Variation of the propagation rate coefficient with pressure and temperature in the free-radical bulk polymerization of styrene

The propagation rate coefficient K_p of the free-radical bulk polymerization of styrene is determined between 30 and 90°C up to a maximum pressure of 2800 bar. The data from pulsedlaser polymerizations and product analyses by gel-permeation chromatography (PLP-GPC) are adequately represented by the expression: The conseaquences of deducing activation volumes and activation energies from k_p/(L · mol^?1 · s^?1) or fromk*_p/(kg · mol^-1 ·. S^?1) are outlined.     

Kinetics of free radical solution polymerization of methyl methacrylate over an extended conversion range

The low and intermediate conversion solution polymerization kinetics of methyl methacrylate in toluene and in 2-butanone are investigated. For this purpose, repetitively applied laser pulses are employed to stimulate initiation, and quantitative near infrared spectroscopy is used to determine methyl methacrylate concentrations. The monomer concentration change per pulse is measured; it is shown how this information may be converted into coupled values of the rate coefficient for propagation k_p, the rate coefficient for termination k_t, and the total efficiency of (photo)initiation. In the present case k_p is already known from independent investigations, which enables our pseudostationary state methods to yield k_t directly. The major thrust of this paper, therefore, concerns the effect of solvent on k_t: it is found that at both low and intermediate conversions, k_t increases as the solvent concentration increases; possible causes of this are discussed. The effect of initial solvent fraction on the variation of k_t with conversion is also reported. Although all our experimental measurements are carried out at an elevated pressure (1000 bar) and a single temperature (30°C), there is no reason for suspecting that our findings regarding solution polymerization termination should not be representative for polymerizations of methyl methacrylate (and other methacrylate monomers) in general.     

Termination kinetics of styrene free‐radical polymerization studied by time‐resolved pulsed laser experiments

The single pulse (SP)‐pulsed‐laser polymerization (PLP) technique has been applied to measure k_t/k_p, the ratio of termination to propagation rate coefficients, for the free‐radical bulk polymerization of styrene at temperatures from 60 to 100°C and pressures from 1800 to 2 650 bar. k_t/k_p is obtained by fitting monomer concentration vs. time traces that are determined via time‐resolved (μs) near infrared monitoring of monomer conversion induced by single excimer laser pulses of about 20 ns width. Styrene is a difficult candidate for this kind of measurements as conversion per pulse is small for this low k_p and high k_t monomer. Thus between 160 to 300 SP signals were co‐added to yield a concentration vs. time trace of sufficient quality for deducing k_t/k_p with an accuracy of better than ± 20 per cent. With k_p being known from PLP–SEC experiments, chain‐length averaged k_t values are immediately obtained from k_t/k_p. At given pressure and temperature, k_t is independent of the degree of overall monomer conversion, which, within the present study, has been as high as 20%percnt;. The k_t value, however, is found to slightly increase with the amount of free radicals produced by a single pulse in laser‐induced decomposition of the photoinitiator DMPA (2,2‐dimethoxy‐2‐phenyl acetophenone). This remarkable observation is explained by DMPA decomposition resulting in the formation of two free radicals which significantly differ in reactivity. Extrapolation of SP–PLP k_t data from experiments at rather different DMPA levels and laser pulse energies toward low primary free‐radical concentration, yields very satisfactory agreement of the extrapolated k_t values with recent literature data from chemically and photochemically induced styrene polymerizations.     

Photo-sensitized polymerization of styrene under high pressure

The polymerization of styrene, photosensitized by azocyclohexane-1,1′-dinitrile, was studied at 30°C over a pressure range of 1 to ca. 1000 bar. The average lifetimes of the growing polymer chains were determined by the rotating-sector method. By measuring the effect of pressure on the individual reactions of styrene polymerization, the activation volumes for the propagation and the termination were obtained as ?17,9 cm³/mol and 13,1 cm³/mol, respectively. The overall activation volume was also measured as ?17,9 cm³/mol and was nearly equal to that calculated from the individual activation volumes (?17,8 cm³/mol). The propagation rate constant increases exponentially with pressure, whereas the termination rate constant decreases rapidly up to 500 bar and then decreases slowly as the pressure increases further.     

