The random copolymers poly(LA-ran-CL) have improved properties of degradability, mechanical strength, elasticity, and permeability over the PLA and PCL homopolymers. However, the synthesis of such copolymers is still a great challenge in polymer chemistry. In this contribution, we develop a simple but well-designed phenoxyimine Al complex (4) with bulky Ph 2 CH groups, which achieves controlled random copolymerization of rac-LA and E-CL in a living manner (Ð 5 1.06-1.09). The reactivity ratios of rac-LA and E-CL (r LA and r CL ) are 1.09 and 1.05 and the average sequence lengths of the lactidyl unit (L LA ) and the caproyl unit (L CL ) are in the range of 1.9-2.0 at different stages of conversion. In marked contrast, Al complexes (1-3) having less bulky substituents on the ligands only produce gradient copolymers. Furthermore, this strategy of catalyst design would be readily extended to other catalytic systems including b-ketiminato Al complex (5). V C 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 611-617

Ring-opening polymerization (ROP) of cyclic esters/lactones by efficient catalysts is a powerful method for synthesis of biodegradable and biocompatible polyesters with well-defined structures. To develop catalytic systems that are fast, selective and controlled is a persistent effort of chemists. In this contribution, we report a binary urea/alkoxide catalytic system that could catalyze ROP of rac-LA in a fast (over 90% conversion within 1–2 min), stereoselective (Pi up to 0.93) and controlled manner, indicated by narrow MW distributions, linear relationship between the monomer conversions and Mns, end-group fidelity, and chain extension experiments. Remarkably, the catalytic system described here is simple, easily prepared, and structurally tunable and thus has versatile catalytic performances.

A series of chromium­(III) complexes, LCrCl3 (Cr1–Cr4: L = 2-(N-R2-benz­imidazol-2-yl)-6-(3,5-R1-pyrazol-1-yl)­pyridine; Cr1: R1 = H, R2 = H; Cr2: R1 = Me, R2 = H; Cr3: R1 = Me, R2 = Me; Cr4: R1 = Me, R2 = Bn), were synthesized and characterized by IR spectroscopy and elemental analysis. The studies of the solid state of Cr2 and Cr3 revealed distorted octahedral geometries around the chromium centers by X-ray diffractions. In the presence of MAO, Cr1–Cr4 exhibited high activities toward ethylene oligomerization (up to 2.17 × 106 g·mol–1(Cr)·h–1) and ethylene polymerization (up to 6.78 × 105 g·mol–1(Cr)·h–1). The oligomers were produced with high selectivity for α-olefins (>99%), confirmed by FT-IR and 13C NMR, and the distributions followed the Schulz–Flory equation. Various reaction parameters including the amount of cocatalyst, reaction temperature, and time were evaluated in detail, and it was evident that the title complexes had good thermal stability and the substituents on imidazole-N of the ligands dramatically impacted the catalytic activities as well as the distribution of the products.

The unsymmetrical salphen ligand (3,5- t Bu-1-OH-C6H2)­CHN-C6H4-NCH­(3-Ph-1-OH-C6H3) ( t Bu‑Ph L­H 2 ) was designed and synthesized to support aluminum complexes. Its symmetric analogues, (3,5- t Bu-1-OH-C6H2)­CHN-C6H4-NCH­(3,5- t Bu-1-OH-C6H2) ( t Bu LH 2 ) and (3-Ph-1-OH-C6H3)­CHN-C6H4-NCH­(3-Ph-1-OH-C6H3) ( Ph LH 2 ), were also explored and compared. The methane elimination reactions between ligands and AlMe3 resulted in formation of t Bu‑Ph L­Al­Me (1), t Bu L­Al­Me (2), and Ph L­AlMe (3) in high yields, which were characterized by elemental analysis, 1H and 13C NMR. The coordination geometries of unsymmetrical and symmetric ligands in 1 and 2 were studied by X-ray diffractions, which revealed a five-coordinated distorted square-pyramidal geometry around Al centers. The aluminum methyl compounds 1–3 reacted with BnOH at 70 °C to give t Bu‑Ph L­Al­OBn (4), t Bu L­Al­OBn (5), and Ph L­Al­OBn (6), respectively, which existed as monometallic species in solution as indicated by NMR studies. However, the aluminum isopropoxide prepared by the reaction of t Bu‑Ph L­H 2 with 1 equiv of Al­(O i Pr)3 contained three species, one monometallic t Bu‑Ph L­Al­O i Pr (7) and two bridged dimers μ-O 2 -( cis - t Bu‑Ph L­Al­O i Pr) 2 (8) and μ-O 2 -( trans - t Bu‑Ph L­Al­O i Pr) 2 (9). The catalytic performances of unsymmetrical 4 for the ring-opening polymerization of racemic lactide (rac-LA) were preliminarily studied and compared to those of symmetric 5 and 6.