Photoinduced decomposition of the polybromostyryl carbanions

The photoinduced decay in THF of the sodium salt of the polybromostyryl carbanions (PS_B?) was investigated in vacuum at ?78°C. Maximal rates of decay were obtained when the wavelength of the incident light corresponded to the absorption maximum of the PS_B? at 495 nm. The quantum yield of the process was found to be 13 eq./Einstein. Products of the photostimulated decompositions at ?78°C were found to be different from those obtained as result of the “dark” thermal decomposition at –20°C. Decomposition of the PS_B? carbanions was accompanied by evolution of Br^? anions and by formation of polyaryl carbanions absorbing in the 270- to 400-nm range. The mechanism of the photoinduced decomposition of PS_B?, though basically analogous to the S_RN1 reaction mechanism, was found to differ significantly from the latter. The intramolecular dissociative electron transfer was assumed to represent its initiation step. Its propagation was attributed to the addition of PS_B? to the initially formed biradicals, ejection of Br^? from the resulting radical anions, and intramolecular electron transfer yielding polybromostyryl radicals. Products of the photoinduced decomposition of PS_B? remained soluble in THF when the intensity of the incident light was relatively weak, though their molecular weight distribution broadened. Insoluble, highly crosslinked polymers were obtained when the intensity of the incident light was very high.     

Organocobalt chelates as initiators of emulsion polymerization of styrene

The kinetics of decomposition of organocobalt chelates in the pH range of 2.2–7.0 has been studied. It has been shown that the rate constant of decomposition of the octyl chelate complex at 20°C changes from ～3 × 10^?3 to ～6 × 10^?6 s^?1 in the above pH range. The rate constants of decomposition of complexes with ethyl, octyl, and cetyl ligands, as estimated at 20°C and pH 8.3, are 1.69 × 10^?4, 1.39 × 10^?4, and 2.42 × 10^?5 s^?1, respectively. As evidenced by emission spectrometry measurements, ～100% of organocobalt chelates with ethyl and isopropyl ligands occur in the aqueous phase, while organocobalt chelates with octyl and cetyl ligands are partitioned between monomer and aqueous phases. The rates of initiation of the emulsion polymerization of styrene have been measured by the inhibited polymerization procedure. It has been demonstrated that among three tested compounds (diphenyl picryl hydrazyl, hydroquinone, and benzoquinone), benzoquinone has been found to be a suitable inhibitor for the polymerization under study. The rates of initiation of styrene polymerization at 30°C for organocobalts with ethyl, octyl, and cyclohexyl ligands are 1.0 × 10^?7, 1.04 × 10^?7, and 3.7 × 10^?6 mol/(l s), respectively. The rate constant of decomposition of the organocobalt complex with the octyl ligand at 30°C is 2.28 × 10^?5 s^?1, and the efficiency of initiation with this complex is 0.95.     

Anionic polymerization and copolymerization of p-bromostyrene

Monodispersed poly(4-bromostyrenes) (PBs) and their block copolymers with styrene, isoprene, and 3-methylbutene were prepared and characterized by GPC and NMR. Polystyryl and α-methylstyryl carbanions act as effective initiators of the anionic polymerization of Bs in THF. The undersirable side reactions, due to thermally or photochemically induced decomposition of the bromostyryl carbanions, PBs^?, may be eliminated by conducting the reaction at ?78°C and in the dark. Under such conditions, the rate constant of propagation, k_p (?78°), is 1.5 × 10³ M^?1 s^?1. Radical anions, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm Bs}^{\mathop - \limits_ \cdot} $\end{document}, formed as result of electron transfer from sodium naphthalenide to Bs, may eject spontaneously bromine ions. This step and reactions involving the respective phenyl radicals compete at ?78°C with the addition steps leading to polymer formation. Electron affinity of Bs seems to be much higher than that of styrene or isoprene, and PBs^? carbanions do not add to the latter monomers. Addition of Bs to polyisoprenyl carbanions leads to formation of the BsIBs block copolymers. BsIBs, prepared in THF, may be converted by preferential hydrogenation of the 1–2 adducts into block copolymers of Bs with poly(2-methylbutene) and isoprenyl segments. The effectiveness of Bs as a flame retarding constituent of polymeric systems seems to be much more pronounced when it is incorporated in a “block” than in a random fashion. A considerably larger fraction of PBs is required to achieve the same LOI value when the respective homopolymers are blended. Spatial distribution of the easily charred microdomains in the block copolymers is believed to be responsible for this phenomenon.     

