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.     

NMR and ESR studies of the solid-state polymerization of vinyl monomers. III. Methacrylic acid

The kinetics of postpolymerization of γ-irradiated methacrylic acid at 77°K has been investigated by wide-line NMR and by ESR spectroscopy. The conversion yield was continuously measured in the temperature range of 260–280°K, from the narrowing of the NMR spectrum due to the progressive “amorphization” of the matrix, releasing the motion of monomer molecules. The rate of postpolymerization decays exponentially with time, independently of the recombination of free radicals. The local concentration of radicals remaining after prolonged annealing is actually the same as initially, showing that no recombination occurs in the microdomains of polymerization. The ¹³C NMR study of the poly(methacrylic acid) formed by solid-state polymerization shows a predominent syndiotactic character, with an increasing contribution of isotactic sequences as the postpolymerization temperature is lowered.     

ESR and NMR study of the γ-ray-induced postpolymerization of vinyl monomers adsorbed on zeolite

The γ-ray-induced postpolymerization of acrylonitrile and methyl methacrylate adsorbed on Linde zeolite 13X irradiated at 77°K has been studied between 303 and 343°K as a function of the amount of adsorbed monomer and of the irradiation dose. The change in the nature and the concentration of free radical with temperature and duration of the postpolymerization was followed by the ESR method, whereas the formation of polymer was monitored continuously by the decay of the ¹H-NMR absorption line of the monomer under high-resolution conditions. It was found that the overall postpolymerization kinetics may be accounted for by assuming an exponential decay of radical propagation and recombination reactions with chain length. The tacticity of the polymer recovered by destroying the matrix in hydrofluoric acid was determined by ¹³C-NMR. The probability of isotactic addition of AN and MMA is larger than in the radical polymerization in solution owing likely to the association of adsorbed monomer molecules in pairs preforming an isotactic diad.     

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.     

Formation of free radicals in photoirradiated cellulose. I. Effect of wavelength

Formation of free radicals in photoirradiated cellulose has been studied by means of ESR spectroscopy at 77°K. Three kinds of light sources with different wavelengths between 2500 and 4000 ÅR were employed. No radicals detectable by ESR were recorded when cellulose was irradiated with light of wavelength longer than 3300–3400 ÅR. Hydrogen atoms that generated a doublet spectrum (ΔH = 508 G) were observed when cellulose was irradiated with light longer than 2800 ÅR. Hydrogen atoms and formly radicals that generated doublet spectra with splitting constants of 508 and 129 G, respectively, were observed when irradiated with light shorter than 2800 ÅR. The scission of the polymer chain in cellulose is evident from decrease of the degree of polymerization, and the results of mass spectrometric analysis indicated H₂, CO, CO₂, and H₂O to be the main volatile products of cellulose upon photoirradiation.     

Change with temperature of the ESR spectra of peroxy radicals trapped in irradiated polytetrafluoroethylene

The ESR spectra of peroxy radicals in irradiated powders and oriented samples of polytetrafluoroethylene (PTEE) have been measured with a K-band spectrometer, and the principal values and directions of the g tensor were determined both at room temperature and at 77°K. In contrast to the spectra of the usual peroxy radicals, those trapped in γ-irradiated PTFE exhibited an ESR spectrum apparently having a larger principal value for g⊥ than for g∥ when measured at room temperature, although the normal principal values were observed at 77°K. As for the directions of the principal axes, g∥ was directed along the chain axis at room temperature and was perpendicular to the chain axis at 77°K. From the temperature change of the g tensor and the line shapes in the oriented samples, it is shown that the observed temperature change of the spectra is due to rapid rotation at room temperature around the chain axis rather than around the C? O bond axis. Assuming this, the apparent principal values of the g tensor at room temperature were calculated from the g tensor obtained at 77°K. for the rigid state, and the results are in good agreement with observations at room temperature. A structure for the peroxy radicals is also proposed. In addition, the spectral line shape function for the uniaxially oriented samples has been derived.     

