Reactivity of monoenic and conjugated diene systems present in unsaturated olefin terpolymers—I. Reactions with t-butoxy radicals (CH3)3 CO•

The relative rate constants for the hydrogen abstraction and the double bond addition reactions of t-butoxy radicals (CH₃)₃ CO^? with the model compounds 5-ethylidennorbornane (I), dihydrodicyclopentene (II), isopropylidendicyclopentene (III) and methylcyclopentadienylnorbornylmethane (IV) have been determined by using as a reference reaction the hydrogen abstraction for iso-octane. With (I), (III) and (IV) the predominant process is the hydrogen abstraction, whilst for (II) both mechanisms are important. The results have been applied for the elucidation of some aspects of the initiating mechanism of peroxide-induced cross-linking of EPDM and EPTM terpolymers containing (I)-(IV) pendants.     

Preparation of nanoparticles by reducing intermediate radicals formed in sonolytical pyrolysis of surfactants

Metal nanoparticles with a narrow size distribution could be prepared by sonolysis of aqueous solutions of metal cations in the presence of surfactants such as sodium dodecyl sulfate, polyethylene glycol monostearate, etc. The role of the surfactans is not only to stabilize formed particles, but also to produce reductive radicals in pyrolysis or hydrogen abstraction of OH radicals from surfactants. Particles with a smaller size could be obtained in a faster reduction rate with dilute metal cations concentration. Pt(IV) is consecutively reduced in two steps to Pt(0)via Pt(II). By comparing the sonolytical reduction withγ-ray radiolysis, two kinds of organic reducing radicals are proposed to contribute to the reduction. One (R_ab) is an intermediate radical which is produced by hydrogen abstraction of OH radical from surfactant and effective only on the reduction of Pt(II) to Pt(0). The other (R_py) is also an intermediate radical which is produced by thermal decomposition of surfactant at the interface between the cavity and bulk solution and effective on the reduction of Pt(IV) to Pt(II).     

Melt polycondensation of L‐lactic acid with Sn(II) catalysts activated by various proton acids: A direct manufacturing route to high molecular weight Poly(L‐lactic acid)

Poly(L ‐lactic acid) (PLLA) was produced by the melt polycondensation of L ‐lactic acid. For the optimization of the reaction conditions, various catalyst systems were examined at different temperature and reaction times. It was discovered that Sn(II) catalysts activated by various proton acids can produce high molecular weight PLLA [weight‐average molecular weight (M_w ) ≥ 100,000] in a relatively short reaction time (≤15 h) compared with simple Sn(II)‐based catalysts (SnO, SnCl₂ · 2H₂O), which produce PLLA with an M_w of less than 30,000 after 20 h. The new catalyst system is also superior to the conventional systems in regard to racemization and discoloration of the resultant polymer. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1673–1679, 2000     

Controlled release of drug via tuning electrospun polymer carrier

Medicated‐fibers have been obtained through electrospinning after rifampin was dissolved in poly (lactic acid)/chloroform solution. The relationship between polymer variables [such as concentration, molecular weight (M_w), and introducing hydrophilic block] and drug release from the electrospun fibers is disclosed. The results show that polymeric concentration and M_w are crucial for producing the medicated fibers, which influence not only the morphology of the medicated‐fiber but also drug release rate from fiber. At the same M_w, the drug release rate decreases with the increase of spinning concentration. At two different M_w blends, drug release behaviors change. When the low M_w content is in a dominant position, drug release rate depends largely on mixing ratio of two M_w contents; on the other hand, drug release rate is also dependent on concentration of spinning fluid. In addition, the block copolymer [poly‐L ‐lactic acid (PLLA)‐polyethylene glycol‐PLLA] shows faster release rate as compared to homopolymer (PLLA). © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Radiation induced oxidative degradation of poly[bis(p-tolylamino)] phosphazene in chlorinated hydrocarbons

