The dynamic structure factor of entangled solutions of very high molecular weight styrene-isoprene (SI) diblock copolymers in the common solvent toluene is studied by photon correlation spectroscopy. The intermediate scattering function S(q,t) with wave vector q spanning the peak position q* of the static structure factor S(q) displays three relaxation processes in the homogeneous disordered state far away from the order-to-disorder (ODT) transition. In this regime, the systems investigated are marginally entangled. To examine the sensitivity of the shape of S(q,t) with regard to polydispersity and composition we report experimental S(q,t) of one symmetric and two asymmetric SI with different distributions. In all cases, chain self-diffusion and two chain conformational motions describe the experimental S(q,t). The main contribution to S(q approximate to q*,t) originates from the slow (reptation-like) chain motion with rate Gamma(1) = O(tau(1)(-1)) where the longest relaxation time sl is obtained by dynamic mechanical shear measurements. The unexpected findings relate to the stronger contribution of fast (Rouse-like) to the S(q approximate to q*, t) of the asymmetric SI rich in I (f(s) = 0.26) and the increase of the S(q similar or equal to 0) due to polydispersity with concentration in the homogeneous state.