Nicolay V. Tsarevsky

Nanostructured functional materials prepared by atom transfer radical polymerization

Atom transfer radical polymerization (ATRP) is the most extensively studied controlled/living radical polymerization (CRP) method, with the interest originating primarily in its simplicity and broad applicability, and in the ability to prepare previously inaccessible well-defined nanostructured polymeric materials. This review illustrates the range of well-defined advanced functional materials that can be prepared by ATRP. We detail the precise synthesis of macromolecules with predetermined molecular weight, designed molecular weight distribution, controlled topology, composition and functionality. The materials include polymers with site-specific functionalities and novel architectures that are starting to find commercial application--such as stars, bottle brushes, block and gradient copolymers. This is followed by discussing their self-assembly into materials with nanoscale morphologies. These macromolecular engineering procedures provide new avenues to nanostructured functional materials for many high-value applications, for example as thermoplastic elastomers, coatings, surfactants, dispersants and as optoelectronic and biomedical materials.

Macromolecular Engineering by Atom Transfer Radical Polymerization

This Perspective presents recent advances in macromolecular engineering enabled by ATRP. They include the fundamental mechanistic and synthetic features of ATRP with emphasis on various catalytic/initiation systems that use parts-per-million concentrations of Cu catalysts and can be run in environmentally friendly media, e.g., water. The roles of the major components of ATRP--monomers, initiators, catalysts, and various additives--are explained, and their reactivity and structure are correlated. The effects of media and external stimuli on polymerization rates and control are presented. Some examples of precisely controlled elements of macromolecular architecture, such as chain uniformity, composition, topology, and functionality, are discussed. Syntheses of polymers with complex architecture, various hybrids, and bioconjugates are illustrated. Examples of current and forthcoming applications of ATRP are covered. Future challenges and perspectives for macromolecular engineering by ATRP are discussed.

Diminishing catalyst concentration in atom transfer radical polymerization with reducing agents

The concept of initiators for continuous activator regeneration (ICAR) in atom transfer radical polymerization (ATRP) is introduced, whereby a constant source of organic free radicals works to regenerate the CuI activator, which is otherwise consumed in termination reactions when used at very low concentrations. With this technique, controlled synthesis of polystyrene and poly(methyl methacrylate) (Mw/Mn < 1.2) can be implemented with catalyst concentrations between 10 and 50 ppm, where its removal or recycling would be unwarranted for many applications. Additionally, various organic reducing agents (derivatives of hydrazine and phenol) are used to continuously regenerate the CuI activator in activators regenerated by electron transfer (ARGET) ATRP. Controlled polymer synthesis of acrylates (Mw/Mn < 1.2) is realized with catalyst concentrations as low as 50 ppm. The rational selection of suitable Cu complexing ligands {tris[2-(dimethylamino)ethyl]amine (Me6TREN) and tris[(2-pyridyl)methyl]amine (TPMA)} is discussed in regards to specific side reactions in each technique (i.e., complex dissociation, acid evolution, and reducing agent complexation). Additionally, mechanistic studies and kinetic modeling are used to optimize each system. The performance of the selected catalysts/reducing agents in homo and block (co)polymerizations is evaluated.

Understanding Atom Transfer Radical Polymerization: Effect of Ligand and Initiator Structures on the Equilibrium Constants

Equilibrium constants in Cu-based atom transfer radical polymerization (ATRP) were determined for a wide range of ligands and initiators in acetonitrile at 22 degrees C. The ATRP equilibrium constants obtained vary over 7 orders of magnitude and strongly depend on the ligand and initiator structures. The activities of the Cu(I)/ligand complexes are highest for tetradentate ligands, lower for tridentate ligands, and lowest for bidentate ligands. Complexes with tripodal and bridged ligands (Me6TREN and bridged cyclam) tend to be more active than those with the corresponding linear ligands. The equilibrium constants are largest for tertiary alkyl halides and smallest for primary alkyl halides. The activities of alkyl bromides are several times larger than those of the analogous alkyl chlorides. The equilibrium constants are largest for the nitrile derivatives, followed by those for the benzyl derivatives and the corresponding esters. Other equilibrium constants that are not readily measurable were extrapolated from the values for the reference ligands and initiators. Excellent correlations of the equilibrium constants with the Cu(II/I) redox potentials and the carbon-halogen bond dissociation energies were observed.

Atom transfer radical polymerization of n‐butyl methacrylate in an aqueous dispersed system: A miniemulsion approach

Ultrasonication was applied in combination with a hydrophobe for the copper-mediated atom transfer radical polymerization of n-butyl methacrylate in an aqueous dispersed system. A controlled polymerization was successfully achieved, as demonstrated by a linear correlation between the molecular weights and the monomer conversion. The polydispersities of the polymers were small (weight-average molecular weight/number-average molecular weight < 1.5). The influence of several factors, including ultrasonication, the amount of the surfactant, and the nature of the initiator, on the polymerization kinetics, molecular weight, and particle size was studied. The polymerization rate and molecular weights were independent of the number of particles and only depended on the atom transfer equilibrium. The final particle size, however, was a function of all the parameters.

