Imidazolidinone Nitroxide-Mediated Polymerization

Temperature Responsive Diblock Polymer Brushes as Nanoreactors for Silver Nanoparticles Catalysis

Metal nanoparticles are widely used in catalysis. Loading metal nanoparticles into polymer brushes has aroused wide attention, but regulation of catalytic performance still needs to be improved. The novel diblock polymer brushes, polystyrene@sodium polystyrene sulfonate-b-poly (N-isopropylacrylamide) (PSV@PSS-b-PNIPA) and PSV@PNIPA-b-PSS with reversed block sequence, were prepared by surface initiated photoiniferter-mediated polymerization (SI-PIMP) and used as nanoreactors to load silver nanoparticles (AgNPs). The block sequence caused the difference of conformation and further affected the catalytic performance. PSV@PNIPA-b-PSS@Ag was found to be able to control the amount of AgNPs exposed to external reactant of 4-nitrophenol at different temperatures to achieve regulation of the reaction rate due to the hydrogen bonds and further physical crosslinking between PNIPA and PSS.

Linear Block Copolymer Synthesis

Block copolymers form the basis of the most ubiquitous materials such as thermoplastic elastomers, bridge interphases in polymer blends, and are fundamental for the development of high-performance materials. The driving force to further advance these materials is the accessibility of block copolymers, which have a wide variety in composition, functional group content, and precision of their structure. To advance and broaden the application of block copolymers will depend on the nature of combined segmented blocks, guided through the combination of polymerization techniques to reach a high versatility in block copolymer architecture and function. This review provides the most comprehensive overview of techniques to prepare linear block copolymers and is intended to serve as a guideline on how polymerization techniques can work together to result in desired block combinations. As the review will give an account of the relevant procedures and access areas, the sections will include orthogonal approaches or sequentially combined polymerization techniques, which increases the synthetic options for these materials.

Attempted Synthesis and Unexpected β-Fragmentation of a Hindered β-Keto Nitroxide

The attempted synthesis of a β-keto imidazolidinone nitroxide by oxidation of the β-hydroxy imidazolidinone precursor with hydrogen peroxide and sodium tungstate led to an unexpected ring-opening reaction to produce 1,4-diazaspiro[4.5]dec-1-en-3-oxo-2-pentanoic acid 1-oxide (13) in high yield. The structure of 13 was confirmed by X-ray crystallographic analysis. A β-fragmentation mechanism is suggested for the oxidative ring-opening reaction.

Progress in reactor engineering of controlled radical polymerization: a comprehensive review

Controlled radical polymerization (CRP) represents an important advancement in polymer chemistry. It allows synthesis of polymers with well-controlled chain microstructures.

Toward living radical polymerization

Radical polymerization is one of the most widely used processes for the commercial production of high-molecular-weight polymers. The main factors responsible for the preeminent position of radical polymerization are the ability to polymerize a wide array of monomers, tolerance of unprotected functionality in monomer and solvent, and compatibility with a variety of reaction conditions. Radical polymerization is simple to implement and inexpensive in relation to competitive technologies. However, conventional radical polymerization severely limits the degree of control that researchers can assert over molecular-weight distribution, copolymer composition, and macromolecular architecture. This Account focuses on nitroxide-mediated polymerization (NMP) and polymerization with reversible addition-fragmentation chain transfer (RAFT), two of the more successful approaches for controlling radical polymerization. These processes illustrate two distinct mechanisms for conferring living characteristics on radical polymerization: reversible deactivation (in NMP) and reversible or degenerate chain transfer (in RAFT). We devised NMP in the early 1980s and have exploited this method extensively for the synthesis of styrenic and acrylic polymers. The technique has undergone significant evolution since that time. New nitroxides have led to faster polymerization rates at lower temperatures. However, NMP is only applicable to a restricted range of monomers. RAFT was also developed at CSIRO and has proven both more robust and more versatile. It is applicable to the majority of monomers subject to radical polymerization, but the success of the polymerization depends upon the selection of the RAFT agent for the monomers and reaction conditions. We and other groups have proposed guidelines for selection, and the polymerization of most monomers can be well-controlled to provide minimal retardation and a high fraction of living chains by using one of just two RAFT agents. For example, a tertiary cyanoalkyl trithiocarbonate is suited to (meth)acrylate, (meth)acrylamide, and styrenic monomers, while a cyanomethyl xanthate or dithiocarbamate works with vinyl monomers, such as vinyl acetate or N-vinylpyrrolidone. With the appropriate choice of reagents and polymerization conditions, these reactions possess most of the attributes of living polymerization. We have used these methods in the synthesis of well-defined homo-, gradient, diblock, triblock, and star polymers and more complex architectures, including microgels and polymer brushes. Applications of these polymers include novel surfactants, dispersants, coatings and adhesives, biomaterials, membranes, drug-delivery media, electroactive materials, and other nanomaterials.

