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-para-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] (1 and 2) [κ2-NNμ-O = bpzte, R = Me 1, CH2SiMe3 2]. 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)]
(3 and 4) [κN:κN-μ-O =
bpzte, R = Me 3, CH2SiMe3 4]. 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. The zinc complexes 1–4, 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-/rac-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(rac-lactide) revealed that the alkyl-aryloxide 3 exerts a moderate level of heteroselectivity (Ps = 0.68). More interestingly, initiators 3 and 4 enabled random copolymerization of ε-CL
and l-LA with near-equal reactivity ratios for the two comonomers
(rCL = 1.15 and 0.92, and rLA = 1.37 and 1.05, for 3 and 4, respectively), resulting in closely similar average sequence lengths
for the caproyl and lactidyl segments (LCL ≈ 2.0; LLA ≈ 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 Tg of various copolymers
produced by 3 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.