The nature of the chain-length dependence of the propagation rate coefficient and its effect on the kinetics of free-radical polymerization. 1. Small-molecule studies

Controversies on the mechanism and kinetics of emulsion polymerization: An updated critical review

Conventional emulsion polymerization (EP) is a process via free radicals whose driving force for its development has been its versatility to generate polymer colloids with ad hoc characteristics for a wide variety of applications, as well as its friendly character to the environment since the continuous medium is water. Although through decades of research, considerable progress has been made in understanding its mechanism and kinetics, some aspects are still not entirely clear. Furthermore, new ideas and experimental results have appeared in the literature that challenge the accepted knowledge about some aspects of EP. This work is a personal vision and an updated critical review on those controversial aspects whose precedent is the review with the same approach published by the author and collaborators almost 20 years ago (J. Macromol. Sci. Part C Polym. Rev., 2004;44:207-229). This review covers advances, aspects that are open to discussion or need improvement regarding what happens in the aqueous phase and in the interface (initiator decomposition, entry and exit of radicals, monomer transport) as well as in the polymer particles (free-radical propagation and termination, swelling, average number of radicals per particle). Special attention is paid to particle formation (nucleation) and its interrelation with colloidal stability and the evolution of the particle size distribution (PSD), which is one of the most fundamental and controversial issues of EP. The Smoluchowski collision rate coefficient to describe diffusion-controlled processes has practically become a paradigm despite the fact that there is evidence that questions its applicability. For this reason, this review also emphasizes this point and the alternatives that have been proposed to mathematically describe the diffusive stages of particle coagulation, the entry of radicals, and the termination reaction. Challenges in improving our understanding of the mechanism and kinetics of emulsion polymerization are pointed out.

Polymer Colloids: Current Challenges, Emerging Applications, and New Developments

Polymer colloids are complex materials that have the potential to be used in a vast array of applications. One of the main reasons for their continued growth in commercial use is the water-based emulsion polymerization process through which they are generally synthesized. This technique is not only highly efficient from an industrial point of view but also extremely versatile and permits the large-scale production of colloidal particles with controllable properties. In this perspective, we seek to highlight the central challenges in the synthesis and use of polymer colloids, with respect to both existing and emerging applications. We first address the challenges in the current production and application of polymer colloids, with a particular focus on the transition toward sustainable feedstocks and reduced environmental impact in their primary commercial applications. Later, we highlight the features that allow novel polymer colloids to be designed and applied in emerging application areas. Finally, we present recent approaches that have used the unique colloidal nature in unconventional processing techniques.

Free-Radical Propagation Rate Coefficients of Diethyl Itaconate and Di-n-Propyl Itaconate Obtained via PLP–SEC

The propagation step is one of the key reactions in radical polymerization and knowledge about its kinetics is often vital for understanding and designing polymerization processes leading to new materials or optimizing technical processes. Arrhenius expressions for the propagation step in free-radical polymerization of diethyl itaconate (DEI) as well as di-n-propyl itaconate (DnPI) in bulk, for which propagation kinetics was yet unexplored, were thus determined via pulsed-laser polymerization in conjunction with size-exclusion chromatography (PLP-SEC) experiments in the temperature range of 20 to 70 °C. For DEI, the experimental data was complemented by quantum chemical calculation. The obtained Arrhenius parameters are A = 1.1 L·mol–1·s–1 and Ea = 17.5 kJ·mol−1 for DEI and A = 1.0 L·mol–1·s–1 and Ea = 17.5 kJ·mol−1 for DnPI.

Evolution of Molar Mass Distributions Using a Method of Partial Moments: Initiation of RAFT Polymerization

We describe a method of partial moments devised for accurate simulation of the time/conversion evolution of polymer composition and molar mass. Expressions were derived that enable rigorous evaluation of the complete molar mass and composition distribution for shorter chain lengths (e.g., degree of polymerization, Xn = N < 200 units) while longer chains (Xn ≥ 200 units) are not neglected, rather they are explicitly considered in terms of partial moments of the molar mass distribution, μxN(P)=∑n=N+1∞nx[Pn] (where P is a polymeric species and n is its’ chain length). The methodology provides the exact molar mass distribution for chains Xn < N, allows accurate calculation of the overall molar mass averages, the molar mass dispersity and standard deviations of the distributions, provides closure to what would otherwise be an infinite series of differential equations, and reduces the stiffness of the system. The method also allows for the inclusion of the chain length dependence of the rate coefficients associated with the various reaction steps (in particular, termination and propagation) and the various side reactions that may complicate initiation or initialization. The method is particularly suited for the detailed analysis of the low molar mass portion of molar mass distributions of polymers formed by radical polymerization with reversible addition-fragmentation chain transfer (RAFT) and is relevant to designing the RAFT-synthesis of sequence-defined polymers. In this paper, we successfully apply the method to compare the behavior of thermally initiated (with an added dialkyldiazene initiator) and photo-initiated (with a RAFT agent as a direct photo-iniferter) RAFT-single-unit monomer insertion (RAFT-SUMI) and oligomerization of N,N-dimethylacrylamide (DMAm).

