Copolymerization of Cyclic Esters Controlled by Chiral NNO-Scorpionate Zinc Initiators
Reaction of chiral alcohol-scorpionate compoundbpzteH [bpzteH = 2,2-bis(3,5-dimethylpyrazol- 1-yl)-1-para-tolylethanol] with [ZnR2] (R = Me, CH2SiMe3) in a 1:2 molar ratio afforded the dinuclear chiral alkyl zinc complexes [Zn(R)(k2-NN μ-O)Zn(R)2] (1–2) [k2-NN μ-O= bpzte, R = Me 1, CH2SiMe32]. Using these trialkyl complexes in an alcoholysis reaction with ArOH (1 equiv; Ar = 2,6-C6H3Me2) yielded the chiral dinuclear zinc complexes [(ZnR)2(kN:kN- μ-O)( μ-EAr)] (3– 4) [kN:kN- μ-O= bpzte, R = Me 3, CH2SiMe34]. The structures of the different compounds were determined by spectroscopic methods and, in addition, the X-ray crystal structures of 1 and 3 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. Trisalkyl and alkyl-aryloxide-containing zinc complexes 1–4 can act as single-component initiators for the ring-opening homopolymerization of e-caprolactone and L-/rac-lactide, affording materials with low molecular weights and narrow monomodal molecular weight distributions under mild conditions in only a few hours. Microstructural analysis of poly(rac-lactide) revealed that the alkyl-aryloxide 3 exerts a moderate level of heteroselectivity (Ps= 0.68). More interestingly, this initiators 3 and 4 also allowed the well-controlled random copolymerization of ε-CL and LLA as indicated by both the values of the reactivity ratios of the two comonomers (rCL = 1.15 and 0.92, and rLA = 1.37 and 1.05, for 3 and 4, respectively), and the average lengths of the caproyl and lactidyl sequences (LCL ~ 2.0; LLA ~ 2.5, for both initiators). The copolymers also showed to possess monomer contents in close agreement with their composition in the initial monomer feed. Inspection by differential scanning calorimetry on the thermal properties such as Tg of various copolymers produced by 3 revealed a strong dependence on monomer content, as these values vary linearly with the molar percentage of L-lactide unit in the copolymer over a temperature range of -30 to 42 ºC.
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