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The products derived from the reaction of sterically hindered sulfur-bridged bisphenols with aluminum trialkyls enable the efficient polymerization of ethylene oxide. The activity of the catalyst is largely determined by the structure of the bisphenol ligand and the size of the aluminum alkyl groups. Bonds of sulfur to aluminum are a common feature of the solid-state structures of compounds examined to this point. In solution, precatalysts of the formula (3-t-Bu-5-Me-2-OAlEt2−C6H2)2S are fluxional, and two processes appear to contribute to the equilibration of all ethyl substituents. The addition of a hindered monomer, 1,2-epoxyhexane, causes the initial formation of solvation complexes with the aluminum precatalysts and effects the disproportionation of the initial dialuminum complex of the bisphenol. One of the polymer end groups appears to be easily replaced by a halide atom during acid workup.