Kinetic and ESR studies on radical polymerization. Radical polymerization behavior of N-octadecylmaleimide in benzene

The polymerization of N-octadecylmaleimide ( 1 ) initiated with azodiisobutyronitrile ( 2 ) was investigated kinetically in benzene. The overall activation energy of the polymerization was calculated to be 94,2 kJ·mol^?1. The polymerization rate (R_p) at 50°C is expressed by the equation, R_p = k[ 2 ]^0,6[ 1 ]^1,7. The homogeneous polymerization system involves ESR-detectable propagating polymer radicals. Using R_p and the polymer radical concentration determined by ESR, the rate constants of propagation (k_p) and termination (k_t) were evaluated at 50°C. k_p (33 L · mol^?1 · s^?1 on the average) is substantially independent of the monomer concentration. On the other hand, k_t (0,3 · 10⁴ – 1,0 · 10⁴ L · mol^?1 · s^?1) is fairly dependent on the monomer concentration, which is ascribable to a high dependence of k_t on the chain length of rigid poly( 1 ). This is the predominant factor for the high order with respect to the monomer concentration in the rate equation. In the copolymerization of 1 (M₁) and St (M₂) with 2 in benzene at 50°C, the following copolymerization parameters were obtained: r₁ = 0,11, r₂ = 0,09, Q₁ = 2,1, and e₁ = +1,4.     

Photoinitiation, 6. Copolymerization of styrene with maleic anhydride photoinitiated by the excited charge-transfer complex styrene-maleic anhydride

The kinetics of styrene (St) copolymerization with maleic anhydride (MA) initiated by light (at λ = 365 nm) was studied in acetone, acetonitrile, chloroform, and N,N-dimethyl formamide at 30°C. With the exception of the chloroform containing system, the copolymerizations took place in homogeneous reaction media. The copolymerization rate R_p = ?d([St] + [MA])/dt was found to be a function of the mole ratio of the comonomers in the reaction mixture. For a given ratio of comonomers R_p and the molecular weight of the resulting copolymer were found to be a function of the donor number of the solvent used for a given rate of initiation. Due to the dependence of R_p on the concentration of an equimolar mixture of both comonomers in acetone on [St] (at constant [MA]), and on [MA] (at constant [St]) the participation of the exciplex {St…acetone}^* in the initiation reaction can be expected. The ratio of the overall rate constants for the propagation (k?_p) and termination (k?_t) reactions, k?_p/2k?_t, determined by a rotating sector technique, was found to depend on the composition of the comonomer mixture. The copolymerization rate is proportional to the square root of the intensity of incident light, which, together with the observed inhibition effect of oxygen points to a radical mechanism of the photoinitiated copolymerization of St with MA. In the presence of the photosensitizer benzophenone in the system St/MA/acetone an increase in R_p was observed, accompanied by a decrease in molecular weight of the copolymer in comparison with the system without benzophenone.     

ESR determination of rate constants for the radical polymerization of methyl phenethyl itaconate

Phenethyl 2-(methoxycarbonylmethyl)acrylate (methyl penethyl itaconate) ( 1 ) was prepared and its polymerization with dimethyl 2,2′-azoisobutyrate ( 2 ) was investigated kinetically in benzene. The polymerization rate (R_p) was found to be expressed by R_p = k·[ 2 ]^0,5·[ 1 ]^1,8 (at 50°C). Further, a higher dependence of R_p (2nd order) on the concentration of 1 was observed at 61°C. The overall activation energy of the polymerization was calculated to be a low value of 50,3 kJ/mol. The initiator efficiency (f) of 2 was determined to be 0,48 to 0,22 at 50°Cand 0,50 to 0,28 at 61°C. f decreases with increasing monomer concentration due to the high viscosity of 1 . The poly( 1 ) radical is stable enough to be observable by ESR at high temperatures (50–60°C). Rate constants of propagation (k_p) and termination (k_t) were estimated using the poly( 1 ) radical concentration determined by ESR. k_p [6,0 to 121/(mol·s) at 50°Cand 7,1 to 15 1/(mol·s) at 61°C] shows a trend to increase with the concentration of 1 . On the other hand, k_t [2,9·10⁴ to 17·10⁴1/(mol·s) at 50°Cand 6,9·10⁴ to 45·10⁴1/(mol. s) at 61°C] decreases with increasing MPI concentration. This behavior is responsible for the high order with respect to monomer concentration. Copolymerization of 1 (M₁) with styrene (M₂) at 50°Cgave the following results: r₁ = 0,36, r₂ = 0,40, Q₁ = 0,82 and e₁ = + 0,59. Using the above results, the rate constants of the cross-propagations were estimated to be k₁₂ = 22 1/(mol·s) and k₂₁ = 308 1/(mol·s) at 50°C.     

