Stereorigid OSSO-Type Group 4 Metal Complexes in the Ring-Opening Polymerization of rac-Lactide

Abstract

The synthesis and characterization of a series of group 4 metal complexes of general formula {OSSO}M(OR) (X = R = Bu, M = Zr (1); X = cumyl, M = Zr, R =Bu (2); X = cumyl, M = Ti, R = Pr (4); X = cumyl, M = Hf, R =Bu (5)) and {OSSO}Zr (X = Cl (3)) supported by o-phenylene-bridged bis(phenolato) ligands (OSSO-H = 6,6'-((1,2-phenylenebis(sulfanediyl))bis(methylene))bis(2,4-di-tert-butyphenol); OSSO-H = 6,6'-((1,2-phenylenebis(sulfanediyl))bis(methylene))bis(2,4-bis(2-phenylpropan-2-yl)phenol); OSSO-H = 6,6'-((1,2-phenylenebis(sulfanediyl))bis(methylene))bis(2,4-dichlorophenol)) are described herein. Complexes 1-5 were readily obtained by σ-bond metathesis reactions between the proligand and the appropriate homoleptic metal precursor. The reaction with OSSO yielded the bis-ligand complex{OSSO}Zr (3) regardless of the OSSO-H/Zr(OBu) molar ratio or experimental conditions. All complexes were characterized in solution using NMR spectroscopy and, in the case of 2, by single-crystal X-ray diffraction experiments. These complexes show a fac-fac ligand wrapping and a cis relationship between the other two monodentate ligands; zirconium and hafnium complexes 1-3 and 5 are configurationally stable, whereas titanium complex 4 is fluxional in solution at room temperature. The complexes tested in the ring-opening polymerization (ROP) of racemic-lactide showed, except in the case of 3, moderate rates and good levels of polymerization control. Upon addition of an exogenous alcohol (isopropyl alcohol or tert-butyl alcohol) efficient binary catalytic systems were achieved. Polymerizations were well-controlled, as testified by the linear growth of the molecular weight as polymerization proceeded, narrow polydispersity indices, and molecular weights close to those expected on the basis of added alcohol amounts. Experimental and theoretical evidence is provided that ROP reactions operate according to an activated monomer mechanism.

Phenylene-Bridged OSSO-Type Titanium Complexes in the Polymerization of Ethylene and Propylene

“…14 These compounds were recently employed by us for the synthesis of a series of alcoholate group IV complexes that mediated the immortal ROP of l -lactide affording polymers with controlled molecular weights and narrow polydispersity indexes. 14…”

“…The compounds 6,6′-((1,2-phenylenebis(sulfanediyl))bis(methylene))bis(2,4-di- tert -butyphenol) (OSSO t Bu -H) and 6,6′-((1,2-phenylenebis(sulfanediyl))bis(methylene))bis(2,4-bis(2-phenylpropan-2-yl)phenol) (OSSO Cum -H) were synthesized following the literature procedure. 14…”

Factors influencing catalytic behavior of titanium complexes bearing bisphenolate ligands toward ring-opening polymerization of L-lactide and ε-caprolactone

“…Since PLA and PCL polymers have wide application in medical field, the minimization of residual hazardous metal in these polymers after synthesis is essential. For this reason as well as its low cost, titanium complexes [8][9][10][11][12][13][14] were chosen as popular catalysts in ROP. Among these titanium complexes, multi-phenol ligands have proven particularly useful because of its ease of synthesis and diverse structural variation [11][12][13][14].…”

Synthesis and Characterization of New Cyclam-Based Zr(IV) Alkoxido Derivatives

“…The latter feature was largely responsible for the extensive use of titanium and zirconium alkoxidos in catalysis [1,2]. The high polarity of M-OR bonds led to the application of several families of early transition metal complexes in the ring opening polymerization (ROP) of cyclic esters [3][4][5][6][7][8][9][10][11][12]. Our previous research on cyclam-based Zr(IV) alkoxido, phenoxido and thiophenoxido derivatives [13][14][15] also disclosed new catalytic systems for the ROP of rac-lactide strongly influenced by the nature of the OR ligands [13].…”