**3.1 Mechanism of cracking reaction**

In general, the following steps of the cracking reaction can be identified:


When the bond between hydrogen and carbon atom is broken, a radical is created (initiation step). In the second step of the process, polymer bond breaks at the *β* position, creating one unsaturated molecule and another radical. Depending on the type of polymer, this cracking can follow one of these types of reactions:


In the case of polyolefins, cracking occurs mainly by random scission and backbiting reactions.

In the last step of the process, the radical can react with another radical, creating saturated alkane (recombination), one alkane and one alkene (disproportionation), create cyclic structure (cyclisation), dehydrogenate and condensate and take part in other reactions. In the end, a mixture of different types of hydrocarbons with varying chain lengths is created. When condensation and dehydrogenation reaction progress, polycyclic aromatic hydrocarbons (PAH) are created, which still react and finally create a coke, that is reach in carbon.
