Synthesis\nof Imine-Phenoxy Ligated Palladium Complexes\nfor Norbornene Homopolymerization

Metal complexes with tunable ligands play a crucial role\nin olefin\npolymerization and impart control over molecular weight, crystallinity,\nand stereoregularity. We report the single-step synthesis of imine-phenoxy\nligands in excellent yields (81–93%). The identity of electronically\ntuned imine-phenoxy ligands was unambiguously ascertained by using\na combination of spectroscopic and analytical methods. These ligands\nwere treated with [Pd(COD)MeCl] in the presence of 2,6-lutidine, resulting\nin the formation of discrete mononuclear palladium complexes Pd1–Pd4\nin excellent yields. 1–2D NMR spectroscopy, mass spectrometry\nanalysis, and single-crystal X-ray diffraction confirmed the identity\nof the palladium complexes. X-ray analysis revealed a distorted square\nplanar geometry around the palladium center. Proton NMR analysis suggested\nthat the Pd1 catalyst was deshielded, indicating electronically deficient\npalladium metal compared to the other complexes. Moreover, the Pd1\ncatalyst showed the highest buried volume percentage (%V<sub>bur</sub> = 44.9). When exposed to norbornene, Pd1–Pd4 were found to\nbe active and produced poly(norbornene) (PNB). High-temperature SEC\nanalysis revealed that the electronically deficient and sterically\nhindered Pd1 catalyst produced the highest molecular weight polymer\n(PNB 37.4 kDa). Boron and aluminum-based cocatalysts were screened,\nand MMAO was found to outperform others with high catalytic activity\n(up to 63.2 × 10<sup>5</sup> g of PNB (mol Pd)<sup>−1</sup> h<sup>–1</sup>).