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By means of computer simulations and solution of the equations of the ModeCoupling Theory (MCT), we investigate the role of the intramolecular barrierson several dynamic aspects of non-entangled polymers. The investigated dynamicrange extends from the caging regime characteristic of glass-formers to therelaxation of the chain Rouse modes. We review our recent work on thisquestion, provide new results and critically discuss the limitations of thetheory. Solutions of the MCT for the structural relaxation reproducequalitative trends of simulations for weak and moderate barriers. However aprogressive discrepancy is revealed as the limit of stiff chains is approached.This disagreement does not seem related with dynamic heterogeneities, whichindeed are not enhanced by increasing barrier strength. It is not connectedeither with the breakdown of the convolution approximation for three-pointstatic correlations, which retains its validity for stiff chains. Thesefindings suggest the need of an improvement of the MCT equations for polymermelts. Concerning the relaxation of the chain degrees of freedom, MCT providesa microscopic basis for time scales from chain reorientation down to the cagingregime. It rationalizes, from first principles, the observed devations from theRouse model on increasing the barrier strength. These include anomalous scalingof relaxation times, long-time plateaux, and non-monotonous wavelengthdependence of the mode correlators.

From caging to Rouse dynamics in polymer melts with intramolecular barriers: a critical test of the Mode Coupling Theory