Degradation induced by ionizing radiations on polyphosphazenes, dissolved in chlorinated methanes, is the subject of this investigation. Both steady gamma and pulse radiolysis techniques have been employed.The labile sites for radical attack in poly[bis(p-tolylamino)] phosphazene were identified as the aniline N-H and benzyl C-H bonds of the pendant groups. All the primary radicals, by themselves or after peroxidation, can give rise to macroradicals by hydrogen abstraction from the polymer molecules. Appreciable crosslinking appeared only in the presence of quite a high concentration of oxygen, whereas, in its absence or after it had been consumed, practically no $\text{M}{}_{\mathrm{<mtext>w</mtext>}}$ changes were detected. Optical spectra of intermediates formed after pulse radiolysis are reported and attributed to radical-polymer charge transfer complexes, anilinium ions and toluidine radicals. p-Toluidine is formed as an end-product by hydrogen abstraction from polymer molecules. In solvents where hydrogen chloride can be formed, the amino functional group is affected.     

Photostabilizing activity of sterically hindered piperidines—II: Radical trapping ability

The abilities of both 2,2,6,6-tetramethylpiperidine (I) and its nitroxyl (II) to trap radicals involved in hydrocarbon photo-oxidations have been studied in cumene and 1,3,5-trimethylcyclohexane at 27° using AIBN, hydroperoxide and dialkylperoxide as initiators: the light was either the band 300–400 nm or 366 nm. Under conditions of photolysis of ROOH (degenerate branching), I is oxidized to II. II is capable of trapping R' radicals, the rate constant being ～50 times lower than that for RO^.₂ formation. RO^.₂ radicals react with neither I nor II. Under the condition of degenerate branching, II is capable of intercepting the radical fragments from decomposing hydroperoxide. The rate constant of this process is ～500 times higher than that for hydrogen abstraction by these fragments. A reaction mechanism is suggested: hydrogen bonded associates formed between an N-containing stabilizer and ROOH play a dominant role. The principal intermediates in this mechanism are represented by >NO^., >NOH and >NOR species.     

The gamma radiolysis of aqueous solutions of ferriciminodiacetate and ferricglycinate

Radiolytic G-values were determined for [−Fe(III)IDA], Fe(II), (−IDA) and CHO-COOH in deoxygenated aqueous solutions of Fe(III)IDA and Fe(III)gly, and in the presence of scavengers, t-butanol, 2-propanol, methanol, sodium formate and O₂. The metal ion was reduced by e^-_aq, H or secondary radicals HO₂, CO⁻₂, CH₂-OH and (CH₃)₂C-OH, while the OH radical did so indirectly through α-hydrogen abstraction from the ligand followed by intramolecular electron transfer. The rate constant k[OH + Fe(III)IDA], determined by the competition kinetics method at pH 2.0, was 1.7 × 10⁸ M^-1 s^-1.     

Chemical forms of cobalt(0) related to polarogrphic and voltammetric catalytic hydrogen currents

The mechanism of catalytic hydrogen evolution at a mercury electrode in buffer solutions containing both bovine serum albumin (BSA) and Co(II) or Co(III) is discussed on the basis of the amount of Co(0) formed on the electrode as well as the solubility of the Co(0) species. An appropriate potential was times t, in ammonia buffer solutions in the presence of Co(III). After the mercury drop was washed with 0.1 M ammonia buffer (pH 9.3) and 1 M hydrochloric acid or concentrated hot hydrochloric acid, the amount of cobalt remaining in and/or on the mercury drop was determined by flameless atomic absorption spectrometry. Amalgamation of Co(0), which is insoluble in the ammonia buffer and 1 M hydrochloric acid, was formed on the electrode, regardless of the electrolysis potential, when t was short. With increase of t, Co(0) changed its characteristics, yielding a Co(0) species [Co(0)_{Group 2}] soluble in 1 M hydrochloric acid but insoluble in the buffer, even though the time required for the change depended on the electrolysis potential. It is demonstrated that Co(0)_{Group 2} plays an important role in the appearance of such catalytic hydrogen currents as the first and second Brdička currents, the P-current of Anzenbacher and Kalous, and i_c of Kolthoff and Kihara.     