Free radical polymerization in the presence of non-solvents, 1. Styrene

During styrene (STY) polymerization, initiated by radicals formed by thermal or photochemical decomposition of 2,2′-azoisobutyronitrile (AIBN) the overall polymerization rate constant K defined by relation K = R^p/([AIBN]^0,5 [STY] η) and the ratio k_p/(2k_t0) increase with decreasing styrene concentration by hexane or benzene (R_p is the polymerization rate and η_MIX the viscosity of the reaction system). In the thermally initiated polymerization K = k_p (2f k_d/(2k_t0))^0,5 and in the photochemically initiated polymerization K = k_p (2,303 ? I₀? d/(2k_t0))^0,5 where k_d, k_p, and k_t0 are respectively, the rate constants of AIBN decomposition, of propagation, and of termination (for a system of the viscosity 1 mPa·s) reactions, ? is the quantum yield of radicals entering into reaction with the monomer, I₀ the intensity of the incident light, ? the molar absorption coefficient of AIBN, and d the path length of the light. The increase of K and of k_p/(2k_t0) with decreasing monomer concentration is more marked for the system styrene/hexane than for styrene/benzene and this increase is greater at 30°C than at 60°C. For Θ-systems formed by binary mixtures like styrene/hexane, styrene/decane and styrene/C₁ – C₄ alcohols the values of k_p and k_t0 at 30°C range between 57 and 91 dm³·mol^?1·s^?1 and (0,9 to 2,2)·10⁷ dm³·mPa·mol^?1, i.e. they are in principle identical with the tabulated values of these rate constants for styrene bulk polymerization.     

Kinetic and electron paramagnetic resonance studies on radical polymerization. Radical-initiated polymerization of methyl N-phenylitaconamate in N,N-dimethylformamide

The polymerization of methyl N-phenylitaconamate(methyl 2-methylenesuccinanilate ( 1 )) with dimethyl 2,2′-azodiisobutyrate ( 2 ) was studied in N,N-dimethylformamide (DMF) kinetically and by means of electron paramagnetic resonance (EPR) spectroscopy. The polymerization rate (R_p) at 55°C is given by the equation: R_p = k[ 2 ]^0,58 · [ 1 ]^1,6. The overall activation energy of the polymerization was calculated to be 54,2 kJ/mol. The number-average molecular weight of poly( 1 ) was in the range between 5000 and 17000. From an EPR study, the polymerization system was found to involve the EPR-detectable propagating polymer radical of 1 at practical polymerization conditions. Using the concentration of polymer radicals, the rate constants of propagation (k_p) and termination (k_t) were determined for 55°C. The rate constant of propagation k_p (between 8,4 and 12 L · mol^?1 · s^?1) tends to somehow increase with increasing monomer concentration. On the other hand, k_t (between 1,9. 10^?5 L · mol^?1 · s^?1) increases with decreasing monomer concentration, which results from a considerable dependence of k_t on the polymer-chain length. Such monomer-concentration-dependent k_p and k_t values are responsible for the high dependence of R_p on the monomer concentration. Thermogravimetric results showed that thermal degradation of poly( 1 ) occurs rapidly at temperatures higher than 200°C and the residue at 500°C amounts to 26% of the initial polymer. For the copolymerization of 1 (M₁) with styrene (M₂) at 55°C in DMF the following copolymerization parameters were found: r₁ = 0,52, r₂ = 0,31, and Q, e values Q₁ = 1,09 and e₁ = +0,55.     

