Midchain Radicals Research Articles

Chain‐Length‐Dependent Termination in Radical Polymerization of Acrylates

AbstractThe technique of SPPLP EPR, which is single‐pulse pulsed‐laser polymerization (SPPLP) in conjunction with electron paramagnetic resonance (EPR) spectroscopy, is used to carry out a detailed investigation of secondary (chain‐end) radical termination of acrylates. Measurements are performed on methyl acrylate, n‐butyl acrylate, and dodecyl acrylate in bulk and in toluene solution at −40 °C. The reason for the low temperature is to avoid formation of mid‐chain radicals (MCRs), a complicating factor that has imparted ambiguity to the results of previous studies of this nature. Consistent with these previous studies, composite‐model behavior for chain‐length‐dependent termination (CLDT) rate coefficients, $k_{{\rm t}}^{i,i} $, is found in this work. However, lower and more reasonable values of αs, the exponent for variation of $k_{{\rm t}}^{i,i} $ at short chain lengths, are found in the present study. Most likely this is because of the absence of MCRs, thereby validating the methodology of this work. Family‐type termination behavior is observed, with the following average parameter values adequately describing all results, regardless of acrylate or the presence of toluene: αs = 0.79, αl = 0.21 (long chains) and ic ≈ 30 (crossover chain length). All indications are that these values carry over to termination of acrylate chain‐end radicals at higher, more practical temperatures. Further, these values largely make sense in terms of what is understood about the physical meaning of the parameters. Variation of the rate coefficient for termination between monomeric radicals, $k_{{\rm t}}^{{\rm 1,1}} $, is found to be well described by the simple Smoluchowski and Stokes–Einstein equations. This allows easy prediction of $k_{{\rm t}}^{{\rm 1,1}} $ for different alkyl acrylates, solvent, and temperature. Through all this the unrivalled power of SPPLP EPR for measuring and understanding (chain‐length‐dependent) termination rate coefficients shines through. magnified image

Kinetic and mechanistic similarities between reversible addition fragmentation chain transfer intermediate and acrylate midchain radicals

This contribution highlights the mechanistic and kinetic similarities as well as differences between midchain radicals occurring in both acrylate free radical polymerization as well as reversible addition fragmentation chain transfer mediated processes.

Free‐Radical Acrylic Polymerization Kinetics at Elevated Temperatures

AbstractFree‐radical acrylic homo‐ and copolymerization kinetics are reviewed, focusing on secondary reactions that impact the polymerization rate and polymer molecular weight (MW) under industrially‐relevant synthesis conditions. Dependent on the monomer type (acrylate, methacrylate, styrene), mechanisms that must be considered include chain‐end depropagation, monomer self‐initiation, formation and reaction of midchain radicals, and incorporation of macromonomers formed by β‐scission of midchain radicals. The relative importance of these reactions varies with temperature and, for copolymerization, monomer composition. A comprehensive treatment of these complexities has been completed for polymerizations conducted up to 180 °C, but further work is required to extend the applicability of the model to even higher temperatures.

Reducing the Degree of Branching in Polyacrylates via Midchain Radical Patching: A Quantitative Melt-State NMR Study

Degrees of branching were measured with 13C melt-state NMR in poly(n-butyl acrylates) synthesized in both the presence and absence of 1-octanethiol at various temperatures, and their precision calculated based on the signal-to-noise ratio of the quaternary carbon at the branching point. Nitrogen gas was used for all pneumatics. Samples synthesized with thiol were measured in 7 mm rotors at MAS rotational frequencies of 1.8 to 4.5 kHz, while samples synthesized without thiol were measured in 4 mm rotors at 9 kHz MAS. The relative standard deviation SD of DB was calculated from the signal-to-noise ratio of the quaternary carbon SNR. The accuracy of measured degrees of branching was ensured by full relaxation of the relevant signals between pulses. A significant decrease of branching was observed in the presence of a thiol during the polymerization process, experimentally confirming the patching effect of midchain tertiary radicals by the thiol.

