Structural and kinetic matrix effects were observed in acceptor-catalytic copolyesterification and esterification in the presence of graphite as a filler. The reactivity of the phenolic OH groups decreases in the presence of the filler, whereas the reactivity of the alcoholic OH groups increases. As a consequence, the block length of the resulting copolymers decreases. With an increase in the specific surface and content of graphite, the value of the heterogeneous matrix effect increases. A mechanism of this phenomenon is connected with selective adsorption of intermediate active complexes of phenols and tertiary amines (general basic catalysis) or acyl ammonium complexes of acid chlorides (nucleophilic catalysis). Some poly(heteroarylenes) were found to have a heterogeneous matrix effect and an influence on the microstructure of the resulting copolymers. Depending on the structure and concentration of polyheteroarylenes in the reaction system under the conditions of acceptor-catalytic copolyesterification, both block and random copolyesters can be obtained. Polymer composites obtained in situ appeared to have enhanced the thermal and physicomechanical properties (softening temperature, thermal stability, specific impact strength and electric conductivity) in comparison to the mechanically filled composites.