Mechanism and kinetics of dithiobenzoate-mediated RAFT polymerization. I. The current situation

Modern Trends in Polymerization-Induced Self-Assembly

Polymerization-induced self-assembly (PISA) is a powerful and versatile technique for producing colloidal dispersions of block copolymer particles with desired morphologies. Currently, PISA can be carried out in various media, over a wide range of temperatures, and using different mechanisms. This method enables the production of biodegradable objects and particles with various functionalities and stimuli sensitivity. Consequently, PISA offers a broad spectrum of potential commercial applications. The aim of this review is to provide an overview of the current state of rational synthesis of block copolymer particles with diverse morphologies using various PISA techniques and mechanisms. The discussion begins with an examination of the main thermodynamic, kinetic, and structural aspects of block copolymer micellization, followed by an exploration of the key principles of PISA in the formation of gradient and block copolymers. The review also delves into the main mechanisms of PISA implementation and the principles governing particle morphology. Finally, the potential future developments in PISA are considered.

Solvent-Free Chemical Recycling of Polymethacrylates made by ATRP and RAFT polymerization: High-Yielding Depolymerization at Low Temperatures

Although controlled radical polymerization is an excellent tool to make precision polymeric materials, reversal of the process to retrieve the starting monomer is far less explored despite the significance of chemical recycling. Here, we investigate the bulk depolymerization of RAFT and ATRP-synthesized polymers under identical conditions. RAFT-synthesized polymers undergo a relatively low-temperature solvent-free depolymerization back to monomer thanks to the partial in situ transformation of the RAFT end-group to macromonomer. Instead, ATRP-synthesized polymers can only depolymerize at significantly higher temperatures (>350 °C) through random backbone scission. To aid a more complete depolymerization at even lower temperatures, we performed a facile and quantitative end-group modification strategy in which both ATRP and RAFT end-groups were successfully converted to macromonomers. The macromonomers triggered a lower temperature bulk depolymerization with an onset at 150 °C yielding up to 90 % of monomer regeneration. The versatility of the methodology was demonstrated by a scalable depolymerization (≈10 g of starting polymer) retrieving 84 % of the starting monomer intact which could be subsequently used for further polymerization. This work presents a new low-energy approach for depolymerizing controlled radical polymers and creates many future opportunities as high-yielding, solvent-free and scalable depolymerization methods are sought.

The Nanostructured Self-Assembly and Thermoresponsiveness in Water of Amphiphilic Copolymers Carrying Oligoethylene Glycol and Polysiloxane Side Chains

Amphiphilic copolymer self-assembly is a straightforward approach to obtain responsive micelles, nanoparticles, and vesicles that are particularly attractive for biomedicine, i.e., for the delivery of functional molecules. Here, amphiphilic copolymers of hydrophobic polysiloxane methacrylate and hydrophilic oligo (ethylene glycol) methyl ether methacrylate with different lengths of oxyethylenic side chains were synthesized via controlled RAFT radical polymerization and characterized both thermally and in solution. In particular, the thermoresponsive and self-assembling behavior of the water-soluble copolymers in water was investigated via complementary techniques such as light transmittance, dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS) measurements. All the copolymers synthesized were thermoresponsive, displaying a cloud point temperature (T_cp) strongly dependent on macromolecular parameters such as the length of the oligo(ethylene glycol) side chains and the content of the SiMA counits, as well as the concentration of the copolymer in water, which is consistent with a lower critical solution temperature (LCST)-type behavior. SAXS analysis revealed that the copolymers formed nanostructures in water below T_cp, whose dimension and shape depended on the content of the hydrophobic components in the copolymer. The hydrodynamic diameter (D_h) determined by DLS increased with the amount of SiMA and the associated morphology at higher SiMA contents was found to be pearl-necklace-micelle-like, composed of connected hydrophobic cores. These novel amphiphilic copolymers were able to modulate thermoresponsiveness in water in a wide range of temperatures, including the physiological temperature, as well as the dimension and shape of their nanostructured assemblies, simply by varying their chemical composition and the length of the hydrophilic side chains.

