Recent developments in polymerization reactions utilizing thiocarbonylthio compounds have highlighted the surprising versatility of these unique molecules. The increasing popularity of reversible addition–fragmentation chain transfer (RAFT) radical polymerization as a means of producing well‐defined, ‘controlled’ synthetic polymers is largely due to its simplicity of implementation and the availability of a wide range of compatible reagents. However, novel modes of thiocarbonylthio activation can expand the technique beyond the traditional system (i.e., employing a free radical initiator) pushing the applicability and use of thiocarbonylthio compounds even further than previously assumed. The primary advances seen in recent years are a revival in the direct photoactivation of thiocarbonylthio compounds, their activation via photoredox catalysis, and their use in cationic polymerizations. These synthetic approaches and their implications for the synthesis of controlled polymers represent a significant advance in polymer science, with potentially unforeseen benefits and possibilities for further developments still ahead. This Research News aims to highlight key works in this area while also clarifying the differences and similarities of each system.

A metal-free, cationic, reversible addition-fragmentation chain-transfer (RAFT) polymerization was proposed and realized. A series of thiocarbonylthio compounds were used in the presence of a small amount of triflic acid for isobutyl vinyl ether to give polymers with controlled molecular weight of up to 1×10(5) and narrow molecular-weight distributions (Mw /Mn <1.1). This "living" or controlled cationic polymerization is applicable to various electron-rich monomers including vinyl ethers, p-methoxystyrene, and even p-hydroxystyrene that possesses an unprotected phenol group. A transformation from cationic to radical RAFT polymerization enables the synthesis of block copolymers between cationically and radically polymerizable monomers, such as vinyl ether and vinyl acetate or methyl acrylate.

Cationic degenerative chain-transfer polymerization of vinyl ethers and p-alkoxystyrenes was investigated using a series of thioethers as a reversible chain-transfer agent via the equilibrium between a growing carbocationic species and the resulting sulfonium intermediate in the presence of a small amount of triflic acid (TfOH) as a cationogen. The stable thioether, which was easily prepared from isobutyl vinyl ether (IBVE) and n-butanethiol, efficiently controls the molecular weight of the resulting poly(IBVE) up to M n ∼ 1 × 10 5 with narrow molecular weight distributions (MWDs) (M w /M n ∼ 1.2). Upon increasing the bulkiness of the alkyl substituents in the thiols (R−SH; R: n-Bu < s-Bu < t-Bu) or those in the monomers (CH 2 CHOR′, R′: ethyl < isobutyl < cyclohexyl), the MWDs became broader due to the slower formation of the sulfonium intermediate for the degenerative chain-transfer reaction. For pmethoxystyrene, thioethers derived from bulkier alkylthiols or more electron-rich thiophenols are more effective. A silylprotected difunctional dithioether produced telechelic polymers possessing hydroxyl groups at both chain ends and stable thiol linkers in the middle of the polymer chains. These polymers were subsequently used in chain-extension reactions in conjunction with diisocyanates and diols as chain extenders to be converted into high molecular weight polymers linked via urethane linkages. ■ INTRODUCTIONLiving polymerization is one of the most effective and useful methods for not only controlling the molecular weight of synthetic polymers but also enabling the synthesis of complex polymeric architectures such as block, end-functional, telechelic, and star polymers by design. 1 Recently, there has been great progress in controlled/living polymerization techniques, most of which are based on the transient reversible deactivation of the propagating chain end into a dormant species, and as such, IUPAC recommends the name reversible deactivation polymerization (RDP). 2 This class of polymerization has been most actively studied in radical polymerization systems and can be classified into at least three different mechanisms, 3 i.e., dissociation−combination, 4−7 atom transfer, 8−15 and degenerative chain transfer, 16−23 according to the kinetic treatments.The concept of RDP originates from several living ionic polymerizations 24−27 such as group transfer polymerization (GTP) of methacrylates 28,29 and living cationic polymerization of vinyl ethers and isobutene, 30−35 which were accomplished in the mid-1980s. Most living cationic polymerizations rely on a type of atom transfer mechanism, in which the dormant covalent bonds, such as carbon−halogen and carbon−oxygen ester bonds, are reversibly activated by Lewis acid catalysts into a carbocationic species (Scheme 1B). 30−35 The Lewis acid catalysts are generally metal halides with a few exceptions such as iodine, 34 while the dormant species is derived from a relatively weak protonic acid generating a nucleophilic counteranion that can form a covalent bond via an a...

A series of acyclic nucleosides with two hydroxymethyl groups mimicking the 3'- and 5'-hydroxyl groups of the 2'-deoxyribose moiety were prepared and evaluated for their antiherpetic activity. Among those, 9-[[cis-1', 2'-bis(hydroxymethyl)cycloprop-1'-yl]methyl]guanine (3) showed extremely potent antiviral activity against herpes simplex virus type-1 (HSV-1) with good selectivity. Both enantiomers of 3 were synthesized starting from chiral epichlorohydrins, and only one of the enantiomers with 1'S,2'R-configuration (3a) exhibited strong antiherpetic activity (IC50 of 0.020 microg/mL against HSV-1 Tomioka vs 0.81 microg/mL for acyclovir). Enantiomer 3a was also more inhibitory than acyclovir against varicella-zoster virus (VZV) but ineffective against human immunodeficiency virus (HIV). Compound 3a is phosphorylated by HSV-1 thymidine kinase (TK) very efficiently. The relationship between conformation and antiherpetic activity in this series of compounds is discussed.