The growth and interaction of conjugated polymer chains,the unpaired electron localization

An ESR method was used to study the structure of the macroradical of the propagating chain R_p in the low-temperature, solid-phase polymerization of p-diethynylbenzene (DEB). The ESR spectra for γ-irradiated DEB samples and those of DEB deuterated in the ethynyl group showed that in the range 77–230 K, the unpaired electron of the macroradical was localized on one of the monomer links. At 230–310 K, its delocalization in a polyconjugated system took place because of addition of a linear macroradical to a double bond of a polymer molecule. The encounter of the macroradical with double bond probably occurs as polymer chain propagation. © 1994 John Wiley & Sons, Inc.     

Energy transfer and radical formation in the system poly(methyl methacrylate)–2-methyltetrahydrofuran

ESR spectra of γ-irradiated poly(methyl methacrylate) (PMMA) at 77°K and the effect of additives have been studied. γ-Irradiation of PMMA containing a small amount of 2-methyltetrahydrofuran as an additive at 77°K gives MTHF radical. Yields of MTHF radical increase with increasing MTHF concentration and reach a plateau value, and are independent of photobleaching with visible light. Yeilds of polymer radical are reduced by the addition of MTHF. Some elementary processes for the MTHF radical formation are proposed on the basis of the experimental results.     

Internal motion in organosilicon polymers. II. Dimethylpolysilazanes crosslinked through silicon

An investigation of the internal motion in organosilicon polymers by wideline nuclear magnetic resonance has been extended to a pair of dimethylpolysilazanes crosslinked through trifunctional silicon. The data suggest that there is considerable internal motion in all silazanes at 77°K. Evidence is presented for the presence of C₃ rotation of the methyl groups, as well as rotation or torsional oscillation of the Si(CH₃)₂ groups about the polymer backbone. Upon warming the NMR line is seen to narrow, and this is associated with the onset of additional motion, including chain translation and chain flexing or bending. Crosslinking through silicon increases the barrier to chain translation while decreasing the barrier to chain flexing or bending.     

Luminescence studies in polymers—IV. Effect of orientation,tacticity and crystallinity on polystyrene and polyvinylcarbazole fluorescence

The fluorescence spectra of amorphous atactic, amorphous isotactic and crystallized isotactic polystyrene films have been compared. The effect of chain orientation has also been analysed on amorphous atactic samples. The results show that the fluorescence yield increases with crystallinity at room temperature and 77°K. The contribution of excimer fluorescence at 77°K increases according to the sequence: atactic < atactic oriented < isotactic amorphous < isotactic crystallized. An increase of the fluorescence yield with crystallinity was also observed for polyvinylcarbazole samples although the contribution of excimer fluorescence at 77°K is independent of crystallinity for this polymer. The results are interpreted in terms of energy migration.     

γ-radiolysis of acetylated cotton cellulose: An ESR study

The nature and behavior of free radicals induced in acetylated cotton celluloses irradiated with γ-rays have been studied by electron spin resonance (ESR) spectroscopy. Dehydrogenation and deacetylation appear to be responsible for the free radicals observed from samples irradiated at 77°K. The degree of substitution enhanced the yield of acetyl radicals when the samples were irradiated at 77°K and adversely affected the overall radical concentration when irradiation was done at 300°K. In addition, the ESR spectra of samples irradiated under vacuum at 300°K were more intense than those obtained from samples irradiated in air. The nature, yield, and post-irradiation behavior of the primary radicals are discussed in the light of the ultimate chemical effects observed.     

Nuclear magnetic resonance of poly-β-alanine

Nuclear magnetic resonance of poly-β-alanine samples differing in solubility in water was studied over a wide temperature range as part of an investigation of their physical properties. Water-soluble poly-β-alanine has more branches and a lower degree of crystallinity than water-insoluble poly-β-alanine. NMR spectra of poly-β-alanine show one component at 77°K. which splits into two components, broad and narrow, at room temperature. Two transition regions were observed in curves for line width and second moment versus temperature. The higher transition temperature, corresponding to the glass transition of the polymer, appears to decrease with increasing water content. The second moment for the water-soluble polymer differs from that of the water-insoluble polymer at 77°K. This is interpreted in terms of the difference in the degree of crystallinity of the polymers.     

