Monomethylaluminum and dimethylaluminum pyrrolylaldiminates for the ring-opening polymerization of rac-lactide: effects of ligand structure and coordination geometry

Abstract

Two series of aluminum alkyl complexes supported by pyrrolylaldiminate ligands, LAlMe2 (1a-7a) and L2AlMe (1b-7b), were successfully synthesized and characterized by NMR spectroscopy and elemental analysis. Reactions of trimethylaluminum with the corresponding pyrrolylaldiminate ligands in the molar ratios of 1 : 1 and 1 : 2 yielded dimethylaluminum pyrrolylaldiminates (1a-7a) and monomethylaluminum pyrrolylaldiminates (1b-7b), respectively, in good yields. The structure of 3b, determined by single-crystal X-ray diffraction, displayed a distorted trigonal bipyramidal geometry with the τ value of 0.65. Upon addition of 1 equivalent of benzyl alcohol, all complexes promoted the living ring-opening polymerization of rac-lactide with a good control over molecular weights and polydispersities. Complexes 6a and 7a were found to efficiently mediate the immortal polymerization in the presence of excess equivalents of benzyl alcohol (up to 5 equivalents), as evidenced by the narrow PDI values and the good agreement between the experimental M(n) values and monomer/benzyl alcohol ratios. The steric and electronic effects of the imine nitrogen substituents had a strong influence on the polymerization activities both in catalytic activity and polymer microstructure. The catalytic activity decreased as follows: 4-Me-C6H4 (3) > C6H5 (1) ≈ 4-F-C6H4 (2) ≈ 2-Me-C6H4 (5) > 4-OMe-C6H4 (4) ≫ 2-(t)Bu-C6H4 (6) > adamantyl (7). In comparison, the catalytic activity of the monomethylaluminum complex is slightly higher than that of the dimethylaluminum counterpart. The polymerization of rac-lactide by 6b yielded heterotactically enriched polylactide (P(r) = 0.60) whereas the isotactic-enriched polymer (P(m) = 0.74) was obtained from 7b.

An Aluminum Siloxide Complex for Stereoselective Polymerization of Racemic Lactide

“…The τ value ranges from 0 (perfectly square pyramidal) to 1 (perfectly trigonal bipyramidal). [34][35][36] For compound 1, the value is 0.77, which indicates more trigonal bipyramidal geometry in the solid state. It is slightly lower than aluminum Schiff-base isopropoxide (τ = 0.78), 31 howerver, is somewhat higher 13 C NMR, and that, =N-CH 2 -group should show four different signals in the 1 H NMR spectrum and two peaks in the 13 C NMR spectrum, respectively.…”

Aluminium complexes bearing N,O-aminophenol ligands as efficient catalysts for the ring opening polymerization of lactide

“…In addition, a short range CH/HC contact of 2.36 Å plays important role in the crystal packing. These [6,23,45,46,54]. Considering that the reactions were performed at 140 °C for several hours, the catalysts were moderately active towards the ROP of lactides.…”

“…The data was deposited with CCDC with the following CCDC number: 1058124 (1). General procedure for kinetic studies of lactides 23 The kinetic studies for the polymerization of rac-LA using 1, 2 and 3 were carried out in 200:1 ratio at 140 °C under solvent free condition. 0.035 mmol of 1, 2 or 3 was mixed with 1 g of monomer in a sealed tube under argon atmosphere.…”

“…Aluminium complexes have attracted widespread attention among all the metal complexes because of its low toxicity, high Lewis acidity, high abundance, low cost and high stereocontrol over ROP reactions [8][9][10][11][12]. Following the remarkable work of Spassky et al on the stereospecific ROP of rac-LA using chiral aluminium salen complexes [13], several aluminium complexes containing ancillary ligands like tetradentate N, N, O, O ligands and N, N, N, N ligands [14][15][16][17][18][19][20][21][22][23][24][25][26], tridentate N, N, N and N, O, O ligands [27][28][29][30][31][32][33], Schiff base ligands, benzotriazole phenoxide, ketiminate, fluorinated β-diketonate ligands etc. [34][35][36][37][38][39][40] [42].…”

Synthesis and Characterization of Bis(2‐iminopyrrolyl) Iron(II) Complexes by N–H Bond Activation

“…[15,16] The iminopyrrolyl complexes of main group metals were found to be catalytically active for the ring-opening polymerization of lactides. [17] Based on different metals in different oxidation states, there are mono(2-iminopyrrolyl), bis(2-iminopyrrolyl), and tri(2-iminopyrrolyl) metal complexes. Usually, (2-iminopyrrolyl) metal complex was formed via the reaction of metal precursor (in most cases, metal halides) and deprotonated 2-iminopyrrole via N-H bond activation under basic condition.…”