title: 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.
authors.affiliation: Department of Chemistry, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand. fscipph@ku.ac.th.
authors.forename: Sittichoke
authors.initials: S
authors.lastname: Tabthong
authors.forename: Tanin
authors.initials: T
authors.lastname: Nanok
authors.forename: Palangpon
authors.initials: P
authors.lastname: Kongsaeree
authors.forename: Samran
authors.initials: S
authors.lastname: Prabpai
authors.forename: Pimpa
authors.initials: P
authors.lastname: Hormnirun
mesh_terms: D017607:Aluminum Compounds / Q000737:chemistry*; D019905:Benzyl Alcohol / Q000737:chemistry; D002208:Caproates / Q000737:chemistry; D002384:Catalysis; D056831:Coordination Complexes / Q000138:chemical synthesis / Q000737:chemistry; D018360:Crystallography, X-Ray; D004146:Dioxanes / Q000737:chemistry*; D007700:Kinetics; D007783:Lactones / Q000737:chemistry; D008024:Ligands; D008968:Molecular Conformation; D058105:Polymerization; D011758:Pyrroles / Q000737:chemistry
chemical_list: D017607:Aluminum Compounds; D002208:Caproates; D056831:Coordination Complexes; D004146:Dioxanes; D007783:Lactones; D008024:Ligands; D011758:Pyrroles; C121056:caprolactone; C091880:dilactide; D019905:Benzyl Alcohol