Polymerization of aromatic aldehydes. II. Cationic cyclopolymerization of phthalaldehyde

Phthalaldehyde was found to undergo cyclopolymerization with ease by several cationic catalysts and by γ-ray irradiation. The polymer was composed entirely of the dioxyphthalan unit, as confirmed by infrared spectroscopy and ready decomposition to monomer. The enhanced polymerizability of phthalaldehyde as compared with other aromatic aldehydes was explained in terms of the intermediate-type or, preferably, concerted propagation scheme. The conversion reached a saturation value of 87% in about 1 hr in methylene chloride at ?78°C, indicating an equilibrium polymerization. The ceiling temperature of the polymerization was ?43°C, as estimated from the relation between the saturation yield and polymerization temperature. The enthalpy and entropy of propagation were ?5.3 kcal/mole and ?23.0 eu, respectively. Since the molecular weight of the polymer was proportional to conversion, the propagating chain end was considered to be “living” in this system. The rate constant for propagation was calculated to be 0.18 1/mole-sec in methylene chloride at ?78°C with BF₃OEt₂ catalyst.     

Thermal and photo-induced phenomena in γ-irradiated poly(methyl methacrylate)

ESR spectra and optical absorption spectra of γ-irradiated poly(methyl methacrylate) (PMMA) at 77°K and the photo- and thermal bleaching behavior have been studied. γ-Irradiation of PMMA yields a singles spectrum with a line width of 6 G which is bleachable with visible light. This species is assigned to the polymer cation. The unbleached species consists mainly of a four-line spectrum and is assigned to a polymer radical having a different CH₂ conformation from the usual nine-line spectrum. On subsequent warming to room temperature, the spectrum changes into the nine-line spectrum. γ-Irradiation of PMMA containing biphenyl as an additive at 77°K gives biphenyl cation. Yield of polymer radical is reduced by the addition of biphenyl and 2-methyltetrahydrofuran. A mechanism is proposed which is consistent with the experimental results.     

Radiation-induced solid-state polymerization of methacrylic acid. II. In-source polymerization

The in-source polymerization of methacrylic acid in the solid state with γ-rays was studied. The conversion rates at various temperatures were obtained as well as the radical concentrations by the measurements of ESR spectrum. The rate of polymerization was found to be proportional to I^0.65 at 0°C. The results could be interpreted on the basis of the assumption that the rate of propagation is proportional to the concentration of the propagating radical, of the monomer, and of the polymer. The addition of water to the monomer seems to accelerate the polymerization reaction. The change of the line shape of the propagating radical during polymerization was interpreted in terms of the change of the matrix which surrounds the propagating radical.     

Study by electron paramagnetic resonance and spectrophotometry of absorption of the primary processes in photochemistry of frozen aqueous solutions at 77 degree K of aromatic amino acids and polypeptides

Abstract— Neutral, acidic or basic frozen aqueous solutions of aromatic amino acids undergo photoionisation under u.v. irradiation, at 77°K. In neutral or basic solutions, photo-ejected electrons are trapped in the solvent matrix and exhibit a characteristic absorption band in the visible region. In acidic solutions electrons are trapped by protons and ESR signal spectrum of hydrogen atoms may be observed. Hydrogen atoms are also produced in low yield in neutral or basic frozen aqueous solutions, u.v. irradiated at 77°K. In basic media the ESR spectrum of 0^- radical ions is observed. Kinetic studies as a function of light intensity show that photoionisation takes place after absorption of a second photon by the phosphorescent molecule in its lowest triplet state. Recombination of trapped electrons in neutral or basic solutions may be induced using secondary excitation with visible light. In all instances we could record the absorption spectrum of photolytic products of aromatic amino acids and polypeptides which are u.v. irradiated at 77°K.     