The anionic polymerization of styrene under pressure

The kinetics of the anionic polymerization of styrene were investigated under pressure (1≤p/bar < 1800) with Na⁺ as counter ion in tetrahydropyran (THP) as solvent and with Cs⁺ as counter ion in 1,2-dimethoxyethane (DME) as solvent. The results yielded the activation volume of the contact ion pair ΔV and the sum (ΔV + ΔV_cs) of the activation volume of the solvent separated ion pair ΔV and the volume change upon formation of solvent separated ion pairs from contact ion pairs ΔV_cs. The numerical values are negative. The activation volume of the solvent separated ion pairs could be estimated.     

Critically evaluated rate coefficients for free-radical polymerization, 1. Propagation rate coefficient for styrene

Pulsed-laser polymerization (PLP) in conjugation with molecular weight distribution (MWD) measurement has emerged as the method of choice for determining the propagation rate coefficient k_p in free-radical polymerizations. Detailed guidelines for using this technique (including essential internal consistency checks) and reporting the results therefrom are given by the authors, members of the IUPAC Working Party on Modeling of kinetics and processes of polymerization. The results for PLP-MWD k_p measurements from many laboratories for bulk free-radical polymerization of styrene at low conversions and ambient pressure are collated, and are in excellent agreement. They are therefore recommended as constituting a benchmark data set, one that is best fitted by (the confidence ellipsoid for the Arrhenius parameters is also given). These benchmark data are also used to evaluate the merits of several other methods for determining k_p; it is found that appropriately calibrated electron paramagnetic resonance spectroscopy appears to yield reliable values of k_p for styrene.     

On the kinetics of polymerization and copolymerization of poly(oxyethylene) macromonomers and styrene

The homogeneous and the dispersion polymerzation and copolymerization of methacryloyl-terminated poly(oxyethylene) (MMA-PEG) and of p-vinylbenzyl-terminated poly(oxyethylene) (St-PEG) macromonomers and styrene, initiated by a radical initiator, was investigated using conventional gravimetric and NMR methods at 60°C. The batch polymerizations in N,N-di-methylformamide and in ethanol/water were conducted to either low or high conversion. The fractional conversion rates of the solution polymerization and copolymerization indicate that the homopolymerizations of macromonomers involve steady-state conditions, whereas copolymerizations proceed under non-stationary conditions. The ratios of the rate constants for propagation and termination (k_p/k) for polymerization and copolymerization of MMA-PEG and St-PEG are by one order of magnitude higher than that for styrene. The increase in k_p/k is more pronounced in dispersion polymerization, which is ascribed to the decrease of both k_p and k_t. The rates of dispersion polymerization are proportional to the particle concentration. The number of particles increses up to 50% conversion. The particle growth is suggested to proceed via association of partcles and by propagation within polymer particles. The decrese of the number of radicals per particles as conversion proceeds is ascribed to the decrese of the growing radical activity and to the transfer of monomeric radicals to the continuous phase. The molecular weights correlate inversely with the particle size.     

Bulk polymerization of styrene and methyl methacrylate using tetraphenylbisphosphine as an initiator

In this study a controlled polymerization of styrene (S) and methyl methacrylate (MMA) with tetraphenylbisphosphine (TPhBP) as the initiator in bulk was carried out, using UV light as the energy source. The yield of polymerization was up to 15% for PS and up to 62% for PMMA. TPhBP is decomposed to form ∗︁PPh₂ radicals, or is oxidized to form bisphosphine oxide. Only non-oxidized TPhBP initiates the photo-polymerization. For S and MMA an insignificant portion of photo- and thermal-polymerization was detected. In the presence of oxygen the phosphine end-groups of the polymer are oxidized to —PO(Ph)₂ groups. The molecular weights of PMMA were determined from ¹H NMR spectra and the molecular weights of PMMA and PS were determined by gel permeation chromatography (GPC) measurements. With increasing concentration of the initiator the molecular weight decreases and at a high concentration of the initiator the molecular weight decreases with conversion. The polydispersity decreases with increasing concentration of TPhBP. Termination occurs by combination of primary radicals ∗︁PPh₂, by combination of macroradicals, and by chain transfer to the initiator. Because of different types of termination the ratio between the tail —CH₂P(Ph)₂ and the head —C(CH₃,COOCH₃)P(Ph)₂ groups is about 4 to 1, and decreases with increasing concentration of the initiator.     