Kinetic and ESR studies of the radical-initiated polymerization of N-(2,6-dimethylphenyl)itaconimide

The polymerization of N-(2,6-dimethylphenyl)itaconimide (1) with azoisobutyronitrile (2) was studied in tetrahydrofuran (THF) kinetically and spectroscopically with the electron spin resonance (ESR) method. The polymerization rate (R_p) at 50°C is given by the equation: R_p = K [2] ^0,5 · [1] ^2,1. The overall activation energy of the polymerization was calculated to be 91 kJ/mol. The number-average molecular weight of poly (1) was in the range of 3500–6500. From an ESR study, the polymerization system was found to involve ESR-observable propagating polymer radicals of 1 under the actual polymerization conditions. Using the polymer radical concentration, the rate constants of propagation (k_p) and termination (k_t) were determined at 50°C. k_p (24–27 L · mol^?1 · s^?1) is almost independent of monomer concentration. On the other hand, k_t (3,8 · 10⁴–2,0 · 10⁵ L · mol^?1 · s^?1) increases with decreasing monomer concentration, which seems mainly responsible for the high dependence of R_p on monomer concentration. Thermogravimetric results showed that thermal degradation of poly (1) occurs rapidly at temperatures higher than 360°C and the residue at 500°C was 12% of the initial polymer. For the copolymerization of 1 (M₁) with styrene (M₂) at 50°C in THF the following copolymerization parameters were found; r₁ = 0,29, r₂ = 0,08, Q₁ = 2,6, and e₁ = +1,1.     

Effect of [Pyridylmethanimine]/[CuI] Ratio,Ligand, Solvent and Temperature on the Activation Rate Constants in Atom Transfer Radical Polymerization

Summary: The effect of [L]/[Cu^I] ratio, ligand, the choice of solvent and temperature on the activation rate constants in ATRP with catalyst complexes formed with two pyridylmethanimine ligands [PMI, i.e., N‐propyl‐pyridylmethanimine (NPPMI) and N‐octyl‐pyridylmethanimine (NOPMI)] were studied. Maximal values for the apparent rate constants were observed at an [L]/[Cu^I] ratio of ∼2/1 and 1/1 in polar and nonpolar solvents, respectively. This is similar to the results obtained with the Cu^I/bpy system and was attributed to the formation of [Cu^I/L₂]⁺Br⁻ and [Cu^I/L₂]⁺Cu^IBr species. The Cu^I/NPPMI system was only soluble in polar reaction media; however, the Cu^I/NOPMI complex was soluble in both polar and less polar reaction media. The activation rate constants increased with increasing temperature and the value of activation energy (E_a) for Cu^I/NPPMI in the activation process was 58.1 kJ · M ⁻¹ · K⁻¹.

Arrhenius plot of ln k_a versus 1/T for the activation process for NPPMI and bpy in acetonitrile; [Cu^IBr/NPPMI₂]₀ ([Cu^IBr/bpy₂]₀)/[EtBriB]₀/[TEMPO]₀/[TCB]₀ = 20/1/10/2 × 10⁻³ M .     

Polymerization of vinyl ethers by 3d-transition metal oxides

Studies on the relative efficiencies of 3d-transition metal oxides as catalysts for the polymerization of vinyl ethers revealed that V₂O₅ is the most efficient catalyst. Some kinetic studies with the system isobutyl vinyl ether/V₂O₅/benzene were carried out at ambient temperature. It was found that the rate is second order in isobutyl vinyl ether (IBVE) concentration (0,25 to 2,0mol.1^?1) and the inhibition of the polymerization by pyridine suggests a cationic mechanism. R_p increases by the use of benzaldehyde along with V₂O₅, whereas water (3.10^?3mol.1^?1) seems to have no effect on R_p. Over the temperature range of 15 to 60°C, R_p as well as [η] attain a maximum value at about 32°C. [η] is unaffected by varying the amounts of oxides, whereas it increases with increasing IBVE concentration. No stereoregularity in the product polymer was observed by using V₂O₅ alone or in combination with modifiers such as benzaldehyde or carbon disulfide. The kinetics are interpreted in terms of the Hinshelwood-Langmuire mechanism assuming a single point adsorption.     

