# Molecular weight of Polymers

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?

• – Mithoron Sep 20 '16 at 10:57
• Mithoron gave a good link. // The short answer is that polyethylene doesn't have a particular molecular mass, so there is no exact value for $n$. So no matter what the process the resulting polymer will be a mixture of chain lengths. The polymerization process can be "tuned" to give short lengths, medium lengths or long lengths so that the resulting plastic has different properties. – MaxW Sep 20 '16 at 17:32