Chemistry of Magnesium and Zinc Complexes Supported by Bulky Ancillary Ligands and their Applications in the Ring-Opening Polymerization Studies of Cyclic Esters

A series of magnesium and zinc complexes supported by bulky ancillary ligands of the form (TMP)MgnBu(L), (BDI*)Mg nBu(L) and (TMP)Zn(X) were synthesized where TMP = 1,5,9-trimesityldipyrromethene, BDI* = (3-[2,6-diisopropylphenyl)amino]- 5-[(2.6-diisopropylphenyl)imino]-2,2,6,6-tetramethyl-hept-3-ene), L = THF, 2-MeTHF, pyridine and DMAP, X = N(SiMe3)2, OPri and OCMe2CO(O)Et. These were all characterized and applied in the study of ring-opening polymerization (ROP) of cyclic esters. All complexes ring-open and sustain the polymerization of lactide (LA) and epsilon-caprolactone (epsilon-CL) in solution. The magnesium catalyst series was shown to initiate ROP via a beta-H atom transfer generating an alkoxide and sequential elimination of 1- butene. The TMP supported catalyst systems show a good stereoselectivity in the ring-opening polymerization of rac-Lactide (rac-LA) with Pr values ranging between 0.80 - 0.96 with higher values observed in THF solvent. Copolymerization of rac-LA with epsilon- CL, when both monomers are present at the same time in a reaction mixture, only led to the ROP of rac-LA. Initial ROP of epsilon-CL with subsequent addition of the rac-LA resulted in stereoblock PLA-b-PCL but the vice-versa could not be attained. This was shown to be as a result of chelation of the ketonic group to the metal center that upon ROP of LA inhibits the enchainment of epsilon-CL. This was further proved by the synthesis of the model compound (TMP)Zn(OCMe 2COOEt) where its carbonyl group, in solution and solid state, was shown to be chelated.;Role of solvent was investigated and we indeed observed that THF accelerated the ROP of epsilon-CL while suppressing that of LA. A detailed explanation of this phenomenon is given in chapter 4.