Radiation-induced postpolymerization of nitroethylene

Radiation-induced postpolymerization of nitroethylene in 2-methyltetrahydrofuran glass has been studied and discussed in reference to the results obtained from ESR measurements. No postpolymerization occurred at the temperature below ?150°C. In the temperature range between ?135°C and ?78°C, the polymer yield decreased with increasing postpolymerization temperature. The polymer yield increased linearly with the increase of the preirradiation dose in the range below 0.9 × 10⁶ r. The mean value for chain initiation was estimated to be about 1.3. The following correlations were observed between the results of the postpolymerization and ESR measurements. The postpolymerization started in the temperature range between ?140°C and ?135°C, where the ESR spectrum due to the anion radicals of nitroethylene disappeared. The polymer yield of the postpolymerization decreased with the photoirradiation at ?196°C before warming the samples in parallel with the photobleachability of the anion radicals observed in the glassy mixture by the ESR method. It was concluded from these results that the radiation-induced postpolymerization was initiated by the anion radicals of nitroethylene formed by the capture of electons.     

Radiation-induced solid-state polymerization of derivatives of methacrylic acid. III. An electron spin resonance study of radical reactions in irradiated octadecyl methacrylate

The electron spin resonance spectrum of gamma-irradiated octadecyl methacrylate (m.p. ≈ 12°C.) was due to a mixture of three radicals formed by (1) loss of a hydrogen atom from the paraffin chain, (2) addition of a hydrogen atom to the double bond, and (3) addition of a monomer molecule to radicals formed by (1) or (2). On warming monomer added to radicals (1) and (2) between ?170 and ?50°C., and above ?50°C. the spectrum was solely due to propagating methacrylate radicals. The total radical concentration decreased slightly at ?150°C. and was then constant up to ?30°C. A marked decrease in radical concentration occurred from ?30 to +12°C., it took place rapidly and reached an equilibrium value after each successive increase in temperature. Differential thermal analysis indicated a solid—solid phase change at ?30°C. When the sample was kept at 0°C. there was no further decrease in radical concentration even with 50% conversion to polymer. With 2% added chloranil the (chloranil)^? was observed to be of about the same concentration as methacrylate radicals. The initial total radical concentration was lower and decreased to zero by 0°C. on warming. No polymer was obtained.     

Radical polymerization and copolymerization of methyl α-(2-carbomethoxyethyl)acrylate,a dimer of methyl acrylate,as a polymerizable α-substituted acrylate

Polymerization and copolymerization of methyl α-(2-carbomethoxyethyl)acrylate (MMEA), which is known as a dimer of methyl acrylate, were studied in relation to steric hindrance-assisted polymerization. The propagating polymer radical from MMEA was detected as a five-line spectrum and quantified by ESR spectroscopy during the bulk polymerization at 40–80°C. The absolute rate constants of propagation and termination (κ_p and κ_t) for MMEA at 60°C (κ_p = 19 L/mol s and κ_t = 5.1 × 10⁵ L/mol s) were evaluated using the concentration of the propagating radical at the steady state. The balance of the propagation and termination rates allows polymer formation from MMEA. The polymerization rate of MMEA at 60°C was less than that of MMA by a factor of about 4 at a constant monomer concentration. Although no influence of ceiling temperature was observed at a temperature ranging from 40 to 70°C, addition-fragmentation in competition with propagation reduced the molecular weight of the polymer. The content of the unsaturated end group was estimated to be 0.1% at 60°C to the total amount of the monomer units consisting of the main chain. MMEA exhibited reactivities almost similar to those of MMA toward polymer radicals. It is concluded that MMEA is one of the polymerizable acrylates bearing a substituted alkyl group as an α-substituent. Characterization of poly(MMEA) was also carried out. © 1996 John Wiley & Sons, Inc.     

Thermal expansion and compressibility in the chain direction of a polydiacetylene single crystal

An x-ray back-reflection rotating camera has been used to measure the lattice constant, thermal expansion, and compressibility in the polymer chain direction of a polydiacetylene, poly[bis(p-toluene sulfonate) of 2,4-hexadiyne 1,6-diol]. The thermal expansion coefficient of the polymer chain is small and positive (0.9 ± 0.2 × 10^?6 °K^?) at 300°K, but negative below about 70°K. Application of 3.43 kbar hydrostatic pressure at 299°K changed the unit cell dimension in the polymer chain direction by less than 10 ppm.