ESR observation of propagating radical and estimation of propagation rate constant in the radical polymerization of vinyl chloride

Well‐resolved electron spin resonance (ESR) spectra of propagating radicals in radical polymerizations of vinyl chloride have been detected under irradiation for the first time. Structure of propagating radicals of vinyl chloride was investigated on the basis of hyperfine splitting constants (hfc's) of the ESR spectrum. The spectrum was assigned to a chain end propagating radical. The propagation rate constant (k_p) for the radical polymerization of vinyl chloride was also estimated by ESR spectroscopy. The value of k_p for vinyl chloride was (5 400 ± 1 500) M^–1·s^–1at 25°C, which is in agreement within experimental error with one of those obtained by the rotating sector method.     

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.     

Pre-exponential factors of rate constants of chain propagation reactions in free radical polymerization

A method is described for calculating the pre-exponential factor of the chain propagation rate constants in free radical vinyl polymerizations taking place in the liquid phase. The pre-exponential factors calculated for some typical monomers are in satisfactory agreement with the experimental values.     

Capacity flow method for determination of propagation and termination rate constants in radical polymerization

The possibility of determination of the propagation and termination rate constants k_p and k_t, resp., as well as their ratios k_p/k_t and k_p/k in homogeneous radical polymerization is shown using the capacity flow method. A theoretical analysis is carried out and relatively simple equations are introduced. The essential point is that the life time of the growing polymer chain is obtained graphically from the residence time of the reactants in the reactor vessel, which is a given quantity. As an experimental example the polymerization of methyl methacrylate initiated by benzoyl peroxide in benzene is investigated in a flow reactor with perfect mixing at 80°C. It is characteristic that the process can be followed by means usually applied for studying slow reactions. The degree of conversion is measured turbidimetrically and gravimetrically, whereas the initiation rate is analysed iodometrically. Thus, through a numerical linear approximation by the method of least squares k_p/k_t = (2,28±0,45)·10^?5, k_p/k = (1,50±0,22). 10^?1 are found from the experimental data, and hence k^p = (9,95±0,83)·10² l mol^?1 s^?1 and k_t = (4,36±0,49)·10⁷ l mol^?1 s^?1 are obtained.     

A rotating sector determination of the propagation rate constant for benzyl methacrylate in its radical polymerization

The unusually large propagation rate constant for benzyl methacrylate reported in the previous paper was studied in more detail. The decomposition rate of 2,2′-azoisobutyronitrile in this viscous monomer was somewhat smaller than in ordinary solvents. A regular monomer kinetic order could be realized in butanol and in benzyl butyrate whose viscosities were very similar to that of the monomer. A rotating sector experiment in benzyl butyrate gave k_p = 1410 dm³ mol^?1 s^?1 for this monomer, which was essentially identical with the previous result.     

Tacticities of poly(glycidyl methacrylate) obtained by radical polymerization

The tacticities of poly(glycidyl methacrylate) prepared by radical polymerization at temperatures in the range from ?78 to 60°C in several solvents or in bulk are described. The redox system diisopropylperoxy dicarbonate/dimethylaniline was found to polymerize this monomer effectively even at low temperature. The tacticities were determined by NMR spectroscopy. The difference of entropy Δ(ΔS

1 Nippon Oils & Fats Co., Ltd. (Chiyoda-ku, Tokyo, Japan)

) and enthalpy Δ(ΔH ) of activation between isotactic and syndiotactic propagations were calculated. The values of Δ(ΔH ) were found not to be affected essentially by polymerization conditions and to be slightly larger than that of methyl methacrylate polymerization.     

Effect of alkyl groups on the rate constants of propagation and termination in the radical polymerization of dialkyl itaconates

Polymerization of dialkyl itaconates with dimethyl azoisobutyrate ( 5 ) was studied in benzene at 50°C by means of electron spin resonance (ESR). The monomers used are dimethyl ( 1 ), diethyl ( 2 ), dibutyl ( 3 ) and di-2-ethylhexyl ( 4 ) itaconates. All the polymerization systems involve ESR-observable propagating polymer radicals under the actual polymerization conditions. The polymerization rate (R_p) and degree of polymerization of the resulting polymer increase in going from shorter to longer alkyl groups. The ESR-determined rate constants of propagation (k_p) and termination (k_t) decrease as the alkyl chain becomes longer. k_p of 1 is 3,3 times higher than that of 4 , while k_t of 1 is 590 times higher than that of 4 . Thus, the steric effect due to the alkyl group suppresses much more termination than propagation, leading to the fact that R_p increases as the alkyl group becomes larger.     

