Dancing PEO
This animation shows just one of 258 different normal modes of this short, stretched PEO molecule... And the molecule can assume great many different conformations (and every conformation has its own set of normal modes)... And typical length of polymers, which we encounter in everyday life, is not six repeat units, but something of the order of 10000...

A natural question arises -- then how, in the world, can we expect that vibrational spectroscopy will provide any sensible information about the molecule? Aren't we about to be drowned in the millions and billions of vibrational frequencies observed in the spectra?!!

Fortunately, this is not the case. Different chemical units possess characteristic vibrational frequencies ("fingerprints"), often fairly independent of the rest of the structure. These are known as group frequencies...