Electrochemical Responses and Sensitivities of Films Based on Multi-Walled Carbon Nanotubes in Aqueous Solutions

Novel films consisting of multi-walled carbon nanotubes (MWCNTs) were fabricated by means of chemical vapor deposition with decomposition of either acetonitrile (ACN) or benzene (BZ) using ferrocene as catalyst. The electrochemical responses of MWCNT-based films towards the ferrocyanide/ferricyanide, [Fe(CN)₆]^3?/4? redox couple were probed by means of cyclic voltammetry and electrochemical impedance spectroscopy at 25.0?±?0.5?°C. Both MWCNT-based films exhibit Nernstian response towards [Fe(CN)₆]^3?/4? with some slight kinetic differences. Namely, heterogeneous electron transfer rate constants lying in ranges of 2.69?×?10^?2?C1.7?×?10^?3 and 9.0?×?10^?3?C2.6?×?10^?3?cm·s^?1 were obtained at v?=?0.05?V·s^?1 for MWCNT_ACN and MWCNT_BZ, respectively. The detection limit of MWCNT_ACN, estimated to be about 4.70?×?10^?7?mol·L^?1 at v?=?0.05?V·s^?1, tends to become slightly poorer with the increase of the scan rate, namely at v?=?0.10?V·s^?1 the detection limit of 1.70?×?10^?6?mol·L^?1 was determined. Slightly poorer response ability was exhibited by MWCNT_BZ; specifically the detection limits of 1.57?×?10^?6 and 4.35?×?10^?6?mol·L^?1 were determined at v?=?0.05 and v?=?0.10?V·s^?1, respectively. The sensitivities of MWCNT_ACN and MWCNT_BZ towards [Fe(CN)₆]^3?/4? were determined as 1.60?×?10^?7 and 1.51?×?10^?7?A·L·mol^?1·cm^?2, respectively. The excellent electrochemical performance of MWCNT_ACN is attributed to the presence of incorporated nitrogen in the nanotube??s structure.     

Polymerization of Styrene Initiated by 1,4-Dimethyl-1,4-bis(p-anisyl)-2-tetrazene

Abstract

The polymerization of styrene (St) initiated by 1,4-dimethyl-1,4-bis(p-anisyl)-2-tetrazene (1a) was studied kinetically in benzene. The polymerization proceeds through a radical mechanism. The rate of polymerization is proportional to [1a]^0.5 and [St]^1.0. The overall activation energy for the polymerization is found to be 81.2 kJ/mol within the temperature range of 65 to 80°C. The activation parameters for the decomposition of 1a at 70°C are k_d = 1.88 × 10^?5s^?1, δH? = 133.1 kJ/mol, and δS? = 29.9 J/mol·deg.     

