Synergistic Advances in Living Cationic and Radical Polymerizations

Abstract

RecommendationsV inyl polymers are among the major families of synthetic polymers.Although the existence of polymers or macromolecules was not generally accepted 100 years ago, in his seminal paper "U ̈ber Polymerization" published in 1920, 1,2 Staudinger drew the formulafor what we now call polystyrene, which could be one of the first vinyl polymers.Now in the 21st century, vinyl polymers are obtained by radical, anionic, cationic, and coordination polymerizations of various vinyl compounds and are widely used in industry.Studies on vinyl polymerizations are still very active, particularly in the field of controlled polymerization, enabling precision polymer synthesis applicable for functional polymer materials.Precision Vinyl Polymerizations: Three General Methods.Carbon-centered cations and radicals are both electron-deficient species, generally more reactive and less stable than carbanions with closed shell structures.Accordingly, the first living polymerization was discovered by Szwarc in 1956 in the anionic polymerization of styrene, in which propagation occurs via the inherently stable carbanion, 3 whereas both living cationic and living radical polymerizations were long considered difficult due to the irreversible termination and chain-transfer reactions prone to occur with unstable cationic and radical propagating species. 4-6However, leading papers published in Macromolecules broke the ice.This editorial aims to introduce some of them, which took the initiative in living cationic and radical polymerizations, especially focusing on synergistic advances (Figure 1).While those polymerizations differ in terms of monomer, initiator, catalyst, and condition, the method of control is the same, i.e., reversible activation of the dormant species into the propagating species.For reversible activation, there are three general mechanisms, 6 namely, dissociation-combination, atom transfer, and degenerative chain transfer (Figure 2), which were proposed originally for radical polymerizations 7 and have also proven effective in cationic polymerizations, though the cationic versions were found earlier than the radical ones.Mechanistically, there would be some variations for these three approaches 8,9 that are not specifically shown herein for simplicity.This editorial chronologically showcases the first papers for distinct living cationic and living radical polymerization 10 based on each mechanism.Living Cationic Polymerization.In an issue of Macromolecules in 1984, cationic polymerization enabling molecular weight control similar to that in the anionic counterpart was first reported. 11The initiating system consisted of hydrogen iodide (HI) and iodine (I 2 ) and resulted in a linear molecular weight increase in the number-average molecular weight (M n ) with monomer conversion and a narrow molecular weight

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