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Since the first report of cationic reversible addition-fragmentation chain transfer (RAFT) polymerization in 2015, there have been great developments in many aspects of cationic RAFT and degenerative chain transfer (DT) polymerizations, including effective chain-transfer agents, applicable initiators or catalysts, controllable monomers, and controlled architectures of polymers. Cationic RAFT polymerizations exhibit not only many similarities to radical RAFT polymerizations in terms of thiocarbonylthio-based RAFT agents, RAFT mechanisms, and controlled polymer structures but also differences originating from the cationic polymerizations, such as novel RAFT or DT agents, initiation and propagation mechanisms, controllable monomers, and stereostructures of the resulting polymers. This review covers the progress made with cationic RAFT and DT polymerizations, focusing on the similarities and differences between cationic and radical RAFT polymerizations. In total, the characteristics and expanding potentials of cationic RAFT and DT polymerizations are discussed.

A series of acridinium salts with high excited-state oxidative power are employed as photoredox organocatalysts in conjunction with various thioesters, such as trithiocarbonate, dithiocarbamate, and xanthate, and thioacetals for photomediated cationic reversible addition–fragmentation chain-transfer (RAFT) and degenerative chain-transfer (DT) polymerizations of vinyl ethers under visible light. A combination of acridinium salts and trithiocarbonate or thioacetal induces relatively fast and controlled cationic polymerization of various alkyl vinyl ethers under blue, green, and white LEDs, where the photoexcited acridinium salts generate cationic species from the sulfur compounds via the mesolytic cleavage of carbon–sulfur bonds through the formation of radical cations. An acridinium salt with a higher excited-state oxidative power induces faster cationic polymerization. Temporal control of the polymerization is achievable by switching the light on and off, although the polymerization in the dark is not completely halted. The controlled nature of the polymerization further enables the synthesis of block copolymers of alkyl vinyl ethers via sequential monomer addition.

This work was designed as a comprehensive study of cationic and radical polymerizations of exo-methylene six-membered conjugated dienes, i.e., 3-methylenecyclohexenes, which were prepared from bio- and petroleum-based compounds, to determine the effects of substituents on monomer reactivities and thermal properties of the polymers. To clarify the effect of a methyl group on the conjugated diene as well as those of isopropyl substituents at other positions in the biobased monomers, a series of model monomers with or without a methyl group at the 3- or 4-position of the exo-methylene moiety were prepared from petroleum-derived compounds. Cationic homopolymerizations of the model monomers were extremely fast, but radical processes were very slow, which was consistent with results reported for terpenoid-derived exo-methylene-conjugated dienes. Kinetic studies of living cationic copolymerizations involving the model monomers and isobutyl vinyl ether indicated that the methyl group on the conjugated diene moiety strongly influenced the cationic reactivity of the monomers; a methyl group at the 4-position provided the highest reactivity due to formation of a stable conjugated tertiary cationic propagating species, whereas a methyl group at the 3-position resulted in the lowest reactivity due to steric hindrance around the exo-methylene moiety. The monomer reactivity ratios observed for radical copolymerizations of exo-methylene-conjugated dienes with methyl acrylate or styrene revealed that a methyl group at the 3-position decreased the reactivities of the exo-methylene-containing dienes due to steric hindrance around the exo-methylene moiety. In addition, the effects of isopropyl substituents at the other positions in the terpenoid-derived monomers depended on the specific position. The polymers obtained from the terpenoid-derived monomers showed higher glass transition temperatures than those obtained from the corresponding model monomers due to additional isopropyl subs

A new strategy for the synthesis of sequence-regulated vinyl polymers based on a combination of iterative atom transfer radical additions (ATRAs) of vinyl monomers and acyclic diene metathesis (ADMET) polymerization was developed. A molecularly defined vinyl monomer sequence at the single monomer unit level was constructed in telechelic diene sequence-regulated oligomers by iterative metal-catalyzed ATRAs of vinyl monomers to dichloride followed by allylation of both carbon–chloride terminals. ADMET polymerization of the obtained telechelic diene oligomer with a symmetrical monomer sequence resulted in polymers with an embedded vinyl monomer sequence that was repeatedly connected by an olefin formed by olefin metathesis polymerization. The subsequent hydrogenation of the main-chain olefins led to polymers that mimicked sequence-regulated vinyl polymer structures consisting of saturated C–C main chains. In particular, this study enabled the synthesis of sequence-regulated vinyl polymer mimics that have perfect and periodic styrene–acrylate–styrene–ethylene–methylene sequences, no statistical distributions of the comonomer compositions and sequences in polymer chains, relatively high molecular weights, and only carbon–carbon single bonds in the main chain. The thermal properties of the sequence-regulated vinyl polymer mimics were evaluated and compared with those of hypothetical random copolymers of styrene, acrylate, and ethylene units with the same comonomer contents.