title: Syntheses of bis(pyrrolylaldiminato)aluminum complexes for the polymerisation of lactide.
abstract: Seven bis(pyrrolylaldiminato)aluminum methyl complexes were synthesized from the reactions of AlMe3 and two equiv. of the corresponding pyrrolylaldimine ligands. The ligands were modified to have different steric hindrances (C6H5 (1), 2,6-Me2C6H3 (2), 2,4,6-Me3C6H2 (3), 2,6-Et2C6H3 (4) and 2,6-(i)Pr2C6H3 (5)) and electronic contributions (4-CF3C6H4 (6) and 4-OMeC6H4 (7)). Crystal structures of complexes 3-7 were determined and shown to have distorted trigonal bipyramidal geometry (4, 6, 7) and intermediates between trigonal bipyramidal and square pyramidal geometries (3 and 5). The rotation around the N-C(aryl) bond was fast for ligands having small ortho substituents and became slower as the size of the substituents increased. Polymerisations of l-lactide using complexes 1-7 and benzyl alcohol as an initiator were carried out giving the rate dependence on steric hindrance (5 < 4 < 3 < 2 < 1) and electronic contribution (6 < 7 < 1). Larger substituents and electron withdrawing groups were found to suppress the polymerisation rates. Despite having C2 symmetry in the crystal structures of compounds 3-7, only slight enhancement for isotactic enchainment was found in the polymerisation of rac-lactide.
authors.affiliation: Center for Catalysis, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Rama 6 Road, Bangkok, 10400, Thailand. khamphee.pho@mahidol.ac.th.
authors.forename: Supathana
authors.initials: S
authors.lastname: Pracha
authors.forename: Siriwan
authors.initials: S
authors.lastname: Praban
authors.forename: Amornrat
authors.initials: A
authors.lastname: Niewpung
authors.forename: Gulthawach
authors.initials: G
authors.lastname: Kotpisan
authors.forename: Palangpon
authors.initials: P
authors.lastname: Kongsaeree
authors.forename: Saowanit
authors.initials: S
authors.lastname: Saithong
authors.forename: Tossapol
authors.initials: T
authors.lastname: Khamnaen
authors.forename: Phairat
authors.initials: P
authors.lastname: Phiriyawirut
authors.forename: Sumate
authors.initials: S
authors.lastname: Charoenchaidet
authors.forename: Khamphee
authors.initials: K
authors.lastname: Phomphrai
mesh_terms: D000535:Aluminum / Q000737:chemistry*; D056831:Coordination Complexes / Q000138:chemical synthesis* / Q000737:chemistry; D018360:Crystallography, X-Ray; D004146:Dioxanes / Q000737:chemistry*; D008958:Models, Molecular; D058105:Polymerization; D011758:Pyrroles / Q000737:chemistry*
chemical_list: D056831:Coordination Complexes; D004146:Dioxanes; D011758:Pyrroles; C091880:dilactide; D000535:Aluminum