Peroxo salts as initiators of vinyl polymerization, 5. Kinetics of polymerization of N-vinyl-2-pyrrolidone by peroxodisulfate in aqueous solution. Effect of Ag+ and Cu2+ ions

The kinetics of the polymerization of N-vinyl-2-pyrrolidone by peroxodisulfate ion in aqueous solution in the presence and absence of metal ions Ag⁺ and Cu²⁺ is studied. The rate laws for the polymerization are established and the k_p/k_t^1/2 values computed. A negative salt effect is observed for the Ag⁺ catalysed process, indicating the formation of a complex of the type AgS₂O₈^? (aq). Cu²⁺ ions, however, are found to reduce the rate of polymerization.     

Copolymerization of styrene with acrylonitrile initiated by 2-methyl-2-undecanethiol

The copolymerization of styrene (St) with acrylonitrile (AN) initiated with 2-methyl-2-undecanethiol (RSH) was investigated. Kinetic studies revealed that the initial rate of the copolymerization, which does not occur spontaneously in the absence of RSH, obeys the following expression: R_p ∝? [RSH]^x, where x is 0,5 when [RSH] ranges from 3,4.10^?4 to 3,4.10^?3 mol/l and 0 when [RSH] is larger than 3,4.10^?3 mol/l. The initial copolymerization rate, R_p, increases with increasing percentage of AN in the monomer feed. From the slope of ln R_p vs. 1/T, the apparent activation energy, E_app, was calculated to be 17 kJ/mol in the temperature range of 30°C to 50°C. RSH is the only initiating species, and it also plays a role as chain-transfer agent. The value of M _w/M _n, which varies from 1,5 to 2,0, is typical of a free-radical poymerization. The reactivity ratios were calculated to be r_St = 0,42 ± 0,04 and r_AN = 0,02 ± 0,03. It seems to be unnecessary to introduce a penultimate effect for explaining the experimental data.     

PLP‐SEC Study into the Free‐Radical Propagation Rate Coefficients of Partially and Fully Ionized Acrylic Acid in Aqueous Solution

Summary: Propagation rate coefficients, k_p, for acrylic acid (AA) polymerization at 6 °C in aqueous solution were measured via pulsed laser polymerization (PLP) with the degree of ionization, α, varied over the entire range between 0 and 1. These measurements were carried out in conjunction with aqueous‐phase size‐exclusion chromatography (SEC). Strictly speaking, the reported k_p's are “apparent” propagation rate coefficients deduced from the PLP‐SEC data under the assumption that the local monomer concentration at the radical site is identical to overall monomer concentration. At an AA concentration of 0.69 mol · L^?1, the apparent k_p decreases from 111 000 L · mol^?1 · s^?1 at α = 0 to 13 000 L · mol^?1 · s^?1 at α = 1.0. The significant lowering of k_p with higher α is attributed to the repulsion between both monomer molecules and macroradicals becoming negatively charged. Addition of up to 10 mol‐% (with respect to AA) sodium hydroxide to the fully ionized aqueous AA solution leads to an enhancement of k_p up to 57 000 L · mol^?1 · s^?1.

Dependence of apparent k_p values on the degree of ionization of acrylic acid (a) and on pH (b) for aqueous polymerizations of acrylic acid.     

Regulation of Ca channel by intracellular Ca2+ and Mg2+ in frog ventricular cells

The effects of changing the intracellular concentrations of Ca²⁺ or Mg²⁺ ([Ca²⁺]_i, [Mg²⁺]_i) on Ca current (I _Ca) was studied in frog ventricular myocytes using the whole-cell and cell-attached patch clamp techniques. In the physiological range of [Mg²⁺]_i an increase in [Ca²⁺]_i enhancedI _Ca whereas at lower [Mg²⁺]_i I _Ca was suppressed. The increase inI _Ca caused by Ca²⁺ loading was not mediated by phosphorylation since the kinase inhibitors H-8 {N-[2-(methylamino)-ethyl]-5-isoquinolinesulphonamide dihydrochloride}, staurosporine and KN-62 {1-[N,O-bis(5-isoquinoline-sulphonyl)-N-methyl-1-tyrosyl]-4-phenylpiperazine} and a non-hydrolysable adenosine 5-triphosphate analogue ,-methyleneadenosine 5-triphosphate did not prevent the Ca²⁺-inducedI _Ca increase.I _Ca was dramatically increased from 10 ± 6 (n = 4) to 71 ± 7 nA/nF (n = 4) when [Mg²⁺]_i was lowered from 1.0 × 10^–3 to 1.0 × 10^–6 M at a [Ca²⁺]_i of 10^–8 M. The concentration response relation for inhibition of Ca channels by [Mg²⁺]_i is modulated by [Ca²⁺]_i. To account for the experimental results it is postulated that competitive binding of Ca²⁺ or Mg²⁺ to the Ca channel accelerates the transition of the channel from an active to a silent mode. Single-channel recordings support this hypothesis. The regulation may have clinical relevance in cytoprotection during cardiac ischaemia.     

