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

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.