Termination and Transfer Kinetics of Butyl Acrylate Radical Polymerization Studied via SP-PLP-EPR

Butyl acrylate (BA) solution polymerization (1.5 M in toluene) was investigated via single-pulse pulsed laser polymerization in conjunction with electron paramagnetic resonance spectroscopy (SP-PLP-EPR) with emphasis on the termination and transfer kinetics of the system in which two distinctly different types of radicals, secondary chain-end radicals (SPRs) and midchain radicals (MCRs), are present. MCRs are produced by intramolecular hydrogen transfer (backbiting). They may react back to SPRs by monomer addition. The evolution of SPR and MCR concentrations after photoinitiation with an intense laser pulse was measured via highly time-resolved EPR at temperatures between −40 and +60 °C. At very low temperatures the MCR concentration is negligible, enabling the chain-length-dependent rate coefficient of SPR termination, ktss(i,i), to be directly determined. At higher temperatures it was necessary to use PREDICI simulation of the radical concentration vs time traces, a process which yields the chain-length-dependent rate coefficient of SPR termination for monomeric radicals, ktss(1,1), as well as the rate coefficients for backbiting, kbb, for monomer addition to an MCR, kpt, and for SPR-MCR cross-termination, ktst. The composite model adequately represents ktss(i,i), with the power-law exponents αs = 0.85 ± 0.09 and αl = 0.16 ± 0.07 for short-chain and long-chain radicals, respectively, and a crossover chain length between short-chain and long-chain behavior at around ic = 30. The activation energy for both ktss(1,1) and ktst(1,1) is found to be as one would expect for translational diffusion of small molecules.

Relevance of backbiting and beta‐scission reactions in the free radical polymerization of Acrylonitrile

AbstractSummary: In this work, backbiting and beta‐scission reactions are investigated through Quantum Chemistry methods by adopting the Becke 3 parameters and Lee Yang Parr functional (B3LYP) and 6‐31G(d,p) basis set. Namely, the 1:3, 1:5 and 1:7 backbiting reactions are studied for acrylonitrile polymerization. It was found that the backbiting 1:5 is the most favorited because this kinetic event leads to the formation of a 6 membered transition state, while the backbiting 1:3 requires high activation energy due to the formation of a highly strained 4 membered ring. 7:3 backbiting reaction was also examined, since it is an alternative pathway that can explain the formation of defects generated by radicals in the third position. Simulations showed that this kinetic step is characterized by high rate constant because of its low activation energy.The right and left beta‐scission reactions from the mid chain radicals formed by the considered backbiting reactions are also studied. Computational analysis demonstrated that all beta‐scission reactions are endothermic and both the right and left beta‐scission reactions have the same activation energy, which seems to be more influenced by the position of the mid chain radical.

High Temperature Semibatch Free Radical Copolymerization of Styrene and Butyl Acrylate

AbstractSummary: High temperature semibatch free radical solution copolymerizations of n‐butyl acrylate (BA) and styrene (ST) were carried out over a range of copolymer composition. The significant increase in experimental polymer weight‐average molecular weight with time, as well as the shift in the entire polymer molecular weight distribution, is explained by assuming fast β‐scission of BA midchain radicals with an adjacent styrene unit, followed by subsequent addition of the resultant macromonomer to growing radicals. A mechanistic model including backbiting and β‐scission, macronomer incorporation, long‐chain branching, and propagation and termination penultimate effects was constructed in Predici; the model provides a good representation of the experimental data using rate coefficients taken from literature.

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) .

Effect of Intramolecular Transfer to Polymer on Stationary Free Radical Polymerization of Alkyl Acrylates, 4 ‐ Consideration of Penultimate Effect

A penultimate model of acrylate polymerization has been proposed to account for reduced reactivity of radicals formed by monomer addition to midchain radicals. New expressions derived for polymerization rate, number-average degree of polymerization and branching level are used for a comparative analysis of the penultimate and terminal models. The penultimate model has been also implemented into the simulation program PREDICI to conduct a comparative analysis of time dependencies of conversion, cumulative number-degree of polymerization and cumulative branching levels calculated for the different models. The calculations show that, depending on radical reactivity ratio s(i) and monomer concentrations, the predictions from the penultimate model significantly deviate from those of the terminal model.

