This report provides a summary of recommended or suggested values of the rate coefficients for thermal decomposition (k d) and of the efficiencies for radical generation (f g) and initiation of polymerization (f i) for some commercially available dialkyldiazene (also known as azo-compound) initiators. These initiators are one of the most important classes of initiators used in both conventional radical polymerization and reversible-deactivation radical polymerization (RDRP). Although values of k d can be cited with confidence for many initiators, it must also be stated that for most initiators there is insufficient quality data in the open literature to allow a rigorous statistical analysis. The situation is complicated by some initiators existing as a mixture of diastereomers and decomposition rates being subject to small, yet experimentally significant, solvent dependence. Efficiencies for radical generation (f g) are available for some initiators. Efficiencies for initiation of polymerization (f i) are less common and have been demonstrated to be strongly dependent on the initiator, the complexity of the mechanisms for radical generation and for initiation of polymerization, the reaction medium, the monomers being polymerized and their concentration as determined by the amount of solvent, and the conversion of monomer to polymer. Consequently, only general recommendations for initiator efficiency are possible at this stage.

Termination kinetics of radical polymerization of N-vinyl formamide (NVF) in aqueous solution has been measured via SP-PLP-NIR, i.e., single pulse (SP) initiation of pulsed laser polymerization (PLP) in conjunction with microsecond time-resolved near-infrared (NIR) detection of monomer concentration. Experiments are performed at initial NVF weight fractions from 0.20 up to bulk NVF, at monomer conversions up to 40%, and at temperatures from 40 to 70 degrees C as well as pressures from 500 to 2500 bar. Applying high pressure improves signal-to-noise quality. Data obtained upon pressure variation allow for extrapolation toward ambient pressure. The primary quantity from SP-PLP-NIR is k(p)/, i.e., the ratio of propagation rate coefficient, k(p), to apparent chain-length-averaged termination rate coefficient, . With k(p) being available from literature, k(p)/ yields . This quantity is relevant for modeling polymerization rate and polymer properties. Termination in the initial polymerization period turns out to be controlled by segmental diffusion and, at higher degrees of monomer conversion up to 40%, by translational diffusion.

The pulsed laser polymerization-size-exclusion chromatography (PLP-SEC) technique has become the International Union of Pure and Applied Chemistry (IUPAC)-recommended method for the accurate determination of propagation rate coefficients, k(p), for radical polymerization. The fundamental insight provided by Heuts, Gilbert, and Radom into the relevance of transition state dynamics allows for an adequate understanding of the dependence of k(p) on radical chain length, solvent environment, and monomer conversion. The associated entropic effects are particularly pronounced in aqueous solution, but are also found in less polar media and with non-polar monomers. Moreover, these arguments are applicable towards the interpretation of copolymerization reactivity ratios. The addition of sodium hydroxide to aqueous solutions of (meth)acrylic acid yields partially and even fully ionized systems with clearly reduced k(p), which indicates the action of repulsive forces between the equally charged monomer and radical species. Increasing the concentration of monomer as well as the addition of salts may reverse this effect and increase k(p) due to counterion activity.

Studies into the termination step of radical polymerization have mostly been carried out via chain-length-averaged properties. With the advent of pulsed laser polymerization (PLP) induced by single pulses (SPs), the analysis of termination rate coefficients k(t) has significantly improved. In conjunction with highly time-resolved near-infrared (NIR) detection of monomer consumption, SP-PLP-NIR allows for the measurement of chain-length-averaged termination rate coefficients as a function of temperature, pressure and degree of monomer conversion. With the addition of a suitable reversible addition-fragmentation chain-transfer agent, this technique even allows for the determination of chain-length-dependent termination rate coefficients k(t)(i,i), where i refers to the chain length of two terminating radicals of identical size. The decay of k(t)(i,i) with chain length is more pronounced at small radical size below a crossover chain length i(c). At larger chain lengths, k(t)(i,i) decreases to a weaker extent. Thus, the so-called composite model is perfectly suited for representing k(t)(i,i) behaviour across the entire chain-length range. A further clear improvement of k(t)(i,i) determination is provided by the SP-PLP-EPR method, which directly measures the type and concentration of radicals via microsecond time-resolved electron paramagnetic resonance (EPR) spectroscopy. This EPR-assisted technique is particularly valuable for polymerizations where secondary radicals undergo backbiting and thus two types of radicals, secondary and tertiary, are simultaneously present, as is the case with acrylic monomers. (c) 2023 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Industrial Chemistry.