Laser flash photolysis and CIDNP studies of steric effects on coupling rate constants of imidazolidine nitroxide with carbon-centered radicals, methyl isobutyrate-2-yl and tert-butyl propionate-2-yl

Time-resolved chemically induced dynamic nuclear polarization (TR-CIDNP) and laser flash photolysis (LFP) techniques have been used to measure rate constants for coupling between acrylate-type radicals and a series of newly synthesized stable imidazolidine N-oxyl radicals. The carbon-centered radicals under investigation were generated by photolysis of their corresponding ketone precursors RC(O)R (R = C(CH3)2-C(O)OCH3 and CH(CH3)-C(O)-OtBu) in the presence of stable nitroxides. The coupling rate constants kc for modeling studies of nitroxide-mediated polymerization (NMP) experiments were determined, and the influence of steric and electronic factors on kc values was addressed by using a Hammett linear free energy relationship. The systematic changes in kc due to the varied steric (Es,n) and electronic (sigmaL,n) characters of the substituents are well-described by the biparameter equation log(kc/M- 1s(-1)) = 3.52sigmaL,n + 0.47Es,n + 10.62. Hence, kc decreases with the increasing steric demand and increases with the increasing electron-withdrawing character of the substituents on the nitroxide.

Photolysis of an alkoxyamine using intramolecular energy transfer from a quinoline antenna—towards photo-induced living radical polymerization

The photolysis of an alkoxyamine triad comprised of the 1-phenylethyl moiety, 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) and 2-methyl-3-hydroxy quinoline leads to energy transfer from the quinoline moiety (antenna) and cleavage of the C-O alkoxyamine bond, suggesting its possible applications in low temperature living free radical polymerization.

Living Radical Polymerization by the RAFT Process

This paper presents a review of living radical polymerization achieved with thiocarbonylthio compounds [ZC(=S)SR] by a mechanism of reversible addition–fragmentation chain transfer (RAFT). Since we first introduced the technique in 1998, the number of papers and patents on the RAFT process has increased exponentially as the technique has proved to be one of the most versatile for the provision of polymers of well defined architecture. The factors influencing the effectiveness of RAFT agents and outcome of RAFT polymerization are detailed. With this insight, guidelines are presented on how to conduct RAFT and choose RAFT agents to achieve particular structures. A survey is provided of the current scope and applications of the RAFT process in the synthesis of well defined homo-, gradient, diblock, triblock, and star polymers, as well as more complex architectures including microgels and polymer brushes.

Nitroxide-mediated radical processes

In the present short review article recent achievements in nitroxide-mediated radical polymerizations are presented. The basic concept behind these reactions, which is the Persistent Radical Effect (PRE), will be briefly explained. The effect of the nitroxide structure on the polymerization process will be discussed. Moreover, results of nitroxide-mediated radical polymerizations in aqueous dispersions will be summarized. Finally, applications of the PRE to environmentally benign radical chemistry such as nitroxide-mediated alkoxyamine isomerization and carboaminoxylation reactions are presented. Moreover, the potential use of microwave-induced heating to conduct these thermal radical reactions will be discussed.