A Critical Assessment of the Kinetics and Mechanism of Initiation of Radical Polymerization with Commercially Available Dialkyldiazene Initiators (2023)

Table of Contents

Progress in Polymer Science Abstract Graphical abstract Introduction Section snippets Mechanism of Thermal Decomposition of Dialkyldiazenes Kinetics of Thermal Decomposition of Dialkyldiazenes (kd Values) Initiator Efficiency (f) Methods for Evaluation of kd and f Recommended Values for Decomposition Rate Coefficients (kd) Recommended Values for Initiator Efficiency (f) Initiators Dialkyldiazene Initiators in Reversible-Deactivation Radical Polymerization (RDRP) Conventional Radical Polymerization in Heterogeneous Media Polymerization in Supercritical Carbon Dioxide Dialkyldiazenes as Photoinitiators Initiation with Dialkyldiazenes under Microwave Irradiation Initiation with Dialkyldiazenes under an External Magnetic Field Conclusions Acknowledgements References (555) Rate coefficients of free-radical polymerization deduced from pulsed laser experiments Prog Polym Sci Critically evaluated termination rate coefficients for free-radical polymerization: Experimental methods Prog Polym Sci Radical Polymerization Radical addition-fragmentation chemistry in polymer synthesis Polymer RAFT Polymerization to form Stimuli-Responsive Polymers Polym Chem Preparation of poly(methyl methacrylate) by ATRP using initiators for continuous activator regeneration (ICAR) in ionic liquid/microemulsions Polymer Theoretical study on the thermal decomposition of azoisobutyronitrile J Mol Struct: THEOCHEM Direct observation of denitrogenation process of 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) derivatives, using a visible 5-fs pulse laser Chem Phys Lett Further studies on the thermal decomposition of AIBN - Implications as to the mechanism of termination in methacrylonitrile polymerization Eur Polym J Introduction Radical Polymerization in Industry Kinetics of Polymerizations Further Effects of Chain-Length-Dependent Reactivities on Radical Polymerization Kinetics Aust J Chem Termination The Kinetics of Free-Radical Polymerization of Vinyl Monomers in Homogeneous Solution Critically evaluated rate coefficients for free-radical polymerization. 1. Propagation rate coefficient for styrene Macromol Chem Phys Critically evaluated rate coefficients for free-radical polymerization 2. Propagation rate coefficients for methyl methacrylate Macromol Chem Phys Critically evaluated rate coefficients for free-radical polymerization, 3. Propagation rate coefficients for alkyl methacrylates Macromol Chem Phys Critically evaluated rate coefficients for free-radical polymerization, 4 Macromol Chem Phys Critically evaluated rate coefficients for free-radical polymerization, 5 - Propagation rate coefficient for butyl acrylate Macromol Chem Phys Critically evaluated rate coefficients for free-radical polymerization. Part 6: Propagation rate coefficient of methacrylic acid in aqueous solution Pure Appl Chem Critically evaluated rate coefficients in radical polymerization - 7. Secondary-radical propagation rate coefficients for methyl acrylate in the bulk Polym Chem Termination Rate Coefficients for Radical Homopolymerization of Methyl Methacrylate and Styrene at Low Conversion Macromol Chem Phys Glossary of terms related to kinetics, thermodynamics, and mechanisms of polymerization (IUPAC Recommendations2008) Pure Appl Chem Initiation Terminology for reversible-deactivation radical polymerization previously called ‘controlled’ radical or ‘living’ radical polymerization Pure Appl Chem Chain Transfer Activity of ⌉-Unsaturated Methyl Methacrylate Oligomers Macromolecules Living Radical Polymerization by the RAFT Process Aust J Chem Living Radical Polymerization by the RAFT Process - A first Update Aust J Chem Living Radical Polymerization by the RAFT Process - A Second Update Aust J Chem Living Radical Polymerization by the RAFT Process - A Third Update Aust J Chem RAFT Polymerization -Then and Now ACS Symp Ser Precision Polymer Synthesis by Degenerative Transfer Controlled/Living Radical Polymerization Using Organotellurium, Organostibine, and Organobismuthine Chain-Transfer Agents Chem Rev Mechanism-Based Invention of High-Speed Living Radical Polymerization Using Organotellurium Compounds and Azo-Initiators J Am Chem Soc Use of iodocompounds in radical polymerization Chem Rev Initiating systems for nitroxide-mediated “living” free radical polymerizations: Synthesis and evaluation Macromolecules Imidazolidinone nitroxide-mediated polymerization Macromolecules Effect of azoinitiators on nitroxide-mediated photo-living radical polymerization of methyl methacrylate Colloid Polym Sci History of nitroxide-mediated polymerization Reversible deactivation radical polymerization mediated by cobalt complexes: recent progress and perspectives Org Biomolecul Chem Kinetic Modeling of Normal ATRP, Normal ATRP with [CuII]0, Reverse ATRP and SR&NI ATRP Macromol Theory Simul Atom Transfer Radical Polymerization (ATRP): Current Status and Future Perspectives Macromolecules Macromolecular Engineering by Atom Transfer Radical Polymerization J Am Chem Soc Controlled/“Living” Radical Polymerization. Homogeneous Reverse Atom Transfer Radical Polymerization Using AIBN as the Initiator Macromolecules Preparation of gradient copolymers via ATRP using a simultaneous reverse and normal initiation process. I. Spontaneous gradient J Polym Sci Part A Polym Chem Preparation of Linear and Star-Shaped Block Copolymers by ATRP Using Simultaneous Reverse and Normal Initiation Process in Bulk and Miniemulsion Macromolecules Further progress in atom transfer radical polymerizations conducted in a waterborne system J Polym Sci Part A Polym Chem Simultaneous reverse and normal initiation in atom transfer radical polymerization Macromolecules Computer-Aided Optimization of Conditions for Fast and Controlled ICAR ATRP of n-Butyl Acrylate Macromol Theory Simul Tuning Polymer Properties through Competitive Processes ACS Symp Ser Cited by (54) Azobisisobutyronitrile loaded on mesoporous silica particles: A new stressor for solid-state oxidative forced degradation studies Radical-promoted single-unit monomer insertion (SUMI) [aka. reversible-deactivation radical addition (RDRA)] Synthesis of low-molecular weight itaconic acid polymers as nanoclay dispersants and dispersion stabilizers An investigation on chain transfer to monomers and initiators, termination of radicals in EVA copolymerization process based on DFT and microkinetic simulation Superfast desulfurization for protein chemical synthesis and modification Exploring solution ICAR ATRP of methyl acrylate with AIBN as azo initiator: a kinetic modeling and simulation study Recommended articles (6) Organometallic-mediated radical polymerization of ‘less activated monomers’: Fundamentals, challenges and opportunities Homolytically weak metal-carbon bonds make robust controlled radical polymerizations systems for “less-activated monomers” Nitroxide mediated suspension polymerization of methacrylic monomers Dispersion polymerization in environmentally benign solvents via reversible deactivation radical polymerization Nitroxide mediated copolymerization of acrylates, methacrylates and styrene: The importance of side reactions in the polymerization of acrylates Modeling and theoretical development in controlled radical polymerization

