Synthesis of ionic liquid-supported ruthenium complex and ring-opening metathesis polymerization of cycloolefin

Ring-opening metathesis polymerization (ROMP) is a widespread tool for synthesizing well-defined and highly functionalized polymers. The catalyst is an important factor which affects the ROMP of monomers. It is well-known that the ruthenium-based Grubbs' catalysts have good solubility in non-polar solvents such as CH₂Cl₂, benzene and toluene, but their solubility in polar solvents such as alcohol, acetone and ionic liquid was very poor, which limited their applications for ROMP in polar solvents. In this paper, an imidazolium salt-supported ruthenium catalyst was synthesized, and its catalytic performance for ROMP of polar or nonpolar cycloolefinic monomers in ionic liquid [BMIm]BF₄ was investigated. A novel pyridyl ligand 2,3-dimethyl-1-[2-(4-pyridyloxy)ethyl] imidazolium hexafluorophosphate (4) was initially designed and synthesized, and its single crystal was obtained. The crystal is monoclinic with the parameters as follow: P2₁/c, a = 1.2469(5) nm, b = 1.6470(7) nm, c = 0.7943(3) nm, β = 106.691(5)°, V = 1.5625(11) nm³. The pyridyl ligand was then used to coordinate with the second generation Grubbs' catalyst to get the first example of the ruthenium catalyst supported by imidazolium salt-contained pyridyl ligand (IL-cat 5). The characterization of the synthesized ligand and the catalyst was carried out by NMR, ICP and elemental analysis. In ¹H-NMR spectroscopy, the signal of the benzylidene proton shifts from δ_H = 19.2 in the second generation Grubbs'catalyst to δ_H = 16.3 in the novel catalyst IL-cat 5. In ³¹P-NMR spectroscopy, the signal of PF₆ ^- in pyridyl ligand was observed at δ_P = -143.3, while δ_P = 36.6 for PCy₃ group was disappeared, which demonstrated the group of PCy₃ was replaced by the pyridyl ligand completely. The content of IL-cat 5 in the mixture of prepared catalyst contained excess 4 was 14 wt% tested by ICP method. As expected, the novel catalyst IL-cat 5 is soluable in polar solvents including methanol acetone, and ionic liquid, thus ROMP in pure ionic liquid could be carried out conveniently. The ROMP of three different cycloolefins, i. e. nonpolar cyclooctene (monomer 1), medium polar 5-hydroxy-1-cyclooctene (monomer 2), and especially the strong polar monomer 5-norbornene-2-carbonitrile (monomer 3), was explored in ionic liquid [BMIm] BF4. The polymerizations of the three monomers catalyzed by catalyst IL-cat 5 processed successfully, and the conversion reached up to 96.0%, 73.0%, and 51.7% for monomer 1, monomer 2, and monomer 3 under conventional conditions, respectively. It was worth noting that the value of 51.7% for cyano group (-CN) contained monomer 3 was reasonably high, which is surprisingly much larger than that using the pyridine coordinated second generation Grubbs' catalyst for the same monomer in [BDMIm] PF₆ as reported in published literature. The results strongly confirmed that ROMP could be undertaken readily for all functionalized monomers using ruthenium catalyst coordinated with imidazolium salt-contained pyridyl ligand in [BMIm]BF ₄, and fortunately Grubbs' catalyst was actually not poisoned in this imidazolium ionic liquid medium without -CH₃ group in 2-position of imidazolyl ring. The molecular weights of polymers were measured by GPC, which were nearly in according with the calculated ones.