A method for producing an AB block copolymer with PDI is provided. The method comprises steps of: 1) reaction, in the presence of at least one free radical initiator, of a reaction mixture comprising a compound of formula (I), and 2) reaction, at a temperature of 45° C. or above, of a reaction mixture comprising an ethylenically unsaturated monomer B and, independent from that of step 1), a compound of formula (I) and the macroinitiator of step 1) in solvent(s) to obtain the AB block copolymer; wherein the weight ratio of the compound of formula (I) to the macroinitiator in step 2) is at least 1:1000.