A process of preparing a block copolymer, said block copolymer comprising a plurality of units, at least two of said units independently comprise a plurality of polymerized monomers of a cyclic ester, at least one unit of said at least two units comprises a plurality of polymerized monomers of a first cyclic ester, and at least one another unit of said at least two units comprises a plurality of polymerized monomers of a second cyclic ester, said second cyclic ester differing from said first cyclic ester by a stereoconfiguration and/or a chemical composition, the process comprising: sequentially contacting a plurality of monomers of said first cyclic ester and a plurality of monomers of said second cyclic ester with a catalyst system comprising an initiator and a {ONNN}M-X complex, wherein M is a divalent metal and X is a monoanionic ligand, to thereby sequentially effect a ring opening polymerization of said first cyclic ester and of said second cyclic ester, said complex being represented by Formula IA: wherein M is magnesium; X is said monoanionic ligand; A is a bridging moiety of 1 to 12 carbon atoms; R 1 and R 2 are each independently hydrogen, alkyl, cycloalkyl, aryl or alternatively, one or both of R 1 and R 2 form together, optionally with one or more carbon atoms in A, a heteroalicyclic or heteroaromatic, 5 to 7-membered ring; n and m are each independently an integer of from 1 to 6; Ra and Rb are each independently hydrogen, alkyl, cycloalkyl, aryl, heteroaryl, heteroalicyclic, hydroxyl, alkoxy, thiol, thioalkoxy, aryloxy, and amine, wherein when m is other than 1, Ra and Rb in each (CRaRb) unit can be the same or different, and one or both Ra and Rb in one unit can form a 5-, 6- or 7-membered alicyclic, heteroalicyclic, aromatic or heteroaromatic ring with one or both Ra and Rb of an adjacent unit; Rc and Rd are each independently hydrogen, alkyl, cycloalkyl, alkaryl, aryl, heteroaryl, heteroalicyclic, hydroxyl, alkoxy, thiol, thioalkoxy, aryloxy, and amine, wherein when n is other than 1, Rc and Rd in each (CRcRd) unit can be the same or different, and one or both Rc and Rd in one unit can form a 5-, 6- or 7-membered alicyclic, heteroalicyclic, aromatic or heteroaromatic ring with one or both Rc and Rd of an adjacent unit; and R 23 -R 30 are each independently selected from the group consisting of hydrogen, alkyl, cycloalkyl, alkaryl, aryl, halo, alkoxy, aryloxy, silyl, heteroalicyclic, heteroaryl, and amine. sequentially contacting a plurality of monomers of said first cyclic ester and a plurality of monomers of said second cyclic ester with a catalyst system comprising an initiator and a {ONNN}M-X complex, wherein M is a divalent metal and X is a monoanionic ligand, to thereby sequentially effect a ring opening polymerization of said first cyclic ester and of said second cyclic ester, said complex being represented by Formula IA: wherein M is magnesium; X is said monoanionic ligand; A is a bridging moiety of 1 to 12 carbon atoms; R 1 and R 2 are each independently hydrogen, alkyl, cycloalkyl, aryl or alternatively, one or both of R 1 and R 2 form together, optionally with one or more carbon atoms in A, a heteroalicyclic or heteroaromatic, 5 to 7-membered ring; n and m are each independently an integer of from 1 to 6; Ra and Rb are each independently hydrogen, alkyl, cycloalkyl, aryl, heteroaryl, heteroalicyclic, hydroxyl, alkoxy, thiol, thioalkoxy, aryloxy, and amine, wherein when m is other than 1, Ra and Rb in each (CRaRb) unit can be the same or different, and one or both Ra and Rb in one unit can form a 5-, 6- or 7-membered alicyclic, heteroalicyclic, aromatic or heteroaromatic ring with one or both Ra and Rb of an adjacent unit; Rc and Rd are each independently hydrogen, alkyl, cycloalkyl, alkaryl, aryl, heteroaryl, heteroalicyclic, hydroxyl, alkoxy, thiol, thioalkoxy, aryloxy, and amine, wherein when n is other than 1, Rc and Rd in each (CRcRd) unit can be the same or different, and one or both Rc and Rd in one unit can form a 5-, 6- or 7-membered alicyclic, heteroalicyclic, aromatic or heteroaromatic ring with one or both Rc and Rd of an adjacent unit; and R 23 -R 30 are each independently selected from the group consisting of hydrogen, alkyl, cycloalkyl, alkaryl, aryl, halo, alkoxy, aryloxy, silyl, heteroalicyclic, heteroaryl, and amine.