Radical polymerization of monomers with functional groups such as carboxylic acid and amide moi-eties yields materials of significant technical importance. The reactions are mostly carried out in aqueous phase, which provides the additional advantage of using a cheap and benign solvent. In addition to vary-ing monomer concentration, temperature and pressure, the kinetics and thus the polymer properties may be tuned by varying the degree of monomer ionization, by changing pH and ionic strength of the aqueous solution, and by addition of an organic cosolvent. These systems exhibit strong interactions via hydro-gen bonds resulting in large effects on rate coefficients, even for propagation, which for long have been considered as almost insensitive towards solvent environment. The determination of rate coefficients in aqueous solution largely assists the understanding of the impact of intermolecular interactions on poly-merization rate. Despite the enormous importance of polymers produced by radical polymerization in aqueous solution, the associated mechanism and the availability of accurate rate coefficients have been very limited. This situation has improved by applying pulsed-laser techniques, which enable the precise measurement of individual rate coefficients in aqueous solution as required for the simulation of radical polymerization processes. This review primarily addresses the two most important rate coefficients, i.e., those for propagation and termination, with the diffusion-controlled termination step depending on radical chain length. Both rate coefficients have been studied over a wide range of reaction conditions. The enormous improve-ment in data quality reached by using methods such as pulsed-laser polymerization (PLP) - size-exclusion chromatography (SEC) and single pulse (SP) - PLP - electron paramagnetic resonance (EPR) spectroscopy is illustrated. Outlined are results for homopolymerizations of non-ionized monomers, subdivided into monomers which may or may not undergo backbiting. This reaction adds considerable complexity, as backbiting results in the formation of midchain radicals with reactivity differing largely from the one of chain-end radicals. The kinetic investigations have been extended to partially and fully ionized monomers. Examples are given of how the rate coefficients from PLP experiments are used to simulate polymeriza-tion kinetics and polymer properties of continuously-initiated systems. The review demonstrates that the basic kinetic concepts for conventional radical polymerization in organic media also apply towards poly-merization of monomers in aqueous solution. (c) 2023 Elsevier B.V. All rights reserved.

The factors influencing the periodic structure in molar mass distributions (MMDs) generated in pulsed laser polymerization (PLP) experiments are investigated to extend the range of operating conditions under which radical polymerization propagation rate coefficients (k(p)) can be reliably estimated. Specifically, it is shown how k(p) may be determined well into conditions corresponding to the so-called low and high termination rate limits. A new parameter x is introduced to provide a convenient measure of when PLP pseudostationary conditions approach the low (x <= 0.2) and high (x >= 5.0) termination rate limits. In addition, a simple transformation is proposed to detect the PLP structure obscured by the background of the distribution under limiting experimental conditions, with simulations confirming that the methodology provides an estimate of k(p) with reasonable accuracy. The usefulness of the technique is then demonstrated through application to several experimental distributions. The influences of chain transfer and of chain-length-dependent kinetic parameters on the PLP structure are also systematically investigated via simulation, revealing that the principal limitation for detecting PLP structure using the methodology is size-exclusion chromatography (SEC) noise at the low and high termination rate limits.

Multiple methods for initiation and selecting catalyst concentration exist in atom transfer radical polymerization (ATRP). Among them, simultaneous reverse and normal initiation (SR&NI) ATRP and initiators for continuous activator regeneration (ICAR) ATRP are phenomenologically very similar. In both methods, thermal radical initiators are employed to reduce the catalyst in the higher oxidation state and generate Cu-I activator in Situ. SR&NI and ICAR ATRP generally differ in the amount of catalyst used and in the rate of catalyst reduction. Commonly, SR&NI ATRP requires high catalyst loadings and a quick initial reduction of Cu-II, while ICAR ATRP relies on slow and continuous reduction of smaller catalyst loadings. However, these criteria might not be sufficient to universally distinguish among both techniques. This article investigates both methods through kinetic simulations and establishes a borderline kinetic criterion. If the polymerization rate depends on the rate of decomposition of the radical initiator, the system follows ICAR ATRP kinetics, and if it depends on the ATRP equilibrium constant, it follows SR&NI ATRP. The transition from one to the other mechanism occurred continuously with an inflection point at a ratio of rate coefficients of radical initiator decomposition to propagation of about k(dc)/k(p) approximate to 10(-7) M under typical conditions. For faster initiator decomposition and slower propagation ATRP follows SR&NI ATRP, and for slower decomposition and faster propagation it obeys ICAR ATRP kinetics. The analysis to verify which mechanism is in operation is helpful for designing reaction conditions in order to obtain well-defined products.

