Passive advection of light particles carried by a high viscosity fluid in polymer processing equipment exhibits complex behavior due in part to the nonlinearity of the equation of motion. The particle motion also exhibits sensitivity to the initial positions. A fundamental understanding of the influence of initial positions of particles on the quality and rate of distributive mixing is essential for the optimization and design of processing equipment. In this work we analyze a twin-flight single-screw extruder. Based on detailed knowledge of the flow patterns, obtained through 3-D FEM numerical simulations, we study particle motion and, implicitly, mixing in the extruder. In our model particles are massless points that do not affect the flow field or the motion of other particles. We visualize the complexity of the advection by using two-dimensional Poincare sections. We then quantitatively examine distributive mixing for different initial locations of the light particles. Renyi entropies for different values of the parameter beta were calculated and related to the various distributive-mixing characteristics they reveal.