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

摘要

Two series of aluminum alkyl complexes supported by pyrrolylaldiminate ligands, LAlMe_2 (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-C_6H_4 (3) > C_6H_5 (1) ≈ 4-F-C_6H_4 (2) ≈ 2-Me-C_6H_4 (5) > 4-OMe-C_6H_4 (4) ? 2-~tBu-C_6H_4 (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.