The kinetics of propylene polymerization with a TiCl4/Di/MgCl2(Di=internal donor) supported Ziegler-Natta catalyst was studied by measuring the number of active centers using 2-thiophenecarbonyl chloride as quenching agent.Influences of hydrogen and alkoxysilane type external donors on the number of active centers and chain propagation rate constant (kp) were investigated.The number and kp value of three groups of active centers that produce atactic

medium-isotactic and isotactic polypropylene (PP) fractions were determined.The catalyst activity was found to significantly increase with the addition of hydrogen

which was a result of simultaneous increases in the number of active centers and kp value.The three groups of active centers response to hydrogen differently.Hydrogen caused lower degree of increase in the number of active centers that produce atactic PP than the other two groups of active centers

and caused the highest degree of increase in the kp value of active centers that produce medium-isotactic PP.The hydrogen response of the active centers was also influenced by external donor.Increasing the bulkiness of alkyl groups in alkoxysilane donors leads to decreased hydrogen sensitivity

meanwhile the stereo-and regio-selectivities were evidently improved

implying that there are close relations between the hydrogen effect and the stereo-and regio-selectivities of the catalyst.A mechanistic model was proposed to explain the hydrogen effect.