Consideration of Macromonomer Reactions in n‐Butyl Acrylate Free Radical Polymerization

n-Butyl acrylate (BA) starved-feed solution semibatch experiments with varying final polymer content and monomer feed times were carried out at 138 °C. A full mechanistic model of the system implemented in Predici includes intermolecular chain transfer to polymer and macromonomer propagation as well as backbiting, chain scission, and midchain radical propagation and termination. The importance of macromonomer propagation under these conditions of industrial interest is illustrated by experiment and simulation, with the macromonomer reaction responsible for the significant increase in polymer weight-average molecular weight ($\overline M _{\rm w}$) with time. Rate coefficients for macromonomer propagation (k(mac) ) and β-scission (k(β) ) of k(mac) /k(p) = 0.55 and k(β) = 12 s(-1) (with k(p) the rate coefficient for BA chain-end propagation) provide a good representation of experimental $\overline M _{\rm w}$ and macromonomer end group data at 138 °C.

Optimum Reaction Conditions for the Synthesis of Macromonomers Via the High‐Temperature Polymerization of Acrylates

AbstractMacromonomers are valuable synthetic building blocks: They can be copolymerized with low molecular weight monomers to generate brush‐like structures or serve as conjugation substrates in pericylic, metathesis, and thiolene reactions. Based on earlier reports on the facile high temperature formation of macromonomers from acrylates, a complex kinetic model is developed which accounts for the key reactions constituting the macromonomer formation process. On the basis of the kinetic model, the important rate coefficients governing acrylate polymerization (e.g., β‐scission and termination rate coefficients of midchain radicals, backbiting and intramolecular chain transfer rate coefficients) as well as the reaction conditions (e.g., initial monomer concentration, reaction temperature, radical flux) are systematically varied and their influence on the synthetic success is critically evaluated. The systematic coefficient variation reveals that there exist optimum reaction conditions under which the high temperature macromonomers formation may be conducted with maximum success. The present study provides a concise summary of these conditions.magnified image

Detailed mechanistic modeling of high-density polyethylene pyrolysis: Low molecular weight product evolution

A detailed, mechanistic model for high-density polyethylene pyrolysis was created based on the modeling framework developed in our previous work and was used to study the time evolution of low molecular weight products formed. Specifically, the role that unzipping, backbiting, and random scission reaction pathways play in the evolution of low molecular weight species was probed. The model tracked 151 species and included over 11,000 reactions. Rate parameters were adapted from our previous work, literature values, and regression against experimental data. The model results were found to be in excellent agreement with experimental data for the evolution of condensable low molecular weight products. The time evolution curves of specific low molecular weight products indicated that the random scission pathway was important for all species, while the backbiting pathway played a complementary role. Net rate analysis was used to further elucidate the competition between the pathways. Net rate analysis of end-chain radicals showed that the unzipping pathway was not competitive with the other pathways, as expected based on experimental yields of ethylene. The random scission pathway was found to be controlling, with the backbiting pathway playing a more minor role for product formation. By comparing the net rates for formation of specific mid-chain radicals via intramolecular hydrogen shift reactions, the contribution of the backbiting pathway was shown to vary, with radicals formed via the most facile x, x + 4-intramolecular hydrogen transfer reactions being favored.

Propagation Kinetics in Radical Polymerization Studied via Pulsed Laser Techniques

AbstractPulsed laser techniques have enormously improved the quality by which rate coefficients of radical polymerization may be determined. The specific and most important feature of the various types of pulsed laser techniques consists in the almost instantaneous production of a significant radical concentration and in the ease by which radical concentration may be controlled by applying further laser pulses at pre‐selected delay times. The pulsed laser polymerization – size exclusion chromatography (PLP‐SEC) experiment is extremely valuable for the determination of reliable propagation rate coefficients, kp. The present article briefly reviews recent results from PLP‐SEC studies directed toward the solvent dependence of kp in aqueous systems, into kp in copolymerization reactions, and into measurements of propagation rate coefficients in systems containing two types of radicals, as is the case with acrylates, where secondary chain‐end radicals may undergo 1,5‐H shift reactions (backbiting steps) to produce tertiary midchain radicals. By a propagation step, the tertiary radicals are transformed back to secondary ones.