Progress in Polymer Science

Volume 88,

January 2019

, Pages 130-188

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https://doi.org/10.1016/j.progpolymsci.2018.08.003 Get rights and content

Abstract

The dialkyldiazene or azo-compound initiators, which include azonitriles such as azobis(isobutyronitrile) (AIBN), are one of the most important classes of initiator being widely used in both conventional and reversible-deactivation radical polymerization (RDRP). This paper briefly reviews the mechanism of radical generation from dialkyldiazenes and provides a critical assessment and recommended values for the decomposition rate coefficients (k_d) and efficiencies (f) of those that are commercially available. A critique of the methods that have been used for the determination of these kinetic parameters is also provided. In this review, a focus is placed on initiation of radical polymerization of the more reactive monomers such as methyl methacrylate (MMA) and styrene at low monomer conversions; where addition of the initiator-derived radicals to monomer is not rate determining (i.e., k_i>k_p) and, thus, the efficiencies for radical generation (f_g) and for initiation of polymerization (f_i) are similar. However, the dependence of the kinetic parameters on such factors as monomer type, monomer conversion, reaction medium, temperature, photo-irradiation, microwave irradiation and magnetic fields is also discussed. We additionally provide comment on the use of dialkyldiazenes in heterogeneous polymerization (emulsion or dispersion polymerization) and in RDRP. Dialkyldiazenes are the most used initiators in RAFT (reversible addition-fragmentation chain-transfer) polymerization and other RDRP based on degenerative chain transfer, which include iodine transfer polymerization (ITP) and tellurium-mediated polymerization (TERP). They also find significant use in various forms of stable-radical-mediated radical polymerization (SRMP), in particular nitroxide-mediated polymerization (NMP) and cobalt-mediated radical polymerization (CMRP), and have an integral role in certain atom-transfer radical polymerization (ATRP) processes, such as reverse and ICAR (initiators for continuous activator regeneration) ATRP and variations on these procedures. In each of these RDRP methods, knowledge of the parameters that characterize the kinetics and efficiency of dialkyldiazene decomposition and the mechanism of initiation is beneficial to understanding and for optimizing control over the process.

