The Vibrations of an Infinitely Long Chain of CH2-Groups and the Infra-Red Spectrum of Polythene

An attempt has been made to interpret the infra-red spectrum of polythene as the spectrum of an infinitely long chain of CH2-groups. The vibrations of such a chain have been calculated using a potential function which involves valence and angle forces as well as the torsional force resisting a twisting of the C-C bonds. The selection rules and states of polarization of the fourteen possible frequencies are derived. The observed bands are assigned to the calculated vibrations and the force constants are evaluated. It is shown that the oscillations involving the C-C valence force are weakly active in contradiction to the theory In the last section, this activity has been ascribed to the presence of CH3-groups at the ends of the polythene chain.