Synthesis of pyridyl-based ionic liguid supported ruthenium complex and kinetics of ring-opening metathesis polymerization in ionic liguid

Ring-opening metathesis polymerization(ROMP) is a widespread tool to synthesize well-defined and highly functionalized polymers. The catalyst is a key factor which affects the polymerization behavior. The Grubbs' catalysts have good solubility in non-polar solvents such as CH₂Cl₂, benzene, THF, and toluene, but their solubility in polar solvents such as alcohol, acetone, and ionic liquid is usually very poor, which limited their application for ROMP in polar solvents. In this paper, the pyridyl-based imidazolium salt-supported ruthenium catalyst was synthesized and its use in polymerization of polar cyclooctene derivative in ionic liquid [BMIm]BF₄ was investigated. The novel pyridyl-based ligand 2, 3-dimethyl-1-[6-(4-pyridyloxy) hexyl] imidazol-1-ium hexafluorophosphate was first synthesized, and then the pyridyl ligand coordinated with the second generation Grubbs catalyst to generate the imidazolium salt-supported ruthenium catalyst. The characterizations of the synthesized compounds, the ligand, and the catalyst were undertaken by NMR analyses. In ¹H NMR spectroscopy, the signal of the benzylidene proton shifted from δ_H 19.2 in the second generation Grubbs catalyst upfield to δ_H 18.6 in the novel catalyst, which meant the ligand exchange had occurred successfully and the novel catalyst formed indeed. The novel imidazolium salt-supported ruthenium catalyst is soluable in CH₂Cl₂, alcohol, acetone, and ionic liquid, thus, ROMP in pure ionic liquid could be carried out. ROMP of the polar monomer 5-hydroxyl-1-cyclooctene was explored in ionic liquid [BMIm]BF₄. The polymerizations of the monomer performed conveniently under varied reaction conditions, and the conversion of monomer reached to more than 95%. The results also proved that the ionic liquid of [BMIm]BF₄ without group of methyl (-CH₃) in 2-position couldn't result in the poisoning of Grubbs' catalyst, and had no influence on the activity of the catalyst. GPC measurement showed the polymer had controlled molecular weight and relatively low molecular weight distribution with polydispersity index of around 1.5. The kinetics of ROMP of the monomer catalyzed by the imidazolium salt-supported ruthenium catalyst in [BMIm]BF₄ were measured. The plots of ln{[M]₀/[M]} versus reaction time and M_n versus [M]/[I] were found to be linear in ionic liquid, and the results showed the polymerization was conducted with controlled/living characteristics. The reaction kinetic equation and reaction active energy for the polymerization process in ionic liquid were also determined, and the results demonstrated the polymerization followed first-order kinetics.