Chemical recycling of poly(lactic acid)-based polymer blends using environmentally benign catalysts

Typical poly(l-lactic acid) (PLLA)-based polymer blends, PLLA/polyethylene (PE) and PLLA/poly(butylene succinate) (PBS), were degraded into each repolymerizable oligomer using environmentally benign catalysts, clay catalysts and enzymes, with the objective of developing a selective chemical recycling process. Two routes to selective chemical recycling of PLLA/PE blend were tested. One is the direct separation of PLLA and PE first by their different solubilities in toluene, followed by the chemical recycling of PLLA using montmorillonite K5 (MK5). The other is the selective degradation of PLLA in the PLLA/PE blend by MK5 in a toluene solution at 100 °C for 1 h forming the LA oligomer with a molecular weight of M_n = 200-300 g/mol, which is the best M_n for repolymerization. Thus regenerated PLLA had a M_w of greater than 100,000 g/mol. The PE remained unchanged and was quantitatively recovered by the reprecipitation method for material recycling. In a similar procedure, chemical recycling of PLLA/PBS blend was also carried out and compared by two routes. One is the direct separation of PLLA and PBS by solubility in toluene. The other route is the sequential degradation of PLLA/PBS blend using a lipase first to degrade PBS into cyclic oligomer, which was then repolymerized to produce a PBS. Next, PLLA was degraded into repolymerizable LA oligomer by MK5. The former procedure was carried out using a single solvent; however, the latter required mixed solvents, which decreased the efficient recycling use of solvents.     

The reaction of acrolein with acetylperoxyl radicals in the gas‐phase

A rate constant for the epoxidation of acrolein by acetylperoxyl radicals has been determined to be k₄ = (1.3 ± 0.9) × 10⁴ dm³mol⁻¹s⁻¹ at 383 K, which is anomalously fast in comparison with the epoxidation of alkenes. Abstraction of the acyl hydrogen atom from acrolein by acetylperoxyl radicals at 393 K was found to be at least 60 times slower than from acetaldehyde and at least three orders of magnitude slower than abstraction of the acyl hydrogen atom of the epoxide of acrolein. The fast rate for epoxidation of acrolein and the slow rate for hydrogen abstraction provide an explanation for the anomalously slow rate for the autoxidation of acrolein and suggests that acrolein formed during the autoxidation of alkene will react further to give its epoxide, and not exclusively by abstraction of the acyl hydrogen atom as was previously accepted. © 1999 John Wiley & Sons, Inc., Int J Chem Kinet 31: 277–282, 1999     

Manipulation of chain transfer agent and cross-linker concentration to modify latex micro-structure for pressure-sensitive adhesives

N-Dodecyl mercaptan (NDM) chain transfer agent and allyl methacrylate (AMA) cross-linker were used to manipulate latex properties in a starved seeded semi-batch emulsion polymerization of butyl acrylate (BA) and methyl methacrylate (MMA) or with a third monomer, acrylic acid (AA). Latexes with higher gel content and lower sol polymer molecular weight (M_w) were produced by adding only AMA. On the other hand, latexes with lower gel content and M_w were produced by adding only NDM. In addition, at a constant AMA concentration (0.2 phm), the addition of NDM (0.2 phm) decreased gel content, increased molecular weight between cross-linking points (M_c), and decreased M_w. Adding more NDM (to a total of 0.4 phm) further decreased the gel content, while decreasing the tested M_c and increasing M_w. It was also found that using higher concentrations of both AMA and NDM could produce latex with similar gel content, but smaller M_c and M_w, compared to the latex produced at lower concentrations of both NDM and AMA. Regarding the influence of AA, gel content was increased and M_w was significantly decreased with an increase in AA concentration and a decrease in MMA concentration. The performance of the latexes was evaluated for application as a pressure-sensitive adhesive (PSA).     