Electrochemical polymerization of 1H,7H-pyrrolo[2′,3′:4,5]-thieno[3,2-b]pyrrole and 4H-dithieno[3,2-b;2′,3′-d]pyrrole

Electrochemical polymerization of 1H,7H-pyrrolo[2′,3′:4,5]thieno[3,2-b]pyrrole and 4H-dithieno[3,2-b;2′,3′-d]pyrrole in CH₃CN + 0,1 M p-toluenesulfonate and tetraethylammonium perchlorate, respectively, yields polymers with a conductivity of ca. 5 S/cm. Electrochemistry and elemental analysis indicate the presence of 0,6-0,7 anions per monomeric unit, while spectroelectrochemistry agrees with a polypyrrole structure for polypyrrolothienopyrrole and a polythiophene structure for polydithienopyrrole. The redox cycle for polypyrrolothienopyrrole (at ?0,53 V vs. Ag/Ag⁺) is close to that of polypyrrole, while that of polydithienopyrrole (at 0,0 V) is intermediate between those of polythiophene and polypyrrole. N-Alkylsubstituted dithienopyrroles produce soluble polymers with higher conductivity (40 S/cm) and the lowest band-gap (1,7 eV) ever reported for a polythiophene-like polymer.     

Polymerizations of butadiene with Ni(acac)2-methylaluminoxane catalysts

Polymerizations of butadiene (BD) with transition metals (MtX) and methylaluminoxane (MAO) catalysts were studied. Among the catalysts examined, nicke(II) acetylacetonate [Ni(acac)₂] in combination with MAO revealed the highest catalytic activities for the polymerization of BD, giving high molecular weight polymers consisting of mainly cis-1,4-units in the microstructure. With Ni(acac)₂-MAO catalyst, the overall activation energy for the polymerization of BD between 0°C and 60°C was estimated to be 18 kJ/mol. From a kinetic study, the rate equation for the polymerization of BD with the Ni(acac)₂-MAO catalyst at 30°C can be expressed as follows: R_p = [BD]^1.8 [Ni(acac)₂-MAO]^0.9. The polymerization mechanism of the polymerization of BD with the Ni(acac)₂-MAO catalyst is discussed.     

Antidiuretic hormone acts via V1 receptors on intracellular calcium in the isolated perfused rabbit cortical thick ascending limb

The effect of antidiuretic hormone ([Arg]vasopressin, ADH) on intracellular calcium activity [Ca²⁺]_i of isolated perfused rabbit cortical thick ascending limb (cTAL) segments was investigated with the calcium fluorescent dye fura-2. The fluorescence emission ratio at 500–530 nm (R) was monitored as a measure of [Ca²⁺]_i after excitation at 335 nm and 380 nm. In addition the transepithelial potential difference (PD _te) and transepithelial resistance (R _te) of the tubule were measured simultaneously. After addition of ADH (1–4 nmol/l) to the basolateral side of the cTAL R increased rapidly, but transiently, from 0.84±0.05 to 1.36±0.08 (n = 46). Subsequently, within 7–12 min R fell to control values even in the continued presence of ADH. The increase in R evoked by the ADH application corresponded to a rise of [Ca²⁺]_i from a basal level of 155±23 nmol/l [Ca²⁺]_i up to 429±53 nmol/l [Ca²⁺]_i at the peak of the transient, as estimated by intra- or extracellular calibration procedures. The electrical parameters (PD _te and R _te) of the tubules were not changed by ADH. The ADH-induced Ca²⁺ transient was dependent on the presence of Ca²⁺ on the basolateral side, whereas luminal Ca²⁺ had no effect. d(CH₂)₅[Tyr(Me)²]²,Arg⁸vasopressin, a V₁ antagonist (Manning compound, 10 nmol/l), blocked the ADH effect on [Ca²⁺]_i completely (n = 5). The V₂ agonist 1-desamino-[d-Arg⁸]vasopressin (10 nmol/l, n=4), and the cAMP analogues, dibutyryl-cAMP (400 mol/l, n = 4), 8-(4-chlorophenylthio)-cAMP (100 mol/l, n = 1) or 8-bromo-cAMP (200 mol/1, n = 4) had no influence on [Ca²⁺]_i. The ADH-induced [Ca²⁺]_i increase was not sensitive to the calcium-channel blockers nifedipine and verapamil (100 mol/l, n = 4). We conclude that ADH acts via V₁ receptors to increase cytosolic calcium activity transiently in rabbit cortical thick ascending limb segments, possibly by an initial Ca²⁺ release from intracellular stores and by further Ca²⁺ influx through Ca²⁺ channels in the basolateral membrane. These channels are insensitive to L-type Ca²⁺ channel blockers, e.g. nifedipine and verapamil.Supported by DFG GR 480/10     

