116) "Termination kinetics of free-radical polymerization of styrene over an extended temperature and pressure range"
M. Buback, F.-D. Kuchta, Macromol. Chem. Phys., 198, 1455-1480 (1997)
The termination rate coefficient k(t) of the free radical bulk polymerization of styrene is determined between 30 and 90 degrees C up to a maximum pressure of 2 800 bar. The majority of polymerization experiments has been carried out at monomer conversions up to 20 per cent. In this range a single value of k(t) is sufficient to describe termination rate at constant pressure and temperature. Toward higher conversion, significant changes in k(t) are observed. The data are measured by a pulsed laser polymerization technique and partly by conventional chemically initiated experiments, both with 2,2'-azoisobutyronitrile (AIBN) as the initiator. Online spectroscopy is applied toward measurement of styrene conversion. The experimental termination rate coefficients up to 20 per cent monomer conversion are adequately represented by the expression:ln[k(t)(p, T)/(L . mol(-1) . s(-1))] = 20.785 - 1.050 . 10(-3) p/bar + 5.2 . 10(-8) p(2)/bar(2) - 753/T/K + 0.1060/TK . p/barActivation volume and activation energy of k(t) are very close to the corresponding activation parameters that characterize the pressure and temperature dependence of the inverse of styrene monomer viscosity. Varying laser pulse repetition rate has been used to investigate a potential chain-length dependence of k(t) at low conversion. It turns out that effects of this kind are not sufficiently pronounced to be safely established in view of the experimental precision of +/-25 per cent that is reached in the k(t) determinations.
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118) "Critically evaluated rate coefficients for free-radical polymerization, 2 - Propagation rate coefficients for methyl methacrylate"
S. Beuermann, M. Buback, T. P. Davis, R. G. Gilbert, R. A. Hutchinson, O. F.Olaj, G. T. Russell, J. Schweer, A. M. van Herk, Macromol. Chem. Phys. 198, 1545-1560 (1997)
Pulsed-laser polymerization (PLP) in conjunction with molar mass distribution (MMD) measurement is the method of choice for determining the propagation rate coefficient k(p) in free-radical polymerizations. The authors, members of the IUPAC Working Party on Modeling of kinetics and processes of polymerization, collate results from using PLP-MMD to determine k(p) as a function of temperature T for bulk free-radical polymerization of methyl methacrylate at low conversions and ambient pressure. Despite coming from several different laboratories, the values of k(p) are in excellent agreement and obey consistency checks. These values are therefore recommended as constituting a benchmark data set, one that is best fitted byk(p) = 10(6,427) L . mol(-1) . s(-1) exp(-22, 36 kJ . mol(-1)/R . T)The 95% joint confidence interval for these Arrhenius parameters is also given. In so doing, we describe the most appropriate statistical methods for fitting k(p)(T) data and then obtaining a joint confidence interval for the fitted Arrhenius parameters. As well, we outline factors which impose slight limitations on the accuracy of the PLP-MMD technique for determining k(p), factors which may apply even when this technique is functioning well. At the same lime we discuss how such systematic errors in k(p) can be minimized.
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117) "Solvent Dependence of Diacyl Peroxide Decomposition Kinetics Under High Pressure"
M. Buback, C. Hinton, Z. Phys. Chem. (Munich), 199, 229-254 (1997)
The effect of molecular surrounding on the decomposition kinetics of bis(3,5,5-trimethylhexanoyl) peroxide (BTMBP), dioctanoyl peroxide (DOP) and dibenzoyl peroxide (BPO) has been gauged by measurements in various solvents using time-resolved FTIR spectroscopy. Two complementary on-line methods, one discontinuous the other continuous, were used to study decomposition rates at pressures up to 3 kbar and temperatures up to 155 degrees C. The decomposition of BTMHP, the prime diacyl peroxide of this study, has been monitored in nine solvents. In going from n-pentadecane to acetonitrile the rate increases by a factor of 7 at 80 degrees C and 1500 bar. Both the Kirkwood function and the empirical solvent parameter E-T(N) correlate very well the observed rate of decay. The activation energy of BTMHP decomposition in dichloromethane differs in going from low to high temperatures, whereas a single Arrhenius line is observed in n-heptane. The solvent influence on BTMHP and DOP decomposition is very similar, and distinctly different to that for BPO. The kinetic results for BTMHP and DOP from polarity, viscosity, pressure and temperature variation are not in conflict with two-bond homolysis, although this mechanism can not be proven.
