NMR and ESR studies of the solid-state polymerization of vinyl monomers. II. Acrylic acid

The post-polymerization of acrylic acid, γ-irradiated at 77°K, has been studied by broad-line NMR and ESR between 240°K and 286°K (mp). The changes in the structure of NMR spectra during the polymerization has been related to the conversion yield checked by gravimetry of the polymer formed. The very fast reaction occurring above ?270°K has been followed simultaneously by NMR and thermal analysis. This last method indicates an activation energy of chain propagation of 18.6 ± 2 kcal/mole in satisfactory agreement with the value given by the initial slope of the conversion—time curves. ESR experiments show that, while the average radical concentration decays by a factor of about ten between 77°K and 240°K, the local concentration of radicals persisting after prolonged annealing at T > 240°K, remains the same as at 77°K, i.e., 1.4 × 10¹⁹ spins/g. A kinetic scheme, assuming an exponential decay of propagation and termination rate constants with chain length, has been proposed to explain the shape of conversion—time curves as well as the almost constancy of the local concentration of growing polymer chains.     

Radical processes in polymer burning and its retardation. II. An ESR study of flame retardation of polypropylene

Qualitative and quantitative free radical transformations between polypropylene alkyl radicals, oxygen, and brominated flame retardants in the molten and gaseous phases of thermally treated samples are described. By ESR technique it has been proved that, depending on the applied pressure during pyrolysis, highly reactive peroxy radicals or less reactive radicals of the retardant are formed. For flame-retarded samples, compared with polypropylene without additives, the concentration of the primary alkyl radicals transferred from the molten to the gaseous phase is reduced by two orders of magnitude (from 7 × 10¹⁴ to 7 × 10¹² spins/0.02 g), whereas the limiting oxygen index [LOI] is raised from 17–18% to 25–26%. The great variety of physical and chemical processes proceeding in molten polymer in the preflame zone and burning gaseous phase calls for use of different retardant types with a programmed release of Br and HBr with the temperature increase. The chain oxidation of the “fuel,” a product of the endothermic decomposition of polymer, determines the temperature of self-ignition according to the number of initiating alkyl and allyl radicals formed per unit time; the [LOI] index depends more on the length of the kinetic chain of propagation reactions in the stationary process of oxidation at a given pressure.     

Molecular design,synthesis, and magnetic characterization of poly(phenylacetylene) with pi-toporegulated pendant nitronyl nitroxide radicals as models for organic superpara- and ferromagnets

The present article describes the synthesis and magnetic properties of poly(phenylacetylene) ( 3 ) with pi-toporegulated pendant stable 4,4,5,5-tetramethylimidazolin-3-oxide-1-yloxyl radicals topologically participating in the pi-conjugated system of poly(phenylacetylene). Polyradical 3 was prepared by the condensation reaction of poly(p-ethynylbenzaldehyde) with 2,3-bis(hydroxylamino)-2,3-dimethylbutane followed by oxidation with lead dioxide. The spin concentration of 3 determined by the ESR spectroscopic method was 1.1 × 10²¹ spins/g. This value approximately corresponds to 0.5 unpaired electron spin per the repeating unit. A powder ESR spectrum of 3 gave a 100% Lorentzian single line showing spin-exchange narrowing. The vibrating sample magnetometer (VSM) measurement of 3 afforded a straight line with a positive slope, suggesting that 3 has predominant paramagnetic properties within an experimental error.     

Electrochemical Polymerization of Benzene by use of Aluminum Chloride and Copper(I) Chloride

Electrochemical polymerization of benzene has been carried out in a one-compartment cell using indium-tin oxide conducting glass as a working electrode and Pt mesh as a counterelectrode by the use of a composite electrolyte of aluminum chloride and copper(I) chloride in nitrobenzene. On applying a constant voltage at + 1.8 V vs SCE, a brown and flexible polyparaphenylene (PPP) film was smoothly obtained on the ITO anode surface. The electrical conductivity of the film as grown was 1.9 × 10^?5 S/cm. Aluminum and chlorine atoms were detected in the film from XPS studies, so the film might contain AlCl₄ ^? (and Cl^?) as a dopant. Scanning electron micrographs of the growing side surface of the film prepared at a constant potential of + 1.8 V vs SCE showed a fibrillar morphology. The fibril diameter (～100 nm) was similar to that of a polyaniline film. The PPP film exhibited a spin concentration of 1.0 × 10¹⁹ spins/g, which corresponds to 800 phenylene units per spin. The g-value and the peak-to-peak linewidth were 2.0045 and 7 G, respectively.     

