"True" initial polymerization rates can be calculated from the adiabatic temperature rise under isoperibolic conditions. They are much higher than initial polymerization rates measured by standard mass flow meter methods under quasi-steady-state conditions. These high initial rates are followed by a fast apparent deactivation ("attenuation") until a constant ("plateau") activity is reached at low polymerization yields of 2-3 g PP (.) (g catalyst)(-1) caused by an "increasing degree of encapsulation" of active sites. Mass transfer limitations are not observed. Cross sectional SEM images of the polymer samples support these kinetic findings.