Copper toxicity in cultured human skeletal muscle cells: the involvement of Na+/K+-ATPase and the Na+/Ca2+-exchanger

Copper (Cu²⁺) intoxication has been shown to induce pathological changes in various tissues. The mechanism underlying Cu²⁺ toxicity is still unclear. It has been suggested that the Na⁺/K⁺-ATPase and/or a change of the membrane permeability may be involved. In this study we examined the effects of Cu²⁺ on the Na⁺ and Ca²⁺ homeostasis of cultured human skeletal muscle cells using the ion-selective fluorescent probes Na⁺-binding benzofuran isophtalate (SBFI) and Fura-2, respectively. In addition, we measured the effect of Cu²⁺ on the Na⁺/K⁺-ATPase activity. Cu²⁺ and ouabain increase the cytoplasmic free Na⁺ concentration ([Na⁺]_i). Subsequent addition of Cu²⁺ after ouabain does not affect the rate of [Na⁺]_i increase. Cu²⁺ inhibits the Na⁺/K⁺-ATPase activity with an IC₅₀ of 51 M. The cytoplasmic free Ca²⁺ concentration ([Ca²⁺]_i) remains unaffected for more than 10 min after the administration of Cu²⁺. Thereafter, [Ca²⁺]_i increases as a result of the Na⁺/Ca²⁺-exchanger operating in the reversed mode. The effects of Cu²⁺ on the Na⁺ homeostasis are reversed by the reducing and chelating agent dithiothreitol and the heavy metal chelator N,N,N,N,-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN). In conclusion, SBFI is a good tool to examine Na⁺ homeostasis in cultured human skeletal muscle cells. Under the experimental conditions used, Cu²⁺ does not modify the general membrane permeability, but inhibits the Na⁺/K⁺-pump leading to an increase of [Na⁺]_i. As a consequence the operation mode of the Na⁺/Ca²⁺-exchanger reverses and [Ca²⁺]_i rises.The authors thank staff and coworkers of the Department of Neurology of the University Hospital Nijmegen, Nijmegen for their kind cooperation in obtaining muscle biopsies. Mr. Arie Oosterhof is gratefully acknowledged for culturing of the human muscle cells. The Prinses Beatrix Fonds and the Dutch-Chinese scientific exchange program contributed financial support for this study.     

Kinetics of inhibited or retarded polymerization. Investigations on the cerium(IV)/toluene/acrylonitrile system

The kinetics and mechanism of the inhibiting action of toluene on the Ce(IV) initiated polymerization of acrylonitrile were studied together with the effects of [M], [Ce(IV)], [Toluene], [HClO₄], [NaClO₄], and [Acetic acid] on the rate of polymerization. The values of composite rate constant kk_t/k_pk_ok_i for toluene (and substituted toluenes) was calculated by plotting [M]/R_p versus [M]^?1. A very significant observation in the present study was that the plot of [M]/R_p versus [M]^?1 gave a negative intercept, which seems to be a general observation for all inhibiting substrates. On the basis of the experimental data, it could be concluded that the benzyl radical obtained by hydrogen abstraction, and not a chain transfer, is responsible for the inhibition.     

