A fundamental theoretical investigation is conducted for the molecular weighs distribution formed in free-radical and living copolymerizations with macromonomers by using the random sampling technique. General analytical expressions for the number- and weight-average molecular weight developments are obtained. The full molecular weight distribution functions are presented for some simpler cases with low mole fractions of macromonomers. The present theoretical analysis provides a great insight into the complex molecular buildup processes, and thus leads to a better control of the graft copolymers.