Graphical abstract

The dialkyldiazenes are one of the most important classes of initiator used in both conventional and reversible deactivation radical polymerization (RDRP). This paper summarizes the current state of knowledge of mechanism of radical generation and provides a critical assessment decomposition rate constants (k_d) and efficiencies (f) of commercially available initiators.

A Critical Assessment of the Kinetics and Mechanism of Initiation of Radical Polymerization with Commercially Available Dialkyldiazene Initiators (3)

Introduction

Radical polymerization has a long and successful history and applications of the technique account for a very substantial fraction of current world polymer production [1,2]. It might therefore be assumed that the kinetics of radical polymerization is a mature field, but this is not the case. The mechanism is complex, involving thousands of kinetically distinguishable species [3,4], and many details remain to be unravelled. Nonetheless, the classical mechanism introduced in the 1950s provides a basis for discussion and remains useful as a qualitative predictive tool.

This classical mechanism for radical polymerization can be depicted as shown in Scheme 1 [5,6] where A₂ is a symmetrical initiator that decomposes to two radicals A∙, M is a monomer, P_n is a polymer chain of chain length n, P_n∙ is a propagating species of chain length n, and P_n^H and P_m⁼ are the saturated and unsaturated products of termination by disproportionation with chain lengths n and m, respectively. P_n+m is the product of termination by combination with chain length n+m.

In this scheme, the properties of the initiator (A₂) are characterized by two kinetic parameters. The rate coefficient for initiator disappearance (k_d) and the efficiency for initiation of polymerization (f_i). This efficiency term is the fraction of A₂ that is converted to P₁∙ (the radicals A∙ that actually add to monomer to initiate polymerization). As will be discussed in Section 4, the value of f_i is not constant but is dependent on the rate of reaction with monomer, the conversion of monomer to polymer and other factors. The value of f_i will always be less than the efficiency of radical generation (f_g); the fraction of initiator (A₂) that is converted to radicals A∙ that escape the cage and are potentially available for reaction.

The traditional treatment of polymerization kinetics is then based on application of a steady state approximation (Eq. (1)), $\frac{- d [P \cdot]}{d t} = R_{i} - R_{t} = 2 k_{d} f_{i} [A_{2}] - 2 k_{t} {[P \cdot]}^{2} = 0$ where P∙ is a propagating species of unspecified chain length ( $[P \cdot] = \sum_{n = 1}^{\infty} [P_{n} \cdot]$ ), and a long chain approximation (negligible monomer consumption in the initiation or reinitiation steps) to provide a number of useful relationships. Rearrangement of Eq. (1) provides an expression for total concentration of propagating radicals, [P∙] (Eq. (2)). $[P·] = {(\frac{k_{d} f_{i}}{k_{t}})}^{0.5} {[A_{2}]}^{0.5}$

It is then possible to derive expressions for the average kinetic chain length (ν) (Eq. (3)), $ν = \frac{R_{p}}{R_{i}} = \frac{R_{p}}{R_{t}} = \frac{k_{p} [M]}{{(2 k_{d} f_{i} [A_{2}] k_{t})}^{0.5}}$ the mean lifetime of a propagating radical (τ) (Eq. (4)), $τ = {(2 k_{d} f_{i} [A_{2}] k_{t})}^{- 0.5}$

and the number average degree of polymerization (X_n) in the absence of chain transfer (Eq. (5)). $X_{n} = \frac{k_{p} [M]}{(1 + \frac{k_{td}}{k_{t}}) {(2 k_{d} f_{i} [A_{2}] k_{t})}^{0.5}}$

