Ethoxylation of Phenols Catalyzed by Metal-Free Lewis Pairs:Living/Controlled Polymerization in a Slow-lnitiation Mode

Phenols,in stead of con venti onal alcoholic compounds,are used to in itiate ring-openi ng polymerizati on(ROP)of ethyle neoxide and propyle ne oxide in bulk at room temperature in cooperati on with the recently prevaili ng bicompone nt metal-free catalyst comprising an organobase and triethylborane.Experimental and DFT calculation results both point out that due to the lower nucleophilicity of the initiating(phe no late)species than the propagating(alcoholate)species,chai n initiat ion is distinctly slower tha n propagation and the difference becomes eve n larger as the acidity of the phe nolic in itiator increases.Nevertheless,we 11-defi ned polyethers with expected molar mass,low dispersity,and high end-group fidelity are afforded substa ntiating that the ROP is living/c on trolled in spite of the slow initiation.Evolution of product composition indicates that the reaction on alcoholates(i.e.,propagation)is effectively inhib-ited by the unreacted phenols during the initiation step because of the adequately large acidity differenee,which is key to the success of the slow-initiation living/controlled polymerization.The acidity difference betwee n phe nol and alcohol also facilitates the fulfill-ment of highly selective phenolysis of epoxides,a"1+1"type addition reaction between phenol and epoxide(in excess)rather than the"1+n"type ROP reaction,by use of a milder organobase and decreased Lewis acid/base ratio.