NH2 and poly(styryllthium) with 4 occurs smoothly to give carboxyphenoxycyclotriphosphazenes carrying 2.2-4.7 polystyrene chains, 5 and 7, respectively. The polymerization of epsilon-CL with hexakis(4-carboxyphenoxy)cyclotriphosphazene [2, (4-HOOCC6H4O)(6)N3P3] showed that all carboxyl groups participate in the initiation of polymerization to give hexaarmed star-shaped polymers. This implies that the residual carboxyl groups in 5 and 7 could also initiate the polymerization of epsilon-CL. The molecular weights of copolymers obtained from the polymerization of epsilon-CL with 5 and 7 are in agreement with those of calculated values, suggesting the formation of heteroarm star-shaped block copolymers with phosphazene core (6 and 8). When these copolymers were used as a compatibilizer of an immiscible blend, poly(2,6-dimethylphenylene oxide) (PPO) and nylon 6, DSC profiles showed a single transition temperature that shifted to lower temperature from the T-g of PPO. The SEM photograph indicates that PPO disperses regularly and finely with 0.3 mu m size in the nylon domain. Furthermore, the tensile strength and elongation at break of ternary blends increased with increasing the copolymer content. These results indicate that the effective penetration of each block sequence of the heteroarm star-shaped copolymers into the blend components is operative, that is, these copolymers act as an excellent compatibilizer for PPO/nylon 6 blends.