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

Abstract

Single-site achiral and chiral C 3 -symmetric complexes LMOR, where M = Mg and Zn, L = an η 3 -trispyrazolyl- or η 3 -trisindazolyl-borate ligand and R = Et, t Bu, Ph, or SiMe 3, have been synthesized and employed in the ring-opening polymerization of l -, rac -, and meso -lactide in CH 2 Cl 2 at 25 °C 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 ∼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 = SiMe 3 are persistent in air for several days. The rate of polymerization is also significantly influenced by the nature of the η 3 -L spectator 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 η 3 -HB(3-Phpz) 3 MgEt(THF), [η 3 -HB((7 R )- i Pr-(4 R )-Me-4,5,6,7-tetrahydro-2H-indazolyl) 3 ]ZnMe and [η 3 -HB(3- t Bupz)(3,5-(CF 3 ) 2 pz) 2 ]ZnOSiMe 3 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.