Acid-Induced Free-Radical Decomposition of Hydroperoxides and Peroxides: New Low-Temperature Initiator Systems for Vinyl Polymerization

Evidence for an acid-induced free-radical breakdown of tert-butyl hydroperoxide and di-tert-butyl peroxide at room temperature has been obtained from polymerization experiments with methyl methacrylate and styrene.     

The Mechanism of the Decomposition of Peroxides and Hydroperoxides by Mineral Fillers

The interaction of fillers and pigments with free radical initiators has been studied. Clay minerals have a marked influence on both the rate and the mechanism of the decomposition of peroxides and hydroperoxides. Kaolinite is a particularly effective catalyst and causes rapid decomposition even at room temperature. The reaction of cumene hydroperoxide with kaolinite is first-order in peroxide and the rate constant is proportional to the ratio of clay to hydroperoxide. From a study of the products of the reaction and the influence of solvent on the decomposition, a mechanism involving an intramolecular rearrangement or closely associated ion pairs has been proposed. The application of these results to polymer filler composites is discussed.     

Free-Radical Carbonylations: Then and Now

Although known since the 1950s, free-radical carbonylation has not received much attention until only recently. In the last few years the application of modern free-radical techniques has revealed the high synthetic potential of this reaction as a tool for introducing CO into organic molecules. Clearly now is the time for a renaissance of this chemistry. Under standard conditions (tributyltin hydride/CO) primary, secondary, as well as tertiary alkyl bromides and iodides can be efficiently converted into the corresponding aldehydes. Aromatic and α,β-unsaturated aldehydes can also be prepared from the parent aromatic and vinylic iodides. If the reaction is carried out in the presence of alkenes containing an electron-withdrawing substituent, the initially formed acyl radical subsequently adds to the alkene, leading to a general method for the synthesis of unsymmetrical ketones. This three-component coupling reaction can be extended successfully to allyltin-mediated reactions. Thus, β,γ-enones can be prepared from organic halides, CO, and allyltributylstannanes. In a remarkable one-pot procedure alkyl halides can be treated with a mixture of alkene, allyltributylstannane, and carbon monoxide in a four-component coupling reaction that provides β-functionalized δ,?-unsaturated ketones by the formation of three new C? C bonds. The reaction of 4-pentenyl radicals with CO leads to acyl radical cyclization, which provides a useful method for the synthesis of cyclopentanones. Certain useful one-electron oxidations can be combined efficiently with free-radical carbonylations. These findings and others discussed in this article clearly demonstrate that free-radical carbonylation can now be considered a practical alternative to transition metal mediated carbonylation.     

Quantal cumulant dynamics: general theory

The authors have derived coupled equations of motion of cumulants that consist of a symmetric-ordered product of the position and momentum fluctuation operators in one dimension. The key point is the utilization of a position shift operator acting on a potential operator, where the expectation value of the shift operator is evaluated using the cumulant expansion technique. In particular, the equations of motion of the second-order cumulant and the expectation values of the position and momentum operators are given. The resultant equations are expressed by those variables and a quantal potential that consists of an exponential function of the differential operators and the original potential. This procedure enables us to perform quantal (semiclassical) dynamics in one dimension. In contrast to a second-order quantized Hamilton dynamics by Prezhdo and Pereverzev which conserves the total energy only with an odd-order Taylor expansion of the potential [J. Chem. Phys. 116, 4450 (2002); 117, 2995 (2002)], the present quantal cumulant dynamics method exactly conserves the energy, even if a second-order approximation of the cumulants is adopted, because the present scheme does not truncate the given potential. The authors propose three schemes, (i) a truncation, (ii) a summation of derivatives, and (iii) a convolution method, for evaluating the quantal potentials for several types of potentials. The numerical results show that although the truncation method preserves the energy to some degree, the trajectory obtained gradually deviates from that of the summation scheme after 2000 steps. The phase space structure obtained by the truncation scheme is also different from that of the summation scheme in a strongly anharmonic region.     