Observation and Modeling of a Sharp Oxygen Threshold in Aqueous Free Radical and RAFT Polymerization

It is known that oxygen (O₂) stops radical polymerization (RP). Here, it was found that the reaction turn-off occurs abruptly at a threshold concentration of O₂, [O₂]_t, for both free RP and reversible addition-fragmentation chain-transfer polymerization (RAFT). In some reactions, there was a spontaneous re-start of conversion. Three cases were investigated: RP of (i) acrylamide (Am) and (ii) sodium styrene sulfonate (SS) and (iii) Am RAFT polymerization. A controlled flow of O₂ into the reactor was employed. An abrupt turn-off was observed in all cases, where polymerization stops sharply at [O₂]_t and remains stopped when [O₂] > [O₂]_t. In (i), Am acts as a catalytic radical-transfer agent during conversion plateau, eliminating excess [O₂], and polymerization spontaneously resumes at [O₂]_t. In no reaction, the initiator alone was capable of eliminating O₂. N₂ purge was needed to re-start reactions (ii) and (iii). For (i) and (ii), while [O₂] < [O₂]_t, O₂ acts a chain termination agent, reducing the molecular weight (M_w) and reduced viscosity (RV). O₂ acts as an inhibitor for [O₂] > [O₂]_t in all cases. The radical-transfer rates from Am* and SS* to O₂ are >10,000× higher than the initial chain propagation step rates for Am and SS, which causes [O₂]_t at very low [O₂].

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).

Polyacrylic Acid Nanoplatforms: Antimicrobial, Tissue Engineering, and Cancer Theranostic Applications

Polyacrylic acid (PAA) is a non-toxic, biocompatible, and biodegradable polymer that gained lots of interest in recent years. PAA nano-derivatives can be obtained by chemical modification of carboxyl groups with superior chemical properties in comparison to unmodified PAA. For example, nano-particles produced from PAA derivatives can be used to deliver drugs due to their stability and biocompatibility. PAA and its nanoconjugates could also be regarded as stimuli-responsive platforms that make them ideal for drug delivery and antimicrobial applications. These properties make PAA a good candidate for conventional and novel drug carrier systems. Here, we started with synthesis approaches, structure characteristics, and other architectures of PAA nanoplatforms. Then, different conjugations of PAA/nanostructures and their potential in various fields of nanomedicine such as antimicrobial, anticancer, imaging, biosensor, and tissue engineering were discussed. Finally, biocompatibility and challenges of PAA nanoplatforms were highlighted. This review will provide fundamental knowledge and current information connected to the PAA nanoplatforms and their applications in biological fields for a broad audience of researchers, engineers, and newcomers. In this light, PAA nanoplatforms could have great potential for the research and development of new nano vaccines and nano drugs in the future.

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.

Connecting Gas-Phase Computational Chemistry to Condensed Phase Kinetic Modeling: The State-of-the-Art

In recent decades, quantum chemical calculations (QCC) have increased in accuracy, not only providing the ranking of chemical reactivities and energy barriers (e.g., for optimal selectivities) but also delivering more reliable equilibrium and (intrinsic/chemical) rate coefficients. This increased reliability of kinetic parameters is relevant to support the predictive character of kinetic modeling studies that are addressing actual concentration changes during chemical processes, taking into account competitive reactions and mixing heterogeneities. In the present contribution, guidelines are formulated on how to bridge the fields of computational chemistry and chemical kinetics. It is explained how condensed phase systems can be described based on conventional gas phase computational chemistry calculations. Case studies are included on polymerization kinetics, considering free and controlled radical polymerization, ionic polymerization, and polymer degradation. It is also illustrated how QCC can be directly linked to material properties.

Polymer-functionalization of carbon nanotube by in situ conventional and controlled radical polymerizations

Functionalization of carbon nanotube (CNT) with polymers has drawn much attention due to its wide range of applications. Polymer-functionalized CNT could exhibit variety of properties, such as responsivity to environmental stimuli, ability of complexation with metal ions, increased dispersibility in different solvents, higher compatibility with polymer matrix, etc. Chemical and physical methods have been developed for the preparation of polymer-functionalized CNT. Polymer chains are chemically bonded to the CNT edge or surface in the chemical methods, which results in highly stable CNT/polymer composites. "Grafting to", "grafting from", and "grafting through" methods are the most common chemical methods for polymer-functionalization of CNT. In "grafting to" method, pre-fabricated polymer chains are coupled with the either functionalized or non-functionalized CNT. In "grafting from" and "grafting through" methods, CNT is functionalized by polymers simultaneously synthesized by in situ polymerization methods. Conventional free radical polymerization (FRP) and also controlled radical polymerization (CRP) are the most promising methods for in situ tethering of polymer brushes onto the surface of CNT due to their control over the grafting density, thickness, and functionality of the polymer brushes. The main focus of this review is on the synthesis of polymer-functionalized CNT via both the "grafting from" and "grafting through" methods on the basis of FRP and CRP routs, which is commonly known as in situ polymerizations. Finally, the most important challenges and applications of the in situ polymer grafting methods are discussed, which could be interesting for the future works.

