Copolymerization of Cyclic Esters Controlled by Chiral NNO-Scorpionate Zinc Initiators

Reaction of the chiral alcohol-scorpionate compound bpzteH [bpzteH = 2,2-bis­(3,5-dimethylpyrazol-1-yl)-1-<i>para</i>-tolylethanol] with [ZnR₂] (R = Me, CH₂SiMe₃) in a 1:2 molar ratio afforded the dinuclear chiral alkyl zinc complexes [Zn­(R)­(κ²-NNμ-O)­Zn­(R)₂] (<b>1</b> and <b>2</b>) [κ²-NNμ-O = bpzte, R = Me <b>1</b>, CH₂SiMe₃ <b>2</b>]. Subsequent alcoholysis reaction with ArOH (1 equiv; Ar = 2,6-C₆H₃Me₂) yielded the chiral dinuclear zinc complexes [(ZnR)₂(κN:κN-μ-O)­(μ-OAr)] (<b>3</b> and <b>4</b>) [κN:κN-μ-O = bpzte, R = Me <b>3</b>, CH₂SiMe₃ <b>4</b>]. The X-ray crystal structures of <b>1</b> and <b>3</b> confirmed a dinuclear structure in all complexes, with the alkoxide of the heteroscorpionate ligands in a μ-bridging mode between the two Zn­(II) centers. The zinc complexes <b>1</b>–<b>4</b>, bearing trisalkyl and alkyl-aryloxide ligands, are shown to function as highly efficient single-component initiators for the ring-opening homopolymerization of ε-caprolactone and l-/<i>rac</i>-lactide. These initiators enable rapid polymerization at ambient temperatures, affording materials with low molecular weights that exhibit narrow, monomodal molecular weight distributions. Microstructural analysis of poly­(<i>rac</i>-lactide) revealed that the alkyl-aryloxide <b>3</b> exerts a moderate level of heteroselectivity (<i>P</i>_s = 0.68). More interestingly, initiators <b>3</b> and <b>4</b> enabled random copolymerization of ε-CL and l-LA with near-equal reactivity ratios for the two comonomers (<i>r</i>_CL = 1.15 and 0.92, and <i>r</i>_LA = 1.37 and 1.05, for <b>3</b> and <b>4</b>, respectively), resulting in closely similar average sequence lengths for the caproyl and lactidyl segments (<i>L</i>_CL ≈ 2.0; <i>L</i>_LA ≈ 2.5, for both initiators). It was also found that the copolymers had monomer contents in close agreement with the composition of the initial monomer feed. Analysis by differential scanning calorimetry of the thermal properties such as <i>T</i>_g of various copolymers produced by <b>3</b> revealed a strong dependence on the monomer content, as these values vary in a linear fashion with the molar percentage of l-lactide units in the copolymer over a temperature range from −30 to 42 °C