This also enables elimination of the radical concentration from the expression for rate of polymerization (Eq. (6)-(7)). $R_{p} = \frac{- d [M]}{d t} = k_{p} [P \cdot] [M]$ $= {(\frac{k_{d} f_{i} k_{p}^{2}}{k_{t}})}^{0.5} {[A_{2}]}^{0.5} [M]$

Members of the IUPAC subcommittee for Modelling of Polymerization Kinetics and Processes (and its predecessors) have been active in providing critically assessed values of rate coefficients for propagation (k_p) [[7], [8], [9], [10], [11], [12], [13], [14]] and termination (k_t) [15,16] but to date they have not directly tackled the kinetic parameters associated with initiation; specifically the rate coefficients for initiator decomposition (k_d) and the initiator efficiencies (f). The present document seeks to redress that omission with a critical assessment of the kinetics and mechanism of initiation of polymerization with commercially available dialkyldiazene initiators and is primarily concerned with the kinetics of generation of initiating radicals (Scheme 2). The initiating radicals are defined as those initiator-derived (or primary radicals) that add to monomer to initiate polymerization. They will equate in number to the number of propagating radicals formed with a degree of polymerization of 1 (P₁∙) [17,18].

The dialkyldiazenes are one of the most important classes of initiator used in radical polymerization [18,19]. A survey of the literature using SciFinder^® and the terms “AIBN” or “azobisisobutyronitrile” indicates that rate of publication remained remarkably constant between ca 1960 and 1995 but has undergone a significant upsurge since that time, tripling over the last 15 years (Fig. 1). This increase parallels the development and still growing importance of techniques for reversible-deactivation radical polymerization (RDRP) [20]. Each year, at least 80% of the publications mentioning AIBN relate to polymers or polymerization, and about 100 papers per annum relate to reaction kinetics in some form. There is substantially less literature on the use of other dialkyldiazenes (2-17; see Fig. 2 and Table 1), though the general trends with respect to publication rate are similar. It can also be noted that more than half of the references to dialkyldiazenes come from the patent literature. It should also be pointed out that for publications on radical polymerization, the specific initiator used is not necessarially mentioned in SciFinder^®.

The dialkyldiazene initiators see widespread use in many forms of RDRP. They are the most used initiators in reversible addition-fragmentation chain-transfer (RAFT) polymerization [[21], [22], [23], [24], [25], [26], [27], [28]] and other forms of RDRP involving a degenerate chain-transfer mechanism, such as tellurium-mediated polymerization (TERP) [29,30] and iodine transfer polymerization (ITP) [31]. They also find use in stable radical-mediated radical polymerization (SRMP), for example, in nitroxide- (aminoxyl-) mediated polymerization (NMP) [[32], [33], [34], [35]] and cobalt-mediated radical polymerization (CMRP) [36] as one of the preferred pathways for in situ (or separate) generation of the required alkoxyamine or organocobalt initiators, respectively. Dialkyldiazene initiators are also used in some forms of atom-transfer radical polymerization (ATRP) [[37], [38], [39]]. In particular, in so-called reverse ATRP [40], SR&NI ATRP (simultaneous reverse and normal initiation) [[41], [42], [43], [44]] and ICAR ATRP (initiators for continuous activator regeneration) [[45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57]]. In each of these RDRP methods, some knowledge of the mechanism, rate and efficiency of radical generation is useful in attaining an understanding of the kinetics and mechanism and realizing the full potential of the process.

The dialkyldiazenes, or azoalkanes as they are often referred to, are thermal or photochemical sources of alkyl radicals. The last comprehensive review on the kinetics and mechanism of their decomposition was compiled by Paul Engel in 1980 [58]. A 1997 chapter by Koga et al. [59] provides a review of the chemistry undergone by dialkyldiazenes with a focus on mechanism covering the literature through 1995. Non-critical surveys of azo-compound decomposition data are included in compilations such as the Polymer Handbook [60], the Handbook of Free Radical Initiators [61] and the Encyclopedia of Polymer Science [62]. A short review of the kinetics and mechanism of dialkyldiazene reactions in a polymerization context appears in Moad and Solomon’s The Chemistry of Radical Polymerization [18]. Other relevant reviews on initiation of polymerization include those by Bevington [63], Mishra et al. [64] and Moad et al. [65]. The topic is given only limited treatment in general texts that deal with radical polymerization [19,[66], [67], [68], [69], [70]].

