New heat-reversibly crystalline poly-(alkylated phenylene oxide)s are described the oxidative polymerization of 2,5-dimethylphenol catalyzed by (1,4,7-triisopropyl-1,4,7-triazacyclononane) copper dichloride produced poly (2,5- dimethly-1,4-phenylene oxide), which showed heat-reversible crystallinity with a high melting point at ca. 300 degreesC, although the isomeric polymer, poly(2,6-dimethyl-1,4-phenylene oxide), never recrystallizes once melted. The polymerization of 2,5-diethylphenol and 2,5-dipropylphenol gave the polymers consisting of 1,4-phenylene oxide units; the latter polymer possessed heat-reversible crystallinity, however, the former one did not.