Radical Polymerization and Copolymerization of 12‐[(N‐Methacryloyl)carbamoyloxy]octadecanoic Acid

Radical polymerization of 12‐[(N‐methacryloyl)carbamoyloxy] octadecanoic acid ( 1 ) was kinetically and ESR spectroscopically investigated in acetone, using dimethyl 2,2′‐azobisisobutyrate ( 2 ) as initiator. The polymerization rate (R_p) is given by R_p = k [2]^0.7[1]^1.4 at 50°C. Propagating poly( 1 ) radical was observed as a 13‐lines spectrum by ESR under the actual polymerization conditions. The ESR‐determined k_p values (1.8–7.9 L/mol·s) are much lower than those of usual methacrylate monomers. The rate constant (k_t) of termination was determined to be k_t = 1.0–2.7·10⁴ L/mol·s from decay curve of the propagating radical. The Arrhenius plots of k_p and k_t gave the activation energies of propagation (63 kJ/mol) and termination (24 kJ/mol). A significant solvent effect was observed on the radical polymerization of 1 . The copolymerizations of 1 with styrene(St) and acrylonitrile were examined at 50°C. Copolymerization parameters obtained for the 1  (M₁)/St (M₂) system are as follows; r₁ = 0.73, r₂ = 0.57, Q₁ = 0.83, and e₁ = 0.13.     

Hypoxia increases the activity of Ca2+-sensitive K+ channels in cat cerebral arterial muscle cell membranes

The cellular mechanisms mediating hypoxia-induced dilation of cerebral arteries have remained unknown, but may involve modulation of membrane ionic channels. The present study was designed to determine the effect of reduced partial pressure of O₂, PO ₂, on the predominant K⁺ channel type recorded in cat cerebral arterial muscle cells, and on the diameter of pressurized cat cerebral arteries. A K⁺-selective single-channel current with a unitary slope conductance of 215 pS was recorded from excised inside-out patches of cat cerebral arterial muscle cells using symmetrical KCl (145 mM) solution. The open state probability (NP _o) of this channel displayed a strong voltage dependence, was not affected by varying intracellular ATP concentration [(ATP]_i) between 0 and 100 M, but was significantly increased upon elevation of intracellular free Ca²⁺ concentration ([Ca²⁺]_i). Low concentrations of external tetraethylammonium (0.1–3 mM) produced a concentration-dependent reduction of the unitary current amplitude of this channel. In cell-attached patches, where the resting membrane potential was set to zero with a high KCl solution, reduction of O₂ from 21% to < 2% reversibly increased the NP _o, mean open time, and event frequency of the Ca²⁺-sensitive, high-conductance single-channel K⁺ current recorded at a patch potential of + 20 mV. A similar reduction in PO₂ also produced a transient increase in the activity of the 215-pS K⁺ channel measured in excised inside-out patches bathed in symmetrical 145 mM KCl, an effect which was diminished, or not seen, during a second application of hypoxic superfusion. Hypoxia had no effect on [Ca²⁺]_i or intracellular pH (pH_i) of cat cerebral arterial muscle cells, as measured using Ca²⁺- or pH-sensitive fluorescent probes. Reduced PO₂ caused a significant dilation of pressurized cerebral arterial segments, which was attenuated by pre-treatment with 1 mM tetraethylammonium. These results suggest that reduced PO₂ increases the activity of a high-conductance, Ca²⁺-sensitive K⁺ channel in cat cerebral arterial muscle cells, and that these effects are mediated by cytosolic events independent of changes in [Ca²⁺]_i and pH_i.     

A kinetic study of butyl acrylate free radical polymerization in benzene solution

The free radical polymerization of butyl acrylate has been studied in benzene solutions ranging from 1 to 5 mol·L^–1 at 50°C using 2,2′‐azobisisobutyronitrile as initiator. Under the conditions of our experiments, both the effective rate coefficient for initiation, 2 f k_d , and the coupled parameter, k_p/k_t^1/2, (where k_p and k_t are the constants for propagation and termination reactions, respectively) are dependent on the monomer concentration. The 2 f k_d value shows little increase with monomer concentration. The variation of the k_p/k_t^1/2 parameter has been correlated with the chain length dependence of the termination rate coefficient. This effect is also responsible for the high dependence of the overall polymerization rate, R_p, onthe monomer concentration (1.49).