Intermediate Radical Termination Research Articles

Ubiquitous Nature of Rate Retardation in Reversible Addition-Fragmentation Chain Transfer Polymerization.

Reversible addition-fragmentation chain transfer (RAFT) polymerization is one of the most powerful reversible deactivation radical polymerization (RDRP) processes. Rate retardation is prevalent in RAFT and occurs when polymerization rates deviate from ideal conventional radical polymerization kinetics. Herein, we explore beyond what was initially thought to be the culprit of rate retardation: dithiobenzoate chain transfer agents (CTA) with more active monomers (MAMs). Remarkably, polymerizations showed that rate retardation occurs in systems encompassing the use of trithiocarbonates and xanthates CTAs with varying monomeric activities. Both the simple slow fragmentation and intermediate radical termination models show that retardation of all these systems can be described by using a single relationship for a variety of monomer reactivity and CTAs, suggesting rate retardation is a universal phenomenon of varying severity, independent of CTA composition and monomeric activity level.

Kinetic importance of the missing step in dithiobenzoate-mediated RAFT polymerizations of acrylates

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.

Experimental and Theoretical Comparison of Addition–Fragmentation Pathways of Diseleno- and Dithiocarbamate RAFT Agents

Theoretical studies suggest that identically substituted dithio- and diselenocarbamate reversible addition–fragmentation chain transfer (RAFT) agents will have similar reactivity but that diselenocarbamate RAFT agents are more likely to undergo intermediate radical termination. These results are supported by an experimental study of polymerizations of vinyl acetate mediated by Se-cyanomethyl N,N-dimethyldiselenocarbamate (1Se) and its sulfur analog, S-cyanomethyl N,N-dimethyldithiocarbamate (1S). While the two polymerizations had similar interchain transfer constants (3.2 for 1S and 3.9 for 1Se), significant retardation was observed for 1Se-mediated polymerizations. External chain transfer constants were also measured for 1S (5.4) and 1Se (45). The low apparent value of the chain transfer constant of 1S is attributed to the selective formation of a single monomer adduct in the early stages of the reaction.

Modeling of RAFT Polymerization Processes Using an Efficient Monte Carlo Algorithm in Julia

A kinetic Monte Carlo model of a reversible addition–fragmentation chain transfer (RAFT) process is presented. The algorithm has been developed and implemented in Julia for the three main RAFT theories under current discussion (slow fragmentation, intermediate radical termination, and intermediate radical termination with oligomers). Julia is a modern programming language designed to achieve high performance in numerical and scientific computing. Thanks to a careful optimization of the code, it is possible to simulate a RAFT reaction scheme in short computing times for any of the three theories. The code is benchmarked against other programming languages (MATLAB, Python, FORTRAN, and C), showing that Julia presents advantages for this particular system. The model offers an efficient method for predicting average properties and molecular weight distributions of the polymer species, including the bivariate molecular weight distribution of the intermediate two-arm adduct. The proposed model can also be emplo...

The influence of intermediate radical termination and fragmentation on controlled polymer synthesis via RAFT polymerization

Reversible addition-fragmentation chain transfer (RAFT) polymerization allows the design of tailored polymers by inducing addition and fragmentation reactions via a powerful kinetic mechanism; the process, however, is not well understood. A parametric analysis was conducted in conjunction with experimental data to provide insights into the RAFT mechanism, accounting for the key kinetics events in RAFT polymerization of styrene. The effects of overall, forward and backward fragmentations of intermediate radicals were analysed with experimental data and sensitivity investigations. Our results show that the intermediate radicals undergo relatively fast fragmentation and that the radical cleavage is symmetrical. These factors, critical in design and control, were found to profoundly influence the process kinetics and the final polymer product attributes.

