DETERMINATION OF END-TO-END DISTANCES IN OLIGOMERS BY PULSED EPR

Conformations of end-labeled alkanes in a rigid polymer matrix were in vestigated using the pulsed double electron-electron resonance (DEER) method. To this end, dipolar spectra of a series of aliphatic dicarbon ic acids with 8-20 methylene units and chain ends bifunctionalized wit h nitroxide radicals were recorded in a polystyrene matrix. The biradi cals are derived from hindered amine light stabilizers (HALS), used as polymer additives. Analysis of the dipolar spectra allows the determi nation of the end-to-end distances of the oligomer biradicals. The exp eriments show that the ''flexible'' alkane chains are almost completel y stretched. This observation is consistent with the results of force field calculations.