Chain transfer for vinyl monomers polymerized in N-allylstearamide

Apparent transfer constants have been determined for styrene, methyl methacrylate vinyl acetate, and diethyl maleate polymerized in N-allylstearamide at 90°C. Regression coefficients for transfer were: methyl methacrylate, 0.301 × 10^?3; styrene, with no added initiator, 0.582 × 10^?3; styrene, initiated with benzoyl peroxide, 0.830 × 10^?3; vinyl acetate, 62.01 × 10^?3; and diethyl maleate, 2.24 × 10^?3. Rates of polymerization were retarded for both styrene and methyl methacrylate. Vinyl monomer and comonomer disappearance followed an increasing exponential dependence on both initiator and monomer concentration. Although degradative chain transfer probably caused most of the retardation, the cross-termination effect was not eliminated as a contribution factor. Rates for the vinyl acetate copolymerization were somewhat retarded, even though initiator consumption was large because of induced decomposition. The kinetic and transfer data indicated that the reactive monomers added radicals readily, but that rates were lowered by degradative chain transfer. Growing chains were terminated at only moderate rates of transfer. Unreactive monomers added radicals less easily, producing reactive radicals, which transferred rapidly, so that molecular weights were lowered precipitously. Although induced initiator decomposition occurred, rates were still retarded by degradative chain transfer. A simple empirical relation was found between the reciprocal number-average degree of polymerization, 1/X?_n1 and the mole fraction of allylic comonomer entering the copolymer F₂, which permitted estimation of the molecular weight of copolymers of vinyl monomers with allylic comonomers. This equation should be applicable when monomer transfer constants for each homopolymer are known and when osmometric molecular weights of one or two copolymers of low allylic content have been determined.     

The effect of organic nitriles on the radiation induced polymerization of styrene

The effect of a range of 10 organic nitriles on the radiation-induced polymerization of styrene was studied. A dose rate of 4.4 rad s^?1 was used. A rate of polymerization of styrene (1.744 mol L^?1 of toluene solution) of 5.0 × 10^?7 mol L^?1 s^?1 was found. With organic nitriles present (styrene:nitrile ratio of 1:0.28) the rate of polymerization increased. Rates in the range of 5.5 × 10^?7 ?5.2 × 10^?6 mol L^?1 s^?1, depending on the nitrile present, were obtained. The polymers were partially characterized and evidence of involvement of each of the nitriles in the polymer chains was revealed. The increase in rate of polymerization has been attributed to the part played by nitrile radicals in the initiation of styrene polymerization. Radical yield values [as G(nitrile radical)] were derived from the relevant rate expressions. Values ranged from 2.7 to 49.5, depending on the particular nitrile. Corresponding values of G(nitrile radical) in the range of 5.1–129.4 were obtained by the manipulation of number-average molar mass data. Values of k_pk_t of approximately 2 × 10^?5 L mol^?1 s^?1 were found. Trommsdorff types of effect are absent from these systems.     

The Polymerization of Free Enols with Electron-Deficient Comonomers

Abstract

The balance between kinetics and thermodynamics is illustrated herein by the first direct polymerization of vinyl alcohol, the thermodynamically unstable tautomer of acetaldehyde, at a rate faster than it can tautomerize. Vinyl alcohol was formed through the acid catalyzed hydrolysis of ketene methyl vinyl acetal. With excess water present, the kinetics of tautomerization first order dependence upon vinyl alcohol (k_obs = 2.73 × 10^?4 s^?1). Under water starved conditions, however, the kinetics now show a zero order dependence upon the concentration of vinyl alcohol (k_obs = 3.5 × 10^?6 M/s). Under these latter conditions, the half life of vinyl alcohol is nearly 24 hours at room temperature. Although cationic and homo free radical polymerization of vinyl alcohol failed, we found that this meta-stable species could be quantitatively polymerized in a copolymerization (AIBN, hυ, -10 to 25°C) with maleic anhydride. The k_obs for copolymerization was found to be 4.41 × 10^?4 sec^?1 at ?10°C. Since the rate of polymerization is far greater than that of tautomerization under these conditions (ca. 30 times faster at ?10°C), there is no significant increase in acetaldehyde concentration during polymerization.     

