Two novel sulfonate phenol ligands-3,3'-di-tert-butyl-2'-hydroxy-5,5',6,6'-tetramethyl-biphenyl-2-y l 4-X-benzenesulfonate (X=CF3, L-CF3-H, and X=OCH3, L-OMe-H)-were prepared through the sulfonylation of 3,3'-di-tert-butyl-5,5',6,6'-tetramethyl-biphenyl-2,2'-diol with the corresponding 4-substituted benzenesulfonyl chloride (1 equiv.) in the presence of excess triethylamine. Magnesium (Mg) complexes supported by sulfonate phenoxide ligands were synthesized and characterized structurally. The reaction of (MgBu2)-Bu-n with L-H (2 equiv.) produces the four-coordinated monomeric complexes (L-CF3)(2)Mg (1) and (L-OMe)(2)Mg (2). Complexes 1 and 2 are efficient catalysts for the ring-opening polymerization of epsilon-caprolactone (epsilon-CL) and trimethylene carbonate (TMC) in the presence of 9-anthracenemethanol; complex 1 catalyzes the polymerization of epsilon-CL and TMC in a controlled manner, yielding polymers with the expected molecular weights and narrow polydispersity indices (PDIs). In epsilon-CL polymerization, the activity of complex 1 is greater than that of complex 2, likely because of the greater Lewis acidity of Mg2+ metal caused by the electron-withdrawing substitute trifluoromethyl (-CF3) at the 4-position of the benzenesulfonate group. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3564-3572, 2010