Organometallic complexes of sequential tetradentate monoanionic ligands and uses thereof in ring opening polymerization of cyclic esters

A process of ring opening polymerization of a cyclic ester, the process comprising contacting the cyclic ester with a catalyst system comprising an organometallic complex represented by Formula I:

• wherein:
• the dashed line represents a coordinative bond;
• M is a divalent metal;
• X is a monoanionic ligand;
• A, BA and BB are each independently a bridging moiety of 1 to 12 carbon atoms;
• R1 and R2 are each independently hydrogen, alkyl, cycloalkyl, aryl or alternatively, one or both of R1 and R2 form together, optionally with one or more carbon atoms in A, a heteroalicyclic or heteroaromatic, 5-membered, 6-membered or 7-membered ring;
• and
• R3 and R4 are each independently hydrogen, alkyl, cycloalkyl, aryl or alternatively, one or both of R3 and R4 form together with one or more carbon atoms in BB, a heteroalicyclic or heteroaromatic, 5-membered, 6-membered or 7-membered ring, and a co-catalyst.