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(t)amd [Hphbp(t)amd = N,N′-di-p-tolylbis(3,5-di-tertbutylpyrazole-1-yl)acetamidine] with one equiv. of ZnMe(2) proceeds in high yield to the mononuclear alkyl zinc complex [ZnMe(κ(3)-phbp(t)amd)] (1). Alternatively, the treatment of the corresponding lithium precursor [Li(phbp(t)amd)(THF)] with ZnCl(2) yielded the halide complex [ZnCl(κ(3)-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 κ(3)-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.