β‐Hydrogen transfer from poly(methyl methacrylate) propagating radicals to the nitroxide SG1: Analysis of the chain‐end and determination of the rate constant

Methyl methacrylate (MMA) was polymerized in bulk at 70 °C in the presence of an alkoxyamine initiator with low dissociation temperature (the so‐called BlocBuilder?) and increasing amounts of free N‐tert‐butyl‐N‐(1‐diethylphosphono‐2,2‐dimethylpropyl) nitroxide (SG1). Low final monomer conversions were reached, indicating a loss in radical activity due to side reactions such as irreversible homoterminations between the propagating radicals and β‐hydrogen transfer (also called disproportionation) from a propagating radical to a free‐SG1 nitroxide. Proton NMR and MALDI‐TOF mass spectrometry were used to analyze the polymer chain‐ends and to clearly identify the main mechanism of irreversible termination. In particular, it was shown that all polymer chains were terminated by an alkene function in the presence of a large excess of free SG1, meaning that β‐hydrogen transfer from PMMA propagating radicals to the nitroxide SG1 was the major chain‐stopping event. On the other hand, for a low excess of free SG1, the two termination modes coexisted. Kinetic modeling was then performed using the PREDICI software, and the rate constant of β‐hydrogen transfer, k_βHtr, was estimated to be 1.69 × 10³ L mol^?1 s^?1 at 70 °C. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6333–6345, 2008     

Double nucleophilic reaction of amines to the imidazole nucleus and selective synthesis of 5-aminoimidazoles

Reaction of 2-(1-chloro-2,2-dimethylpropyl)-1-methyl-1H-imidazole with an excess of N,N-dimethylamine at room temperature gave an abnormal adduct, trans-4,5-bis(dimethylamino)-1-methyl-2,2-dimethylpropyl-2-imidazoline, which was derived from a serial, double nucleophilic addition into the imidazole nucleus in 74% yield together with a normal S_N product, 1-methyl-2-(1-dimethylamino-2,2-dimethylpropyl)-1H-imidazole in 15% yield. The former was easily converted to 1-methyl-5-(dimethylamino)-2-(2,2-dimethylpropyl)-1H-imidazole by only reflux in toluene in 90% yield. The scope, mechanism and limitation of these reactions are discussed.     

Facile synthesis of thermoresponsive block copolymers of N-isopropylacrylamide using heterogeneous controlled/living nitroxide-mediated polymerizations in supercritical carbon dioxide

A new protocol for preparation of thermoresponsive poly(N-isopropylacrylamide, NIPAM) containing block copolymers is described. It involves two successive heterogeneous controlled/living nitroxide-mediated polymerizations (NMPs) in supercritical carbon dioxide (scCO₂) using N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)]nitroxide (SG1), as the nitroxide. Precipitation NMPs give narrow dispersity macroinitiators (MIs), and a first report of the controlled/living polymerization of N,N-dimethylacrylamide (DMA) in scCO₂ is described. The MI is then used in an inverse suspension NMP of NIPAM in scCO₂ resulting in the efficient preparation of block copolymers containing DMA, tert-butyl acrylate and styrene. Aqueous cloud point temperature analysis for poly(DMA)-b-poly(NIPAM) and poly(acrylic acid)-b-poly(NIPAM) shows a significant dependence on poly(NIPAM) chain length for a given AB block copolymer.     

Nitroxide-mediated precipitation polymerization of styrene in supercritical carbon dioxide: Effects of monomer loading and nitroxide partitioning on control

Nitroxide-mediated solution and precipitation polymerizations of styrene in toluene and supercritical carbon dioxide (scCO₂), respectively, using the nitroxides N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)]nitroxide (SG1) and 2,2,5-trimethyl-4-phenyl-3-azahexane-3-nitroxide (TIPNO) are presented. Solution polymerizations are compared with simulations using PREDICI software, revealing that differences in the polymerization behaviours between the SG1- and TIPNO-mediated systems cannot be rationalized based on literature rate coefficients and the ideal mechanism for nitroxide-mediated polymerization. Nitroxide and monomer partitioning between the polymer particles and the continuous phase play important roles in the precipitation polymerizations in scCO₂. Loss of control (broader molecular weight distributions) as a result of nitroxide partitioning is accentuated at low monomer loading, and is significantly more pronounced for TIPNO than SG1. However, at higher monomer loading the level of control was superior in scCO₂ compared to in the corresponding solution polymerizations for both nitroxides, most likely caused by an increase in the number of activation-deactivation cycles experienced by any given chain during its growth.     

