Group 3 metal initiators with an [OSSO]-type bis(phenolate) ligand for the stereoselective polymerization of lactide monomers.

A series of 1,ω-dithiaalkanediyl-bridged bis(phenols) of the general type [OSSO]H(2) with variable steric properties and various bridges were prepared. The stoichiometric reaction of the bis(phenols) 1,3-dithiapropanediyl-2,2'-bis(4,6-di-tert-butylphenol), 1,3-dithiapropanediyl-2,2'-bis[4,6-di(2-phenyl-2-propyl)phenol], rac-2,3-trans-propanediyl-1,4-dithiabutanediyl-2,2'-bis[4,6-di(2-phenyl-2-propyl)phenol], rac-2,3-trans-butanediyl-1,4-dithiabutane diyl-2,2'-bis[4,6-di(2-phenyl-2-propyl)phenol], rac-2,3-trans-hexanediyl-1,4-dithiabutanediyl-2,2'-bis[4,6-di(2-phenyl-2-propyl)phenol], 1,3-dithiapropanediyl-2,2'-bis[6-(1-methylcyclohexyl)-4-methylphenol] (C(1), R=1-methylcyclohexyl), and 1,4-dithiabutanediyl-2,2'-bis[6-(1-methylcyclohexyl)-4-methylphenol] with rare-earth metal silylamido precursors [Ln{N(SiHMe(2))(2)(3)(thf)(x)] (Ln=Sc, x=1 or Ln=Y, x=2; thf=tetrahydrofuran) afforded the corresponding scandium and yttrium bis(phenolate) silylamido complexes [Ln(OSSO){N(SiHMe(2))(2)}(thf)] in moderate to good yields. The monomeric nature of these complexes was shown by an X-ray diffraction study of one of the yttrium complexes. The complexes efficiently initiated the ring-opening polymerization of rac- and meso-lactide to give heterotactic-biased poly(rac-lactides) and highly syndiotactic poly(meso-lactides). Variation of the ligand backbone and the steric properties of the ortho substituents affected the level of tacticity in the polylactides.