Quantitative Product Spectrum Analysis of Poly(butyl acrylate) via Electrospray Ionization Mass Spectrometry

Electrospray ionization mass spectrometry (ESI-MS) is used to analyze poly(butyl acrylate) samples at full conversion obtained by bulk free radical polymerization in the temperature range of 60−140 °C in the presence of the chain transfer agent (CTA) 1-octanethiol at concentrations varying between 0 and 0.4 mol·L−1. Termination by combination products carrying initiator and transfer-derived fragments as well as three β-scission products were identified in the accessible mass range of up to 2000 m/z. The resulting mass spectra are subsequently quantitatively evaluated via integration to assess product distributions as a function of varying reaction conditions, that is, different temperatures and CTA concentrations. The present study provides for the first time quantitative product distribution data for butyl acrylate polymerization. The main species of interest, defined to be those most sensitive to variations in temperature, CTA concentration, or both, are identified to be thiol-capped polymers (TCPs) produced by the interference of the CTA in the polymerization and βI, the β-scission product produced by chain cleavage of so-called midchain radicals formed via transfer to polymer reactions. Relatively high concentrations of 1-octanethiol produce a very uniform thiol-capped polymer, whereas low concentrations of thiol produce a very large number of β-scission products. Increasing the reaction temperature increases the proportions of β-scission products, and increasing the CTA concentration can suppress and control the amounts and types of species formed to the point where the product spectrum is almost quantitatively constituted of the TCP product. Other species found include conventional termination via combination products and small amounts of β-scission products with initiator end groups. The quantity of combination products has no significant variation under the range of conditions studied, and they are thus considered minor products. The data provided in the present study can constitute the basis for the further quantitative evaluation of reliable rate coefficients for midchain radical formation and depletion pathways.

The role of mid‐chain radicals in acrylate free radical polymerization: Branching and scission

AbstractThe past 5 years have seen a significant increase in the understanding of the fate of so‐called mid‐chain radicals (MCR), which are formed during the free radical polymerization of monomers that form highly reactive propagating radicals and contain an easily abstractable hydrogen atom. Among these monomers, acrylates are, beside ethylene, among the most prominent. Typically, a secondary propagating acrylate‐type macroradical (SPR) can easily transfer its radical functionality via a six‐membered transition state to a position within the polymer chain (in a so‐called backbiting reaction), creating a tertiary MCR. Alternatively, the radical function can be transferred intramolecularly to any position within the chain (also forming an MCR) or intermolecularly to another polymer strand. This article aims at providing a comprehensive overview of the up‐to‐date knowledge about the rates at which MCRs are formed, their secondary reactions as well as the consequences of their occurrence under variable reaction conditions. We explore the latest aspects of their detection (via electron spin resonance spectroscopy) as well as the characterization of the polymer structures to which they lead (via high resolution mass spectrometry). The presence of MCRs leads to the formation of branched polymers and the partial formation of polymer networks. They also limit the measurement of kinetic parameters (such as the SPR propagation rate coefficient) with conventional methods. However, their occurrence can also be used as a synthetic handle, for example, the high‐temperature preparation of macromonomers. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7585–7605, 2008

Specific Polymerization Mechanism Involving β-Scission of Mid-Chain Radical Yielding Oligomers in the Free-Radical Polymerization of Vinyl Ethers

Butyl vinyl ether (BVE) was polymerized radically, and the resultant oligomeric poly(BVE)s were characterized mainly using MALDI-TOF-MS spectrometry. For a further structural identification, the poly(BVE) obtained in bulk was subsequently subjected to 1H NMR measurement. Notably, an intensive peak assignable to −CH2C(═O)CH2− protons was observed unexpectedly although a peak assignable to −CHO proton as the terminal end-group generated via β-scission of growing polymer radical as a presumed chain-end forming reaction appeared only weakly. Thus, a plausible chain-end forming reaction would be the intramolecular (backbiting) hydrogen abstraction to form mid-chain radical which subsequently undergoes β-scission resulting in oligomeric poly(BVE) with carbonyl group in the polymer backbone. Then, the specific polymerization mechanism involving β-scission of mid-chain radical yielding oligomers is discussed. The hydrogen abstraction ability of growing poly(BVE) radical was checked by using toluene as a solvent h...

