In this work we used quantum mechanics hybrid models (QM/QM) to investigate the high-temperature polymerization of styrene with the aim of elucidating its elementary chemistry. High and low quality quantum mechanics calculations were performed at the B3LYP/6-31g(d,p) and PM3 levels, respectively. Reaction kinetic constants were calculated for oligomers composed of up to 6 styrene units with classic transition state theory on potential energy surfaces determined at the QM/QM level. The capability of this approach to predict kinetic constants of elementary processes relevant to styrene polymerization was validated through the satisfactory comparison with well-known experimental data. One of the main results of this study is that a new kinetic route is proposed to describe the polymer growth mechanism, in which the succession of backbiting and beta scission reactions plays a critical role. In particular we found that the 7:3 backbiting reaction can proceed fast and influence significantly the polymer weight distribution. Finally, the so evaluated kinetic constants were introduced in a polymerization reactor model and used to compare the distribution of the produced oligomers measured experimentally with that predicted from first principles. The agreement with experimental data is good, suggesting that the proposed approach provides a valuable tool to investigate the kinetics of polymerization systems for which experimental data on elementary reactions are not available.