Single Site Metal Catalysts For Ring Opening Polymerization Of Lactides

Author : Khamphee Phomphrai
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (532 download)

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Download or read book Studies of Single-site Zinc, Magnesium and Calcium Catalyst Precursors for Ring-Opening Polymerization of Lactides written by Khamphee Phomphrai and published by . This book was released on 2003 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Abstract: The preparation and characterization of a series of closely related magnesium and zinc compounds active for ROP of lactides are reported using three [beta]-diiminate ligands L 1, L 2 and L 3 where L 1 = CH(CMeNC 6 H 3 -2,6- i Pr 2) 2, L 2 = CH(CMeNC 6 H 4 -2- t Bu) 2 and L 3 = CH(CMeNC 6 H 4 -2-OMe) 2 . For the L 1 ligand, L 1 Mg(N i Pr 2)·(THF), L 1 Zn(N i Pr 2), L 1 Mg(O t Bu)·(THF), L 1 Zn(O t Bu) and L 1 Zn(OSiPh 3)·(THF) have been synthesized. All compounds initiate and sustain ROP of lactides. For a related series L 1 MX(THF) n, where n = 0 or 1, the reactivity follows the order M = Mg> Zn and X = O t Bu> N i Pr 2> N(SiMe 3) 2> OSiPh 3 . Heterotactic polylactide (PLA) is produced from ROP of rac-lactide by zinc catalysts in CH 2 Cl 2 . Magnesium catalysts produce atactic and heterotactic PLA in CH 2 Cl 2 and THF, respectively. The resting states for Zn and Mg are proposed to be L 1 Zn(nu 2 -OCHMeC(O)OP) and L 1 Mg(mu-OP) 2 MgL 1 . For the L 2 ligand, L 2 MgN i Pr 2 ·(THF) and L 2 ZnN i Pr 2 are prepared. In solution, L 2 ZnN i Pr 2 exists as a mixture of syn- and anti-rotamers while L 2 MgN i Pr 2 ·(THF) exists only as a syn-rotamer. In the zinc compound, the syn-conformer is shown to be more reactive than the anti-conformer and is responsible for the polymerization. Heterotactic PLA is obtained in CH 2 Cl 2 for the zinc compound and in THF for the magnesium compound. For the L 3 ligand, L 3 MgN(SiMe 3) 2, [L 3 MgO t Bu] 2, L 3 ZnN(SiMe 3) 2, L 3 ZnO i Pr and L 3 CaN(SiMe 3) 2 ·(THF) are prepared. The OMe groups of ligand L 3 show little affinity toward zinc and in the ROP of rac-lactide give PLA with only moderately enhanced heterotactic tetrads. The OMe groups are shown to bind to Mg reversibly and display significant enhancement of heterotactic PLA. The first single-site calcium complexes of L 1 and tris(pyrazolyl/indazolyl)borate ligands are reported and shown to initiate and sustain ROP of lactide: L 1 CaNR 1 2 ·(THF), Tp R2 CaNR 1 2 ·(THF) n, Tp Men CaNR 1 2, (9-BBN)Bp (+)-Cam CaNR 1 2 ·2THF where Tp R2 = [nu 3 -HB(3-R 2 pz) 3], R 1 = SiMe 3, R 2 = t Bu or i Pr, n = 0 or 1, Men = menthone-derived indazole and Cam = camphor-derived indazole. All calcium complexes polymerize rac-lactide in THF giving mostly atactic PLA except for Tp tBu CaNR 1 2 where heterotactic PLA is produced. Tp tBu CaNR 1 2 reacts with 2,6-diisopropylphenol giving Tp tBu Ca(O-2,6- i Pr 2 C 6 H 3). The order of heterotactic selectivity is THF> PO approx CHO> CH 2 Cl 2 . The single-site calcium complexes are not active for ROP of epoxides and the compound Tp tBu Ca(O-2,6- i Pr 2 C 6 H 3)·(PO) is reported and structurally characterized by a single crystal X-ray diffraction study.