Chlorine atoms, thermal, addition-elimination

The second step, between 400°C and 800°C is exothermic. In this step, elimination of HCI takes place from the polyene at a lower rate since chlorine atoms linked to unsaturated carbon atoms are involved which gives a more thermally stable bond than chlorine-saturated carbon of the original chloroparaffin. Intermolecular addition, responsible for the exothermic effect, takes places between double bonds leading to cross-linked products46 and possibly to aromatization. 

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