Most dialkyldiazenes used as polymerization initiators have general structure 1 (Scheme 2). They are symmetrically substituted about the nitrogen-nitrogen double bond and have tertiary alkyl substituents that possess functionality (X) that determines both the rate of decomposition of the dialkyldiazene and the properties of the produced radicals. These initiators are commercially available from AkzoNobel [71], Arkema [72], DuPont [73], Otsuka [74] and Wako [75,76], amongst others, all of whom provide some data on rates of azo-compound decomposition through their product literature. A summary of the commercially available initiators considered in this document is provided in Table 1 . These compounds (1) include compounds where X is nitrile (e.g., 1-8), ester (9), amidinium salt (11, 13), or amide (14, 15). Those dialkyldiazenes (1) where X is alkyl – 2,2’-azobis(2,2,4-trimethylpentane or azoisoctane (16) and 2,2’-azobis(2-methylpropane) or azo-t-butane (17) – are high temperature initiators.

This paper sets out to provide critically assessed values of the rate coefficients for decomposition (k_d) and efficiencies for initiation (f_i) or radical generation (f_g). A greater focus on (E)-2,2'-(diazene-1,2-diyl)bis(2-methylpropanenitrile) better known as azobis(isobutyronitrile) or AIBN (4) and (E)-dimethyl 2,2'-(diazene-1,2-diyl)bis(2-methylpropanoate) better known as azobis(methyl isobutyrate) or AIBMe (9) is dictated by the relative mass of literature data with respect to these initiators. A few commercial initiators, not listed in Table 1, have been excluded from consideration because of the paucity of suitable data to analyze. Some of the initiators listed in Table 1 do not appear in the current catalogues of the companies mentioned.

Section snippets

Mechanism of Thermal Decomposition of Dialkyldiazenes

Other than molecular nitrogen, all isolable products from the thermal decomposition of dialkyldiazenes 1, including polymer end-groups, are attributable to the reactions of the derived alkyl radicals. Nonetheless, there has been some discussion on whether the decomposition of dialkyldiazenes involves two sequential (1-bond) scissions, with the intermediacy of a diazenyl radical (refer Scheme 2), or concerted (2-bond) scission and direct formation of a molecule of nitrogen and two alkyl

Kinetics of Thermal Decomposition of Dialkyldiazenes (k_d Values)

Thermolysis rate coefficients (k_d) of dialkyldiazenes (1) show a marked dependence on the nature of the substituents. In general, k_d for tertiary dialkyldiazenes (1) is dramatically accelerated by α-substituents X capable of delocalizing the free spin of the incipient radical. For example, Timberlake [99] found that k_d increases in the series where X is CH₃<-OCH₃<-SCH₃<-CO₂R∼-CN<-Ph<CH = CH₂. For the initiators currently under consideration, the value of k_d at 60 °C varies by more than two

Initiator Efficiency (f)

There are two relevant definitions of initiator efficiency that need to be considered. The efficiency of radical generation (f_g) is the fraction of radicals formed from an initiator that escape the primary solvent cage and are potentially available for reaction with a substrate, f_g is defined as shown in Eq. (12). $f_{g} = \frac{(r a t e o f r a d i c a l p r o d u c t i o n)}{n (r a t e o f i n i t i a t o r d i s a p p e a r a n c e)}$ where n is the number of moles of radicals generated per mole of initiator. The value of f_g is relativly independent of

Methods for Evaluation of k_d and f

A wide range of techniques have been used in the determination of values of the decomposition rate coefficients (k_d) and initiator efficiencies (f_g or f_i) for the dialkyldiazenes. A brief description of these, with some discussion on advantages and potential limitations, follows with particular reference to their application in the study of AIBN decomposition. The reader is also referred to Stickler’s survey of methods for studying polymerization kinetics [115], which only covers the literature

Recommended Values for Decomposition Rate Coefficients (k_d)

Recommended values for rate coefficients (k_d) and the associated Arrhenius parameters for decomposition of dialkyldiazenes are summarized in Table 3. These data are in most cases derived by reanalysis of the reported k_d values. One criterion for providing a recommended value should be that several groups have provided consistent experimental values for k_d using what appears to be reliable methodology. However, for most systems the datasets are quite small and low errors can be indicative of

