The dominant distributive mixing mechanism in polymer processing is convection. Convection, involving movement of fluid elements fi om one spatial location in the system to another, results in the creation of interfacial area. Length stretch distributions can be wed as a measure of distributive mixing efficiency. We have studied the dynamics of distributive mixing in single and tangential twin screw extruders by means of tracking the evolution of particles originally gathered as clusters. Length stretch distributions and the dynamics of the average length stretch indicate that the tangential twin screw extruder is a better mixing device by comparison with the single screw extruder. The chaotic features of flow in these devices, studied by means of Poincare sections and Lyapunov exponents, correlate well with the distributive mixing efficiency of the equipment.