Polyamine macromonomer possessing a polymerizable vinyl group at the chain end was synthesized by lithium amide catalyzed anionic self-polyaddition reaction of N,N′-diethyl-N-(4-vinylphenethyl)ethylenediamine. To elucidate the reaction mechanism for the macromonomer synthesis, the quantitative analyses were carried out on this reaction using double-wavelength UV method as well as UV fourth order derivative spectra method. The number average molecular weight of the macromonomer was also measured with these UV methods. From these studies, it was concluded that one to one complex consisting of >NH and >NLi played an important role in the successive addition reactions. When the ratio of [>NH] to [>NLi] became smaller to approach unity, there occurred a break point at which the rate of the addition reaction decreased sharply. The number average molecular weight of the macromonomer at the point was expressed by a simple equation (cf. eq 7). Thus, it was possible to control the molecular weight of the polyamine macromonomer by adjusting the relative concentrations of >NH to >NLi in the initial mixture for the polyaddition reaction.