Photosensitized polymerization of butyl methacrylate under high pressure

The propagation and termination rate constants for butyl methacrylate polymerization were measured at 30°C over a pressure range of 1 to 1 000 bar at a 250 bar pressure interval. By determining the effect of pressure on the individual reactions of polymerization, the activation volumes for the propagation and the termination were obtained as ?23,2 cm³/mol and 17,8 cm³/mol, respectively. The overall activation volume was ?17,4 cm³/mol. The effect of pressure on the individual rate constants for butyl methacrylate polymerization was compared with the results for methyl methacrylate polymerization. The comparison shows that, for the propagation reaction, the increase in the size of the ester group brings about a diminution in the value of the activation volume, and that the effect of pressure on the termination reaction was somewhat smaller than that for methyl methacrylate polymerization.     

Anionic polymerization of butyl cyanoacrylate by tetrabutylammonium salts, 2. Propagation rate constants

Anionic polymerizations of butyl cyanoacrylate were initiated in tetrahydrofuran (plus a few experiments in 1,2-dimethoxyethane) by the salts tetrabutylammonium hydroxide, bromide, acetate and three substituted acetates. The hydroxide gives near-ideal ‘living polymerization’ kinetics, with k_p close to 10⁶l·mol^?1. s^?1 at 20°C. The kinetics of the reactions initiated by the acetates and bromide are analysed by the slow-initiation-no-termination theory, using values of the initiation rate constants evaluated in Part 1. The k_p values derived are in the same range as those from the OH-initiated reactions and those of the zwitterionic polymerizations initiated with covalent bases, i.e., tertiary phosphines and amines. A ca. 4-fold variation of k_p with concentration of active species is given a speculative analysis in terms of dissociation from paired to free ions, yielding tentative estimates for k ≈ 10⁵ and k ≈ 10⁷l·mol^?1·s^?1 in THF at 20°C. Molecular weights were all high M _n ≈ 10⁶, with M _w/M _n ≈ 2.     

Study of radical polymerization by means of ESR, 2. Homogeneous polymerization of methyl methacrylate and vinyl acetate

A very efficient ESR spectrometer was applied to the measurement of the stationary concentration of macroradicals in homogeneous polymerization. From these data the rate constants of chain propagation and chain termination for the polymerizations of methyl methacrylate and vinyl acetate were found in a direct way. Activation energies of these elementary reactions were measured.     

Solvent effect on the propagation rate constant in the radical polymerization of bis(2-ethylhexyl) itaconate

Polymerization of bis(2-ethylhexyl) itaconate ( 1 ) with dimethyl azobis(isobutyrate) ( 2 ) was carried out at 50°C in various solvents. Polar solvents caused a significant decrease in the polymerization rate (R_p) and the molecular weight of resulting poly( 1 ). The propagating poly( 1 ) radical could be observed as a five-line ESR spectrum in the actual polymerization systems used. The stationary concentration of poly( 1 ) radical was determined by ESR to be 4,2–6,4 · 10^?6 mol · L^?1 at 50°C when the concentrations of 1 and 2 were 1,03 and 3,00 · 10^?2 mol · L^?1. Using R_p the monomer concentration and the polymer radical concentration, the propagation rate constant (K_p) was estimated to be 1,4–6,8 L · mol^?1 · s^?1, depending on the solvents used. The k_p value was smaller in more polar solvents. The solvent effect is explained in terms of the solvent affinity for the propagating polymer chain.     

Molecular weight distribution in the activated monomer polymerization with nonequal propagation rate constants

Equations are derived to calculate the molecular weight distribution (MWD) of polymer chains for polymerizations with nonequal propagation rate constants. The presented set of equations allows the determination of the relative propagation rate constants provided the mole fractions of the chains with different degrees of polymerization are known. Furthermore, the MWD function for a polymerization with linearly increasing propagation rate constants is given. Application of the presented relations to analyse the MWD of polymers obtained by activated monomer polymerization is proposed.     

Free radical polymerization kinetics of n-lauryl methacrylate

The kinetics of free radical polymerization of n-lauryl methacrylate (dodecyl methacrylate) were studied at 60 and 80°C in bulk and in benzene solution. The analysis of the data showed the kinetic results deviated from classical behaviour and that this deviation could be explained by the chain length dependence of the diffusion controlled termination rate constant, K_t. Primary radical termination is explicitly considered and rejected as being insignificant in this work. Because of its lessened flexibility poly(n-lauryl methacrylate) shows a greater chain length dependence for K_t than does poly(methylmethacrylate).     

Propagation rate coefficients in free-radical homopolymerizations of butyl methacrylate and dodecyl methacrylate

Propagation rate coefficients (k_p) for the homopolymerizations of butyl methacrylate (BMA) and dodecyl methacrylate (DMA) were determined by the pulsed laser polymerization (PLP)/size-exclusion chromatography (SEC) technique. At amibient pressure, the polymerizations of BMA and DMA were studied between ?20 and 50°C and between 9 and 60 °C, respectively. For DMA the pressure dependence of propagation rate has been measured up to 1500 bar. The propagation rate coefficients of three major methacrylates (MMA, BMA, DMA) are available over an extended pressure and temperature range. The k_p data at identical pressure p and temperature T are close to each other and exhibit a very pronounced family-type behaviour with respect to both pressure and temperature dependence. The uncertainty of experimental data is estimated to be ± 10 per cent. The imperfect knowledge of the Mark-Houwink coefficients may give rise to an increased error for the BMA and DMA data which should, however, stay below ± 20 per cent.     