Controlled (Co)Polymerization of Methacrylates Using a Novel Symmetrical Trithiocarbonate RAFT Agent Bearing Diphenylmethyl Groups

Herein, we report a novel type of symmetrical trithiocarbonate chain transfer agent (CTA) based diphenylmethyl as R groups. The utilization of this CTA in the Reversible Addition-Fragmentation chain Transfer (RAFT) process reveals an efficient control in the polymerization of methacrylic monomers and the preparation of block copolymers. The latter are obtained by the (co)polymerization of styrene or butyl acrylate using a functionalized macro-CTA polymethyl methacrylate (PMMA) previously synthesized. Data show low molecular weight dispersity values (Đ < 1.5) particularly in the polymerization of methacrylic monomers. Considering a typical RAFT mechanism, the leaving groups (R) from the fragmentation of CTA should be able to re-initiate the polymerization (formation of growth chains) allowing an efficient control of the process. Nevertheless, in the case of the polymerization of MMA in the presence of this symmetrical CTA, the polymerization process displays an atypical behavior that requires high [initiator]/[CTA] molar ratios for accessing predictable molecular weights without affecting the Đ. Some evidence suggests that this does not completely behave as a common RAFT agent as it is not completely consumed during the polymerization reaction, and it needs atypical high molar ratios [initiator]/[CTA] to be closer to the predicted molecular weight without affecting the Đ. This work demonstrates that MMA and other methacrylic monomers can be polymerized in a controlled way, and with "living" characteristics, using certain symmetrical trithiocarbonates.

Multicatalytic Transformation of (Meth)acrylic Acids: a One-Pot Approach to Biobased Poly(meth)acrylates

Shifting from petrochemical feedstocks to renewable resources can address some of the environmental issues associated with petrochemical extraction and make plastics production sustainable. Therefore, there is a growing interest in selective methods for transforming abundant renewable feedstocks into monomers suitable for polymer production. Reported herein are one-pot catalytic systems, that are active, productive, and selective under mild conditions for the synthesis of copolymers from renewable materials. Each system allows for anhydride formation, alcohol acylation and/or acid esterification, as well as polymerization of the formed (meth)acrylates, providing direct access to a new library of unique poly(meth)acrylates.

Polymers via Reversible Addition-Fragmentation Chain Transfer Polymerization with High Thiol End-Group Fidelity for Effective Grafting-To Gold Nanoparticles

End-group fidelity is the most important property for end-functional polymers. Compared to other controlled living polymerization methods, reversible addition-fragmentation chain transfer (RAFT) polymerization often yields polymers with a lower end-group fidelity, which greatly affects their applications. Herein, we report a staged-thermal-initiation RAFT polymerization for the synthesis of polymers with high thiol end-group fidelity and their high efficiencies for grafting to various gold nanoparticles (GNPs). We experimentally prove that the decrease of end-group fidelity with their molecular weight is caused by the gradual decomposition of the initiator rather than the degradation of chain-transfer agents. We show that the staged-thermal-initiation RAFT polymerization is more effective for synthesis of polymers with high thiol end-group fidelity. The grafting-to assays for various GNPs illustrate the positive correlation between the end-group fidelity of polymers and grafting-to efficiency. This work highlights the prospects for synthesis of high end-group fidelity polymers and their application in the preparation of nanoparticles-polymer hybrid materials.

Achieving molecular weight distribution shape control and broad dispersities using RAFT polymerizations

Metered additions of chain transfer agents are used to control molecular weight distribution (MWD) features in reversible addition-fragmentation chain-transfer polymerizations, giving polymers with tailored MWD shapes and dispersities as high as 6.2.

How does the single unit monomer insertion technique promote kinetic analysis of activation and initiation in photo-RAFT processes?

The single unit monomer insertion technique provides a simple platform for the kinetic investigation of early stage of photo-RAFT process that comprises photo-activation of initial RAFT agents and addition of RAFT leaving radicals to the monomers.

Influence of structure and solubility of chain transfer agents on the RAFT control of dispersion polymerisation in scCO2

Reversible addition-fragmentation chain transfer (RAFT) dispersion polymerisation of methyl methacrylate (MMA) is performed in supercritical carbon dioxide (scCO2) with 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) present as chain transfer agent (CTA) and surprisingly shows good control over PMMA molecular weight. Kinetic studies of the polymerisation in scCO2 also confirm these data. By contrast, only poor control of MMA polymerisation is obtained in toluene solution, as would be expected for this CTA which is better suited for acrylates. In this regard, we select a range of CTAs and use them to determine the parameters that must be considered for good control in dispersion polymerisation in scCO2. A thorough investigation of the nucleation stage during the dispersion polymerisation reveals an unexpected "in situ two-stage" mechanism that strongly determines how the CTA works. Finally, using a novel computational solvation model, we identify a correlation between polymerisation control and degree of solubility of the CTAs. All of this ultimately gives rise to a simple, elegant and counterintuitive guideline to select the best CTA for RAFT dispersion polymerisation in scCO2.