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103) "Quantitative IR-Spectroscopic Analysis of Ethylene-Acrylate Copolymers"
M. Buback, M. Busch, Th. Dröge, F.-O. Mähling, C. Prellberg, Eur. Polym. J. 33, 375-379 (1997)
An IR spectroscopic technique for the quantitative analysis of composition of three ethylene (E)-acrylate copolymer systems (poly(ethylene-co-methyl acrylate), poly(ethylene-co-butyl acrylate), poly(ethylene-co-2-ethylhexyl acrylate)) is presented. Based on a simple model which explicitly considers vibrational band intensities characteristic for CH and for C=O, copolymer composition may be derived from the ratio of C=O and CH integrated absorbances with a precision of +/-3 mol %. The optical path length of the copolymer samples which are subjected to IR analysis as pressed films need not be known. (C) 1997 Elsevier Science Ltd.
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115) "Polymer Modification of Ethylene-Acrylic Copolymers in Near-Critical Water"
M. Buback, U. Elstner, M. McHugh, F. Rindfleisch, Macromol. Chem. Phys. 198, 1189-1196 (1997)
Modification reactions are reported for fully-hydrogenated butadiene-acrylonitrile (35.8 mol-% AN) copolymer, ethylene(E)-butyl acrylate (4.7 mol-% BA) copolymer, and ethylene(E)-methyl acrylate (45 mol-% MA) copolymer in dense, near-critical water at 250 degrees C and 300 degrees C and at 300 bar and 1500 bar. Nitrile, amide, and eater moieties can be converted into COOH groups. Kinetic analysis of the ester to acid transformations suggests autocatalytic activity of the acid groups.
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114) "Simultaneous determination of free-radical propagation and transfer rates from a novel type of PLP-SEC experiment"
M. Buback, R. A. Lämmel, Macromol. Theory Simul. 6,145-150 (1997)
A novel type of pulsed laser polymerization (PLP)-size exclusion chromatography (SEC) experiment is presented in which laser pulse sequences are applied at considerable dark time intervals between each pulse package. The pulse sequences give rise to the characteristically structured molecular weight distribution (MWD) from which k(p) may be derived. Polymer produced during the extended dark time intervals determines the MWD at high molecular weights and allows for the measurement of chain transfer rates. The method by which propagation and transfer rates are accessible from one experiment is illustrated by simulations using the program package PREDICI.
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113) "Free-radical polymerization kinetics in extended ranges of temperature, pressure, and monomer conversion"
M. Buback, Macromol. Symp. 111, 229-242 (1996)
Free-radical polymerizations have been studied within extended ranges of pressure, temperature, and monomer conversion, including reactions in supercritical phase, where polymerization rate and polymer properties may be continuously tuned. The methods by which propagation (k(p)) and termination (k(t)) rate coefficients of homopolymerizations may be determined as a function of P and T, partly up to 3000 bar and 250 degrees C, are based on using pulse lasers for inducing polymerization (PLP) either in conjunction with size exclusion chromatography (SEC), for k(p) measurement, or in conjunction with online spectroscopic analysis of monomer conversion, for k(t)/k(p) measurement. With k(p) from PLP-SEC, the k(t)/k(p) data allow to deduce also individual rate coefficients for the termination step. k(p) primarily depends on temperature and pressure with the values being close to each other within a monomer family, such as the acrylic ester or the methacrylic ester family. Termination rate, which refers to a radical-radical process, depends on P, T, monomer conversion, solvent type, and on chain length. The major changes of k(t) with conversion are illustrated using a model that takes segmental diffusion, translational diffusion, and reaction diffusion into account.
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112) "High-Pressure Free-Radical Copolymerization of Ethene and 2-Ethylhexyl acrylate"
M. Buback, T. Dröge, A. van Herk, F.-O. Mähling, Macromol. Chem. Phys. 197, 4119-4134 (1996)
The free-radical copolymerization of ethene (E) and 2-ethylhexyl acrylate (EHA) is studied between 150 and 250 degrees C and at pressures from 1500 to 2500 bar. The reactions which were induced either thermally or laser-photochemically are carried out in two types of continuously operated devices and also within a batch reactor. The precision in the determination of reactivity ratios is considered in detail for the continuous experiments taking the error structure of the copolymer composition measurement into account. The reactivity ratios r(E) and r(EHA) at 220 degrees C and 2000 bar are found to be 0,050 +/- 0,002 and 3,9 +/- 0,9, respectively. From the ethene rich reaction mixtures r(E) is available with higher precision than is r(EHA). The E/EHA reactivity ratios are found to be very close to literature data for the E/butyl acrylate (BA) copolymerization. Fitting of the combined data set, for E/EHA and E/BA, yields for the activation energy and activation volume of the ethene reactivity ratio E(A)(r(E)) = (11,9(-1,4)(+1,5) kJ . mol(-1) and Delta V-not equal(r(E)) = -(8,2 +/- 3,5) cm(3) . mol(-1), respectively.