Chain-Length Dependence of Propagation Rate Coefficient for Methyl Acrylate Polymerization at 25 °C Investigated by PLP-SEC Method

When applying the pulsed laser polymerization–size exclusion chromatography (PLP-SEC) method to determine the propagation rate coefficient (kp) for bulk methyl acrylate at 25 °C, the characteristic ratio L1/L2 of chain...

Egalitarian Kinetic Models: Concepts and Results

In this paper, two main ideas of chemical kinetics are distinguished, i.e., a hierarchy and commensuration. A new class of chemical kinetic models is proposed and defined, i.e., egalitarian kinetic models (EKM). Contrary to hierarchical kinetic models (HKM), for the models of the EKM class, all kinetic coefficients are equal. Analysis of EKM models for some complex chemical reactions is performed for sequences of irreversible reactions. Analytic expressions for acyclic and cyclic mechanisms of egalitarian kinetics are obtained. Perspectives on the application of egalitarian models for reversible reactions are discussed. All analytical results are illustrated by examples.

Impact of side reactions on molar mass distribution, unsaturation level and branching density in solution free radical polymerization of n-butyl acrylate under well-defined lab-scale reactor conditions

The paper describes the influence of side reactions in isothermal solution free-radical polymerization of n-butyl acrylate accounting for chain-length dependent diffusional limitations on termination.

Progress in Reaction Mechanisms and Reactor Technologies for Thermochemical Recycling of Poly(methyl methacrylate)

Chemical or feedstock recycling of poly(methyl methacrylate) (PMMA) by thermal degradation is an important societal challenge to enable polymer circularity. The annual PMMA world production capacity is over 2.4 × 10⁶ tons, but currently only 3.0 × 10⁴ tons are collected and recycled in Europe each year. Despite the rather simple chemical structure of MMA, a debate still exists on the possible PMMA degradation mechanisms and only basic batch and continuous reactor technologies have been developed, without significant knowledge of the decomposition chemistry or the multiphase nature of the reaction mixture. It is demonstrated in this review that it is essential to link PMMA thermochemical recycling with the PMMA synthesis as certain structural defects from the synthesis step are affecting the nature and relevance of the subsequent degradation reaction mechanisms. Here, random fission plays a key role, specifically for PMMA made by anionic polymerization. It is further highlighted that kinetic modeling tools are useful to further unravel the dominant PMMA degradation mechanisms. A novel distinction is made between global conversion or average chain length models, on the one hand, and elementary reaction step-based models on the other hand. It is put forward that only by the dedicated development of the latter models, the temporal evolution of degradation product spectra under specific chemical recycling conditions will become possible, making reactor design no longer an art but a science.

From n-butyl acrylate Arrhenius parameters for backbiting and tertiary propagation to β-scission via stepwise pulsed laser polymerization

A stepwise method to estimate the Arrhenius parameters for backbiting, tertiary propagation, and β-scission in acrylate radical polymerization.

Coupled stochastic simulation of the chain length and particle size distribution in miniemulsion radical copolymerization of styrene and N-vinylcaprolactam

Kinetic Monte Carlo modeling is applied for the coupled simulation of the chain length and particle size distribution in isothermal batch miniemulsion copolymerization of styrene and N-vinylcaprolactam.

Access to the β-scission rate coefficient in acrylate radical polymerization by careful scanning of pulse laser frequencies at elevated temperature

A novel method to estimate the β-scission rate coefficient in acrylate radical polymerization is presented.

Ab initio based kinetic Monte Carlo analysis to unravel the propagation kinetics in vinyl acetate pulsed laser polymerization

The radical propagation kinetics of vinyl acetate in pulsed laser polymerization (PLP) is studied by combining ab initio calculated rate coefficients with kinetic Monte Carlo modeling of PLP spectra.

Efficiency assessment of single unit monomer insertion reactions for monomer sequence control: kinetic simulations and experimental observations

On-line microreactor/ESI-MS experiments and kinetic simulations on single unit monomer insertions are combined to assess the efficiency of the SUMI process.