No abstract

A series of group 4 metal complexes 1–4 (1 = (t-BuOS)2Ti(O-i-Pr)2; 2 = (t-BuOS)2Zr(O-t-Bu)2; 3 = (t-BuOS)2Hf(O-t-Bu)2; 4 = (CumOS)2Zr(O-t-Bu)2) supported by two phenolate bidentate ligands (t-BuOS–H = 4,6-di-tert-butyl-2-phenylsulfanylphenol and CumOS–H = 4,6-dicumyl-2-phenylsulfanylphenol) were synthesized by the reaction of appropriate metal tetra(alkoxide)s with 2 equiv of the ligands. The X-ray structure of 2 revealed that the two ligands were κ2-chelated to the metal center with two phenoxy groups in trans positions and the two thioether moieties in cis positions. VT NMR analysis revealed a fast inversion of configuration at the metal center. Complexes 1–4 promoted the ring-opening polymerization (ROP) of lactide and ε-caprolactone. Complex 4 exhibited the highest activity with a pseudo-first-order rate constant of 0.061 ± 0.003 min–1 at 100 °C, which compares favorably with those reported for the most active group 4 complexes. Polymerizations were well-controlled, giving predictable molecular weights and narrow molecular weight distributions. Essentially atactic PLA were obtained in the ROP of rac-lactide. A DFT study on ROP promoted by these complexes highlighted the importance of the coordinative flexibility of the ancillary ligand to promote monomer coordination at the reactive metal center. In presence of isopropanol, lower PDI indexes and molecular weights of the PLAs proportional to the equivalents of isopropanol suggested that adequate conditions for effective “immortal” polymerizations were achieved. More interestingly, these catalysts promoted the copolymerization of l-lactide and ε-caprolactone. The microstructure disclosed by 13C NMR analysis and the thermal behavior exhibited in DSC studies indicated a random distribution of the two monomer along the polymer chain

Group 4 complexes 1-3 [1 5 (t-BuOS) 2 Ti(O-i-Pr) 2 ; 2 5 (t-BuOS) 2 Zr(O-t-Bu) 2 ; 3 5 (t-BuOS) 2 Hf(O-t-Bu) 2 ] supported by two phenolate bidentate ligands (t-BuOS-H 5 4,6-di-tert-butyl-2phenylsulfanylphenol) promote the well-controlled ring opening polymerization of rac-b-butyrolactone. In presence of isopropanol, low dispersities and molecular weights proportional to the equivalents of isopropanol are achieved. Moreover, the zirconium complex is effective in the copolymerization of rac-bbutyrolactone with rac-lactide. The 13 C nuclear magnetic reso-nance analysis revealed that the obtained copolymers have a tapered diblock microstructure consisting of an initial block composed of lactide sequences and a terminal block composed of butyrolactone sequences.

a Titanium and zirconium complexes 1-3 (1 = ( t-Bu OS)2Ti(O-i-Pr)2; 2 = ( t-Bu OS)2Zr(O-t-Bu)2; 3 = ( Cum OS)2Zr(O-t-Bu)2) supported by two phenolate bidentate ligands ( t-Bu OS-H = 4,6-di-tert-butyl-2phenylsulfanylphenol and Cum OS-H = 4,6-di-cumyl-2-phenylsulfanylphenol) promoted the copolymerization of L-lactide with ε-caprolactone. The reactivity displayed by the two monomers during the copolymerization experiments and the microstructure disclosed by 13 C NMR analysis indicated a gradient distribution of the two monomers along the polymer chain. Copolymers with high ε-caprolactone content showed a large scale formation of crystalline spherulites prone to perfection of the crystallinity upon thermal annealing at 50 °C. Differently L-lactide rich copolymers revealed a thin film morphology consisting of small rigid domains of L-lactide segments of about 15 nm embedded in a soft matrix of the counterpart. Copolymers with comparable mole fractions of the two monomers were entirely amorphous.

The selective and effective synthesis of organic carbonates under mild conditions, starting from carbon dioxide and oxiranes, catalyzed by metal complexes is currently a focus of interest for both industrial and academic researchers. We recently developed a novel thioether-triphenolate iron(III) catalyst (Ct-BU) that has proven to be highly active for the coupling of CO2 with epoxides, resulting in cyclic organic carbonates under solvent-free conditions. In the current work, the properties of this novel class of catalysts were extensively investigated. In particular, the steric properties of the ligand were modulated by changing the substituents of the aromatic rings in order to obtain a deeper knowledge of the relationship between the complex structure and catalytic performance/selectivity for these iron complexes. Notably, the less steric demanding iron(III) CH complex synthesized shows, when activated by n-tetrabutylammonium bromide, an impressive turnover frequency (TOF) of 3800 h(-1) for the formation of propylene carbonate and glycerol carbonate which are, by far, the highest reported for an iron based catalyst and compares well with the most active catalyst based on other metals.