Answer: -72 kJ/mol
In the given polymerisation reaction, one mole of ethylene involves breaking of one C=C double bond and formation of three C-C single bonds. However, in the complete polymer chain, the number of single C-C bonds is two per C=C double bond broken.
Energy required in breaking of one mole of C=C double bond = 590 kJ
Energy released in the formation of two moles of C-C single bonds = $2\times 331 = 662$ J
$\therefore$ Net energy releases per mole ethylene = 662 - 590 = 72 kJ
i.e., $\Delta H = -72 $ kJ/mol