DFT computations have been performed to investigate the mechanism of H-2-assisted chain transfer strategy to functionalize polypropylene via Zr-catalyzed copolymerization of propylene and p-methylstyrene (pMS). The study unveils the following: (i) propylene prefers 1,2-insertion over 2,1-insertion both kinetically and thermodynamically, explaining the observed 1,2-insertion regioselectivity for propylene insertion. (ii) The 2,1-inserion of pMS is kinetically less favorable but thermodynamically more favorable than 1,2-insertion. The observation of 2,1-insertion pMS at the end of polymer chain is due to thermodynamic control and that the barrier difference between the two insertion modes become smaller as the chain length becomes longer. (iii) The pMS insertion results in much higher barriers for subsequent either propylene or pMS insertion, which causes deactivation of the catalytic system. (iv) Small H-2 can react with the deactivated [Zr]-pMS-PPn facilely, which displace functionalized pMS-PPn chain and regenerate [Zr]H active catalyst to continue copolymerization. The effects of counterions are also discussed. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 576-585