Recent Advances in Living Cationic Polymerization with Emerging Initiation/Controlling Systems

While the conventional living cationic polymerization (LCP) provided opportunities to synthesizing well-defined polymers with predetermined molecular weights, desirable chemical structures and narrow dispersity, it is still important to continuously innovate new synthetic methods to meet the increasing requirements in advanced material engineering. Consequently, a variety of novel initiation/controlling systems have be demonstrated recently, which have enabled LCP with spatiotemporal control, broadened scopes of monomers and terminals, more user-friendly operations and reaction conditions, as well as improved thermomechanical properties for obtained polymers. In this work, recent advances in LCP is summarized with emerging initiation/controlling systems, including chemical-initiated/controlled cationic reversible addition-fragmentation chain transfer (RAFT) polymerization, photoinitiated/controlled LCP, electrochemical-controlled LCP, thionyl/selenium halide-initiated LCP, organic acid-assisted LCP, and stereoselective LCP. It is hoped that this summary will provide useful knowledge to people in related fields and stimulate new ideas to promote the development and application of LCP in both academia and industry.

Reversible-deactivation radical polymerization of vinyl acetate mediated by tralen, an organomediator

An organic compound, tralen, has been developed as a mediator to control the radical polymerization of vinyl acetate, methyl acrylate, and N-vinyl pyrrolidone via the reversible termination mechanism.

Visible light-induced free radical promoted cationic polymerization using organotellurium compounds

In the present study, visible light and sunlight-induced cationic polymerization of vinyl and cyclic ethers using organotellurium compounds is demonstrated.

Synthesis of Multivalent Organotellurium Chain-Transfer Agents by Post-modification and Their Applications in Living Radical Polymerization

Functionalized or multivalent organotellurium chain-transfer agents (CTAs) for living radical polymerization were synthesized by post-modification, which involved the condensation between a carboxylic-acid-functionalized CTA and various amines in excellent yields without affecting the reactive tellurium moiety. The CTAs exhibited high synthetic versatility for radical polymerization and gave structurally well-controlled polymers, such as multiarmed polymers, from various monomers. Because all new CTAs are easily available on a large scale by simple purification, the current method significantly facilitates macromolecular engineering based on organotellurium-mediated radical polymerization (TERP).

A well-defined block copolymer synthesis via living cationic polymerization and nitroxide-mediated polymerization using carboxylic acid-based alkoxyamines as a dual initiator

A series of carboxylic acid-based alkoxyamines associated with SnBr₄, a Lewis acid, have been used as protonic acids of a binary initiating system to control the cationic polymerization of vinyl ether.

Expanding the Scope of Controlled Radical Polymerization via Cobalt-Tellurium Radical Exchange Reaction

Cobalt-mediated radical polymerization (CMRP) and tellurium-mediated radical polymerization (TERP) were combined for the first time, offering new perspectives in the precision design of macromolecular structures. In particular, the present work highlights the benefits of this strategy for the synthesis of novel poly(vinyl acetate)-based block copolymers. A range of well-defined poly(vinyl acetate)s (PVAc) were first produced via CMRP using the bis(acetylacetonato)cobalt(II) complex (Co(acac)₂) as a regulating agent. Substitution of a methyltellanyl moiety for Co(acac)₂ at the ω-chain end of the precursor was then achieved upon treatment with dimethylditelluride. In contrast to the PVAc prepared by TERP, the ones produced by sequential CMRP and Co/Te exchange reaction almost exclusively consist of regular head-to-tail-TeMe chain-end species that can be activated by TERP. Ultimately, a series of monomers problematic in Co(acac)₂-mediated radical polymerization including N-isopropylacrylamide (NIPAM), 2-(dimethylamino)ethyl acrylate (ADAME), n-butyl acrylate (BA), isoprene (IP), and vinylimidazole (NVIm) were polymerized by TERP from the PVAc-TeMe macroinitiators leading to novel diblock copolymers that cannot be made by each technique used separately.

Organotellurium-mediated living radical polymerization under photoirradiation by a low-intensity light-emitting diode

A low-intensity (6 W) light-emitting diode (LED) effectively activated an organotellurium chain transfer agent and the dormant species, promoting well-controlled radical polymerization. The use of the LED provided many advantages over the previously reported high-intensity Hg lamp, including high energy efficiency during the polymerization, and easy availability of the low-cost light source. Structurally well-defined poly(methyl methacrylate), poly(methyl acrylate), and polystyrene, with narrow molecular weight distributions, were synthesized under LED irradiation with or without a neutral density filter.

Highly Controlled Organotellurium-Mediated Living Radical Polymerization (TERP) in Ionic Liquids (ILs). The New Role of ILs in Radical Reactions

Ionic liquids (ILs) served perfectly as solvents for the organotellurium-mediated living radical polymerization (TERP) of methyl methacrylate (MMA), methyl acrylate (MA), and styrene. The reaction rate of polymerizing MMA and MA was significantly increased as previously reported, and the controllability of the polydispersity index (PDI) was also improved by a great margin. The TERP of MMA can now give poly(methyl methacrylates) (PMMAs) with PDIs less than 1.1 and nearly full conversion in a half hour without the presence of (TeMe)₂. The kinetic study revealed that the improved control could be ascribed to a faster degenerative chain transfer (DT) reaction which plays a key role in the control of PDI for TERP. Besides the polar effect of ILs, the existence of Lewis acid-base interaction between ILs and the Te atom was proven by UV-vis spectroscopy and ¹²⁵Te NMR results. Such positive interaction lowered the activation energy of the DT process.