Studies of single-site zinc, magnesium and calcium catalyst precursors for ring-opening polymerization of lactides
Polyesters derived from the ring-opening polymerization (ROP) of lactides formed from inexpensive renewable resources constitute one important class of biodegradable and biocompatible polymers. Herein, the preparation and characterization of a series of closely related magnesium and zinc compounds that are active for ROP of lactides are reported using three ?-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 the zinc catalysts in CH 2 Cl 2 . The 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(? ^2 -OCHMeC(O)OP) and L ^1 Mg(?-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. Herein, 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 = [? ^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 ? 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.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:lactide polymerization single site catalyst calcium magnesium heterotactic polylactide stereoselective
Date of Publication:01/01/2003