A series of bis-(aryl)-R-diimine ligands were synthesized bearing a range of electrondonating and -withdrawing substituents to systematically investigate the ligand electronic effects on late transition metal olefin polymerization catalysts. Their palladium(II) complexes were prepared and characterized. Electronic perturbations were verified by analysis of the 1 H and 13 C NMR chemical shifts of the corresponding methyl chloride complexes and the CO stretching frequencies of the corresponding cationic carbonyl complexes, which were found to correlate strongly with the Hammet substituent constant (σ p ) of the substituent on the ligand. The palladium(II) complexes of the functionalized R-diimine ligands were employed in the polymerization of ethylene and the copolymerization of ethylene with methyl acrylate. For ethylene homopolymerization, higher molecular weight was obtained with catalysts bearing more strongly electron-donating ligands. It was observed for the first time that the ligand electronic structure of the catalysts had a significant effect on topology of the polyethylene formed. More dendritic polyethylene was obtained with catalysts bearing more strongly electron-withdrawing ligands. This provides a fundamentally different approach (catalyst approach) to control polyolefin branching topology, which complements our previous strategy of controlling polymer topology by polymerization conditions. For the copolymerization of ethylene with methyl acrylate, strong correlations were observed in the incorporation ratio of methyl acrylate; catalysts bearing strongly electron-donating ligands afforded copolymers with higher incorporation of the polar comonomer. These catalysts also exhibited far greater activity in the presence of polar monomers than catalysts bearing more weakly donating ligands possessing withdrawing substituents, which were deactivated completely when methyl acrylate concentrations were sufficiently high.

Polar olefins were copolymerized with ethylene using a cyclophane-based palladium(II) α-diimine catalyst. The incorporation levels of both methyl acrylate and tert-butyl acrylate are unusually high compared to what is obtained with the corresponding acyclic catalyst. Mechanistic studies by low-temperature NMR spectroscopy revealed that the differences between the insertion barriers for ethylene and MA (ΔΔG ⧧) are similar for the acyclic and cyclophane catalysts. The equilibration of comonomers, fast on experimental time scales for the acyclic catalyst, was too slow to be measured for the cyclophane catalyst. These results suggest that ligand substitution is significantly retarded by the bulky cyclophane ligand. The reduction of the rate of monomer exchange with respect to monomer insertion reduces the catalyst's ability to discriminate between comonomers, hence resulting in the high incorporation levels of acrylates in copolymerizations with ethylene.

The synthesis of Ni(II) and Pd(II) cyclophane-based α-diimine olefin polymerization catalysts bearing a range of electron-donating or -withdrawing groups is described. Substituent effects were confirmed by measurement of CO infrared stretching frequencies in Pd(II) carbonyl complexes. Polymerizations with ethylene were investigated in detail involving determination of catalyst productivity and thermal stability, especially at elevated temperature, as well as analysis of polymer molecular weight and microstructure. The Ni(II) catalysts, formed by in situ treatment of tetraarylborate salts of [Ni(diimine)(acac)]+ with triisobutylaluminum, exhibited little variation in productivity or thermal stability across the substitution series, but the resulting polymers showed an increase in both molecular weight and branching density for catalysts with increasingly electron-withdrawing character. The chloride-substituted Pd(II) analogue, however, was notable in its markedly higher productivity and thermal stability at elevated temperature compared to the other substituted Pd(II) cyclophane catalysts, which otherwise showed little variation, like the Ni(II) catalysts. Unlike the previously studied acyclic Pd(II) α-diimine catalysts, a strong tendency toward higher molecular weight polymer with more electron-deficient catalysts was noted. An alternative dissociative chain transfer mechanism is proposed to account for this difference, as well as the generally lower than expected molecular weight of the cyclophane catalysts. As a further explanation for the unusual behavior of these catalysts, NMR evidence of a stabilizing ligand−metal H-agostic interaction in the case of the Pd(II) system is also reported.

The effect of electron-donating and -withdrawing groups on the ligands of Pd(II) α-diimine olefin polymerization catalysts on catalyst stability, activity, and polymer molecular weight is investigated. The polyethylene molecular weight and the productivity of catalysts bearing substituted bis(aryl)dimethyldiazabutadiene (Me2DAB) and bis(aryl)acenaphthenequinonediimine (BIAN) ligands were analyzed over time at room temperature and 40 °C to monitor catalyst stability and chain transfer processes. The introduction of electron-donating groups led to a dramatic increase in polymer molecular weight, with polymer chains still growing after 24 h of polymerization. The amino-substituted Me2DAB analogue afforded polymer of more than twice the molecular weight compared to the polymer made with the unsubstituted analogue after 24 h of polymerization. The unsubstituted catalysts and those bearing electron-withdrawing groups, however, reached a maximal molecular weight, generally lower, after a comparatively short time, which was presumably due to higher chain transfer rates. Electron-donating groups also provided increased stability to the catalysts leading to longer catalyst lifetimes. Both of these effects are likely due to stabilization of the reactive, electron-deficient, and coordinatively unsaturated alkyl agostic intermediate, the reactivity of which is key to both chain transfer and decomposition processes.

The synthesis and characterization of Ni(II) and Pd(II) α-diimine olefin polymerization catalysts bearing a fluorinated cyclophane-based ligand were performed. Fluorine was placed in such a manner as to interact with the metal center from the axial direction. The catalysts were active in the polymerization of ethylene, showing substantial differences in both catalytic behavior and polymer size and structure as compared to their nonfluorinated analogues. Both catalysts afforded polymer of comparatively low branching density and high molecular weight. The Ni(II) catalysts, from precursor [Ni(acetylacetonato)(F-Cyc)]+ salts (F-Cyc = fluorinated cyclophane), exhibited enhanced thermal stability by remaining active after 70 min with little loss in polymerization activity at 105 °C. The Pd(II) catalysts from salts of [Pd(F-Cyc)Me(NCR)]+ (NCR = nitrile) afforded polymer of molecular weights far higher than the nonfluorinated analogue. Additionally, polymerization activity was directly related to ethylene feed pressure for the Pd(II) system, and NMR analysis could not detect the presence of bound olefin, indicating that the polymerization proceeded via different kinetics involving an olefin-free 14 e− complex as the catalyst resting state. Furthermore, NMR 1H−19F coupling data provide clear evidence that the fluorine atoms were indeed interacting with the metal axial site. The unusual properties of these new complexes are thus attributed to stabilization of the highly reactive 14 e− intermediate by donation of the fluorine lone pair to the metal center.