Enthalpy of polymerization

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Homework Statement

The polymerization of ethylene to linear polyethylene is represented by the following reaction
$nCH_{2}=CH_{2} \rightarrow (-CH_{2}-CH_{2}-)_{n}$
where n=large integer value
Calculate the enthalpy of polymerization per mol of ethylene at 298K if average bond enthalpies of bond dissociation for C=C and C-C are 590 and 331kJ/mol respectively

The Attempt at a Solution

For one mole
$ΔH_{poly} = BDE of C=C+4\times BDE C-H - BDE of C-C - 4\times BDE C-H$
=590-331
=259

But this is not correct.