Recommended Values for Initiator Efficiency (f)

The efficiency (f_g) for radical production is the fraction of radicals that escape the cage. For high concentrations of monomers with reactivity sufficient to ensure that the reaction of initiating radicals with monomer is not rate determining (k_i>k_p), such as MMA and styrene, f_g will equate with f_i for initiation of polymerization. The monomer concentration must also be sufficient for all initiator-derived radicals to react initially with monomer to form propagating species and that none are

Initiators

Data from experiments carried out in air have been excluded from consideration (see comments in Section 5 above). Where Arrhenius values only are given, data may be shown on the figures but do not contribute to the analysis unless indicated otherwise. The data analysis was performed and the Arrhenius plots were produced using Mathematica^® version 10.4.

Dialkyldiazene Initiators in Reversible-Deactivation Radical Polymerization (RDRP)

Reversible-deactivation radical polymerization (RDRP) [20] is a class name for processes that can provide living character to radical polymerization by allowing all chains to grow simultaneously. RDRP works by maintaining most living chains in a dormant form and providing a mechanism for equilibration of active and dormant chains that is rapid with respect to the rate of propagation. The three most common mechanisms for RDRP are stable-radical-mediated radical polymerization (SRMP), radical

Conventional Radical Polymerization in Heterogeneous Media

Many radical polymerizations are carried out in heterogeneous media, most often an aqueous dispersion. Specifically, much commodity production of high molecular weight polymers makes use of suspension or emulsion polymerization. The advantages over bulk or solution polymerization lie in more effective dissipation of the heat of polymerization (better temperature control), the avoidance of organic solvents, the ease of handling a particulate product rather than a glass or a viscous mass, and the

Polymerization in Supercritical Carbon Dioxide

Supercritical CO₂ is a good solvent for many low molar mass monomers. However, only a few polymers, such as those based on certain fluoro-olefins, possess significant CO₂ solubility and polymers formed by radical polymerization are generally insoluble in supercritical CO₂. Thus, radical polymerizations in supercritical CO₂ taken to high conversion are seldom solution polymerizations. Most often, they are precipitation polymerizations, (inverse) emulsion polymerizations or some other form of

Dialkyldiazenes as Photoinitiators

Photodecomposition of dialkyldiazene initiators has been widely applied and studied but is not commonly used in initiating polymerization. The diazene chromophore in dialkyldiazenes absorbs in the near UV with λ_max = 350-370 nm and ε = 2-50 M^-1 cm^-1 [34,58] (see Table 48). Quantum yields for radical production are typically in the range 0.4-0.5 (Table 46, see also [58]). It has been proposed [58] that the main light-induced reaction is trans-cis isomerization and that dissociation to radicals

Initiation with Dialkyldiazenes under Microwave Irradiation

There has been much discussion in the literature on the acceleration of radical polymerization under microwave irradiation and the potential causes of such acceleration. There are recent reviews on microwave-assisted polymerization [[533], [534], [535], [536]] and two reviews specifically on microwave-assisted RAFT polymerization [537,538]. Amongst explanations advanced for apparent microwave effects on radical (and RAFT) polymerization) have been the possible enhancement of rate and/or

Initiation with Dialkyldiazenes under an External Magnetic Field

When a bond connecting two groups undergoes homolysis, a primary radical pair is produced with conservation of spin.[548] Only singlet radical pairs will undergo in-cage reaction by recombination or disproportionation. A triplet radical pair will generally not self-react without one radical first undergoing intersystem crossing to form a singlet radical pair. In cases where radicals are produced as a triplet radical pair (e.g., when a triplet sensitizer is used), it is possible that

Conclusions

This review has provided a critical assessment and recommended values for decomposition rate coefficients (k_d) and efficiencies of radical formation (f_g) and for initiation of polymerization (f_i) with respect to commercially available dialkyldiazene polymerization initiators. In doing this, the advantages and limitations of the various methods used in the determination of these kinetic parameters have been reviewed and their suitability assessed. The dependence of the kinetic parameters on such