Rate coefficients of the free-radical polymerization of 1,3-butadiene

Rate coefficients of termination and transfer in the free-radical polymerization of 1,3-butadiene in chlorobenzene were determined in the temperature range 318 K < T < 333 K. On the basis of an earlier published temperature dependence of the rate coefficient of propagation, for the termination reaction the Arrhenius equation K_t = 1,13 · 10¹⁰ · exp(? 711 K/T) L · mol^?1 · s^?1 was obtained. For the transfer to monomer the experiments yielded the Arrhenius equation K_tr,M = 4,22 · 10⁶ · exp(? 5140 K/T) L · mol^?1 · S^?1 and for the transfer to the solvent K_tr,S = 2,25 · 10⁸ · exp(? 7050 K/T) L · mol^?1 · S^?1.     

Influence of small additions of hexamethylphosphoric triamide on the kinetics of polymerization of styrene in THF

Unsaturated polyamides from methyl-fumaric (2-methyl-fumaric and 2.3-dimethyl-fumaric) acid dichlorides and diamines, i.e. 1.6-hexamethylenediamine, 1.10-decamethylenediamine, m-xylylenediamine, piperazine and trans-2.5-dimethylpiperazine were synthesized by low-temperature polycondensation. The polymer structures and properties were investigated by IR, X-ray, NMR, DSC, and TGA analyses. All polyamides were found to be soluble in several volatile organic solvents: furthermore – unlike polyamides derived from unsubstituted fumaric acid – polyamides from substituted fumaric acid derived from primary diamines showed melting or softening points.     

Thermally initiated emulsion polymerization of styrene

The thermal polymerization of styrene in emulsion was studied by determination of the number of particles, the rate of polymerization, and the average molecular weight of the polymer formed as a function of the emulsifier concentration. For the number of particles the relation N ～ [E]^1,4 was found, whereas the rate of polymerization showed a dependence of v_br ～ [E]^0,7, only up to an emulsifier content of 0,2 mol. dm^?3, but remained constant at higher concentrations. A transfer of low molecular weight radical species from the organic to the aqueous phase is discussed as a prerequisite of polymerization in the latex particles. The deviations from the Smith-Ewart theory are explained by the low rate of radical formation, which does not provide a constant radical concentration per particle.     

The asymmetric-selection polymerization of styrene oxide

The asymmetric-selection polymerization of styrene oxide was investigated by using diethylzinc/menthol system as catalyst. Monomer consumption was found to follow a second order kinetic, whereas developing of the optical rotation of the remaining monomer followed a first order reaction scheme. This phenomenon is accounted for by the assumption that the polymerization consists of rapid initiation followed by slow stepwise growth. Two moles of monomer participate in the polymerization, one of which is polymerized and the other is responsible for activation of the catalyst. The asymmetric ability of the catalyst is arising from the menthoxyl group but not from propylene oxide complexed to the catalyst.     

Emulsion polymerization of styrene using phospholipids with polymerizable chain as an emulsifier

Emulsion polymerizations of styrene with various phospholipids as emulsifiers were carried out to obtain latices bearing the lipids. Using 1-palmitoyl-2-(4-vinylbenzoyl)-sn-glycero-3-phospho-choline (2a) and 1-[5-(4-vinylphenyl)pentanoyl]-sn-glycero-3-phosphocholine (3) as emulsifiers, latices of narrow size distributions were produced in high yields with a water-soluble initiator, K₂S₂O₈. The lipids were found to be quantitatively immobilized onto the latex surface. With a hydrophobic initiator, 1-palmitoyl-2-[5-(4-vinylphenyl)pentanoyl]-sn-glycero-3-phosphocholine (2c) were bound densely on the latex surface, although the yields were not high. Phospholipase C was found to hydrolyze 2b and 2c attached to the surface.     

