Univalent Gallium Salts of Weakly Coordinating Anions: Effective Initiators/Catalysts for the Synthesis of Highly Reactive Polyisobutylene

Abstract

The scope of the univalent gallium salts [Ga(C6H5F)2]+[Al(ORF)4]− and the new completely characterized [Ga(1,3,5-Me3C6H3)2]+[Al(ORF)4]− (RF = C(CF3)3) was investigated in terms of initiating or catalyzing the synthesis of highly reactive poly(2-methylpropylene)—highly reactive polyisobutylene (HR-PIB)—in several solvents. A series of polymerization reactions proved the high efficiency and quality of the univalent gallium salts for the polymerization of isobutylene. The best results were obtained using very low concentrations of [Ga(C6H5F)2]+[Al(ORF)4]− (down to 0.007 mol%) while working at reaction temperatures of up to ±0 °C and in the noncarcinogenic and non-water hazardous solvent toluene. Under these conditions, HR-PIB with an α-content of terminal olefinic double bonds up to 91 mol% and a molecular weight of 1000–2000 was obtained in good yields. Upon changing [Ga(C6H5F)2]+[Al(ORF)4]− for the electron richer [Ga(1,3,5-Me3C6H3)2]+[Al(ORF)4]−, polymerization temperatures could be increased to +10 °C. The reactivity of the gallium(I) cations therefore seems to be tunable through ligand exchange reactions. Experimental results, density functional theory calculations, and mass spectrometric investigations point toward a coordinative polymerization mechanism.