Structure,thermal stability and decomposition of bis-allyl-zinc compounds

The structure, thermal stability and decomposition of solutions of diallylzinc (I), bis(2-methylallyl)zinc (II), bis(3-methylallyl)zinc (III) and bis(3,3-dimethylallyl)zinc (IV) in deuterated solvents, have been investigated by¹H NMR and by kinetic measurements at temperatures between ?125 and +180°C. At room temperature I, II, III and IV are dynamic systems and are best described as being rapidly equilibrating mixtures of all isomeric σ-allyl forms; the NMR spectra are averages weighted according to the relative concentrations of the respective forms. I displays a¹H NMR spectrum of a static σ-allyl system only below ?125°C and II only below ?115°C. At temperatures above 100°C the thermal decomposition of I–IV results in coupling of the allyl groups, decomposition via radicals being the major process. The coupled products exhibit CIDNP, in which the multiplet polarisations confirm a decomposition via randomly diffusing allyl radicals. In the allyl radicals CH₂CR¹CR²R³ an alternating spin density was proved experimentally. The thermal stability decreases in the order I > II > III > IV.     

On the mechanism of reaction of tert-butoxyl radicals with dialkylphosphites

It has been shown by ESR spectroscopy that the title reaction involves abstraction of hydrogen from the phosphite, since at ?10°C the reaction has a kinetic deuterium isotope effect, k_H/k_D, or ～3. The rate constant for hydrogen abstraction is c. 2 × 10⁴ M^?1 s^?1. There is no significant addition of alkoxyl radicals to the phosphite.     

Bistrifluoromethylcyclopropenes

Hexafluorobut-2-yne reacts with dichlorocarbene [generated by thermal decomposition of the silane CC1₃SiF₃] to give 3,3-dichlorobistrifluoromethyl- (I) and 1,3-dichlorobistrifluoromethyl-cyclopropene (II).

The 3,3-dichloro-compound (I) is isomerised to (II) by heat, light, or chemical catalysis, but high yields of either pure cyclopropene may be obtained by modification of the reaction conditions. The cyclopropenium ion (III) is formed when either (I) or (II) is treated with antimony pentafluoride.Both (I) and (II) undergo free-radical addition of halogens, but the slow reaction of (II) with trifluoromethyl radicals gives a mixture of products. Nucleophilic substitution of chloride ion from both cyclopropenes occurs very readily, and with ethoxide ions both mono- and di-ethoxybistrifluoromethylcyclopropenes are obtained. The cyclopropenes also react with fluoride ion, iodide ion, or Grignard compounds.     

Use of homogeneous competition for OH radicals in electrochemical studies of H abstraction from organic molecules

The influence on the N₂O photocurrent of homogeneous competition for OH radicals between two organic solutes which form (as the result of hydrogen abstraction) radicals, one of which is reduced by the electrode, the other being oxidised, is considered theoretically. Such competition can be employed to investigate the kinetics of hydrogen abstraction in a case in which uncompetitive homogeneous destruction of OH radicals by a solute has no effect on the photocurrent. The influence of incomplete oxidation of alcohol radicals when a light pulse of short duration is employed is discussed, together with complications caused by adsorption of the organic solute. Competition for OH radicals between phenol and methanol points to a rate constant for H abstraction from phenol of ca. 2.8 × 10⁹ l mol^?1 s^?1 and to rapid heterogeneous reduction of most of the C₆H₄OH radicals throughout the accessible potential range.     