EPR Study into Cross‐Termination and Fragmentation with the Phenylethyl–Phenylethyl Dithiobenzoate RAFT Model System

AbstractThe reversible addition–fragmentation chain transfer (RAFT) equilibrium constant of the phenylethyl–1‐phenylethyl dithiobenzoate model system is deduced via EPR spectroscopy to be 31 ± 4 L mol−1 at 110 °C. The difference in activation energies of addition and fragmentation, Ea(kad) – Ea(kβ) ≈ −45.6 kJ mol−1, is close to that of the polystyryl–polystyryl dithiobenzoate system. Significant amounts of products from cross‐termination and from “missing step” reaction of the cross‐termination product with the phenylethyl radical demonstrate that intermediate radical termination, rather than slow fragmentation of the RAFT intermediate radical, is responsible for the rate retardation in the dithiobenzoate‐mediated styrene polymerization.magnified image

Thioketone‐Mediated Polymerization with Dithiobenzoates: Proof for the Existence of Stable Radical Intermediates in RAFT Polymerization

A novel dithioester control agent [dimethyltetrathioterephtalate (DMTTT)] is presented for the thioketone-mediated radical polymerization (TKMP) of n-butyl acrylate. The rate of polymerization is significantly decreased in the presence of DMTTT indicating formation of dormant radical species. During polymerization, molar masses increase linearly with monomer conversion with reasonably narrow initial molar mass distributions (PDI between 1.3 and 1.8), whereas the dispersity increases during the course of the polymerization due to irreversible termination of both propagating and dormant radicals. The present results thus highlight the possibility of a mixed mechanism operating in RAFT polymerization, which combines slow fragmentation (long-lived intermediates) and intermediate radical termination.

“Assessing the RAFT Equilibrium Constant via Model Systems: An EPR Study”—Response to a Comment

We have presented an EPR-based approach for deducing the RAFT equilibrium constant, K(eq), of a dithiobenzoate-mediated system [Meiser, W. and Buback M. Macromol. Rapid Commun. 2011, 32, 1490]. Our value is by four orders of magnitude below K(eq) from ab initio calculations for the identical monomer-free system. Junkers et al. [Macromol. Rapid Commun. 2011, 32, 1891] claim that our EPR approach would be model dependent and our data could be equally well fitted by assuming slow addition of radicals to the RAFT agent and slow fragmentation of the so-obtained intermediate radical as well as high cross-termination rate. By identification of all side products, our EPR-based method is shown to be model independent and to provide reliable K(eq) values, which demonstrate the validity of the intermediate radical termination model.

Full Molecular Weight Distribution in RAFT Polymerization. New Mechanistic Insight by Direct Integration of the Equations

AbstractThe full molecular weight distribution (FMWD) equations in RAFT polymerization are solved by direct integration for three models: intermediate radical termination (IRT), slow fragmentation (SF), and IRT with oligomers (IRTO). It is shown that the quasi steady state approximation (QSSA) is always applicable to living radicals, and also applicable to intermediate radicals in the IRT and IRTO models. The application of the QSSA allows for considerable simplification of the numerical solution of the equations. A marked bimodality is found for the FMWD of the SF model only. This suggests model discrimination by radical trapping experiments followed by GPC measurement of the total MWD. magnified image

RAFT‐Mediated Polymerization—A Story of Incompatible Data?

The mechanistic interpretation of kinetic anomalies in reversible addition-fragmentation chain transfer (RAFT)-mediated polymerization is critically reviewed. The main conclusion of this exercise is that available data do not allow model discrimination between the two prevailing mechanistic schemes, i.e., the slow fragmentation model and the intermediate radical termination model. However, assessment of the rate parameters reveals that the incompatibilities may not be as large as previously reported in literature. Dedicated kinetic studies on model compounds should be performed to shed further light on the seemingly incompatible data that currently exists in literature.

Intermediate Radical Termination in Reversible Addition-Fragmentation Chain Transfer-Mediated Polymerization: Identification of Termination Products

Rate retardation in reversible addition-fragmentation chain transfer (RAFT)-mediated polymerizations is the topic of an ongoing debate. One of the main explanations for this rate retardation is termination of the intermediate radical. The use of a RAFT agent with a UV label in the leaving group acts as a means to identify products of intermediate radical termination (IRT). In the analysis of the polymeric samples with size exclusion chromatography (SEC), two different UV absorbance traces were compared (i.e., the UV absorbance of the label used and that of the thiocarbonyl thio moiety of the RAFT agent). Deviations in the ratio of the CS of the RAFT moiety to the UV label indicate that IRT products could be formed. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry was used to identify these IRT products formed during a “normal” RAFT-mediated polymerization (not using forced conditions). These experimental findings support the postulated IRT model and therefore help to elucidate the fate of the intermediate radical during a RAFT mediated polymerization.

