Modification of polymers by radical initiated addition of reactive monomers has two limiting cases : grafting of long chains accompanied by homopolymer formation and isolated substitution by the monomer units, respectively. In both cases (which in simplified form can be treated similarly) the efficiency, i.e. the monomer conversion after decomposition of a given amount of the initiator is a practically important characteristic of the process. Knowing the temperature dependent rates of the reactions, this conversion is calculable.