Michael Buback

Rate coefficients of free-radical polymerization deduced from pulsed laser experiments

Abstract Pulsed laser techniques have enormously improved the quality by which rate coefficients of individual steps in free-radical polymerization may be measured. Pulsed laser initiated polymerization (PLP) in conjunction with size-exclusion chromatography (SEC) yields the propagation rate coefficient, kp. The PLP-SEC-technique has been applied to a wide variety of homopolymerizations and copolymerizations, either in bulk or in solution. In addition to reporting kinetic data, experimental details of PLP, of SEC, and of the limitations associated with the accurate determination of the MWD are discussed. The single pulse (SP)-PLP method, which combines PLP with time-resolved NIR spectroscopy, allows for a very detailed insight into the termination rate coefficient, kt, for homo- and copolymerizations. kt data are reported as a function of temperature, pressure, monomer conversion, solvent concentration, and partly also of chain length. This review considers literature up to December 2000.

Pressure dependence of propagation rate coefficients in free-radical homopolymerizations of methyl acrylate and dodecyl acrylate

Propagation rate coefficients (k p ) for the homopolymerization of methyl acrylate (MA) and dodecyl acrylate (DA) were determined by the pulsed laser polymerization (PLP)/size-exclusion chromatography (SEC) technique at pressures between 100 and 2000 bar. The activation volumes for the propagation step, ΔV#(k p ), of MA and DA are identical within experimental accuracy, but are clearly different from the previously reported ΔV#(k p ) of methyl and dodecyl methacrylate. For both the alkyl acrylates and methacrylates a pronounced family-type behaviour of k p is seen, with k p being slightly enhanced toward larger ester size.

Free-radical propagation rate coefficients for cyclohexyl methacrylate, glycidyl methacrylate and 2-hydroxyethyl methacrylate homopolymerizations

Propagation rate coefficients (k p ) for the homopolymerization of cyclohexyl methacrylate (CHMA), glycidyl methacrylate (GMA) and 2-hydroxyethyl methacrylate (HEMA) were determined by the pulsed laser polymerization (PLP)/size-exclusion chromatography (SEC) technique. Temperature was varied between -10 and 90°C and pressure up to 2500 bar. k p values for CHMA and GMA agree within the experimental accuracy of ± 20 per cent. They are by about a factor of 2.5 below k p of HEMA. The activation energies and activation volumes of k p of methacrylates studied so far, including linear and branched alkyl methacrylates, are close to each other: E A (k p ) = (22 ± 2) kJ.mol -1 and ΔV ( k p ) = - (16 ± 2) cm 3 . mol -1 , respectively, indicating a pronounced family-type behavior.

EPR Analysis of n-Butyl Acrylate Radical Polymerization.

Via electron paramagnetic resonance (EPR) spectroscopy, concentrations of secondary propagating radicals (SPRs) and tertiary mid-chain radicals (MCRs) in n-butyl acrylate solution polymerization were measured. The EPR spectrum is dominated by the 4-line spectrum of SPRs at -50 °C and by the 7-line spectrum of MCRs at +70 °C. At intermediate temperatures, a third spectral component is seen, which is assigned to an MCR species with restricted rotational mobility. The MCR components are produced by 1,5-hydrogen shift (backbiting) of SPRs. The measured ratio of MCRs to SPRs allows for estimating the rate coefficient k pt for monomer addition to a mid-chain radical. For 70 °C, k pt is obtained to be 65.5 L · mol(-1)  · s(-1) .

Critically evaluated rate coefficients for free-radical polymerization. Part 6: Propagation rate coefficient of methacrylic acid in aqueous solution

Critically evaluated propagation rate coefficients, kp, for free-radical polymerization of methacrylic acid, MAA, in aqueous solution are presented. The underlying kp values are from two independent sources, which both used the IUPAC-recommended technique of pulsed-laser-initiated polymerization (PLP) in conjunction with molar mass distribution (MMD) analysis of the resulting polymer by size-exclusion chromatography (SEC). Different methods of measuring the MMD of the poly(MAA) samples have, however, been used: (i) direct analysis via aqueous-phase SEC and (ii) standard SEC with tetrahydrofuran as the eluent carried out on poly(methyl methacrylate) samples obtained by methylation of the poly(MAA) samples from PLP. Benchmark kp values for aqueous solutions containing 15 mass % MAA are presented for temperatures between 18 and 89 °C. The Arrhenius pre-exponential and activation energy of kp at 15 mass % MAA are 1.54 × 106 L mol-1 s-1 and 15.0 kJ mol-1, respectively. Also reported are critically evaluated kp values for 25 °C over the entire MAA concentration range from dilute aqueous solution to bulk polymerization.

