In a variety of amorphous polymers such as poly(propylene), poly(viny1 acetate) and poly(isobutylene), various backbone conformations are detected by magic-angle-spinning (MAS) NMR in the solid state, exploiting the y-gauche effect on the isotropic chemical shifts. In poly(propylene), the populations of the conformations are related to specific configurations determined from solution ''C NMR. Above the glass transition temperature, dynamical exchange between the conformations is observed. The non-exponentiality of the correlation functions and the motional rates are similar of those of the diffuse reorientations observed by 2D exchange NMR with slow MAS or on static samples at the same temperatures. Although trans and gauche define the preferred conformations within the distribution of rotational angles for poly-(propylene), the dynamics are not suitably described by motions ofthe atoms on a diamond 1attice.This is demonstrated in particular by the absence of long term orientational memory in 3D exchange NMR experiments. To characterize the nature of the motion further, correlation functions of orders L = 2 and L = 4 have been calculated from 2D exchange spectra without MAS. A behavior intermediate between large-angle jumps and isotropic rotational diffusion is observed.

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