Radicals are produced in a special reaction for starting a radical polymerization. Because free radicals are reactive intermediates that possess only very limited lifetimes, radicals are generally produced in the presence of a monomer that is to be polymerized. They react very rapidly with the monomer present. The rate of the reaction of initially formed free radicals with the monomer (the initiation step) is high compared with the rate of radical formation; thus, the latter process is rate determining. Therefore, radical generation by respective initiators is a very characteristic and important feature of radical initiation.