Ozone reduction at Nafion modified Au electrode and the measurement of ozone concentration in highly resistive solutions

The mass transport and charge transfer kinetics of ozone reduction at Nafion coated Au electrodes were studied in 0.5 mol/L H2SO4 and highly resistive solutions such as distilled water and tap water. The diffusion coefficient and partition coefficient of ozone in Nafion coating are 1.78×10-6 cm2·s-1 and 2.75 at 25℃ (based on dry state thickness), respectively. The heterogeneous rate constants and Tafel slopes for ozone reduction at bare Au are 4.1×10-6 cm·s-1, 1.0×10-6 cm·s-1 and 181 mV, 207 mV in 0.5 mol/L H2SO4 and distilled water respectively and the corresponding values for Nafion coated Au are 5.5×10-6 cm·s-1, 1.1×10-6 cm·s-1 and 182 mV, 168 mV respectively. The Au microelectrode with 3 μm Nafion coating shows good linearity over the range 0-10 mmol/L ozone in distilled water with sensitivity 61 μA·ppm-1 ·cm-2, detection limit 10 ppb and 95% response time below 5 s at 25℃. The temperature coefficient in range of 11-30℃ is 1.3%.     

Trityl salt—initiated polymerization of α-methylstyrene

The initiation reaction of the polymerization of α-methylstyrene by trityl tetrachloroferate and tritylhexachloroantimonate in 1,2-dichloroethane at 20°C was studied. The rate constants were 14 × 10^?3 and 27 × 10^?3 L mol^?1s^?1, respectively. The dissociation constants of tritylterachloroferate (K_d = 0.88 × 10^?4M^?1) and tritylhexachloroantimonate (K_d = 2.64 × 10^?4M^?1) was determined. The effect of electron acceptors and donors on the dissociation equilibrium and initiation rate was investigated. It was shown that in strongly dissociated ion pairs such as stable carbenium salts the electron donors and acceptors have no appreciable effect on the magnitude of the dissociation. The temperature dependence of the rate constants in the ?20–+20°C range yielded the following thermodynamic parameters for trityltetrachloroferate: E_i = 8.54 kcal/mol; A = 3.2 × 10⁴ mol^?1s^?1; ΔH* = 8 kcal/mol; and S* = ?39.8 eu.     

Cationic polymerization of styrenes by activated covalent species. Direct 1H-NMR observation of complexes of 1-phenylethyl acetates with lewis acids

Complexes of boron trichloride, boron tribromide, and ethylaluminumdichloride with various acetates were directly observed by ¹H-NMR. Complexes of secondary and tertiary acetates which model macromolecular active species in polymerization of styrene and isobutene are stable at ?75°C, but decompose at temperatures above ?30°C to yield corresponding chlorides or bromides. The stability of complexes depends on the Lewis acid, the alkyl group in the ester, and the structure of acetate. Rates of the bimolecular exchange of complexes with excess acetate were calculated from dynamic NMR to be k_ex = 2 × 10¹ L mol^?1 s^?1 (?65°C) and k_ex = 5 × 10⁴ L mol^?1 s^?1 (?75°C) for 1-phenylethyl acetate with BCl₃ and EtAlCl₂, respectively.     

Mechanism and kinetics of the imidazolidinone nitroxide‐mediated free‐radical polymerization of styrene

Styrene radical polymerizations mediated by the imidazolidinone nitroxides 2,5‐bis(spirocyclohexyl)‐3‐methylimidazolidin‐4‐one‐1‐oxyl (NO88Me) and 2,5‐bis(spirocyclohexyl)‐3‐benzylimidazolidin‐4‐one‐1‐oxyl (NO88Bn) were investigated. Polymeric alkoxyamine (PS‐NO88Bn)‐initiated systems exhibited controlled/living characteristics at 100–120 °C but not at 80 °C. All systems exhibited rates of polymerization similar to those of thermal polymerization, with the exception of the PS‐NO88Bn system at 80 °C, which polymerized twice as quickly. The dissociation rate constants (k_d) for the PS‐NO88Me and PS‐NO88Bn coupling products were determined by electron spin resonance at 50–100 °C. The equilibrium constants were estimated to be 9.01 × 10^?11 and 6.47 × 10^?11 mol L^?1 at 120 °C for NO88Me and NO88Bn, respectively, resulting in the combination rate constants (k_c) 2.77 × 10⁶ (NO88Me) and 2.07 × 10⁶ L mol^?1 s^?1 (NO88Bn). The similar polymerization results and kinetic parameters for NO88Me and NO88Bn indicated the absence of any 3‐N‐transannular effect by the benzyl substituent relative to the methyl substituent. The values of k_d and k_c were 4–8 and 25–33 times lower, respectively, than the reported values for PS‐TEMPO at 120 °C, indicating that the 2,5‐spirodicyclohexyl rings have a more profound effect on the combination reaction rather than the dissociation reaction. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 327–334, 2003     