Interpretation of Reversible Addition−Fragmentation Chain-Transfer Polymerization Mechanism by MALDI-TOF-MS

Several polystyrene polymers were prepared by reversible addition-fragmentation chain-transfer (RAFT) polymerization of styrene, using two different RAFT agent-initiator systems, and then further characterized by NMR and SEC as well as MALDI-TOF-MS techniques. The data indicate that most of the polymer chains are terminated by the active groups (Ph-C(=S)-S-) derived from RAFT agents, and few of the polymer chains bear initiator fragments at one end. Most importantly, the structures arising from the intermediate RAFT radicals and their cross-termination adducts were detected. Also, the MALDI-TOF-MS analysis shows that the combination termination between two macromolecular radicals is minor, and the amount of dead chains is quite low. Thus, narrow molecular weight distribution is obtained. This analysis confirms the operation of the Rizzardo mechanism including the Monteiro intermediate radical termination model for the RAFT polymerization.

A Mechanistic Interpretation of Initialization Processes in RAFT‐Mediated Polymerization

AbstractSummary: The presence of strong chain length selectivity behavior during initialization in the RAFT process is an experimental observation. The mechanistic underpinning of the observation is, however, open to interpretation. The rates and concentration profiles for the cyanoisopropyl‐dithiobenzoate‐mediated polymerization of styrene at 70 °C can be relatively well reproduced by either the slow fragmentation or intermediate radical termination models. The use of modeling that provides a fit to the data is an important tool, but should be considered as additional evidence in support of experimental data rather than as experimental evidence in and of itself.Comparison of the time dependencies of the concentrations of AD, AMD, and AM2D by PREDICI modeling for the cyanoisopropyl‐mediated polymerization of styrene at 70 °C generated by the IRT model (lines) versus experimental data.magnified imageComparison of the time dependencies of the concentrations of AD, AMD, and AM2D by PREDICI modeling for the cyanoisopropyl‐mediated polymerization of styrene at 70 °C generated by the IRT model (lines) versus experimental data.

Design strategies for controlling the molecular weight and rate using reversible addition–fragmentation chain transfer mediated living radical polymerization

AbstractLiving radical polymerization has allowed complex polymer architectures to be synthesized in bulk, solution, and water. The most versatile of these techniques is reversible addition–fragmentation chain transfer (RAFT), which allows a wide range of functional and nonfunctional polymers to be made with predictable molecular weight distributions (MWDs), ranging from very narrow to quite broad. The great complexity of the RAFT mechanism and how the kinetic parameters affect the rate of polymerization and MWD are not obvious. Therefore, the aim of this article is to provide useful insights into the important kinetic parameters that control the rate of polymerization and the evolution of the MWD with conversion. We discuss how a change in the chain‐transfer constant can affect the evolution of the MWD. It is shown how we can, in principle, use only one RAFT agent to obtain a polymer with any MWD. Retardation and inhibition are discussed in terms of (1) the leaving R group reactivity and (2) the intermediate radical termination model versus the slow fragmentation model. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3189–3204, 2005

High Pressure 'Living' Free-Radical Polymerization of Styrene in the Presence of RAFT

Reversible addition-fragmentation chain transfer (RAFT) polymerization of styrene was studied at high pressure, employing two dithioester RAFT agents with an isopropylcyano (5) and a cumyl (6) leaving group, respectively. The high-pressure reaction resulted in low polydispersity polymer. It was found that controlled polymerizations can be performed at increased pressures with a high degree of monomer conversion, which signifies that high-pressure polymerizations can be utilized for the production of higher molecular weight polystyrene of controlled microstructure. Retardation of styrene polymerization was also observed at high pressure in the presence of RAFT agents (5) and (6). It is postulated that the retarding potential of these two RAFT agents is associated with an intermediate radical termination mechanism. High-pressure free-radical polymerizations open the way to producing living polymers with high rates, and thus lower impurities such as 'dead' polymer that are formed through bimolecular termination reactions.

Intermediate Radical Termination as the Mechanism for Retardation in Reversible Addition−Fragmentation Chain Transfer Polymerization

Intermediate Radical Termination as the Mechanism for Retardation in Reversible Addition−Fragmentation Chain Transfer Polymerization