The mole percentage of dead chains (T-mol %) was quantified in a normal Cu-mediated atom transfer radical polymerization (ATRP) of methyl acrylate with tris(2-pyridylmethyl)amine and tris[2-(dimethylamino)ethyl]amine as the ligands in acetonitrile. The value for T-mol % significantly exceeded the values predicted for conventional termination between two radicals. The main reason for the additional loss of chain-end functionality was identified as a Cu-I-induced catalytic radical termination (CRT). The mechanism proposed for this CRT involves the formation of highly active R-Cu-II/L and/or H-Cu-II/L intermediates which react with radicals to form dead chains and regenerate the Cu-I activator. The contribution of CRT to the loss of terminal functionality can be significantly decreased by reducing [Cu-I] to <1 mM, according to recently developed new ATRP processes.

High-pressure atom transfer radical polymerization (ATRP) of n-butyl acrylate (BA) is performed in acetonitrile (MeCN) with Cu(I) Br/TPMA [TPMA: tris(2-pyridylmethyl)-amine] as the catalyst up to 5 kbar. Increasing either pressure or temperature significantly enhances the rate of polymerization, while retaining control over the polymerization. The polymerizations under high pressure could be efficiently performed with very low levels of Cu catalyst in the absence of any reducing agents. For example, 100 ppm Cu is sufficient to catalyze the polymerization of BA with targeted degree of polymerization (DPT ) = 1000. The conversion reached 79% in 3.0 h at 80 °C providing PBA with Mn = 112 000, Mw /Mn = 1.12. Since the initial Cu(I) -to-initiator molar ratio is 0.05:1, the molar percentage of terminated chains should remain <5%. For DPT = 10 000 using only 50 ppm Cu catalyst, a polymer with molecular weight Mn = 612 000 (DP = 4800) was obtained at 67% conversion.

Atom transfer radical polymerization (ATRP) equilibrium constants (KATRP) were measured during polymerization of methyl acrylate (MA) with CuIBr/CuIIBr2 in either dimethyl sulfoxide (DMSO) or acetonitrile (MeCN) in the presence of either tris(2-pyridylmethyl)amine (TPMA) or tris[2-(dimethylamino)ethyl]amine (Me6TREN) as the ligand and with ethyl 2-bromopropionate as the initiator. The ln(KATRP) values changed linearly with the volume fraction of solvents in the reaction medium, allowing extrapolation of the values for KATRP to bulk conditions, which were 2 × 10-9 and 3 × 10-8 for TPMA and Me6TREN ligands at 25 °C, respectively. The temperature effect on KATRP values was studied in MA/MeCN = 1/1 (v/v) with TPMA as the ligand in the temperature range from 0 to 60 °C. The KATRP values increased with temperature providing ΔH = 36 kJ mol-1 in MeCN.

Atom transfer radical polymerization (ATRP) equilibrium constants (KATRP) were measured during polymerization of methyl acrylate (MA) with CuIBr/CuIIBr2 in either dimethyl sulfoxide (DMSO) or acetonitrile (MeCN) in the presence of either tris­(2-pyridylmethyl)­amine (TPMA) or tris­[2-(dimethylamino)­ethyl]­amine (Me6TREN) as the ligand and with ethyl 2-bromopropionate as the initiator. The ln­(KATRP) values changed linearly with the volume fraction of solvents in the reaction medium, allowing extrapolation of the values for KATRP to bulk conditions, which were 2 × 10–9 and 3 × 10–8 for TPMA and Me6TREN ligands at 25 °C, respectively. The temperature effect on KATRP values was studied in MA/MeCN = 1/1 (v/v) with TPMA as the ligand in the temperature range from 0 to 60 °C. The KATRP values increased with temperature providing ΔH = 36 kJ mol–1 in MeCN.

Polymerization of nonionized acrylic acid (AA) in aqueous solution has been studied via single pulse–pulsed laser polymerization in conjunction with electron paramagnetic resonance spectroscopy. Termination of two types of radicals, secondary chain-end radicals (SPRs) and midchain radicals (MCRs), as well as intramolecular chain transfer (backbiting) of SPRs have been studied between 5 and 40 °C at initial AA concentrations of 10 and 50 wt % in the presence of 15 wt % poly(AA). Predici modeling of the measured SPR and MCR concentration vs time traces after single-pulse initiation at t = 0 yields rate coefficients for backbiting, kbb, and for propagation from an MCR, kpt. These rate coefficients increase toward lower AA-in-water concentration. Estimates for termination of two SPRs, kts,s, and of an SPR and an MCR, kts,t have been obtained by assuming the reported composite-model behavior for acrylates to also hold for acrylic acid polymerization.

