Hydrogenolysis of a series of model Ziegler-Natta (Z-N) catalysts to form Ti-H bond was studied within DFT. We focused our efforts on Ti species attached to the (110) lateral cut of MgCl 2 which exist as different centres including Ti-C 2H 5, Ti-CH 2CH(CH 3) 2, and Ti-CH(CH 3)CH 2CH 3 in ethylene and propylene polymerization. In the next step, reactivity of Ti-H bond towards ethylene and propylene (1,2- and 2,1-) insertion was investigated. Results showed that insertion of ethylene and propylene into Ti-H bond has less barrier, in comparison with their insertion in Ti-C bond, however, ethylene and propylene 2,1- insertion lead to Ti-C 2H 5 and Ti-CH(CH 3) 2 centres respectively, which were stable due to strong β-agostic interactions. Finally, by considering different possible reactions of active centre, activity depression in ethylene polymerization and activity increase in propylene polymerization were explained in detail. © 2012 Elsevier B.V.