  We present molecular dynamics simulations of a simple model for polymer melts
with intramolecular barriers. We investigate structural relaxation as a
function of the barrier strength. Dynamic correlators can be consistently
analyzed within the framework of the Mode Coupling Theory (MCT) of the glass
transition. Control parameters are tuned in order to induce a competition
between general packing effects and polymer-specific intramolecular barriers as
mechanisms for dynamic arrest. This competition yields unusually large values
of the so-called MCT exponent parameter and rationalize qualitatively different
observations for simple bead-spring and realistic polymers. The systematic
study of the effect of intramolecular barriers presented here also establishes
a fundamental difference between the nature of the glass transition in polymers
and in simple glass-formers.