A rapid method for the spectrophotometric determination of palladium(II) and osmium(VIII)

Propionyl promazine phosphate is proposed as a new reagent for the rapid spectrophotometric determination of microgram amounts of Pd(II) and Os(VII). PPP instantaneously forms an orange-red 1:1 complex with Pd(II) in sodium acetate-hydrochloric acid buffer of pH 0.8 to 4.0 at room temperature. The reagent also forms an orange-red radical cation with Os(VIII) in 0.5 to 2.0 M hydrochloric acid. The Pd-PPP complex exhibits an absorption maximum at 490–500 nm with molar absorptivity of 7.1 × 10³ liter mol^?1 cm^?1. The Os-PPP radical cation has an absorption maximum at 505–515 nm with molar absorptivity of 2.21 × 10⁴ liters mol^?1 cm^?1. The Sandell sensitivity is 0.022 μg/cm² (Pd) and 0.008 μg/ cm² (Os). Beer's law is valid over the concentration range 0.2 to 21 ppm (Pd) and 0.2 to 4.2 ppm (Os). The proposed method offers the advantages of simplicity, rapidity, and without the need for heating or extraction. The reagent is used for the determination of Pd in the synthetic mixtures corresponding to Pd alloys used in jewelery and Os in osmiridium alloy.     

Properties of poly(p-phenylene)s and the perdeuterated analog

ESR data for poly(p-phenylene) (PPP) prepared from benzene and catalyst-oxidant systems were compared with data for PPP obtained by a variety of other methods. The results were similar, except for the Yamamoto preparation, and comparable to the spin densities for heterocyclic polymers of thiophene, pyrrole, and selenophene. Deuterated PPP from C₆D₆ showed significant differences in properties, in comparison to ordinary PPP, in relation to molecular weight, color, solubility, and spins/g in electron spin resonance (ESR). The implications are discussed with regard to the mechanism of benzene polymerization by AlCl₃-CuCl₂.     

Oligomers from biphenyl,biphenyl-d10, or p-terphenyl with aluminium chloride-cupric chloride: Mechanism,ESR, and conductivity

Biphenyl and biphenyl-d₁₀ were coupled by aluminum chloride-cupric chloride. For the two systems, the yields of p-quaterphenyl, p-sexiphenyl, and sublimation residue were compared. The results, which have mechanistic significance, are similar to those reported earlier for benzene versus benzene-d₆; that is, a decrease in DP for the deuterated substrate. p-Sexiphenyl, obtained from biphenyl or p-terphenyl and aluminum chloride-cupric chloride, displayed a concentration of 3 × 10¹⁷ spins/g in ESR. There was little or no change in spin density after sublimation or crystallization. Even after drastic purification by a variety of techniques the radical character persisted. Biphenyl was polymerized by aluminum chloride-cupric chloride at temperatures in the 50–155°C range. Processable polymers with m- and p-phenylene linkages were produced. When doped with AsF₅, the highest conductivity obtained was 7.3 × 10^?2 Ω^?1 cm^?1.     

Emulsifier-free emulsion polymerization of tetrafluoroethylene by radiation. II. Effects of reaction conditions on polymer particle size and number

The size, distribution, and number of PTFE particles formed by radiation-induced emulsifier-free polymerization were measured by electron microscope and automatic particle analyzer (centrifugation method). From the electron micrographs we found that the particles are formed within 5 min. The change in the number of polymer particles (n_p) with reaction time (t) depends on the relative concentration of growing polymer chains to stabilizing species produced by the radiolysis of water and monomer; that is, it was governed by TFE pressure/dose rate ratio and classified into three cases: case I, dn_p/dt = 0 (e.g., at 3 × 10⁴ rad/hr and 20 kg/cm²); case II, dn_p/dt < 0 (e.g., at dose rate below 1.9 × 10⁴ rad/hr and 20 kg/cm²); case III, dn_p/dt > 0 (e.g., at 3 × 10⁴ rad/hr and 2 kg/cm²). The polymer molecular weight above 10⁶ is almost independent of the particle size. The polymerization loci are mainly on the surface of polymer particles dispersed in the aqueous phase in cases I and II except in the initial stage. In case III new particles are formed successively during polymerization. Therefore the polymerization loci are mainly in the aqueous phase. Especially in case I, we concluded that after the generation of particles the propagation proceeds mainly on the surface of polymer particles like the core shell model proposed by Granico and Williams.     