Pulsed-laser-assisted methods have enormously improved the understanding of kinetics and mechanism of the basic steps of radical polymerisation, i.e. of initiation, propagation, and termination. The decarboxylation kinetics of primary radicals from peroxyester dissociation was studied on a picosecond time scale by UV-pump - VIS/NIR/IR probe experiments. The application of sequences of laser pulses and the subsequent analysis of the structure on the molar mass distribution of so-obtained polymer allows for the reliable measurement of propagation rate coefficients as a function of temperature, pressure and solvent environment. The SP-PLP-EPR technique has emerged as the leading method for addressing the complexity of radical termination. Unrivalled insight into the kinetics is provided by the highly time-resolved measurement of reacting species, i.e. of radicals at sub-micromolar concentrations via electron paramagnetic resonance spectroscopy. The impact of chain length on termination rate may be accurately determined within a wide range of monomer conversion and, via characteristic EPR bands, even for several radical species, which occur whenever backbiting takes place. Application of laser-assisted techniques extends chemical kinetics to very large species and to reactions under heavy diffusion control. The novel techniques demonstrate that polymerisation kinetics obeys the fundamental principles established for small species.

The effect of retardation exhibited in dithiobenzoate-mediated polymerizations is perhaps the most controversial kinetic phenomenon discussed in polymer chemistry in the last 20 years. However, the Missing Step reaction (MSR), which involves the termination between a propagating radical and a 3-arm star product, has emerged as a hypothesis to explain the full kinetic picture for acrylate polymerizations. Here, a kinetic model, the most detailed of its kind, is developed to describe and study the MSR. The model is based on the method of moments and includes RAFT pre-equilibrium and equilibrium, backbiting, cross-termination, MSR and chain-length dependent radical termination. After validation through comparison with experimental data available in the literature, the model is used to demonstrate that the kinetic behavior of the propagating and intermediates radicals can be described by the quasi-steady state assumption and that backbiting does not significantly influence predicted conversion profiles and product molecular weights. It is also shown that the induction period observed experimentally can be represented by either slow initiation/reinitiation of primary radicals or by assuming that radical intermediates formed by addition of primary radicals to the RAFT agent have increased stability; however, the associated rate constants for these reactions had to be adjusted from literature values. A comparison between the MSR and Intermediate Radical Termination (IRT) representations RAFT-mediated acrylate polymerization identifies chain end-group functionality (EGF) as a means to further discriminate between these theories.

Equilibrium constants, K-eq, for the reversible addition-fragmentation chain transfer (RAFT) polymerization of butyl acrylate mediated by trithiocarbonate and dithiobenzoate RAFT agents have been estimated by quantum chemical calculations, namely by a combination of density functional structure optimizations, corrections for solvent effects plus the double harmonic approximation as well as high-level ab-initio local correlation methods. Individual contributions to K-eq are analyzed. The results are compared to experimental K-eq measured by microsecond time-resolved electron paramagnetic resonance (EPR) spectroscopy. Dithiobenzoate (DTB) RAFT agents are of particular interest, as earlier quantum chemical calculations resulted in K-eq values differing by several orders of magnitude from the numbers deduced via EPR measurements. This mismatch between theory and experiment is overcome by new quantum chemical estimates. The major factors behind the improved agreement are the application of dispersion-corrected density functional theory (DFT) functionals for the equilibrium structures and full-system coupled cluster calculations. The so-obtained ab initio K-eq data provide clear evidence for rate retardation with DTB-mediated acrylate polymerizations being due to cross-termination rather than to slow fragmentation of the RAFT intermediate radical.