Molecular Weight Distribution (Soluble and Insoluble Fraction) in Emulsion Polymerization of Acrylate Monomers by Monte Carlo Simulations

A Monte Carlo simulation model for the semibatch emulsion polymerization of acrylate monomers was developed. The model accounts for the complex kinetics of acrylate monomers (presence of chain-end and midchain radicals), the compartmentalization of emulsion polymerization systems and the development of the entire molecular weight distribution (MWD) as well as the branching density. It was found that the MWD produced in the semibatch process was bimodal (a sharp and extremely high Mw peak and a broad and lower Mw mode), the midchain radicals were predominant during the polymerization, and most of the branches were short and produced by the backbiting mechanism. Interestingly, the bimodality of the MWD could be avoided under certain experimental conditions (those who led to a decrease of particles with more than two radicals), but this did not prevent the formation of polymer chains of molecular weights above 107 g/mol (which are typically insoluble). The model also shows the importance of the presence of midchain radicals in the microstructure of the polymer formed. Thus, the fraction of termination by combination and disproportionation of the midchain radicals had a significant influence on the location of the high molecular weight peak. The predictions of the Monte Carlo model were in good agreement with experimental data and a previously developed deterministic mean-field theory model for the seeded semibatch emulsion polymerization of n-butyl acrylate.

Particle Size Distributions in Electrosterically Stabilized Emulsion Polymerization Systems: Testing the “Mid-Chain-Radical” Hypothesis

It has been found [Thickett, S. C.; Gaborieau, M.; Gilbert, R. G. Macromolecules 2007, 40, 4710-4720] that rate and characterization data on the seeded growth of latex particles electrosterically stabilized by a hairy layer of anchored poly(acrylic acid) (pAA) can be qualitatively and quantitatively explained by mechanistic events involving mid-chain radicals (MCRs). A considerable range of data was found to be consistent with the supposition that these MCRs are formed by abstraction on the pAA during polymerization, and are slow to propagate but quick to terminate, and thus are a cause of radical loss; they may also undergo P-scission leading to secondary particle formation. Results are reported here for four additional types of experiments on the evolution of the particle size distribution (PSD) in these systems; all these experiments have the potential to refute the mechanistic hypothesis. The experiments comprise (i) evolution of the PSD of small particles, (ii) evolution of the PSD in a bimodal system (competitive growth), (iii) growth rates of separate experiments of small and large particles with the same total amount of pAA, and (iv) the amount of secondary nucleation (formation of small particles) with both pAA and also with poly(ethylene oxide) steric stabilizers. In each case, the data are qualitatively and quantitatively consistent with the predictions of the mid-chain-radical mechanisms and thus support the general applicability of this effect.

Living Star Polymer Formation: Detailed Assessment of Poly(acrylate) Radical Reaction Pathways via ESI-MS

The generation of star polymers via living polymerization protocols is well documented; however, the impact of midchain radicals (MCRs) on the precise formation pathways under operation in living acrylate star polymerizations is still poorly understood. In the present study, electrospray ionization−mass spectrometry (ESI-MS) technology has been applied to map the products generated in R-group approach reversible addition fragmentation chain transfer (RAFT) methyl acrylate (MA) star polymerizations in order to gain insight into the precise formation pathways under operation in such systems. The polymerizations were conducted at 65 °C using the tetrafunctional RAFT agent 1,2,4,5-tetrakis(2-phenylthioacetylsulfanylmethyl)benzene and 2,2′-azobis(isobutyronitrile) (AIBN) as the thermally decomposing initiator. Initiator fragment derived linear chains, ideal stars, star−star couples, and other terminated star products formed as a result of combination and disproportionation reactions were successfully imaged. Additionally, MCR derived products that lie outside of the conventional R-group approach RAFT star polymerization mechanistic scheme were identified. Products associated with termination reactions involving intermolecularly formed MCRs on star arms and linear chains were observed; specifically, structures formed from MCR termination with propagating stars or radical carrying star cores, or with initiator fragments or propagating initiator derived linear chains. Additionally, structures produced via repropagation of intermolecularly formed MCRs on star arms were also identified. The products generated from MCR-derived reaction pathways were imaged from a degree of polymerization (DPn) as low as one, indicating that MCRs can form upon molecules carrying only a single monomer unit.

Controlling the Fate of Midchain Radicals in Acrylate Polymerization

Thomas Junkers studied chemistry at the Georg-August-Universitat Gottingen, where he worked for his Ph.D. thesis on laser singte-pulse -initiated polymerization kinetics under the supervision of Michael Buback. In 2006, he joined the CAMD team at the University of New South Wales, where he focussed on novel controlled polymerization methods and mass spectrometric analysis of polymers. He is now a research staff member in the group of Christopher Barner-Kowollik at the University of Karlsruhe.