main chain (backbone)

of a polymer
That linear chain to which all other chains, long or short or both, may be regarded as being pendant.
Where two or more chains could equally be considered to be the main chain, that one is selected which leads to the simplest representation of the molecule.
PAC, 1996, 68, 2287. 'Glossary of basic terms in polymer science (IUPAC Recommendations 1996)' on page 2294 (