Acknowledgements

This manuscript was prepared under the auspices of the IUPAC subcommittee for Modelling of Polymerization Kinetics and Processes and the task group 2009-050-1-400, “Critically evaluated rate coefficients associated with initiation of radical polymerization”, whose members have included Mathieu Ahr (Akzo, Deventer, Netherlands), Sabine Beuermann (Clausthal-Zellerfeld, Germany), Michael Buback (Gottingen, Germany), Michelle L. Coote (ANU, Canberra, Australia), Klaus-Dieter Hungenberg (BASF,

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Cited by (54)

Azobisisobutyronitrile loaded on mesoporous silica particles: A new stressor for solid-state oxidative forced degradation studies
2023, Journal of Pharmaceutical and Biomedical Analysis
A new approach for testing drug sensitivity to autooxidative degradation in the solid state is demonstrated in this work. A novel solid-state form of stressing agent for autooxidation has been proposed, based on azobisisobutyronitrile loaded into mesoporous silica carrier particles. The new solid-state form of the stressing agent was applied in degradation studies of two active pharmaceutical ingredients: bisoprolol and abiraterone acetate. The effectiveness and predictivity of the method were evaluated by comparing impurity profiles with those obtained by traditional stability testing of commercial tablets containing the investigated APIs. The results obtained by the new solid-state stressor were also compared with those obtained by an existing method for testing peroxide oxidative degradation in the solid state using a complex of polyvinylpyrrolidone with hydrogen peroxide. It was found that the new silica particle-based stressor was able to effectively predict which impurities could be formed by autooxidation in tablets and that this new approach is complementary to methods for testing peroxide oxidative degradation known from the literature.
Radical-promoted single-unit monomer insertion (SUMI) [aka. reversible-deactivation radical addition (RDRA)]
2023, Progress in Polymer Science
We survey progress in the development of the processes for radical-promoted single-unit monomer insertion (SUMI) or reversible deactivation radical addition (RDRA), focussing on aminoxyl- [nitroxide-] mediated SUMI (NM-SUMI), reversible-addition-fragmentation chain transfer-SUMI (RAFT-SUMI) and atom-transfer radical addition (ATRA). Radical-promoted thiol-ene processes are also briefly discussed. We detail the strategies for achieving selectivity with respect to single unit insertion vs oligomerization and look critically at progress towards discrete oligomer synthesis by consecutive SUMI reactions. We examine the use of SUMI to install α-, ω- or mid-chain-functionality in RDRP-synthesized polymers. Finally, we examine the prospects for using radical-promoted SUMI in the synthesis of sequence-defined polymers where monomer placement is precisely defined to the level of the individual monomer units in the polymer chain.
Synthesis of low-molecular weight itaconic acid polymers as nanoclay dispersants and dispersion stabilizers
2023, Polymer
Citation Excerpt :
In general, these variations may a) originate from changes in the chemical structure of the V-50 end groups which may be localized on amidine moiety and occur during synthesis of itaconic oligomers, or b) arise as an artifact during the ESI-MS analysis process. V-50 is a popular water-soluble initiator and its chemistry appears to be well understood [35,47,48]. It is known that amidine groups of V-50 undergo hydrolysis at pH > 7 [48], but stability at acidic aqueous conditions was not studied in the cited work.
Itaconic oligomers with an average molecular weight of M_n=1500–2200g/mol and different end-groups were prepared by 2,2′-azobis(2-methyl-propionamidine) dihydrochloride (V-50) – induced radical polymerization of itaconic acid (IA) in aqueous solution at 65°C for 24h. Mercaptoethanol, thioglycerol, and sodium hypophosphite were used as chain transfer agents (CTA) with the molar ratio ranging from 30:3:1 to 30:3:4 of IA, V50, and CTA, respectively. The structure of the oligomers was confirmed by electrospray ionization mass spectrometry (ESI-MS) and tandem mass spectrometry (ESI-MS/MS). The excellent dispersing efficiency of the oligomers at very low concentrations for Laponite® sols was proved by oscillatory rheology and small-angle X-ray scattering (SAXS) measurements.
An investigation on chain transfer to monomers and initiators, termination of radicals in EVA copolymerization process based on DFT and microkinetic simulation
2022, Polymer
Citation Excerpt :
Induced decomposition transfers radical from monomer to initiator and forms a primary radical again. Primary radicals may couple each other back to stable molecules and may not decompose again in a cage of monomers and polymers [21]. Actually, physical and chemical properties are often determined by defect units such as head-to-head connection and end groups.