The effects of aromatic solvents on the photopolymerization rates of isoprene,ethyl methacrylate,and styrene

The photochemical polymerization rates of isoprene, ethyl methacrylate, and of styrene in various aromatic solvents were measured. The average lifetimes of propagating radicals were measured by the rotating sector method. The polymerization rate constants, K_p, were determined and compared with dipole moments (μ) and Hammett σ constants for the aromatic solvents. Linear correlations of log(k_p/k_{p, benzene}) vs. μ and σ were obtained.     

Intramolecular Hydrogen Bonding: The Case of β‐Phosphorylated Nitroxide (= Aminoxyl) Radical

Alkoxyamines and persistent nitroxide (= aminoxyl) radicals are important regulators of nitroxide‐mediated radical polymerization. Since polymerization times decrease with the increasing homolysis rate constant of the C? ON bond homolysis between the polymer chain and the aminooxy moiety, the factors influencing the cleavage rate constant are of considerable interest. It has already been shown that the value of the homolysis rate constant k_d is very sensitive to the stabilization of both released radical species. X‐Ray, EPR, and kinetic data showed that the intramolecular H‐bonding radical in the 1‐(diethoxyphosphoryl)‐2,2‐dimethylpropyl 2‐hydroxy‐1,1‐dimethylethyl nitroxide ( 3a ) (homologue of 2‐hydroxy‐1,1‐dimethylethyl 1‐phenyl‐2‐methylpropyl nitroxide ( 2a )) did not occur with the nitroxide moiety as expected but with the phosphoryl group. However, the polymerization rate of styrene (= ethenylbenzene) was significantly enhanced.     

Features of the kinetics of the liquid-phase oxidation of cyclohexanol

The kinetics of oxygen uptake in the cumyl peroxide-initiated oxidation of cyclohexanol (373 K, o-dichlorobenzene) is studied. The parameters of the oxidizability of k _p (2k _t )^?0.5 (which depend on [RH]) and the rate constants of the bi- and trimolecular reactions of chain initiation (k ₀ = 1.25 × 10^?8 L/(mol s) and k′₀ = 2.5 × 10^?9 L²/(mol² s), respectively) are determined by solving the inverse kinetic problem. It is demonstrated that the quadratic-law recombination of peroxyl radicals during cyclohexanol oxidation also occurs without chain termination. The recombination rates of peroxyl radicals with and without chain termination (k′/k _t ) are found to grow with increasing [RH], reaching their maxima at [RH] = 1.0 mol/L, and to diminish subsequently. We conclude that this can be attributed to changes in the ratio between the propagating peroxyl radicals (hydroperoxyl and 1-hydroxycyclohexylperoxyl) in the reaction medium.     

Nitroxide-mediated miniemulsion polymerization of n-butyl acrylate: synthesis of controlled homopolymers and gradient copolymers with styrene

Controlled free-radical homopolymerization of n-butyl acrylate and its copolymerization with styrene have been studied in aqueous miniemulsion, using an acyclic β-phosphonylated nitroxide as a mediator, the N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl) nitroxide, also called SG1. Polymerization kinetics have been studied and characterization of the (co)polymers has been performed, demonstrating the successful synthesis of well-defined poly(n-butyl acrylate) homopolymers and poly(n-butyl acrylate-co-styrene) gradient copolymers.     

