Studies of single-site zinc, magnesium and calcium catalyst precursors for ring-opening polymerization of lactides

Biodegradable and biocompatible polymers formed from inexpensive renewable resources are attracting considerable current attention in both academic and industrial circles.Polyesters derived from the ring-opening polymerization (ROP) of lactides and lactones constitute one important class of such 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 β-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 raclactide by the zinc catalysts in CH 2 Cl 2 .The polymers produced by the magnesium catalysts are highly solvent dependent where atactic and heterotactic PLA is produced in CH 2 Cl 2 and THF, respectively.The resting states for Zn and Mg are proposed to be L 1 Zn(η 2 -OCHMeC(O)OP) and L 1 Mg(µ-OP) 2 MgL 1 .Reactivities with epoxides and CO 2 are also reported.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) ii exists only as a syn-rotamer.Both compounds are active for ROP of rac-lactides.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 have significant effects in the polymerization of rac-lactide giving 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 = [η 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 TptBu CaNR 1 2 where heterotactic PLA is produced.Tp tBu CaNR 1 2 reacts with 2,6-diisopropylphenol giving Tp tBu Ca(O-2,6i Pr 2 C 6 H 3 ) which is solvent dependent in the polymerization.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.iii Dedicated to my family iv