This paper investigates the effects of increase of reaction mixture viscosity on the kinetics of linear polymer creation in a bulk heteropolyaddition process of 2,4-toluene diisocyanate (TDI) and 1,4-butanediol (BD). The paper presents a method for solving a system of bulk polyaddition of 2,4-toluene diisocyanate and 1,4-butanediol process balance equations, allowing the determination of the process kinetic parameters. Determination of polymerisation reaction kinetic parameters was also made possible by the use of the so-called partial reaction rate constant. Such an approach enabled a significant simplification of the mathematical expressions describing the heteropolyaddition process and provided an opportunity to associate kinetic parameters with the average molar mass of the mixture and, thus, with the viscosity. The method presented herein facilitates an analysis of the linear polymers heteropolyaddition process.