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

• Main Authors: Marta Navarro, Andrés Garcés, Luis F. Sánchez-Barba, Felipe de la Cruz-Martínez, Juan Fernández-Baeza, Agustín Lara-Sánchez
• Format: Article
• Language: English
• Published: MDPI AG 2021-07-01
• Series: Polymers
• Subjects: homogenous catalysis; zinc scorpionates; <i>rac</i>-lactide; heterotactic poly(<i>rac</i>-lactide)s
• Online Access: https://www.mdpi.com/2073-4360/13/14/2356

The direct reaction of the highly sterically demanding acetamidinate-based NNN′-scorpionate protioligand Hphbp^tamd [Hphbp^tamd = <i>N</i>,<i>N</i>′-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(<i>κ</i>³-phbp^tamd)] (<b>1</b>). Alternatively, the treatment of the corresponding lithium precursor [Li(phbp^tamd)(THF)] with ZnCl₂ yielded the halide complex [ZnCl(<i>κ</i>³-phbptamd)] (<b>2</b>). The X-ray crystal structure of <b>1</b> confirmed unambiguously a mononuclear entity in these complexes, with the zinc centre arranged with a pseudotetrahedral environment and the scorpionate ligand in a <i>κ</i>³-coordination mode. Interestingly, the inexpensive, low-toxic and easily prepared complexes <b>1</b> and <b>2</b> resulted in highly efficient catalysts for the ring-opening polymerisation of lactides, a sustainable bio-resourced process industrially demanded. Thus, complex <b>1</b> behaved as a single-component robust initiator for the living and immortal ROP of <i>rac</i>-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 <i>M</i>_n values and monomer/benzyl alcohol ratios, as well as enhanced levels of heteroselectivity, reaching <i>P</i>_s values up to 0.74.