Phase Transfer Activation of Fluorous Analogs of Grubbs' Second-Generation Catalyst: Ring-Opening Metathesis Polymerization
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Grubbs second-generation alkene metathesis catalyst and the fluorous analog (H 2IMes)((R f8(CH 2) 2) 3P)(Cl) 2Ru(=CHPh) (1; H 2IMes/R f8 = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene/(CF 2) 7CF 3) catalyze ring-opening metathesis polymerizations of norbornene at essentially identical rates (CDCl 3, RT). However, dramatic accelerations can be observed with 1 in the presence of the fluorous solvent perfluoro(methylcyclohexane) (PFMC). The fluorous phosphine (R f8(CH 2) 2) 3P must first dissociate from 1 to generate the 14-valence-electron intermediate that begins the catalytic cycle and should be scavenged by the PFMC phase (PFMC/toluene partition coefficient >99.7:<0.3). This would allow alkenes to more effectively compete for active catalyst. However, faster rates are seen only when 1 (partition coefficient 39.6:60.4) is added as a PFMC solution or a PFMC/CDCl 3 biphase mixture, as opposed to CDCl 3 solution, and possible additional contributing factors are analyzed. Analogous effects are observed with a 7-oxanorbornene-based N-butylsuccinimide. The molecular weights, polydispersities, glass transition temperatures, and cis/trans C=C linkage ratios of the polynorbornene produced under monophasic and biphasic conditions are compared and are usually similar. 2011 American Chemical Society.