Kinetics of ring-opening metathesis polymerization of cyclooctene derivative in ionic liquid

Ring-opening metathesis polymerization (ROMP) is a widespread tool to synthesize well-defined and highly functionalized polymers. Recently, the ROMP of 5-substituted cyclooctene with the pendant of imidazolium salt catalyzed by Grubbs second generation catalyst [RuCl₂(PCy₃)(SIMes) (CHPh)] in CH₂ Cl₂ and ionic liquid was reported. The polymerization, which was carried out in CH₂Cl₂, afforded much higher number average molecular weights (M_n) and a much broader molecular weight distribution (PDI) than those expected for a living polymerization. These results imply that, with functionalized cyclooctene monomer propagation is significantly faster than initiation resulting in a poor initiation efficiency. Additionally, the broadening PDI of polymers may be due to its propensity to perform intramolecular backbiting reactions. This is a general phenomenon in ROMP initiated with [RuCl₂(PCy ₃)(SIMes)(CHPh)] in conventional organic solvents. However, the polymerization, which was carried out in ionic liquid, afforded improved control over the molecular weight and polydispersity of the resultant product (PDI < 1.4). This polymerization showed many of the characteristics of a living system which prompted us to look more closely at the kinetics of this particular system. The first investigation for kinetics of the initiation and propagation of ring-opening metathesis polymerization of 3-methyl-1-[(5-cyclooctenyl) oxycarbonylmethylene]-imidazol-1-ium tetrafluoroborate catalyzed by [RuCl ₂(PCy₃)(SIMes)(CHPh)] in ionic liquid and CDCl₃ was carried out by ¹H-NMR spectroscopy measurement. The decrease in the monomer concentration over time was tracked by the monitoring of the decrease in the resonance of the alkenes protons on the cyclooctenes ring which appeared at δ = 5.5-5.7 relative to an internal standard of CDCl ₃. These proton signals shifted upfield to approximately δ= 5.38-5.39 as the undertaking of reaction, this indicated that the polymerization had occurred, and the monomer had turned to be polymer. The polymerization performances were much different in CDCl₃ and ionic liquid. ¹H-NMR studies have revealed that the initiating carbene, which shows a signal for the alkylidine proton at δ = 19.2 in CDCl₃, is converted into the propagating carbine with a remarkable peak change from broad to sharp over time. The multiplicity and broadening of the resonances are consistent with a multiplicity of overlapping environments associated with irregular living chain terminal. The polymerization was also monitored by ¹H-NMR in ionic liquid, within the limits of NMR sensitivity, signals attributed to the propagating carbine (δ = 19.2) maintained a relatively same intensity throughout the polymerization which meant a constant concentration for the living propagating chain. The plot of In[M] ₀/[M] versus reaction time was found to be linear in ionic liquid, and the results demonstrated the absence of chain termination on the timescale of the polymerization and showed the polymerization was conducted with living characteristics. The reaction kinetic equation for the polymerization process in ionic liquid was also determined. The exact nature of the propagating species in ROMP is unknown, but we assume that the living characterization is ascribed to the relatively slow propagation of monomer because of the interaction between the charge of monomer and medium of ionic liquid.