Lanthanide alkoxide complexes stabilized by a novel salen-type Schiff-base ligand: Synthesis, structure, and catalysis for the polymerization of lactide

Synthesis and catalytic performance of wood cellulose nanofibers grafted with polylactic acid in rare-earth complexes based on tetrazole carboxylic acids

Stannous octanoate [Sn(Oct)2] and 4-dimethylamino pyridine (DMAP) were used to catalyze the synthesis of amphiphilic cellulose-based graft copolymers, but the acute toxicity of tin ions and DMAP prompts the need for the application of less harmful catalysts. Herein, green catalyst complexes 1-3 [M(H0.5L)2(H2O)5]·2(H2O) (M = Sm, 1; M = Nd, 2; M = Eu, 3; H2L = 4-(3-(tetrazol-5-yl)pyridin-5-yl)benzoic acid) were synthesized, and their properties were systematically investigated. Single-crystal X-ray diffraction showed that the complexes possessed a zero-dimensional structure, while the thermogravimetry and scanning electron microscopy results confirmed their stability after heating at 110 °C for 10 h. Using complexes 1-3 and DMAP as the catalysts, CNFs were grafted with l-lactide via homogeneous ring-opening polymerization to form wood cellulose nanofibers grafted with l-lactide (WGLAs), and the effects of the ratio of wood cellulose nanofibers (WCNFs) to l-lactide ([AGU]/[LA]) and catalyst dosage were studied. The polymerization followed the coordination-insertion mechanism. Under comparable reaction conditions, the grafting ratio of WGLA-1 reached 84.7 %, and the grafting ratio of complex 1 was found to be higher than those achieved using DMAP. WGLAs demonstrated good thermal stability without cytotoxicity, and the residual catalysts in the WGLAs exhibited fluorescence characteristics. Overall, amphiphilic cellulose-based materials with fluorescence emission offered a promising modification strategy to prepare high-performance polymer composites for agriculture and biomedical application.

Influences of polarizability effect of alkyl group and homoring competition effect of substituents on the NMR spectra of salen-type Schiff base

Salen-type Schiff bases are a kind of important compounds and are widely used. In order to explore the effect of alkyl groups and substituents attached to aromatic ring on the chemical shifts, 63 title compounds were synthesized. Their ¹ H NMR and ¹³ C NMR spectra were obtained; and the effects of the alkyl chain length and substituents on the chemical shifts (δ_H (CHN), δ_C (CHN), δ_H (OH), and δ_C (COH)) were studied. The results show that (1) the alkyl polarizability effect index (PEI) has an important influence on the chemical shifts of the above four atoms, with the increase of PEI, the values of δ_H (CHN) and δc(CHN) decrease, and the values of δ_H (OH) and δ_C (COH) increase. (2) The influence of substituent X attached to aromatic ring on the chemical shift is related to its position by taking OH or CHN as reference. As for the effect of substituent on the chemical shifts, the effect of Hammett constant σ(X)_OH and excited-state substituent parameter σ CC ex X  OH with OH as reference are different from that ofσ(X)_CHN and σ CC ex X  CH  N with CHN as reference, and there is a "homoring competition effect" of the substituent. (3) The effect of the cross-interaction between X and OH on the chemical shift is also significantly different due to the different position of X. Quantitative correlation equations against chemical shifts were built for the four atoms, and the stability and prediction ability of the obtained equations were confirmed by leave-one-out cross validation.

Ring-opening polymerisation of l- and rac-lactide using group 4 permethylpentalene aryloxides and alkoxides

A new family of group 4 permethylpentalene (C₈Me₆^2-; Pn*) aryloxide and alkoxide complexes have been synthesised and fully characterised by multinuclear NMR spectroscopy and single-crystal X-ray diffraction; (η⁸-C₈Me₆)Zr(OR)₂ (R = ^tBu (1), 2,6-Me-C₆H₃ (2), 2,6-ⁱPr-C₆H₃ (3) and 4-OMe-C₆H₄ (4)), (η⁸-C₈Me₆)Zr (OR) (R = 2,6-^tBu-C₆H₃ (5) and 2,6-^tBu-4-Me-C₆H₂ (6)), (η⁸-C₈Me₆)ZrCp(OR) (R = ^tBu (7), 2,6-Me-C₆H₃ (8) and 2,6-ⁱPr-C₆H₃ (9)), (η⁸-C₈Me₆)TiCp(O-2,6-Me-C₆H₃) (10) and (η⁸-C₈Me₆)ZrCp^Me(OR) (R = 2,6-Me-C₆H₃ (11), 2,6-ⁱPr-C₆H₃ (12) and 2,4-^tBu-C₆H₃ (13)). 2, 3, 6, 7, 9, 10 and 12 were studied as initiators for the ring-opening polymerisation (ROP) of l-lactide, and 2, 3, 6, 7 and 10 were studied as initiators for the ROP of rac-lactide. 3 was found to be the most active initiator for the ROP of l-lactide (k_obs = 0.35 h^-1) and 2 for the ROP of rac-lactide (k_obs = 0.21 h^-1). These initiators produced isotactic PLA for the ROP of l-lactide and moderately heterotactic enriched (maximum P_r of 0.69) or atactic PLA for the ROP of rac-lactide with polymer chains consisting of polylactic acid repeat units with -OR and -OH end groups.

Salen complexes of zirconium and hafnium: synthesis, structural characterization and polymerization studies

Salen complexes of zirconium and hafnium were synthesized and used as effective catalysts for the polymerization of lactide and ε-CL and homopolymerization, copolymerization and coupling of epoxides with CO₂.