  We present computer simulations of a simple bead-spring model for polymer
melts with intramolecular barriers. By systematically tuning the strength of
the barriers, we investigate their role on the glass transition. Dynamic
observables are analyzed within the framework of the Mode Coupling Theory
(MCT). Critical nonergodicity parameters, critical temperatures and dynamic
exponents are obtained from consistent fits of simulation data to MCT
asymptotic laws. The so-obtained MCT $\lambda$-exponent increases from standard
values for fully-flexible chains to values close to the upper limit for stiff
chains. In analogy with systems exhibiting higher-order MCT transitions, we
suggest that the observed large $\lambda$-values arise form the interplay
between two distinct mechanisms for dynamic arrest: general packing effects and
polymer-specific intramolecular barriers. We compare simulation results with
numerical solutions of the MCT equations for polymer systems, within the
polymer reference interaction site model (PRISM) for static correlations. We
verify that the approximations introduced by the PRISM are fulfilled by
simulations, with the same quality for all the range of investigated barrier
strength. The numerical solutions reproduce the qualitative trends of
simulations for the dependence of the nonergodicity parameters and critical
temperatures on the barrier strength. In particular, the increase of the
barrier strength at fixed density increases the localization length and the
critical temperature. However the qualitative agreement between theory and
simulation breaks in the limit of stiff chains. We discuss the possible origin
of this feature.
