Ethoxylation of Phenols Catalyzed by Metal‐Free Lewis Pairs: Living/Controlled Polymerization in a Slow‐Initiation Mode^†

Abstract

Main observation and conclusion Phenols, instead of conventional alcoholic compounds, are used to initiate ring‐opening polymerization (ROP) of ethylene oxide and propylene oxide in bulk at room temperature in cooperation with the recently prevailing bicomponent 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 (phenolate) species than the propagating (alcoholate) species, chain initiation is distinctly slower than propagation and the difference becomes even larger as the acidity of the phenolic initiator increases. Nevertheless, well‐defined polyethers with expected molar mass, low dispersity, and high end‐group fidelity are afforded substantiating that the ROP is living/controlled in spite of the slow initiation. Evolution of product composition indicates that the reaction on alcoholates (i.e., propagation) is effectively inhibited by the unreacted phenols during the initiation step because of the adequately large acidity difference, which is key to the success of the slow‐initiation living/controlled polymerization. The acidity difference between phenol and alcohol also facilitates the fulfillment 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.

Organoboron-mediated polymerizations

“…Organoboron-catalyzed anionic ROP/ROCOP follows an immortal manner in that chain transfer is much faster than propagation. Successful examples demonstrated that phenols, 287 alcohols, 233,241 carboxylic acids, 288 and carboxylate salts 180,286 were all effective chain transfer agents. Therefore, well-defined star and block polymers could be directly synthesized when multifunctional chain transfer agents or macromolecular chain transfer agents 289 were used.…”

Controlled Ring-Opening Polymerization of Epoxides Catalyzed by Metal-Free Phosphazenium Salts (P5+): Using Carboxylic Acid as an Initiator to Prepare Esterified Polyethers

“…Hydroxyl compounds have been the most commonly used initiators in the ROP of epoxides, resulting in aliphatic ether α-end junctions and main-chain monomeric units that are difficult to selectively cleave or modify. Therefore, significant efforts have been dedicated to expanding the structural diversity and the heterofunctional initiators for ROP of epoxides, including amide, , carboxylic acid, , phenols, − and carbamates . Furthermore, the nonalcoholic initiator-mediated-controlled ROP of epoxides will contribute to the advancement of novel functionalities in polyethers and their derivatives, thereby providing enhanced opportunities for selective cleavage/modification of the initiating moieties.…”