Free-Radical Copolymerizations of N-Phenyl Maleimide

Free radical-initiated copolymerization of N-phenyl maleimide (NPMI) with styrene (St), vinyl acetate (VAc) and methyl meth-acrylate (MMA) at 35°C in benzene solution initiated by AIBN was studied. The copolymerization of NPMI and St yields a “nearly equimolecular” alternating copolymer, irrespective of monomer feed. Reactivity ratios of NPMI with St, VAc, and MMA were determined by a curve-fitting method which has the advantage of delivering values not involving personal judgement. Q₁ and e₁ values of NPMI were also calculated. Tentative explanations have been proposed to Interpret the “nearly alternating” copolymerization between NPMI and St. In addition, thermal stabilities of copolymers were studied by using a programmed thermo-gravimetric analysis technique. Copolymers of St, VAc, and MMA show a considerable increase in thermal stability with increasing content of NPMI. The glass transition temperatures of copolymers of NPMI with MMA and St were measured by differential scanning calorimetry. In both series of copolymers the glass transition temperature increases markedly with increasing in NPMI content. In the case of NPMI-St copolymers, the relative thermal stability as well as glass transition temperature also corroborated the nearly alternating behavior observed.     

Catalytic Systems Based on Magnesium and Zinc Compounds in the Oxidation Reactions of Alkylarenes and the Decomposition Reactions of the Corresponding Hydroperoxides

The oxidation of aromatic hydrocarbons in the presence of magnesium and zinc 2-ethylhexanoates and a mixed catalyst based on these compounds is studied. It is shown that magnesium and zinc carboxylates are active catalytic systems which catalyze the decomposition of hydroperoxides representing the primary alkylarene oxidation products alongside with activation of oxygen.     

Photoinitiated Free-Radical Polymerization of 4,5,6,7-Tetrahalogenated Fluoresceins

We demonstrated the photoredox catalytic performances of fluorescein derivatives, bearing heavy halogen atoms (Br or I) on a benzoic acid group, using photoinitiated free-radical polymerization. 4,5,6,7-Tetrabromofluorescein and 4,5,6,7-tetraiodofluorescein were used as visible-light-photoredox catalysts to initiate polymerization of poly(ethylene glycol) diacrylate and N-vinylpyrrolidone in the presence of triethanolamine under aerobic conditions. Their photocatalytic performances were evaluated by the hydrogelation of photopolymerization both on the surface of an agarose film and in a liquid solution. The polymerization degree increased considerably in the following order: tetraiodofluorescein<tetrabromofluorescein<fluorescein. This result was different from fluorescein derivatives containing the heavy halogen atoms on a xanthene core ring. Consequently, the location of the heavy halogen atoms was crucial in the photocatalytic performance of fluorescein derivatives.     

Convenient Synthesis and Free-Radical Copolymerization of p-Chloromethylstyrene

p-Chloromethylstyrene, a useful monomer for a starting material in preparation of functional polymers, is conveniently synthesized by the chloromethylation of 2-phenyl-ethyl bromide followed by dehydrobromination. This procedure is simpler and the yield is higher than those of methods previously described. Radical copolymerizations of this compound with styrene and methyl methacrylate were studied at 60°C. From the copolymerization with styrene, Q and e values for p-chloromethylstyrene were calculated as Q = 1.13 and e = ?0.58. Similarly, the copolymerization parameters with methyl methacrylate were obtained as Q = 1.16 and e = ?0.69.     

NIR-Sensitized Activated Photoreaction between Cyanines and Oxime Esters: Free-Radical Photopolymerization

Cyanines comprising either a benzo[e]- or benzo[c,d]indolium core facilitate initiation of radical photopolymerization combined with high power NIR-LED prototypes emitting at 805 nm, 860 nm, or 870 nm, while different oxime esters function as radical coinitiators. Radical photopolymerization followed an initiation mechanism based on the participation of excited states, requiring additional thermal energy to overcome an existing intrinsic activation barrier. Heat released by nonradiative deactivation of the sensitizer favored the system, even under conditions where a thermally activated photoinduced electron transfer controls the reaction protocol. The heat generated internally by the NIR sensitizer promotes generation of the initiating reactive radicals. Sensitizers with a barbiturate group at the meso-position preferred to bleach directly, while sensitizers carrying a cyclopentene moiety unexpectedly initiated the photosensitized mechanism.     

