A combined QM/MM study of the ethylene/1-hexene copolymerisation with bisiminepyridine iron(II) is presented. It has been found experimentally that these catalysts do not copolymerise ethylene with 1-hexene. Based on the mechanism of propagation and termination processes proposed by Deng et al., we have performed calculations for the ethylene and 1-hexene monomers in order to give a suitable explanation to the experimental findings. The propagation reaction is divided into two fundamental steps: the backside monomer capture and the backside monomer insertion. The energy barriers for these steps are, respectively, 2.11 and 0.59 kcal/mol for the ethylene monomer and 6.62 and 5.43 kcal/mol for 1-hexene. Additionally, the backside pi-complex formation for 1-hexene is an endothermic process by 0.72 kcal/mol. Therefore, the ethylene propagation reaction is very favourable as compared to the 1-hexene propagation. In the same way, the termination reaction is also divided into two elementary steps: the frontside monomer capture and the beta-hydrogen transfer steps. The associated energy barriers for these two processes are, respectively, 5.83 and 6.55 kcal/mol for the ethylene monomer and 6.03 and 8.38 kcal/ mol for 1-hexene. So, the differences between the rate-limiting step of the propagation and termination energy barriers are 4.44 kcal/mol for the ethylene and 1.76 kcal/mol for the 1-hexene monomer. These facts are in good agreement with concurrent experimental results. Furthermore, the role of the bulky ancillary ligands has been analysed.