Group 3 Metal Initiators with an [OSSO]‐Type Bis(phenolate) Ligand for the Stereoselective Polymerization of Lactide Monomers

Abstract

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.

Ring‐opening polymerization of rac‐ and meso‐lactide initiated by indium bis(phenolate) isopropoxy complexes

“…and ). Control over molar mass dispersity of the poly( rac ‐lactides) is similar to the control given by recently reported bis(phenolate) complexes of scandium and yttrium complexes bearing a rigid 1,3‐dithiapropanediyl bridge in the backbone ( M w / M n = 1.05 for yttrium and M w / M n = 1.13 for scandium) . Figures show linear dependance of M n with the initial monomer:initiator ratio for the polymerization of rac ‐lactide initiated by 1 in CH 2 Cl 2 and THF and initated by 2 in CH 2 Cl 2 , consistent with controlled polymerization.…”

“…Chiral initiator 3 gave heterotactic poly( rac ‐lactides) in THF ( P r = 0.77), showing that 3 is more stereoselective than initiators 1 and 2 . The tacticities of the polymers are similar to those for PLA prepared by [Sc(cyccp){N(SiMe 2 H) 2 }] ( P r = 0.74) and [Y(cyccp){N(SiMe 2 H) 2 }] ( P r = 0.68) . Ligand‐free indium initiators were found to be more active and more selective than indium bis(phenolate) precursors .…”

“…1–3 polymerized meso ‐lactide syndioselectively, with 2 ( P s = 0.93) showing syndioselectivity slightly higher than 1 ( P s = 0.89) and 3 ( P s = 0.89). Rare earth metal bis(phenolate) initiators produced poly( meso ‐lactides) with comparably high syndiotacticity but with broader polydispersities . Bis(phenolate) complexes of titanium and zirconium gave poly( meso ‐lactides) with a lower syndiotacticities but higher polydispersities …”

Stereoselective Ring‐Opening Polymerization of Heterocyclic Monomers

“…The polymerization proceeds in living fashion, although detailed initiation mechanism is not clear. Okuda reported that dithiobis(phenolate) scandium complexes such as 59 show high activity and selectivity for heterotactic polymerization of rac-lactide (P r up to 95%) (152)(153)(154). The coordination geometry of the tetradentate ligand ( or ) changes depending on the chirality of the ultimate repeating unit.…”

“…These polymerizations proceed via chain-end control mechanism. Recently, Okuda revealed Y, Sc, Ti, and Zr complexes with dithobis(phenolate) ligand such as 59 and 66 afford syndiotactic polymer from meso-lactide (154,165,166). Sc catalyst shows high syndioselectivity (P r = 0.92).…”