The kinetics of the heat release of the reaction of radical styrene polymerization that is carried out in a glass batch microreactor under conditions of weak heat exchange with the surrounding medium is experimentally studied. The recording of the intensity of light scattering of a laser light source was conducted simultaneously with the recording of the temperature during the reaction. It is found that in the range of the values of the determining parameters that correspond to high parametric sensitivity, there is a sharp qualitative and quantitative change in the temperature and the intensity of light scattering during the reaction, as well as in the molecular weight characteristics and residual monomer content. The mechanism of the influence of the thermal-kinetic and diffusion-viscosity parameters on the regularities of the process and the characteristics of a product is proposed. A set of equations that models the process under study is numerically solved. The results of the calculation and experimental data are compared.