Addition polymerisation is defined in my textbook as the process in which a molecule or compound is formed by combination of small molecules (monomers) implicitly having greater molecular mass than either of the monomers.
"A molecule is the smallest part of a substance which can exist independtly."
By this definition, one should find THAT part (Smallest freely existing part) of any polymer and then extract a molecular formula for the polymer. But it does not happen in actual. For example, polythene is assigned a molecular formula $(C_2H_4)_n$.
I just want to know what kind of molecule is determined by this kind of molecular formula as far as the definition of molecule is concerned?
And by what means I can find out its (polythene's) molecular mass?