Alkaline Earth Metal-Mediated Highly Iso-selective Ring-Opening Polymerization of rac-Lactide

Highly efficient Ti-catalyst for deoxygenative reduction of esters at ambient conditions: experimental and mechanistic insights from DFT Study

In this paper, we report the synthesis of dianionic amidophosphineborane–supported titanium chloride [{Ph2P(BH3)N}2C6H4}TiCl2] (1) and TiIV alkyl complex [{Ph2P(BH3)N}2C6H4}Ti(CH2SiMe3)2] (2) using salt metathesis reaction. The TiIV complex 1 was obtained...

Efficient Bulky Organo-Zinc Scorpionates for the Stereoselective Production of Poly(rac-lactide)s

The direct reaction of the highly sterically demanding acetamidinate-based NNN'-scorpionate protioligand Hphbp^tamd [Hphbp^tamd = N,N'-di-p-tolylbis(3,5-di-tertbutylpyrazole-1-yl)acetamidine] with one equiv. of ZnMe₂ proceeds in high yield to the mononuclear alkyl zinc complex [ZnMe(κ³-phbp^tamd)] (1). Alternatively, the treatment of the corresponding lithium precursor [Li(phbp^tamd)(THF)] with ZnCl₂ yielded the halide complex [ZnCl(κ³-phbptamd)] (2). The X-ray crystal structure of 1 confirmed unambiguously a mononuclear entity in these complexes, with the zinc centre arranged with a pseudotetrahedral environment and the scorpionate ligand in a κ³-coordination mode. Interestingly, the inexpensive, low-toxic and easily prepared complexes 1 and 2 resulted in highly efficient catalysts for the ring-opening polymerisation of lactides, a sustainable bio-resourced process industrially demanded. Thus, complex 1 behaved as a single-component robust initiator for the living and immortal ROP of rac-lactide under very mild conditions after a few hours, reaching a TOF value up to 5520 h^-1 under bulk conditions. Preliminary kinetic studies revealed apparent zero-order dependence on monomer concentration in the absence of a cocatalyst. The PLA materials produced exhibited narrow dispersity values, good agreement between the experimental M_n values and monomer/benzyl alcohol ratios, as well as enhanced levels of heteroselectivity, reaching P_s values up to 0.74.

Alkali and Alkaline Earth Metal Complexes as Versatile Catalysts for Ring-Opening Polymerization of Cyclic Esters

Biodegradable polyesters such as poly(ϵ-caprolactone) (PCL) and poly(lactic acid) (PLA) have been considered for use in several areas, such as drug delivery devices, sutures, tissue engineering, and GBR membranes, due to its bio-renewability, biodegradability, and biocompatibility. Several synthetic techniques for the preparation of polyesters have been reported in the literature, amongst which the ring-opening polymerization (ROP) of cyclic esters is the most efficient. A convenient approach to access iso-selective PLAs is polymerization of racemic lactide (rac-LA), which shows excellent stereoregularity without the need for costly chiral auxiliaries or ligands. In this personal account, we review a series of methods that have been practiced to the synthesis of biodegradable polyesters from various cyclic monomers using alkali and alkaline earth metal complexes as efficient catalysts.

Stereoselective polymerization of rac-lactide catalyzed by zwitterionic calcium complexes

Two zwitterionic calcium complexes L¹CaN(SiMe₃)₂(THF) (1) and L²CaN(SiMe₃)₂ (2) via protolysis reaction were synthesized. At −75 °C, 1 gave a heterotactic sequence enriched polylactide, whilst 2 produced an isotactic sequence enriched polymer.