Kinetic and ESR studies on radical polymerization of methyl methacrylate in the presence of fullerene

The effect of fullerene (C₆₀) on the radical polymerization of methyl methacrylate (MMA) in benzene was studied kinetically and by means of ESR, where dimethyl 2,2′-azobis(isobutyrate) (MAIB) was used as initiator. The polymerization rate (R_p) and the molecular weight of resulting poly(MMA) decreased with increasing C₆₀ concentration ((0–2.11) × 10⁻⁴ mol/L). The molecular weight of polymer tended to increase with time at higher C₆₀ concentrations. R_p at 50°C in the presence of C₆₀ (7.0 × 10⁻⁵ mol/L) was expressed by R_p = k[MAIB]^0.5[MMA]^1.25. The overall activation energy of polymerization at 7.0 × 10⁻⁵ mol/L of C₆₀ concentration was calculated to be 23.2 kcal/mol. Persistent fullerene radicals were observed by ESR in the polymerization system. The concentration of fullerene radicals was found to increase linearly with time and then be saturated. The rate of fullerene radical formation increased with MAIB concentration. Thermal polymerization of styrene (St) in the presence of resulting poly(MMA) seemed to yield a starlike copolymer carrying poly(MMA) and poly(St) arms. The results (r₁ = 0.53, r₂ = 0.56) of copolymerization of MMA and St with MAIB at 60°C in the presence of C₆₀ (7.15 × 10⁻⁵ mol/L) were similar to those (r₁ = 0.46, r₂ = 0.52) in the absence of C₆₀. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2905–2912, 1998     

Poly(carbon suboxide): A photosensitive paramagnetic ladder polymer

Carbon suboxide was found to give, on photo- and thermal-polymerization, a photosensitive paramagnetic polymer. Studies of the kinetics of the ESR signal growth accompanying the polymerization process complement documented results obtained from monitoring the rate of polymer production and monomer disappearance. The spin concentration of the polymer increases with higher reaction temperature, reaching 2 × 10¹⁹ spin/g at a polymerization temperature of 105°C. The paramagnetism of poly(carbon suboxide) follows the Curie—Weiss law. Relaxation behavior at room temperature and g values for the spin systems have been obtained. The 105°C polymer shows a Weiss constant equal to 17°K and an extremely narrow ESR line width, ca. 10 mG, at 5°K. The ¹³C coupling constant from the selectively labeled polymer indicates π-electron delocalization over the ladder polymer. The polymer paramagnetism can be further reversibly enhanced by visible light irradiation. The steady-state concentration of the photo-ESR signal is proportional to the square root of incident light intensity, with a quantum yield at room temperature for charge accumulation equal to 5% at an incident light level of 10¹⁵ photons/sec-cm². Fluorescence and excitation spectra of the soluble fraction of poly(carbon suboxide) are presented together with the quantum yield. The polymer has also been found to be an effective photopolymerization initiator at wavelengths longer than 340 nm.     

Radical/cation transformation polymerization and its application to the preparation of block copolymers

p-Methoxystyrene (MOS), butyl vinyl ether (BVE), and N-vinylcarbazole (VCZ) were polymerized in high yield by azoinitiators such as 2, 2'-azodiisobutyronitrile (AIBN) in the presence of electron acceptors such as Ph₂I⁺PF₆⁻. An electron paramagnetic resonance (ESR) study of the model radicals of the propagating radical showed the transformation of the radical to the corresponding cation in the presence of the electron acceptors. In the case of BVE, the polymer formation was caused by cationic species produced by the transformation of the initiating radical. The polymerizations of MOS and VCZ were ascribed to the transformation of the growing radical to the corresponding cation during the propagation step which was classified as the radical/cation transformation polymerization. Block copolymers of MOS/cyclohexene oxide (1, 2-epoxycyclohexane) (CHO) and VCZ/CHO were effectively prepared by the radical/cation transformation polymerization of the appropriate monomers in the presence of AIBN, electron acceptor and CHO. The formation of block copolymers was characterized by turbidimetry, thin-layer chromatography, and solubility tests.     

