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Issue 34, 2010
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Amidinate aluminium complexes: synthesis, characterization and ring-opening polymerization of rac-lactide

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Abstract

A series of aluminium alkyl complexes {PhC(NR′)(NR′′)}AlR2 (4a–n, R′ = 2,6-iPr2C6H3, 2,6-Me2C6H3; R′′ = aryl groups with various ortho-, para- or meta-substituents, tert-butyl; R = methyl, ethyl) bearing non-symmetrically N-substituted benzamidinate ligands were synthesized via the reaction of trialkylaluminium and the corresponding benzamidine proligands. Complex 5 bearing symmetric amidinate ligand was also obtained for comparison purposes. The X-ray diffraction studies of complexes 4b, 4c and 5 show in each case a distorted tetrahedral geometry around the aluminium center. All the amidinate aluminium complexes were found to catalyze the ring-opening polymerization (ROP) of rac-lactide with moderate activities. The steric and electronic characteristics of the ancillary ligands have a significant influence on the polymerization performance of the corresponding aluminium complexes. The introduction of electron-withdrawing substituents at the ortho-positions of N-phenyl ring of the ligands resulted in an obvious increase in catalytic activity. Complex 4b showed the highest activity among the investigated aluminium complexes due to the high electrophilicity of the metal center induced by the ortho-chloro substituents on the phenyl ring. The existence of ortho-substituents of small steric bulkiness is also beneficial for the increase of activity of these catalysts. However, further increase of steric hindrance of the ligands by introducing bulky ortho-substituents onto the phenyl moieties resulted in a decrease of activity and an increase in the isotactic bias of the obtained polylactides. The broad molecular weight distributions (PDI = 1.13–2.02) of the polymer samples indicated that the ROP of rac-lactide initiated by these complexes was not well-controlled.

Graphical abstract: Amidinate aluminium complexes: synthesis, characterization and ring-opening polymerization of rac-lactide

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Publication details

The article was received on 07 Apr 2010, accepted on 28 May 2010 and first published on 27 Jul 2010


Article type: Paper
DOI: 10.1039/C0DT00272K
Citation: Dalton Trans., 2010,39, 8071-8083
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    Amidinate aluminium complexes: synthesis, characterization and ring-opening polymerization of rac-lactide

    F. Qian, K. Liu and H. Ma, Dalton Trans., 2010, 39, 8071
    DOI: 10.1039/C0DT00272K

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