A consistent model for the thermal decomposition of NCN3 and the singlet– triplet relaxation of NCN

The thermal decomposition of cyanogen azide (NCN₃) and the subsequent collision‐induced intersystem crossing (CIISC) process of cyanonitrene (NCN) have been investigated by monitoring excited electronic state ¹NCN and ground state ³NCN radicals. NCN was generated by the pyrolysis of NCN₃ behind shock waves and by the photolysis of NCN₃ at room temperature. Falloff rate constants of the thermal unimolecular decomposition of NCN₃ in argon have been extracted from ¹NCN concentration–time profiles in the temperature range 617 K <T< 927 K and at two different total densities: k(ρ ≈ 3 × 10^?6 mol/cm³)/s^?1=4.9 × 10⁹ × exp (?71±14 kJ mol^?1/RT) (± 30%); k(ρ ≈ 6 × 10^?6 mol/cm³)/s^?1=7.5 × 10⁹ × exp (‐71±14 kJ mol^?1/RT) (± 30%). In addition, high‐temperature ¹NCN absorption cross sections have been determined in the temperature range 618 K <T< 1231 K and can be expressed by σ /(cm²/mol)= 1.0 × 10⁸ ?6.3 × 10⁴ K^?1 × T (± 50%). Rate constants for the CIISC process have been measured by monitoring ³NCN in the temperature range 701 K <T< 1256 K resulting in k_CIISC (ρ ≈ 1.8 ×10^?6 mol/cm³)/ s^?1=2.6 × 10⁶× exp (‐36±10 kJ mol^?1/RT) (± 20%), k_CIISC (ρ ≈ 3.5×10^?6 mol/cm³)/ s^?1 = 2.0 × 10⁶ × exp (?31±10 kJ mol^?1/RT) (± 20%), k_CIISC (ρ ≈ 7.0×10^?6 mol/cm³)/ s^?1=1.4 × 10⁶ × exp (?25±10 kJ mol^?1/RT) (± 20%). These values are in good agreement with CIISC rate constants extracted from corresponding ¹NCN measurements. The observed nonlinear pressure dependences reveal a pressure saturation effect of the CIISC process. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 45: 30–40, 2013     

Methyl methacrylate as solvent for the thermal decomposition of the cyclic molecule pinacolone diperoxide: Toward the polymerization process

The decomposition rate constant (k_d) of pinacolone diperoxide (PDP, 3,6‐diterbutyl‐3,6‐dimethyl‐1,2,4,5‐tetraoxacyclohexane) in methyl methacrylate (MMA) is determined by the kinetic study of its thermal decomposition at temperatures from 110 °C to 140 °C. The calculated k_d values for PDP are higher than the corresponding values previously determined and reported for diethyl ketone triperoxide (DEKTP, 3,3,6,6,9,9‐hexaethyl‐1,2,4,5,7,8‐hexaoxacyclononane), for example, at 140 °C the k_d for PDP is 75.4 × 10^?5 s^?1, while for DEKTP, it is 50.6 × 10^?5 s^?1. The difference in the k_d between 130 °C and 140 °C indicates that the decomposition mechanism, sequential and/or concerted, is a function of temperature. The conformations of both initiators justify the higher k_d for PDP in MMA than DEKTP, where one single conformer is found for PDP, whereas 212 conformers are found for DEKTP. Bulk polymerization of MMA using PDP as the initiator reveals also the presence of an induction period, such as in DEKTP case. This work provides mechanistic insights into the interactions among the bifunctional cyclic peroxide PDP and the MMA monomer and their influence on the polymerization kinetics. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 997–1007     