Adsorption of toluene-soluble polymers at the toluene–water interface

The adsorption of several toluene-soluble polymers at the toluene–water interface has been investigated by using the duNouy ring method of measuring interfacial tension γT /W . Polystyrene and poly(ethylene-co-vinyl acetate) (11.1 mole-% vinyl acetate) have little affinity for this interface at 29°C, but poly(methyl methacrylate) (PMMA) (M?_n = 420,000) and ethyl cellulose (EC) (M?_n = 50,100; 49.1% ethoxyl) adsorb significantly at concentrations as low as 1.0 × 10^?4 g/100ml. A plot of interfacial tension lowering versus initial logarithm of initial bulk phase polymer concentration is linear from 1.0 × 10^?4 to 1.0 × 10^?1 g/100 ml for EC and 1.0 × 10^?4 to 1.0 × 10^?2 g/100 ml for PMMA. When the PMMA concentration increases to 1.15 × 10^?1 g/100 ml, its adsorption behavior changes markedly. Prolonged time effects occur and adsorption becomes dependent upon dissolved water content of the toluene prior to formation of the toluene/water interface. Such effects are not observed with the other solutions studied. Increasing temperatures have variable effects on values of γT /W for the polymer solutions studied. Experiments with various polymer mixtures indicate that the polymer lowering T /W the most is preferentially adsorbed at the toluene–water interface and rapidly displaces less strongly adsorbed polymers.     

Initiation of cationic polymerization of n-butyl vinyl ether by stable cation radicals of substituted phenothiazines

3,7-Diethyl- 10-phenylphenothiazine (DEPPT), a phenothiazine derivative whose 3,7- and 10-positions are blocked, was synthesized. Potentiostatic electrolysis of DEPPT in acetonitrile (ACN) in the presence of 0.1M of LiClO₄ at 0.7 V (vs. Ag/Ag/Cl) yielded the stable cation radical of DEPPT (DEPPT^+·) which was characterized by cyclic voltammetry, UV-visible spectroscopy, and ESR spectrometry. Stable cation radicals of 10-phenylphenothiazine and 3,7-diethyl-10-methylphenothiazine were also prepared. The cationic polymerization of n-butyl vinyl ether was initiated by these cation radicals, including DEPPT^·+. The electron transfer mechanism for the initiation step, which we proposed previously, was supported by the fact that DEPPT^·+ was capable of initiating the polymerization; dimerization of DEPPT^·+ by releasing protons is precluded because 3,7- and 10-positions are all blocked. © 1994 John Wiley & Sons, Inc.     

Radical/Cation Transformation Polymerization and Its Application to the Preparation of Block Copolymers

Abstract

ESR study on the primary radicals obtained by decomposition of azo-compounds showed that primary radicals with electron donating substituents were transformed to the corresponding cations in the presence of electron acceptors such as ph₂I⁺PF^? ₆. Accordingly, propagating radicals are transformed to the corresponding cations in the polymerization of p-methoxy-styrene (MOS), n-butyl vinyl ether (BVE), and N-vinylcarbazole (VCZ) with azoinitiators such as AIBN in the presence of electron acceptors such as Ph₂I⁺PF^? ₆. In the case of BVE, the polymer formation was caused by cationic species produced by the transformation of the initiating radical. The polymerizations of MOS and VCZ were ascribed to the transformation of the growing radical to the corresponding cation during the propagation step which was classified as the radical/cation transformation polymerization. Block copolymers of MOS/cyclohexene oxide (CHO) and VCZ/CHO were effectively prepared by the radical/cation transformation polymerization of the appropriate monomers in the presence of AIBN, electron acceptor and CHO. The formation of block copolymers was characterized by turbidimetry, thin-layer chromatography, and solubility tests.     

NMR and ESR studies of the solid-state polymerization of vinyl monomers. I. Acrylamide

The kinetics of the solid-state polymerization of acrylamide, γ-irradiated at 77°K, has been studied by wide line NMR and by ESR at temperatures above 300°K. This reaction may be followed by the growth of a narrow line superimposed on the NMR spectrum of the monomer. The amplitude of this narrow line has been found to be proportional to the polymer yield. By long continued annealing, the conversion yield approaches a limiting value, generally lower than 50% under most of our experimental conditions. The activation energy of post-polymerization is 19 ± 1 kcal/mole. The radiochemical yield of radicals, determined by ESR, is 0.27 ± 0.03 at 77°K before warming. In the course of annealing above 300°K, the overall concentration of radicals is reduced to about one tenth of its initial value at 77°K. However, the local concentration of the remaining radicals is constant and equal to 1.2 × 10¹⁹ spins/g, i.e., more than 100 times their overall concentration. The recombination of radicals does not seem to intervene in the kinetics of post-polymerization, so that it may be assumed that only the radicals remaining after annealing, have actually contributed to the reaction.     