Incubation period in the 2,2,4,4‐tetramethyl‐1‐piperidinyloxy‐mediated thermal autopolymerization of styrene: Kinetics and simulations

Mechanisms and simulations of the induction period and the initial polymerization stages in the nitroxide‐mediated autopolymerization of styrene are discussed. At 120–125 °C and moderate 2,2,4,4‐tetramethyl‐1‐piperidinyloxy (TEMPO) concentrations (0.02–0.08 M), the main source of radicals is the hydrogen abstraction of the Mayo dimer by TEMPO [with the kinetic constant of hydrogen abstraction (k_h)]. At higher TEMPO concentrations ([N?] > 0.1 M), this reaction is still dominant, but radical generation by the direct attack against styrene by TEMPO, with kinetic constant of addition k_ad, also becomes relevant. From previous experimental data and simulations, initial estimates of k_h ≈ 1 and k_ad ≈ 6 × 10^?7 L mol^?1 s^?1 are obtained at 125 °C. From the induction period to the polymerization regime, there is an abrupt change in the dominant mechanism generating radicals because of the sudden decrease in the nitroxide radicals. Under induction‐period conditions, the simulations confirm the validity of the quasi‐steady‐state assumption (QSSA) for the Mayo dimer in this regime; however, after the induction period, the QSSA for the dimer is not valid, and this brings into question the scientific basis of the well‐known expression k_th[M]³ (where [M] is the monomer concentration and k_th is the kinetic constant of autoinitiation) for the autoinitiation rate in styrene polymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6962‐6979, 2006     

The role of excess nitroxide in the SG1 (N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide)-mediated polymerization of methyl methacrylate

Methyl methacrylate (MMA) polymerizations have been conducted in the presence of large excesses of N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl) nitroxide] (SG1) at 110°C. It is demonstrated that such a protocol does not improve control/livingness in the nitroxide mediated polymerization (NMP) of this monomer, instead substantial levels of disproportionation between the nitroxide and propagating radical (PMMA^•) results. The extent of the disproportionation reaction increased with the SG1 concentration, eventually becoming the sole end forming event. Significant disproportionation between SG1 and PMMA^• was also observed at substantially lower temperatures (45°C) in the presence of large excesses of SG1. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2194–2203, 2007     

Free-radical chain oxidation of 1,4-dioxane and styrene in the presence of fullerene C60

The antiradical activity of fullerene C₆₀ was studied for the oxidation of 1,4-dioxane and styrene initiated by azobisisobutyronitrile and benzoyl peroxide as model reactions. The effective rate constants of the reaction of peroxyl radicals with fullerene C₆₀ (k ₇) and the stoichiometric inhibition factor (f _eff) were determined in air ( $P_{O_2 }$ = 0.21 atm) and oxygen ( $P_{O_2 }$ = 1.0 atm). The rate of the liquid-phase oxidation of 1,4-dioxane does not depend on $P_{O_2 }$ , and the effective rate constant of inhibition is k ₇ = (2.4 ± 0.2) × 10⁴ L mol^?1 s^?1. Chain termination in the oxidation of styrene occurs when C₆₀ reacts with both the peroxyl radicals (k ₇ = (1.2 ± 0.1) × 10³ L mol^?1 s^?1) and alkyl (k ₈ = 1.07 × 10⁷ L mol^?1 s^?1) radicals.     

Recombination of 1,1-dimethylpropyl peroxy radicals in polar solvents

The kinetics of 1,1-dimethylpropyl peroxy radicals recombination in polar solvents—water, methanol, and their mixtures—was studied by EPR spectroscopy in combination with the stopped-flow method, and the rate constants of this reaction were determined. Peroxyl radicals were generated by mixing solutions of Ce⁴⁺ sulfate and 1,1-dimethylpropyl hydroperoxide. The observed EPR signal of the peroxyl radical is a singlet with a g-factor of 2.015 ± 0.001, and a line width of ΔH = (1.36 ± 0.02) × 10^?3 T for methanol and ΔH = (9.7 ± 0.2) × 10^?4 T for water. The measured rate constants of (CH₃)₂C(O₂^·)CH₂CH₃ radical recombination at 298 K are 2k_t = (3.9 ± 0.4) × 10⁴ L mol^?1 s^?1 for water and 2k_t = (5.2 ± 0.5) × 10³ L mol^?1 s^?1 for methanol. A linear relationship between ln(2k_t) and the Kirkwood function (ε?1)/(2ε + 1), where e is the dielectric constant of the medium, has been established, indicating an important role of nonspecific solvation in the recombination of tertiary peroxyl radicals.     