Heterogeneous reactions of solid nickel(II) complexes XXIV

The Stoichiometry of thermal decomposition was studied for the following compounds: Ni(NCS)₂(pip)₄ (I), (pip=piperidine), Ni(NCS)₂(pip)₂py·H₂O (II), (py=piridine), Ni(NCS)₂(4-Mepip)₃ (III), Ni(NCS)₂(3-Mepip)₃ (IV) and Ni(NCS)₂(3.5-Me₂pip)₃ (V). In complexes I, II, III and IV the loss of the volatile ligands (on the TG curve to 300 °C) occurs in three steps and in complex V in two steps. The loss of the last molecules of volatile ligands is accompanied by the decomposition of NCS groups. Spectral data and magnetic moment values for the initial complexes I and II (together with the defined intermediates) indicated pseudooctahedral configuration while pentacoordination for complexes III, IV and V. Structural changes of the complexes studied in thermal decomposition reactions are discussed.     

Experimental evidence of consecutive electron transfers for the Sb(III)−Sb(Hg) couple in 4 M H(Cl,ClO4) at low chloride concentrations

A dc polarographic and cyclic voltammetric study has been made of the reduction of Sb(III) ions from 0.01 M HCl+3.99 M HClO₄ and 0.001 M HCl+3.999 M HClO₄ supporting electrolytes in which a quasi-reversible, respectively irreversible behaviour is observed. It is shown that the Sb(III) reduction can be explained on the assumption of a reaction mechanism that consists of three successive one-electron transfers. Along the reduction wave the Sb(III)→Sb(II) and Sb(II)→Sb(I) step are rate determining, respectively at more negative and more positive potentials. Kinetic parameters were determined and the rate constants are shown to increase with chloride ion concentration.     

Understanding the Initial Decomposition Pathways of the n‐Alkane/Nitroalkane Binary Mixture

Addition of nitroalkanes into n‐alkanes can lower the activation barriers of free‐radical production and accelerate the decomposition of n‐alkanes at relatively low temperatures. Four initial decomposition mechanisms of the n‐butane/nitroethane binary mixture were proposed for the promoting effect and considered theoretically at the B3LYP, BB1K, BMK, MPW1K, and M06‐2X levels with MG3S basis set. Energetics above was compared to high‐level CBS‐QB3 and G4 calculations. Calculated results confirm the feasibility of the four initial decomposition pathways: (I) the C? NO₂ bond rupture of nitroethane to produce ethyl and ·NO₂, (II) HONO elimination from nitroethane followed by decomposition to ·OH and ·NO, (III) rearrangement of nitroethane to ethyl nitrite which further dissociates into CH₃CH₂O· and ·NO, and (IV) direct hydrogen‐abstraction of nitroethane with n‐butane.     

Styrene polymerization catalyzed by metal porphyrin complex/MAO for in situ synthesizing polystyrene containing air stable π cation radicals

A novel catalyst system based on nickel(II) tetraphenylporphyrin (Ni(II)TPP) and methylaluminoxane for styrene polymerization was developed. This catalyst system has a high thermal stability and show fairly good activity. The obtained polystyrene (PS) was isotactic‐rich atactic polymer by ¹³C NMR analysis, and its molecular weight distribution was rather narrow (M_w/M_n ≈ 1.6, by GPC analysis). ESR revealed that Ni(II)TPP π cation radicals were formed in the polymerization and could remain in the resulting PS stably. The mechanism of the polymerization was discussed and a special coordination mechanism was proposed. The PS product containing Ni(II)TPP π cation radicals can be used as a potential functional material. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1240–1248, 2008     

An indirect measurement of the absolute rate constant of the self-reaction of tert-butoxy radicals

No reliable rate constant is available for the self-reaction of tert-;butoxy radicals. We have set up a competition between hydrogen abstraction and self-reaction of tert-butoxy radicals in a flash photolysis electron spin resonance study to extract this information. Experimental values of hydrogen abstraction product radical concentrations under various hydrogen donor concentrations were then compared with theoretically calculated values with different values of 2k₄ to obtain the best fit. Hydrogen donors such as cyclopentane, anisole, methyl tert-butyl ether, and methanol were chosen for the study. A value of (1.3 ± 0.5) × 10⁹M^?1 sec^?1 for the rate constant of the self-reaction of tert-butoxy radicals has been determined at 293°K.