The copolymerization of a precursor of poly(p-phenylene benzobisthiazole) (PBZT) with aromatic polyamides was attempted. Two types of copolymers, randomlike and blocklike, which have different properties, were synthesized according to the copolymerization process. The copolymers were suitable for use as the reinforcing polymer in molecular composite because of the improved intermolecular hydrogen bridges between the matrix polymers, such as aromatic polyamides, and then could be converted to the PBZT copolymers by heat treatment of the molecular composite. In particular, the possibility that the fine phase structure of the molecular composite was maintained was shown, even after heat treatment at above the melting temperature of the thermoplastic matrix polymer, due to the use of a PBZT copolymer as the reinforcing polymer, introduced a fragment which had the same molecular structure as in the matrix polyamides.