Relaxation Studies of Emulsion Polymerization of Styrene Initiated by Ultraviolet Light (using α-sodium anthraquinone sulfonate/triethyl amine as photosensitizer)

Abstract

This paper presents the results of relaxation studies of the seeded emulsion polymerization of styrene initiated by UV-light with a water-soluble photosensitizer. The relaxation kinetic runs were performed at 45, 50, 55, and 60°C using a small diameter rotary dilatometer with an inner magnetic agitator. The entry rate coefficient of thermally induced free radicals into the latex particles, p ₀; the exit rate coefficient of free radicals from latex particles, k; and the average number of free radicals per particle in the thermally induced background polymerization, n _ss(thermal) were determined. The propagating rate coefficient kp was determined with the data of monomer concentration in a particle, C _M, obtained from experiments with chemical initiation. The Arrhenius formula with p ₀, k, and k_p was also obtained.     

Gas‐phase radical–radical recombination reactions of nitroxides with substituted phenyl radicals

Fourier‐transform ion cyclotron resonance mass spectrometry has been used to examine gas‐phase reactions of four different nitroxide free radicals with eight positively charged pyridyl and phenyl radicals (some containing a Cl, F, or CF₃ substituent). All the radicals reacted rapidly (near collision rate) with nitroxides by radical–radical recombination. However, some of the radicals were also able to abstract a hydrogen atom from the nitroxide. The results establish that the efficiency (k_reaction/k_collision) of hydrogen atom abstraction varies with the electrophilicity of the radical, and hence is attributable to polar effects (a lowering of the transition‐state energy by an increase in its polar character). The efficiency of the recombination reaction is not sensitive to substituents, presumably due to a very low reaction barrier. Even so, after radical–radical recombination has occurred, the nitroxide adduct was found to fragment in different ways depending on the structure of the radical. For example, a cationic fragment was eliminated from the adducts of the more electrophilic radicals via oxygen anion abstraction by the radical (i.e., the nitroxide adduct cleaves heterolytically), whereas adducts of the less electrophilic radicals predominantly fragmented via homolytic cleavage (oxygen atom abstraction). Therefore, differences in the product branching ratios were found to be attributable to polar factors. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 216–229 2004     

Copolymerization of methyl 2-acetamidoacrylate and styrene in the presence of stannic chloride

Continuous variation method in UV revealed that methyl N-acetylaminoacrylate (MNA) and SnCl₄ formed the 1:1 complex. The copolymerization of MNA with styrene in tetrahydrofuran was carried out at 50 °C in the presence of SnCl₄. The resulting monomer reactivity ratios decreased with an increasing concentration of SnCl₄ added. This finding suggests that SnCl₄ participates in the propagation step of the copolymerization. Therefore, the copolymerization was analyzed by assuming terpolymerization of free MNA (M₁), complexed MNA (M₂), and styrene (M₃). The absolute copolymerization parameters were obtained as follows: k₁₁/k₁₂=0.165, k₁₁/k₁₃=3.04, k₂₂/k₂₁=0.32, k₂₂/k₂₃=0.103, k₃₃/k₃₁=0.058, k₃₃/k₃₂=0.001, Q₁=6.03, e₁=0.52, Q₂=88.57, and e₂=2.23. The complexed MNA is more reactive to polymer radicals with free MNA and styrene as the terminal unit than the free MNA. Very small values of k₂₂/k₂₃ and k₃₃/k₃₂ suggests that the copolymerization of the complexed MNA and styrene proceeds alternatingly.     

