Novel aromatic main-chain benzoxazine copolymer oligomers are synthesized by the Mannich reaction. Cured main-chain benzoxazine copolymers possess improved processing and high-frequency dielectric properties in comparison with corresponding main-chain benzoxazines, owing to the decreased molecular polarity and molecular weight. It should be noted that the polar/nonpolar heterogeneous solvent indeed plays a key role in the preparation of main-chain copolymer oligomers. The copolymers prepared in the optimized heterogeneous solvent condition has the least activation energy, the minimum high-frequency dielectric constants (2.81, 5 GHz; 2.75,10 GHz), and a relatively high glass transition temperature (260 degrees C). Therefore, this work not only provides a simple path for improving processing and high-frequency dielectric properties of main-chain benzocaine but also gives some insight into the effects of solvent on the formation of copolymers.