Determination of free-radical propagation rate coefficients for methyl methacrylate and butyl acrylate homopolymerizations in fluid CO2

Free-radical polymerizations of methyl methacrylate (MMA) and of butyl acrylate (BA) in fluid CO₂ were studied by means of pulsed laser polymerization (PLP) and molecular weight analysis of the resulting polymer using size-exclusion chromatography (SEC). With the PLP-SEC technique propagation rate coefficients, k_p, were determined. Depending on the monomer concentration, k_p values are up to 40% smaller than corresponding data for bulk polymerizations of MMA and BA.     

Electron paramagnetic resonance investigation of the nature of the propagating species in methyl methacrylate polymerization

The electron paramagnetic resonance (EPR) spectrum of methyl methacrylate polymerizing in a range of systems is simulated using a consistent set of parameters for two rotamers of a single free radical. The 9-line spectrum observed in bulk and emulsion polymerization was fitted by a superposition of two spectra corresponding to two rotamers of a long-chain species (each a somewhat hindered, non-rotating macroradical), having anisotropic methylene proton hyperfine couplings which can be treated as isotropic to a good approximation. The 13-line spectrum observed at low conversion with very high radical flux was simulated as a superposition of the spectra of two very similar rotamers, the EPR spectra of which are indistinguishable; either rotamer is part of a very short chain (primarily an initiator fragment which has propagated once) undergoing free rotation in a low-viscosity medium. Thus the EPR spectra can be explained without having to invoke earlier suggestions that there are two types of free radicals (“trapped” and “untrapped”) in these systems, corresponding to the 9-line and 13-line spectra. The existence of an enormous proportion of very short free radicals under conditions of high radical flux is supported by quantitative calculations of the radical chain-length distribution. It is suggested that the two rotamers have significantly different propagation rate coefficients because of differences in the hindered rotations in their transition states.     

Chain propagation rate constants for gas-phase polymerization of propene and 1-butene with Ziegler-Natta catalysts

¹³C NMR analysis of propene/1-butene block copolymers obtained by gas-phase polymerization with Ziegler-Natta catalysts allows the determination of the propagation rate constants for the homopolymerization of the two monomers. They are very similar for the Solvay type catalyst δ-TiCl₃/AlMe₃ and the supported catalyst TiCl₄/MgCl₂/phthalate/AlMe₃. The constant of propene polymerization is three times higher than that of 1-butene polymerization. The high value of the constant found for propene polymerization is in agreement with the literature value determined by the stopped flow polymerization method.     

Propylene polymerization on titanium-magnesium catalysts determination of the number of active centers and propagation rate constants

By use of the quenching technique with ¹⁴CO and ¹⁴CO₂ the number of active centers and the propagation rate constants (k_p) were determined for the propylene polymerization on different titanium-magnesium catalysts in the presence and absence of an organoaluminium cocatalyst. The k_p values at 70°C were found to be 500–1000 1·mol^?1·s^?1, which were confirmed by independent data of molecular mass measurements of the isotatic polymer after a short polymerization time (5 s). Similar isotactic and atactic k_pvalues were found. The maximum number of active centers for supported titanium-magnesium catalysts can reach about 10% of the titanium content in the catalyst. The k_p values of ethylene polymerization on catalysts active without an organoaluminium cocatalyst were also determined (≈ 10⁴ l·mol^?1·s^?1 at 70°C).     

Electron spin resonance spectroscopy does not reveal hydroxyl radical production in activated natural killer lymphocytes

We investigated hydroxyl radical (OH) production by human natural killer (NK) cells, using electron spin resonance (ESR) spectroscopy and 5.5 dimethyl-1-pyrroline-N-oxide (DMPO), a spin trap specific for OH. production. We confirmed that hydroxyl radical scavengers, n-propyl gallate and catechin, inhibited NK cell-mediated cytotoxicity (NK-CMC) in a dose-dependent manner and demonstrated that DMPO also inhibited NK-CMC. Polymorphonuclear leukocytes (PMNL) activated by opsonized zymosan (2.4 mg/ml) and mixed with DMPO (0.14 M) showed an early increase in hydroxyl radical production, leading to a net production of free radical of almost 400 pMol/10(6) cells. We then mixed NK cells with K562, an NK-sensitive tumor cell, at a 1:1 ratio and added DMPO (0.14 M). We pelleted the cells to increase EC to TC binding before taking the sample readings. Activated NK cells showed no increase in OH. production, leading to a net production of free radicals less than 1% that of activated PMNL. These data strongly suggest that hydroxyl radical production does not play a role in the early events of NK cell activation; they indicate a need to reevaluate the mechanism of inhibition of NK-CMC by OH. scavengers.