The current state of scientific analysis of high-pressure radical polymerization kinetics is presented. The techniques for measuring rate coefficients up to 3 kbar are illustrated and selected examples of initiation, propagation, and termination rate coefficients obtained as a function of pressure are given. High-pressure ethene homo- and copolymerization are specifically addressed. Further, examples of reversible deactivated polymerization up to high pressure are illustrated. The applicability of supercritical (sc) CO2 as a reaction medium for homogeneous-phase polymerization of conventional monomers and of fluoroolefins is demonstrated, accompanied by information on the influence of compressed CO2 on polymerization kinetics.

Free-radical batch polymerization (FRP) of N-vinyl pyrrolidone (NVP) and N-vinyl formamide (NVF) monomers in aqueous solution as well as NVP polymerization in organic (n-butanol) solution has been studied. The differences found in rate of monomer conversion with monomer and solvent choice correlates well with the differences in values of the propagation rate coefficients (k(p)) and their variation with monomer concentration measured in independent pulsed-laser polymerization studies, a result demonstrating that a generalized understanding of water-soluble vinyl monomers can be obtained once their k(p) differences have been accounted for. A reasonable representation of polymer molecular mass averages and the complete molecular mass distributions for the three systems was obtained by assuming that the rate coefficient for transfer to monomer, polymer, and organic solvent also vary as a function of monomer concentration.

The termination kinetics of methacrylic acid (MAA) radical polymerization in aqueous solution containing 10 wt % MAA was measured via the single pulse-pulsed laser polymerization-electron paramagnetic resonance (SP-PLP-EPR) method from 0 to 50 degrees C. The termination rate coefficient, k(u) was determined as a function of radical chain length. Addition of high-molecular-mass poly(methacrylic acid), which mimicks monomer conversion, leads to a reduction of k(u), which is however far below the associated decrease in fluidity, eta(-1).

Pulsed laser polymerization (PLP) with subsequent analysis of molecular mass distribution (MMD) is used to determine the rate coefficient of chain transfer to an agent A, k(trA), by varying pulse repetition rate such that the contributions of PLP-induced and chain-transfer-induced peaks to the MMD change to a significant extent. It is shown by simulation that the relative heights of these peaks may be used to estimate k(trA). The method is applied to evaluation of the rate coefficient of chain transfer to dodecylmercaptan with butyl methacrylate polymerizations at -11, 0, 20 and 40 degrees C. The Arrhenius parameters for this coefficient are determined to be: A(k(trA)) (2.2 +/- 0.6) x 10 6 L.mol(-1).s(-1) and E-a(k(trA)) (22.1 +/- 0.7) kJ.mol(-1).

Termination kinetics of 1-vinylpyrrolidin-2-one radical polymerization in aqueous solution has been studied at 40 degrees C between 20 and 100 wt.-% VP. The /k(p) values from laser single-pulse experiments with microsecond time-resolved NIR detection of monomer conversion, in conjunction with k(p) from literature, yield chain-length-averaged termination rate coefficients, . Because of better signal-to-noise quality, experiments were carried out at 2 000 bar, but also at 1 500, 1 000, and 500 bar, thus allowing for estimates of at ambient pressure. The dependence of on monomer conversion indicates initial control by segmental diffusion followed by translational diffusion and finally reaction diffusion control. To assist the kinetic studies, viscosities of VP-water mixtures at ambient pressure have been determined.

Aqueous-phase free-radical batch and semibatch polymerizations of NVP have been carried out with varying initial monomer and initiator concentrations. The rate of conversion was observed to increase as the initial monomer concentration was lowered, a result explained by the dependence of the propagation rate coefficient, k(p), on monomer concentration. A kinetic model with termination and conversion-dependent k(p), rate coefficients taken from independent studies provides a good description of the conversion profiles. A reasonable representation of polymer molecular-weight averages and the complete MWD was obtained by assuming that the rate coefficient for transfer to monomer also varies as a function of monomer concentration and that a small amount of chain transfer to polymer occurs.