Molecular design of single-site metal alkoxide catalyst precursors for ring-opening polymerization reactions leading to polyoxygenates.: 1.: Polylactide formation by achiral and chiral magnesium and zinc alkoxides, (η3-L)MOR, where L = trispyrazolyl- and trisindazolylborate ligands

Single-site achiral and chiral C-3-symmetric complexes LMOR, where M = Mg and Zn, L = an eta (3)-trispyrazolyl- or eta (3)-trisindazolyl-borate ligand and R = Et, Bu-t, Ph, or SiMe3, have-been synthesized and employed in the ring-opening polymerization of L-, rac-, and meso-lactide in CH2Cl2 at 25 degreesC and below. The polymerization occurs by acyl cleavage and gives rise to polylactide, PLA, with PDI of 1.1-1.25 up to 90% conversion. Studies of the kinetics of polymerization reveal first order behavior in both lactide and metal catalyst. For L = tris(3-tert-butylpyrazolyl)borate, ((t)Bupz)(3)BH, polymerization of similar to 500 equiv of L-lactide proceeds to 90% conversion within 1 h and 6 d for the magnesium and zinc catalysts, respectively. The zinc complexes are, however, more tolerant to air and moisture and solid samples where R = SiMe3 are persistent in air for several days. The rate of polymerization is also significantly influenced by the nature of the eta (3)-L spectator li,ligand. Chiral C-3-symmetric catalysts, where L = tris(indazolyl)borates derived:from camphor and menthone, show only slight enantioselectivity in their polymerization of rac-lactide but do show significant diastereoselectivity in their ability to preferentially polymerize meso-lactide from a mixture of rac- and meso-lactide. The poly(mesolactide) shows a modest preference for syndiotactic junctions, RSRSRS. The molecular structures, deduced from single-crystal X-ray crystallography, are reported for eta (3)-HB(3-Phpz)(3)MgEt(THF), [eta (3)-HB((7R)-Pr-i-(4R)-Me-4,5,6,7-tetrahydro-2H-indazolyl)(3)]ZnMe and [eta (3)-HB(3-(t)Bupz)(3,5-(CF3)(2)pz)(2)]-ZnOSiMe3 and serve as structural models for activated and ground-state configurations of the metal ions during the polymerization reaction. The molecular structure of meso-lactide is also reported. These results are compared with polymerizations of lactide by other coordinate catalysts.