Vinyl-addition polymerization of norbornene catalyzed by (pyridyl)imine Ni(II), Pd(II), Co(II), and Fe(II) complexes.
Authors:   , van der Westhuizen Barend Danielle 
Published: February 2024

The thesis reports the syntheses, and structural characterization of (pyridyl)imine transition metal complexes and their applications as pre-catalysts in the vinyl-addition polymerization of norbornene. The bidentate ligand (E)-N-(1 phenylethyl)-1-(pyridin-2-yl)methanimine (L1) was synthesized by reactions of 2-pyridine carboxaldehyde with (R)-(+)-α-methyl benzylamine in the presence of p-TsOH and obtained in moderate yields of 64%. On the other hand, the tridentate ligand (E)-1-(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanimine (L2) was obtained by reacting 2-pyridine carboxaldehyde with 2-picolylamine in the presence of p-TsOH in high yields of 90%. Reactions of bidentate ligand L1 and tridentate ligand L2 with NiCl2, [Pd(COD)Cl2], FeCl2, and CoCl2 metal salts gave the corresponding complexes [Ni(L1)2Cl2] (Ni1), [Ni(L2)Cl2] (Ni2), [Pd(L1)Cl2] (Pd1), [Co(L1)3][2PF6] (Co1), [Co(L2)Cl2] (Co2), [Fe(L1)3][2PF6] (Fe1), [Fe(L2)Cl2] (Fe2) in low to moderate yields (18% - 60%). The identities of the isolated complexes were confirmed by characterization with FT-IR spectroscopy, mass spectrometry, elemental analysis, single crystal X-ray crystallography for Co1, and nuclear magnetic resonance where applicable. The solid state structure of complex Co1 was established as tris-chelated, containing three ligands to give an octahedral coordination environment. The metal complexes were evaluated as pre-catalysts in the vinyl-addition polymerization of norbornene to produce poly(2,3-bicyclo[2.2.1]heptene) using modified methyl aluminoxane (MMAO) as the co-catalyst. Complex Ni1 was the most active with catalytic activity of 22.7 g×10³(PNBE).mol(M)-¹.h-¹ followed by complex Pd1 which showed catalytic activity of 17.6 g×10³(PNBE).mol(M)-¹.h-¹ whereas complex Co1 showed 0.7 g×10³(PNBE).mol(M)-¹.h-¹ and complex Fe1 showed catalytic activity of 0.3 g×10³(PNBE).mol(M)-¹.h-¹ concluding that the choice of metal center is of absolute importance to achieve high catalytic activity. The number of electron donor atoms in the ligand structure influenced catalytic activity as bidentately chelated complex Ni1 showed catalytic activity of 22.7 g×10³(PNBE).mol(M)-¹.h-¹ whereas the tridentately chelated Ni2 showed catalytic activity of 81.9 g×10³(PNBE).mol(M)-¹.h-¹. The influence of reaction parameters were investigated using Ni1 and Ni2 as pre-catalysts and it was concluded that monomer/metal ratios, co-catalyst/metal ratios, reaction temperatures, reaction times, and solvent choice influenced catalytic activity. A higher monomer/metal ratio of 1250 resulted in catalytic activity of 17.5 g×10³(PNBE).mol(M)-¹.h-¹ compared to the value of 6.8 g×10³(PNBE).mol(M)-¹.h-¹ obtained from a lower monomer/metal ratio of 625. An optimum co-catalyst/metal ratio of 1500 was established and recorded catalytic activity of 33.7 g×10³(PNBE).mol(M)-¹.h-¹. Polymerization reactions at room temperature gave higher monomer conversions of 70% as opposed to lower conversions of 17% obtained at 50 °C. The choice of solvent influenced catalytic activities of the complexes, with the more polar o-chlorobenzene solvent giving the highest monomer conversion of 70% in comparison to 31% obtained in toluene solvent. Polymers formed from all complexes were of the vinyl type with possible ring-opening metathesis polymeric inserts present in the polymer backbone. Thermal gravimetric analysis and differential scanning calorimetry of the formed polymers demonstrated that polymers formed from complex Co2 and complex Fe2 displayed degradation temperatures of 492 °C and 478 °C respectively opposed to polymers formed from complex Ni2 and complex Pd2 which gave values of 478 °C and 462 °C.