Chain transfer and termination in copolymerization between ethylene and vinyl acetate (VAc) were investigated with DFT method. Chain transfer becomes more active in copolymerization than homopolymerization due to an incorporation between low propagationreactivity of alkyl radical and high chain transfer reactivity of VAc. Competition between combination and disproportionation were detailed for self-termination and cross-termination, and most radical chains terminate through combination. Combination involving alkyl radical or tail radical shows much lower energy barrier than combination involving head radical. Effect of head-to-head defect on termination and chain transfer is also clarified. Chain transfer to initiators and termination of primary radicals were studied for BPO and AIBN. The both, especially AIBN, shows reactivity in direct addition of radicals to initiators, which is revealed by this work at first time. Based on kinetics from DFT, a microkinetic simulation was conducted to extend the research from electron scale to micro scale. Relationship between copolymerization conditions (including temperature, monomer composition and concentration, and initiator concentration) and rates of elemental reactions, radical concentrations, DP and PDI was discussed.
Superfast desulfurization for protein chemical synthesis and modification
2022, Chem
Highly effective yet chemoselective chemical transformation strategies enable the facile access and precise modification of complicated biomacromolecules. In particular, the application of desulfurization chemistry expands the dimension of chemical protein synthesis with the cysteine-based peptide ligation. Considering the existing peptide desulfurization methods, a milder, faster, and easier strategy is still required for the increasing complexity of proteins by chemical synthesis. Herein, we report a superfast desulfurization strategy based on tetraethylborate for effectively and chemoselectively desulfurizing peptides/proteins containing cysteine or penicillamine in an add-and-done manner. This strategy can be simply applied under ambient conditions without requirement of inert atmosphere protection, irradiation, heating, or exogenous thiol additives. Such desulfurization can even overcome a certain amount of radical scavengers. Various peptide and protein substrates were examined, and a practical one-pot native chemical ligation (NCL)-desulfurization was developed for the synthesis of leukocyte-associated immunoglobulin-like receptor 1 (LAIR1) cytoplasmic domain and semisynthesis of serotonylated histone H3 (H3Q5ser) protein.
Exploring solution ICAR ATRP of methyl acrylate with AIBN as azo initiator: a kinetic modeling and simulation study
2022, Results in Materials
Citation Excerpt :
Azo initiators have the generic formula R–NN-R′, where R and R′ can be aryl (aromatic) or alkyl (aliphatic) functional groups, respectively [1,6–9]. A critical review on the kinetics and mechanism of initiation of radical polymerization with azo initiators was done recently by Graeme Moad (2019) [12]. The polymerization rate in ICAR ATRP can be adjusted by the chemical nature of the azo initiator and its initial concentration in the reaction system [1,6–9].
Supplementary techniques to atom transfer radical polymerization (ATRP) have been studied to reduce the concentration of catalytic complexes used, aiming at a less costly and more environmentally friendly process for synthesizing polymeric materials. Initiators for continuous activator regeneration ATRP (ICAR ATRP) is one technique in which a source of primary free radicals allows the reactivation of catalytic species used in the conventional ATRP. Due to the technological relevance of the topic, this work aims to expand the contribution to literature with kinetic modeling and simulation study for an ICAR ATRP system not yet explored. For this purpose, it was investigated solution ICAR ATRP of methyl acrylate with copper-based catalysts and azobisisobutyronitrile as ICAR initiator. The method of moments was employed to derive material balance equations. We find that the kinetic mechanism that best represented the trends of experimental data from literature was the one that considered specific side reactions of acrylates, such as backbiting and radical catalytic termination. In the model fitting procedure, a nonlinear optimization process had to be developed to estimate the values of kinetic rate constants not yet reported in the literature. The values obtained for the missing kinetic rate constants are consistent with similar experimental systems found in the literature. The deterministic model obtained was then employed to study the concentration profile maps. Simulations allowed verifying the slow release of primary free radicals from ICAR initiator to reduce the deactivators in activators species continuously. In addition, it was possible to confirm that the ICAR ATRP can be thermoregulated. Parametric analyses showed that variations in kinetic rate constants and reaction stoichiometry can shape monomer conversion and the molecular properties of the polymer (e.g., molecular weight, dispersity, and end functionality).

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