Quantification of Ion‐Pairing Effects on the Nucleophilic Reactivities of Benzoyl‐ and Phenyl‐Substituted Carbanions in Dimethylsulfoxide

Second‐order rate constants for the reactions of acceptor‐substituted phenacyl (PhCO?CH^??Acc) and benzyl anions (Ph?CH^??Acc) with diarylcarbenium ions and quinone methides (reference electrophiles) have been determined in dimethylsulfoxide (DMSO) solution at 20 °C. By studying the kinetics in the presence of variable concentrations of potassium, sodium and lithium salts (up to 10^?2 mol L^?1), the influence of ion‐pairing on the reaction rates was examined. As the concentration of K⁺ did not have any influence on the rate constants at carbanion concentrations in the range of 10^?4–10^?3 mol L^?1, the acquired rate constants could be assigned to the reactivities of the free carbanions. The counter ion effects increase, however, in the series K⁺<Na⁺<Li⁺, and the sensitivity of the carbanion reactivities toward variation of the counter ion strongly depends on the structure of the carbanions. The reactivity parameters N and s_N of the free carbanions were derived from the linear plots of log k₂ against the electrophilicity parameters E of the reference electrophiles, according to the linear‐free energy relationship log k₂(20 °C)=s_N(N+E). These reactivity parameters can be used to predict absolute rate constants for the reactions of these carbanions with other electrophiles of known E parameters.     

Controlled radical polymerization of styrene mediated by the C‐phenyl‐N‐tert‐butylnitrone/AIBN pair: Kinetics and electron spin resonance analysis

Kinetics of the free radical polymerization of styrene at 110 °C has been investigated in the presence of C‐phenyl‐N‐tert‐butylnitrone (PBN) and 2,2′‐azobis(isobutyronitrile) (AIBN) after prereaction in toluene at 85 °C. The effect of the prereaction time and the PBN/AIBN molar ratio on the in situ formation of nitroxides and alkoxyamines (at 85 °C), and ultimately on the control of the styrene polymerization at 110 °C, has been investigated. As a rule, the styrene radical polymerization is controlled, and the mechanism is one of the classical nitroxide‐mediated polymerization. Only one type of nitroxide (low‐molecular‐mass nitroxide) is formed whatever the prereaction conditions at 85 °C, and the equilibrium constant (K) between active and dormant species is 8.7 × 10^?10 mol L^?1 at 110 °C. At this temperature, the dissociation rate constant (k_d) is 3.7 × 10^?3 s^?1, the recombination rate constant (k_c) is 4.3 × 10⁶ L mol^?1 s^?1, whereas the activation energy (E_a,diss.), for the dissociation of the alkoxyamine at the chain‐end is ～125 kJ mol^?1. Importantly, the propagation rate at 110 °C, which does not change significantly with the prereaction time and the PBN/AIBN molar ratio at 85 °C, is higher than that for the thermal polymerization at 110 °C. This propagation rate directly depends on the equilibrium constant K and on the alkoxyamine and nitroxide concentrations, as well. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1219–1235, 2007     

Polymerization of methyl methacrylate by a charge transfer mechanism with urea and iron (III) complex

Methyl methacrylate (MMA) can be polymerized by a charge transfer complex formed by the interaction of urea, methyl methacrylate, and carbon tetrachloride (CCl₄) in a nonaqueous solvent like dimethylsulfoxide (DMSO). The rate of polymerization can be accelerated by Lewis acids like Fe³⁺. This article reports the polymerization of MMA initiated by urea and CCl₄ and accelerated with hexakisdimethylsulfoxide iron (III) perchlorate, [Fe(DMSO)₆](ClO₄)₃, and A at 60°C. Definite induction periods were observed for the polymerization reaction initiated by urea and CCl₄ alone, but the induction period completely vanished when the molar ratio of urea to A reached 6:1. The molecular weights of the polymers with 6:1 molar ratio of urea to A were higher than with urea alone. The rate constant for the polymerization of MMA in the presence of [Fe(urea)₆]³⁺ was 1.03 × 10^?5 1 mol^?1 s^?1 at 60°C. The transfer constant for CCl₄ for polymerization with urea alone is 2.43 × 10^?3 at 60°C.     