The precise knowledge of rate coefficients is of key importance for the understanding and the application of radical polymerization processes. Propagation rate coefficients, k(p), of n-pentyl methacrylate (PnMA) radical polymerization are measured in bulk and partly in toluene solution over an extended temperature range via pulsed laser polymerization (PLP) in conjunction with size-exclusion chromatography (PLP-SEC). Rate coefficients, k(t)(i)(,)(i), for termination of two radicals of chain length i are determined as a function of chain length by single-pulse PLP in conjunction with electron paramagnetic resonance spectroscopy (SP-PLP-EPR). The as-obtained data that allow for modeling PnMA polymerization kinetics and product properties at moderate degrees of monomer conversion are compared with reported data for several other alkyl methacrylates. A distinct family behavior of this group of monomers is seen.

Methyl acrylate (MA) and dodecyl acrylate (DA) homopolymerizations (1.5 M in toluene) were investigated via single pulse-pulsed laser polymerization in conjunction with detection of the type and of the concentration of radicals by electron paramagnetic resonance spectroscopy (SP-PLP-EPR). The evolution of secondary propagating (SPR) and midchain (MCR) radicals, after instantaneous laser-induced production of an intense burst of primary radicals, was measured with a time resolution of microseconds from 0 to 60 degrees C. With the kinetics of chain-length-dependent termination and of propagation for SPRs being already known from experiments at subzero temperature, the rate coefficients for the formation of MCRs from SPRs by backbiting, k(bb), of propagation from MCRs, k(p)(t), and of cross-termination between an SPR and an MCR, K-t(st), were deduced. The present paper aims at elucidating the impact of the size of the ester side chain on k(bb), k(p)(t), and k(t)(st). In passing from MA to DA (in 1.5 M solution of toluene), the rate coefficients are lowered by factors of about 1.5 for k(bb), 3.7 for k(p)(t), and 8 for k(t)(st), depending on polymerization temperature. Together with the earlier results for SPRs, the reported data provide a comprehensive set of rate coefficients for the simulation of MA and DA homopolymerizations in solution. The data may be used for estimating, by interpolation, the rate coefficients of acrylates with intermediate sizes of the alkyl side chain.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Single pulse-pulsed laser polymerization in conjunction with the highly time-resolved detection of radical concentration by electron paramagnetic resonance (SP-PLP-EPR) was applied for the first time toward measuring termination kinetics up to high degrees of monomer conversion with styrene bulk polymerization up to 80% conversion taken as an example. Monomer conversion was mimicked by the addition of polystyrene. The composite model turns out to provide an adequate representation of the termination kinetics. A constant value of the power-law exponent for small chain radicals, alpha(s), holds up to about 50% conversion, in which region the model parameter k(t)(1,1), the rate coefficient for termination of two radicals of size unity, exhibits a minor decay. Above 50% polymer content, both alpha(s) and k(t)(1,1) decrease significantly. Above 80% polymer, the chain-length dependence almost disappears, and termination runs under reaction-diffusion control. As the region of large radicals is not accessible at higher monomer conversion, the knowledge from the composite model has been used to represent the chain-length dependence of large radicals. To check for the quality of the measured and predicted rate parameters, PREDICI estimates on the basis of this data have been compared to experimental results for chemically initiated bulk styrene polymerizations up to almost full conversion. The satisfactory comparison suggests that the data obtained by the SP-PLP-EPR studies are suitable for kinetic simulations of styrene bulk polymerization over the entire conversion range.