An NMR,ESR, ENDOR and TRIPLE resonance investigation of a cation radical formed from 1,2,4,5-tetramethoxybenzene

Several methods have been established for preparing cation radicals from 1,2,4,5-tetramethoxybenzene that allow highly resolved ESR spectra to be recorded. Precise values of the hyperfine coupling constants for the aromatic and methoxy protons have been obtained; the values are 0.2268±0.0004 and 0.0863±0.0002 mT, respectively, with dichloromethane as solvent. No temperature dependence is evident. TRIPLE resonance experiments showed that both coupling constants have the same sign. NMR experiments provided contact shift and line broadening measurements; these proved that both the above constants are positive and led to a value of 3.1 (±0.3)×10⁸M^?1 s^?1 at 23°C for the rate constant for electron exchange between the cation radical and the parent compound.     

Polymerization of (p‐vinylphenyl)hydrogalvinoxyl and formation of a stable polyradical derivative

4‐[(3,5‐Di‐tert‐butyl‐4‐hydroxyphenyl)(3,5‐di‐tert‐butyl‐4‐oxo‐cyclohexa‐ 2,5‐dienylidene)methyl]styrene (abbreviated as (p‐vinylphenyl)hydrogalvinoxyl) was polymerized using AIBN as an initiator to give a bright yellow polymer with M w = 3.2 × 10⁴. The polymer was oxidized to give the corresponding polyradical derivative, whose spin concentration could be increased up to about 70 mol % depending on oxidative conditions. ESR signal line‐width in the solid state was greatly increased below 200 K for the polyradical with a high spin concentration (> 50 mol %). The magnetization and magnetic susceptibility indicated weak antiferromagnetic interaction among the radical sites. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 189–198, 1999     

Applications of the channel flow cell for UV-visible spectroelectrochemical studies. Part 2: Transient signals

The use of the channel flow cell for ultraviolet-visible (UV-vis) spectroelectrochemical experiments has been developed to determine the diffusion coefficients of electrogenerated species by means of monitoring the transient absorbance response resulting from a potential step at a working electrode immediately upstream of the incident spectrophotometric beam. The technique is applied to measure the diffusion coefficient of tris(4-bromophenyl)amine (TBPA) radical cation in acetonitrile at 25°C. It is shown that the diffusion coefficient of electrogenerated TBPA radical cation (1.64 ± 0.02 × 10⁻⁵ cm² s⁻¹) is very close to that of the parent molecule (1.57 ± 0.03 × 10⁻⁵ cm² s⁻¹).     

CHLOROPHYLL PHOTOCHEMISTRY IN CONDENSED MEDIA—I. TRIPLET STATE QUENCHING AND ELECTRON TRANSFER TO QUINONE IN CELLULOSE ACETATE FILMS*

Chlorophyll-a was incorporated into cellulose acetate films and the triplet state decay kinetics and electron transfer from triplet to p-benzoquinone in aqueous solution was studied using laser flash photolysis and EPR. The triplet was found to decay by first order kinetics with a rate constant which was independent of Chl concentration. The triplet yield, however, was concentration dependent. These properties are due to quenching which occurs only at the singlet state level. In the presence of quinone, the triplet is quenched and, when the quinone is in an aqueous solution in contact with the film, Chl cation radical (C^±) as well as the semiquinone anion radical (Q^±) can be observed. The C decays by second order kinetics with a rate constant of 1.5 × 10⁶M^-1 s^-1. Although triplet conversion to radicals is slightly lower in the films as compared to fluid solutions (? 3 times), the lifetimes of the radicals are greatly increased (? 10³ times).     

Precise architecture of 1:1 alternating copolymers between germylenes and p-benzoquinone derivatives: First clear- cut evidence of biradical mechanism in polymerization chemistry

Germylenes bearing a bulky amide substituent have been copolymerized with various p-benzoquinone derivatives without any added catalyst to give novel class of germanium-containing polymers having a tetravalent germanium unit and a p-hydroquinone unit alternatingly in the main chain (“oxidation-reduction alternating copolymerization”). The resulting copolymers have high molecular weight (Mw>5.5×10⁴) and are soluble in common organic solvents. A novel biradical mechanism involving a germyl radical and a semiquinone radical is proposed on the basis of ESR analysis of the propagating polymer end as well as trapping experiments using a disulfide or TEMPO radical.