Copolymerization Parameters of Itaconic Anhydride in Free-Radical Polymerization

The copolymerization of itaconic anhydride (M₁) with four different monomers (M₂), namely: vinyl acetate, 2-chloro-ethyl acrylate, acrylonitrile, and styrene, was studied. Values of r₁ and r₂ were determined for copolymerizations in benzene and for two of these systems, namely those with vinyl acetate and 2-chloroethyl acrylate. Values of r₁ and r₂ were also determined for copolymerizations in tetra-hydrofuran. The value of ri ranged from 0.53 to 4.8, but it was always very much greater than r2. The Q-e values for itaconic anhydride in all the systems studied were also calculated and the average values for Q₁ = 8.2, e₁ = 1.45 were determined.     

A Free-Radical Prompted Barrierless Gas-Phase Synthesis of Pentacene

A representative, low-temperature gas-phase reaction mechanism synthesizing polyacenes via ring annulation exemplified by the formation of pentacene (C₂₂H₁₄) along with its benzo[a]tetracene isomer (C₂₂H₁₄) is unraveled by probing the elementary reaction of the 2-tetracenyl radical (C₁₈H₁₁^.) with vinylacetylene (C₄H₄). The pathway to pentacene—a prototype polyacene and a fundamental molecular building block in graphenes, fullerenes, and carbon nanotubes—is facilitated by a barrierless, vinylacetylene mediated gas-phase process thus disputing conventional hypotheses that synthesis of polycyclic aromatic hydrocarbons (PAHs) solely proceeds at elevated temperatures. This low-temperature pathway can launch isomer-selective routes to aromatic structures through submerged reaction barriers, resonantly stabilized free-radical intermediates, and methodical ring annulation in deep space eventually changing our perception about the chemistry of carbon in our universe.     

Phase-Transfer Catalytic Alkylation of Hydroperoxides: A Convenient Route to Mixed Dialkyl Peroxides

The solid-liquid phase transfer catalytic alkylation of hydroperoxides using solid potassium hydroxide as a base and TEBAC as a phase transfer catalyst is reported. When the alkylating agent is a primary bromide, this reaction represents a simple and quick method for the synthesis of mixed dialkyl peroxides in fair yields.     

Organic Syntheses via Free-Radical Displacement Reactions of Organoboranes

In the few short years since the discovery that organoboranes undergo facile free-radical substitution, application of these reactions has become a major new area of synthetic utility. Organoboranes undergo a wide variety of free-radical reactions such as autoxidation to the alcohol or hydroperoxide, 1, 4 addition to α,β-unsaturated derivatives, addition to disulfides, and oxygen-induced radical coupling. It is evident that organoboranes constitute a versatile new source of free radicals and that these reactions can be readily controlled to give very clean syntheses. The application of these reactions will be discussed.     

Free-Radical Addition of Dialkyl Phosphites to Branched Fluoro-Olefines

Abstract

We found that under UV-radiation, and without radical initiators, some higher fluoro-olefines regioselectively add dialkyl phosphites to give adducts with high yields. The intermediate phosphonyl substituted fluoroalkyl radicals were observed by ESR-technique and their reactivity was studied:     

Study on Aromatic Phosphines for Novel Fluorometry of Hydroperoxides (II) -The Determination of Lipid Hydroperoxides with Diphenyl-1-Pyrenylphosphine -

Abstract

A new fluorometric determination of hydroperoxides was proposed. Non-fluorescent diphenyl-1-pyrenyl-phosphine (DPPP) was oxidized quantitatively by hydroperoxides to a strong fluorescent DPPP oxide. A sensitive assay for lipid hydroperoxides was developed based on the reaction. The reaction of lipid hydroperoxides with DPPP was conducted in the dark in the mixture of chloroform and methanol at 60°C within 60 min. The contents of hydroperoxides showed linear relation to the fluorescence intensities in a wide concentration range. The sensitivity was 10,000 times higher than conventional iodometry. The correlation constant between peroxide values obtained by iodometry and by this method was 0.9993 (n=10). The sample size was minimized to less than 3 mg per tube.