Dosimetry of therapeutic photon beams using an extended dose range film

For intensity modulated radiation therapy (IMRT) dose distribution verification, multidimensional measurements are required to quantify the steep dose-gradient regions. High resolution, two-dimensional dose distributions can be measured using radiographic film. However, the photon energy response of film is known to be a function of depth, field size, and photon beam energy, potentially reducing the accuracy of dose distribution measurements. The dosimetric properties of the recently developed Kodak EDR2 film were investigated and compared to those of Kodak XV film. The dose responses of both film types to 6 MV and 18 MV photon beams were investigated for depths of 5 cm, 10 cm, and 15 cm and field sizes of 4x4 cm2 and 15x15 cm2. This analysis involved the determination of sensitometric curves for XV and EDR2 films, the determination of dose profiles from exposed XV and EDR2 films, and comparison of the film-generated dose profiles to ionization chamber measurements. For the combinations of photon beam energy, depth, and field size investigated here, our results indicate that the sensitometric curves are nearly independent of field size and depth of calibration. For a field size of 4x4 cm2, a single sensitometric curve for either EDR2 and XV film can be used for the determination of relative dose profiles. For the larger field size, the sensitometric curve for EDR2 film is superior to XV film in regions where the dose falls below 20% of the central axis dose, due to the effects that the increased low energy scattered photon contributions have on film response. The limited field size and depth dependence of sensitometric data measured using EDR2 film, along with the inherently wide linear dose-response range of EDR2 film, makes it better suited to the verification of IMRT dose distributions.     

Initiator efficiencies in 2,2′-azoisobutyronitrile-initiated free-radical polymerizations of styrene

Results are presented from a comprehensive study of the variation with conversion, temperature and pressure of the initiator efficiency of 2,2′-azoisobutyronitrile in styrene bulk polymerizations. These efficiencies were measured using a new method involving the use of infra-red spectroscopy to monitor directly the concentrations of various nitrile-group containing species. Our results are shown to be satisfactorily fitted by a simple model based on the idea that initiation follows as a result of primary radical fragments diffusing away from each other. Closer microscopic inspection of the variations with temperature and pressure of the entire range of our initiator efficiencies implies the possibility that capture by monomer may also contribute as a mechanism of out-of-cage escape of primary radicals. Taking a wider view, the experimental method of this paper is shown to be of use for simultaneous determination of other important kinetic parameters: it yields rate coefficients of initiator decomposition, and also, when initiator decomposition is stimulated by pulse-laser irradiation, the method is capable of delivering individual values of the rate coefficients for termination and propagation.     

Mechanism of catalytic chain transfer in the free-radical polymerisation of methyl methacrylate and styrene

The mechanism of catalytic chain transfer with bis(boron difluorodimethylglyoximate) cobaltate(II) (COBF) has been studied in the homopolymerisations of methyl methacrylate and styrene. The chain transfer constants were measured using both the Mayo and Chain Length Distribution (CLD) methods over a range of temperatures (40–70°C). The two methods generally agree within 10%. The high values of the chain transfer rate coefficients, k_tr (∼10⁷ for MMA), suggest the possibility that the reaction is approaching diffusion control. This is also supported by the high values obtained for the frequency factor (A ∼ 10¹⁰). The chain transfer rate coefficients for styrene are approximately two orders of magnitude lower than those obtained for MMA, which can be explained in terms of the formation of cobalt-carbon bonds and the accessibility of β-H sites for hydrogen abstraction from the two different radical chain ends in the case of styrene. High conversion, solution polymerisation experiments on methyl methacrylate in toluene reveal behaviour inconsistent with a simple catalytic mechanism and may suggest deactivation of the catalyst by solvent. On the assumption that the kinetics of catalytic chain transfer can be explained by a classical free-radical mechanism, it is possible to derive information on the chain length dependence of the average termination rate coefficient, 〈k_t〉. Applying this approach to methyl methacrylate and styrene at different temperatures, we have found that the chain length effect on 〈k_t〉 appears to be independent of both temperature and monomer type.     

Polymer-supported rare-earth catalysts for polymerization of styrene

A neodymium complex supported on an ethylene-vinyl alcohol copolymer (EVA · Nd) was prepared for the first time and the polymerization of styrene in the presence of a new type of catalyst system composed of this complex was studied. The new catalytic system [EVA · Nd Al(i-Bu)₃ CCl₄] (i-Bu = isobutyl) is characterized by its catalytic activity in comparison with that of conventional rare-earth catalysts. When [Nd] = 10⁻³ mol/L, [Styrene] = 5 mol/L, [Al]/[Nd] = 120 and [CCl₄]/[Nd] = 35, then the conversion to polystyrene (PS) was 55% in 6 h, and the catalytic activity reached 1972 g PS/g Nd which is higher by a factor of 45 than that of conventional rare-earth catalysts. The polymerization reaction shows a short induction period where the rate is first order with respect to the monomer concentration. The activation energy is 27,6 kJ/mol. This polymerization may proceed according to a free-radical or a Ziegler-type coordination mechanism.