Critically evaluated rate coefficients in radical polymerization – 7. Secondary-radical propagation rate coefficients for methyl acrylate in the bulk

Propagation rate coefficient (kp) data for radical polymerization of methyl acrylate (MA) in the bulk are critically evaluated and a benchmark dataset is put forward by a task-group of the IUPAC Subcommittee on Modeling of Polymerization Kinetics and Processes. This dataset comprises previously published results from three laboratories as well as new data from a fourth laboratory. Not only do all these values of kp fulfill the recommended consistency checks for reliability, they are also all in excellent agreement with each other. Data have been obtained employing the technique of pulsed-laser polymerization (PLP) coupled with molar-mass determination by size-exclusion chromatography (SEC), where PLP has been carried out at pulse-repetition rates of up to 500 Hz, enabling reliable kp to be obtained through to 60 °C. The best-fit – and therefore recommended – Arrhenius parameters are activation energy EA = 17.3 kJ mol−1 and pre-exponential (frequency) factor A = 1.41 × 107 L mol−1 s−1. These hold for secondary-radical propagation of MA, and may be used to calculate effective propagation rate coefficients for MA in situations where there is a significant population of mid-chain radicals resulting from backbiting, as will be the case at technically relevant temperatures. The benchmark dataset reveals that kp values for MA obtained using PLP in conjunction with MALDI-ToF mass spectrometry are accurate. They also confirm, through comparison with previously obtained benchmark kp values for n-butyl acrylate, methyl methacrylate and n-butyl methacrylate, that there seems to be identical family-type behavior in n-alkyl acrylates as in n-alkyl methacrylates. Specifically, kp for the n-butyl member of each family is about 20% higher than for the corresponding methyl member, an effect that appears to be entropic in origin. Furthermore, each family is characterized by an approximately constant EA, where the value is 5 kJ mol−1 lower for acrylates.

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

Full Paper: The single pulse (SP)-pulsed-laser polymerization (PLP) technique has been applied to measure κ t /κ 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 2650 bar. κ t /κ 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 κ p and high κ t monomer. Thus between 160 to 300 SP signals were co-added to yield a concentration vs. time trace of sufficient quality for deducing κ t /κ p with an accuracy of better than ±20 per cent. With κ p bein known from PLP-SEC experiments, chain-length averaged κ t values are immediately obtained from κ t /κ p . At given pressure and temperature, κ t is independent of the degree of overall monomer conversion, which, within the present study, has been as high as 20 %. The κ 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-phenylacetophenone). 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 κ 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 κ t values with recent literature data from chemically and photochemically induced styrene polymerizations.

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 CO 2 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, kp, were determined. Depending on the monomer concentration. k p , value; are up to 40% smaller than corresponding data for bulk polymerizations of MMA and BA.

SP–PLP–EPR Investigations into the Chain-Length-Dependent Termination of Methyl Methacrylate Bulk Polymerization

Termination kinetics of methyl methacrylate (MMA) bulk polymerization has been studied via the single pulsed laser polymerization-electron paramagnetic resonance method. MMA-d(8) has been investigated to enhance the signal-to-noise quality of microsecond time-resolved measurement of radical concentration. Chain-length-dependent termination rate coefficients of radicals of identical size, k ti,i, are reported for 5-70 °C and up to i = 100. k ti,i decreases according to the power-law expression

Characterization of branched ultrahigh molar mass polymers by asymmetrical flow field-flow fractionation and size exclusion chromatography

k_{\rm t}^{i,i} = k_{\rm t}^{{\rm 1,1}} \cdot i^{ - \alpha }