Multisegmented Block Copolymers by 'Click' Coupling of Polymers Prepared by ATRP

Multisegmented block copolymers were prepared by the step-growth click coupling of well-defined block copolymers synthesized by atom transfer radical polymerization (ATRP). α,ω-Diazido-terminated polystyrene-block-poly(ethylene oxide)-block-polystyrene was coupled with propargyl ether in N,N-dimethylformamide in the presence of a CuBr/N,N,N´,N´´,N´´-pentamethyldiethylenetriamine catalyst. The preparation of multisegmented block copolymers was also demonstrated by the click coupling of propargyl ether with another diazido-terminated triblock copolymer, poly(n-butyl acrylate)-block-poly(methyl methacrylate)-block-poly(n-butyl acrylate), and a diazido-terminated pentablock copolymer, polystyrene-block-poly(n-butyl acrylate)-block-poly(methyl methacrylate)-block-poly(n-butyl acrylate)-block-polystyrene. The formation of a product of higher molecular weight and broader molecular weight distribution was verified by triple-detection size exclusion chromatography, which revealed that typically five to seven block copolymers were linked together during the click reaction. Differential scanning calorimetry and dynamic mechanical analysis revealed that the amphiphilic block copolymer behaves as a viscoelastic fluid, while its corresponding multiblock copolymer is an elastic material. The multisegmented block copolymers with partially miscible segments exhibit higher glass transition temperatures than their precursors.

Fundamentals of Controlled/Living Radical Polymerization

Introduction Kinetic, thermodynamic and other aspects of radical polymerization Fundamental aspects of living polymerization Controlled/living radical polymerization in the presence of iniferters Controlled/living radical polymerization mediated by stable radicals Organometallic controlled/living radical polymerization Controlled/living radical polymerizations in the presence of tellurium, antimony, and bismuth compounds Reversible addition-fragmentation chain transfer (RAFT) polymerization Degenerative transfer polymerization based on iodine compounds and reversible chain transfer catalyzed polymerization mediated by germanium, tin and phosphorus compounds Atom transfer radical polymerization (ATRP)

New strategy for RDC‐assisted diastereotopic proton assignment using a combination of J‐scaled BIRD HSQC and J‐scaled BIRD HMQC/HSQC

A new strategy to assign diastereotopic protons was developed on the basis of residual dipolar couplings (RDCs) collected in compressed poly(methyl methacrylate) (PMMA) gels. A combination of 2D J‐scaled BIRD HSQC and J‐scaled BIRD HMQC/HSQC NMR experiments was used to collect the RDC data. In the proposed strategy, the first experiment is used to measure 1DCH for methine groups, the sum of 1DCHa + 1DCHb for methylene groups and the average 1DCH3 value for methyl groups. In turn, the small molecule alignment tensor is calculated using these D values without the a priori assignment of CH2 diastereotopic protons. The D values of each individual CH bond (CHa and CHb) of each methylene group in the molecule are then predicted using the calculated alignment tensor and these values compared with the results from the HMQC/HSQC experiment, leading to their unambiguous assignment. This strategy is demonstrated with the alkaloid strychnine that contains five methylene groups with diastereotopic protons, and our results fully agree with the previously reported assignment using combinations of permutated assignments. Copyright

Low-catalyst concentration atom transfer radical polymerization of a phosphonium salt-type monomer

Polymeric phosphonium salts are an important class of polyelectrolytes that are currently gaining increasing interest due to their solution properties and reactivity. The successful controlled/living radical polymerization of a phosphonium salt-type monomer, 4-vinylbenzyltriphenylphosphonium tetrafluoroborate (4VBTPPBF4), under initiators for continuous activator regeneration atom transfer radical polymerization (ICAR ATRP) conditions employing a low concentration of catalyst is reported. After optimization of the reaction conditions, high monomer conversions to the polymeric phosphonium salt, poly4VBTPPBF4, were reached with very good polymerization control. The living character of the polymerizations was further evidenced via chain extension experiments that yielded either higher molecular weight homopolymers or block copolymers. It is also shown that poly4VBTPPBF4 can be efficiently converted to linear poly(4-vinylstyrene) using the Wittig olefination reaction.

Epoxides as Reducing Agents for Low‐Catalyst‐Concentration Atom Transfer Radical Polymerization

Activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) conditions utilizing a low concentration of catalyst are successfully applied for the preparation of well-defined poly(glycidyl methacrylate) without the addition of external reducing agents. The living character of polymerization is evidenced by successful chain extensions with methyl methacrylate and methyl acrylate, again, in the absence of additional reducing agents, yielding block copolymers. The epoxide groups in glycidyl methacrylate or the corresponding polymer can serve as an intrinsic reducing agent to continuously regenerate the Cu(I) -based ATRP activator from the Cu(II) halide complex present in the systems. The reactivity of various epoxides in the reduction of the Cu(II) Br2 complex of tris(2-pyridylmethyl)amine is compared.