In this paper

a new theoretical way to deduce molcular weight distribution functions in the simple copolymer model is given according to the most probable state using the basic principle in statistical physics for two systems of typical linear copolymers

the forming of which obeys the mechanism of free radical termination by combination and disproportionation.In combination with kinetics principle and probability of the free radical copolymerization

the composition equations i.e. the Mayo-Lowis and the Alfrey-Goldfinger copolymer composition equations are obtained through these distribution functions.This study could help to understand the physical foundation of composition equation in the simple copolymer model and offer at the same time a new method to analyze the free radical copolymerization mechanism.