Abstract: A method and system for regulating the isotacticity of polypropylene in a continuous process for manufacturing polypropylene by the polymerization of alpha-olefins in a fluidized bed reactor system using a model which correlates isotacticity values to catalyst productivity values. A catalyst productivity target value is computed using the model based upon a selected xylene solubles or isotactic index. The instantaneous catalyst productivity of the system is then determined by dividing product production rate by catalyst flow rate which is then compared to the catalyst productivity target value. When the comparison between the instantaneous catalyst productivity value and the catalyst productivity differ the molar feed ratio of SCA to alkyl is automatically adjusted until the catalyst productivity, and hence isotacticity, is changed approaching the target values.