The SP-PLP EPR (single pulse pulsed laser polymerization electron paramagnetic resonance) method has been used to measure the rate coefficients of termination, intramolecular transfer, and propagation for the radical polymerization of 20 wt % trimethylaminoethyl acrylate chloride (TMAEA) in the temperature range 0-90 degrees C. The high complexity of this acrylate system is due to water being the solvent, to the monomer being a strong electrolyte, and to both secondary chain-end radicals and tertiary midchain radicals being simultaneously present. The termination kinetics, which was analyzed by a chain-length-dependent scheme, largely differs from the situation met with nonionized radicals. The reliability of the rate coefficients obtained from the SP-PLP-EPR experiments has been demonstrated by the almost perfect agreement of TMAEA conversion vs time data from simulation, on the basis of these coefficients, and from the chemically initiated TMAEA polymerization experiment at 70 degrees C.

Propagation rate coefficient values, k p , reported by several groups for radical polymerization of bulk vinyl acetate are critically evaluated. All data are obtained by the combination of pulsed‐laser polymerization and subsequent polymer analysis by size exclusion chromatography, as recommended by the IUPAC Working Party on Modeling of Polymerization Kinetics and Processes. Although a small (≈15%) increase in k p is observed as laser pulse repetition rate is increased from low (25–100 Hz) to high (300–500 Hz) values, all of the data fulfill the required consistency criteria and thus are combined into a benchmark set covering the temperature range of 5–70 °C. The data are fitted well by an Arrhenius relation resulting in a pre‐exponential factor of 1.35 × 10 7 L mol −1 s −1 and an activation energy of 20.4 kJ mol −1 , with 95% confidence ellipsoids for the parameters also presented. image

Pulsed-laser polymerization (PLP) in conjunction with polymer analysis by size-exclusion chromatography (SEC) is the IUPAC-recommended method for measuring propagation rate coefficients, kp, of radical polymerization. Pulse repetition rate, vrep, and photoinitiator concentration need to be adjusted to the termination rate of the monomer, preferably via the quantity β, the fraction of radicals terminating prior to applying the successive laser pulse. By PREDICI simulation, the prerequisites for successful PLP-SEC work are demonstrated. Slow termination, as with fully ionized monomers, is specifically addressed. Both very high and very low vrep are not suitable for accurate kp measurements.

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.

A speciation analysis for Fe-mesohemin-(MPEG(500))(2)-mediated reversible-deactivation radical polymerization (RDRP) in aqueous solution was carried out by a combination of visible (vis) and Fe-57 Mossbauer spectroscopy. The results were used within kinetic studies of ATRP and OMRP reactions via highly time-resolved EPR spectroscopy. ATRP control was effective with the rate coefficient for deactivation clearly exceeding the one for formation of organometallic species. Deactivation rate coefficients increase by more than 1 order of magnitude in passing from polymerization in 30 to 90 wt % H2O. Media with water contents of and above 70 wt % are well suited for controlled ATRP. The Fe-mesohemin catalyst provides an exceptionally high ATRP equilibrium constant even at ambient temperature, which approaches the one of highly active Cu catalysts.

Acrylamide (AAm) is an important water-soluble monomer. Pulsed-laser polymerization in conjunction with size-exclusion chromatography (PLP-SEC) has been employed to measure the propagation rate coefficient, k(p), in aqueous solution at monomer concentrations from 1 to 30 wt % and temperatures between 5 and 70 degrees C. Laser pulse repetition rate has been extensively varied, from 25 up to 500 Hz, to check for consistency of the measured k(p) values, which increase toward larger water content, e.g., by 50% upon lowering AAm concentration from 30 to 5 wt %. Variation of pH, up to pH 11, and addition of up to 1 mol L-1 NaCl to the polymerizing system do not significantly affect k(p). The activation energy, E-A(k(p)), of about 18 kJ mol(-1) stays more or less constant within the monomer concentration range under investigation, which indicates that the changes of k(p) are primarily of entropic origin. The pre-exponential factor, A(k(p)), increases from 5.7 X 10(7) to 7.7 X 10(7) L mol(-1) s(-1) upon lowering the AAm concentration from 30 to 10 wt %. These high A(k(p)) numbers demonstrate the fluidizing action of water,on the transition state toward internal rotation with AAm being lower than with acrylates.