ESR study on radical polymerization and its application to microemulsion systems

Well-resolved electron spin resonance (ESR) spectra of propagating radicals of vinyl and diene compounds were observed in a single scan by a conventional CW-ESR spectrometry without the aid of computer accumulation and the specially designed cavity and cells. Although solvents which could be used for ESR measurements were restricted to nonpolar solvents, such as benzene, toluene, and hexane, new information on dynamic behavior and reactivity of the propagating radicals in the radical polymerization of vinyl and diene compounds were obtained. Thus, values of propagation rate constants (k_p) for vinyl and diene compounds were determined by an ESR method. Some of the k_p values were in a fair agreement with those obtained by a pulsed laser polymerization (PLP) method. Furthermore, polymer chain effect on apparent k_p was clearly observed in the radical polymerization of macromonomers and in the microemulsion polymerization. In ESR measurement on inclusion polymerization system, important information on the origin of the 9-line spectrum observed in the radical polymerization of methacrylate propagating radicals was obtained.     

Studies on the influence of vibrational excitation of CF2Cl* radicals on the rate of their reaction with HCl

A method is proposed for studying the influence of vibrational excitation of radicals on their reactivity in bimolecular reactions. Investigations of the reaction CF₂Cl + HCl → CF₂ HCl + Cl by this method show for the first time that this reaction is accelerated by vibrational excitation of CF₂Cl* radicals. Under the experimental conditions, it was found that k^*₁/k₁ ? 6.0.     

Magnetism in organic radical ion salts based on nitronyl nitroxide derivatives substituted with heterocyclic aromatic hydrocarbons

Radical cation and anion salts of the neutral organic radicals, 2-imidazolyl nitronyl nitroxide (2-IMNN) and 2-benzimidazolyl nitronyl nitroxide (2-BIMNN), have been prepared and their magnetic properties studied by SQUID magnetometry. The radical salts exhibit one-dimensional (1-d) antiferromagnetic (AFM) intermolecular interactions with the exchange coupling J/k between −0.8 and −6.3 K, which are significantly reduced from those observed in the two neutral radicals, while 2-IMNN shows an AFM interaction with J/k=−88 K within the molecular dimers and 2-BIMNN has quasi 1-d ferromagnetic (FM) intermolecular interactions with J/k=+22 K (intrachain) and zJ′/k=+0.24 K (interchain). The magnetic properties of the nitronyl nitroxide and iminonitroxide derivatives having molecular structure related to 2-IMNN have also been investigated. In 2-benzimidazolyl iminonitroxide (2-BIMIN), the FM interaction observed in 2-BIMNN is replaced by strong 1-d AFM interaction with J/k=−11.7 K.     

Nitroxide‐mediated free‐radical copolymerization of styrene with butyl acrylate

Controlled free‐radical copolymerization of styrene (S) and butyl acrylate (BA) was achieved by using a second‐generation nitroxide, N‐tert‐butyl‐N‐[1‐diethylphosphono‐(2,2‐dimethylpropyl)] nitroxide (DEPN), and 2,2‐azobisisobutyronitrile (AIBN) at 120 °C. The time‐conversion first‐order plot was linear, and the number‐average molecular weight increased in direct proportion to the ratio of monomer conversion to the initial concentration, providing copolymers with low polydispersity. The monomer reactivity ratios obtained were r_S = 0.74 and r_BA = 0.29, respectively. To analyze the convenience of applying the Mayo–Lewis terminal model, the cumulative copolymer composition against conversion and the individual conversion of each monomer as a function of copolymerization time were studied. The theoretical values of the propagating radical concentration ratio were also examined to investigate the copolymerization rate behavior. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4168–4176, 2004     

Determination of the activation rate constants of alkyl halide initiators for atom transfer radical polymerization

The second-order activation rate constants k_A for low-molar-mass alkyl halides catalyzed by cuprous halide complexes Cu(I)X/2L (X = Cl or Br; L = 4,4′-diheptyl-2,2′-bipyridine) were determined by the nitroxide capping method along with ¹H NMR. The k_A for 1-phenylethyl bromide, a typical initiator for atom transfer radical polymerization (ATRP), with the Cu(I)Br complex was found to be close to the known value of the k_A for a polystyryl bromide, being large enough for the initiation to be completed at an early stage of polymerization. It was also found that k_A strongly depends on the kind of halogen and the steric factor of the alkyl halide in question.