Ethoxylation Of Phenols Catalyzed By Metal-free Lewis Pairs

Anionic ring-opening polymerization of epoxides is the most important and commonly used method for the synthesis of aliphatic polyethers because of its simple and efficient features as well as the controllable molecular weight and low dispersity of the products.Expanding the scope of initiator is a key strategy to enrich macromolecular structures and functionalities that a polymerization reaction can deliver.In this study,living/controlled ring-opening polymerization of epoxides from phenols is achieved by using metal-free Lewis pairs as catalysts.The kinetics of polymerization are studied by changing the acidity of the phenols and the basicity of the organobase.Besides,under optimized reaction conditions,phenol can react with an equal molar amount of epoxide,though added in excess.Details are given as followed:(1)Controlled ring-opening polymerization of epoxides initiated by phenols(“1+N”type reaction).Phenols are used to initiate ring-opening polymerization of epoxides,including ethylene oxide and propylene oxide,at room temperature catalyzed by metal-free Lewis pairs comprising an organobase and triethylborane.The chain initiation is significantly slower than propagation,which is attributed to the lower nucleophilicity of the initiating(phenolate)species compared with the propagating(alcoholate)species.However,experimental results show that the polymerization still proceeds in a controlled manner resulting in polyethers with expected molar mass,low dispersity,and high end/central group fidelity,which satisfies the requirement of living/controlled polymerization.Kinetic analysis indicates that phenolate reacts with equimolar epoxides to form alcoholate during the chain initiation process;reaction on alcoholates(chain propagation)is substantially inhibited by unreacted phenols during the initiation process because of the large acidity difference between phenols and alcohols,which is key to the success of this slow-initiation living/controlled polymerization.In addition,with fixed Lewis acid/base ratio and catalyst loading,the rate of initiation shows to be markedly influenced by the acidity of the phenolic initiator and basicity of the organobase,whereas the rate of propagation is only dependent on the latter.(2)Selective phenolysis of epoxides with organobase and triethylborane(Et₃B)as the catalyst(“1+1”type reaction).we have explored optimized conditions that the reaction ceases or significantly slows down after the completion of the initiation stage by varying the organobase,molar ratio of organobase and Et₃B,and solvent.The initiating rate is significantly decreased when using low-polarity or chlorinated solvents.However,the ROP still cannot be blocked in the initiation step with the reaction time prolong.Due to the activation effect of triethylborane on epoxides and the large difference in acidity between phenol and alcoholic species,the chain growth can be dramatically inhibited by reducing the basicity of organobase and the molar ratio of organobase/triethylborane.Among them,epoxides phenolysis shows the best selectivity when DABCO is used as the organobase and the molar ratio of DABCO/Et₃B/Phenol is fixed at 0.05/0.05/1.¹H NMR spectrum shows that less than 5 mol%dimer or oligomers exist under the optimized condition.In summary,our findings thus provide a convenient metal-free strategy for controlled ethoxylation(“1+1”or“1+N”type reaction)of phenol-based substrates and insights for controlling slow-initiation polymerization.