Polymerization of Acrylonitrile Initiated by 1,4-Dimethyl-1,4-bis(p-nitrophenyl)-2-tetrazene

The polymerization of acrylonitrile (AN) initiated by 1,4-dimethyl-1,4-bis(p-nitrophenyl)-2-tetrazene (Ie) was studied in dimethylformamide (DMF) at high temperature. The polymerization proceeds by a radical mechanism. The rate of polymerization is proportional to [Ie]^0.64 and [AN]^1.36. The overall activation energy for the polymerization is 21.5 kcal/mole within the temperature range of 115-130°C. The chain transfer of Ie was also undertaken over the temperature range of 120-135°C. The activation parameters for the decomposition of Ie at 120°C are k_d = 2.78 × 10^?6 sec^?1, ΔH^? = 40.8 kcal/mole, and ΔS^? = 19.5 cal/mole-deg, respectively.     

Photocurrent kinetics at the electron emission from a metal into electrolyte solution: Part II. Solutions without acceptors and solutions of N2O,NO2−, NO3−

A method of measuring the kinetics of currents arising at the electron photoemission from a metal into electrolyte solution when affected by the u.v. laser pulses for 10^?8 s at the frequency of repetitions 10–25 Hz is described. Measurements have been taken in solutions without acceptors and in those containing N₂O and NO₂^?, NO₃^? ions as electron acceptors. The rate constants of capture of the solvated electrons by N₂O ((6±1)×0⁹ mol^?1 s^?1) and NO₂^? ((4.5±1)×10⁹ mol^?1 s^?1) and the diffusion coefficients of OH-radicals ((1.0±0.3)×10^?5 cm² s^?1) and of NO ((1.2±0.3)×10^?5 cm² s^?1) are found. The oxidation rate of NO₃^2? has been shown to decrease from 40 cm s^?1 in the range of potentials ?0.55 to ?1.0 V. The rate constant of bimolecular recombination of the solvated electrons ((1.3±0.4)×10¹⁰ mol^?1 s^?1) has been found from the dependence of the emitted charge on the light intensity.     

Use of samarium (III) acetate as initiator in ring-opening polymerization of trimethylene carbonate

Abstract

In this work was evaluated the activity of samarium acetate (III) (Sm(OAc)₃) as a possible initiator in the polymerization by ring opening of trimethylene carbonate (TMC). All polymerizations were carried out under solvent-free melt conditions in ampoules-like flasks, equipped with a magnetic stirrer. The effects of different parameters of reaction, such as molar ratio monomer to initiator, temperature and reaction time, on typical variables of polymers, e.g., conversion of TMC to poly(trimethylene carbonate) (PTMC), dispersity and molar mass, were analyzed. The molar ratio of monomer to initiator was varied between 0 and 1000?mol/mol and the temperature among 70 and 150?°C. Nuclear Magnetic Resonance (¹H-NMR and HMBC) and Size Exclusion Chromatography (SEC) were used to characterize the polymers. The results indicate that the Sm(OAc)₃ induces the polymerization of TMC to high conversion with number-average molecular weights of 3.11?×?10³ to 38.40?×?10³?Da. Based on the ¹H-NMR end-group analysis of low-molecular-weight PTMC, it was proposed a coordination–insertion mechanism for the polymerization, with a breakdown of the acyl-oxygen bond of the TMC. In according to the kinetic study carried out, the polymerization rate is first-order with respect to monomer concentration with apparent rate constants of k_ap?=?7.02?×?10^?4?mol?×?L^?1?×?h^?1.