Fundamentals of Emulsion Polymerization

A comprehensive overview of the fundamentals of emulsion polymerization and related processes is presented with the object of providing theoretical and practical understanding to researchers considering use of these methods for synthesis of polymer colloids across a wide range of applications. Hence, the overview has been written for a general scientific audience with no prior knowledge assumed. Succinct introductions are given to key topics of background science to assist the reader. Importance is placed on ensuring mechanistic understanding of these complex polymerizations and how the processes can be used to create polymer colloids that have particles with well-defined properties and morphology. Mathematical equations and associated theory are given where they enhance understanding and learning and where they are particularly useful for practical application. Practical guidance also is given for new researchers so that they can begin using the various processes effectively and in ways that avoid common mistakes.

Graft modification of lignin-based cellulose via enzyme-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization and free-radical coupling

In this study, a green approach combining enzyme-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization and free-radical coupling was developed for the modification of jute fiber, which is a typical lignin-based cellulose. Jute fiber surface was covered by rich amount of lignin, which offered great opportunities for further functional modification. The controlled polymerization of vinyl monomers, acrylamide (AM) or butyl acrylate (BA), was carried out by horseradish peroxidase (HRP)-initiated RAFT to form well-defined polymers with well-controlled molecular weights and structures. Enzymatic grafting by HRP occurred between the free radicals of well-defined polymers and free radicals of lignin on jute. Gel permeation chromatography (GPC) analysis indicated the alkyl chain length of polymers prepared via HRP-initiated RAFT polymerization was well-controlled. Other results of flourier transformed infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) revealed that well-controlled alkyl chains prepared via enzymatic catalysis were grafted on the exposed lignin of jute. The study explores a new and eco-friendly modification method for lignin-based materials with the controlled graft chain structure via two different catalysis with HRP.

Synthetic approaches for multiblock copolymers

Multiblock copolymers (MBCs) are an emerging class of synthetic polymers that exhibit different macromolecular architectures and behaviours to those of homopolymers or di/triblock copolymers.

Rigor and reproducibility in polymer nanoparticle synthesis and characterization

Standardized process improvement methods and tools were used to enhance the rigor and reproducibility of diblock copolymer nanoparticle (NP) synthesis and characterization. Models linking design parameters with NP characteristics boosted process control for NP synthesis, which may improve translation and commercialization of NP research.

Novel modeling and process control approaches provide useful insights to improve rigor and reproducibility in polymer nanoparticle synthesis and characterization.

New Aspects on the Modeling of Dithiolactone-Mediated Radical Polymerization of Vinyl Monomers

A kinetic model for the dithiolactone-mediated radical polymerization of vinyl monomers based on the persistent radical effect and reversible addition (negligible fragmentation) was used to calculate the polymerization rate and describe molar mass development in the polymerization of methyl methacrylate at 60 °C, using 2,2-azobisisobutyronitrile (AIBN) as an initiator, as well as dihydro-5-phenyl-2(3H)-thiophenethione (DTL1) and dihydro-2(3H)-thiophenethione (DTL2) as controllers. The model was implemented in the PREDICI commercial software. A good agreement between experimental data and model predictions was obtained.

Suspending Polyrotaxane Dissociation via Photo-Reversible Capping of Terminals

Reversible covalent bonds yield polymeric materials with functional characteristics such as self-healing, shape memory, stress relaxation, and stimuli-responsiveness. Here, photo-reversibly cappable polyrotaxanes are designed and the on-off controlled dissociation of their supramolecular architectures is demonstrated. The polyrotaxanes are synthesized by capping dithiobenzoates at both terminals of polyethylene glycol threaded through multiple α-cyclodextrins. Since dethreading of the α-cyclodextrins is prevented by the dithiobenzoate stoppers, the supramolecular dissociation is induced by their photo-cleavage. Subsequently, the cleaved dithiobenzoates spontaneously re-cap the polyrotaxane terminals in darkness. Thus, the supramolecular dissociation can be modulated by photo-reversible capping of the dithiobenzoate stoppers. These polyrotaxanes with dithiobenzoate stoppers are promising functional materials for photo-controlling physical properties and structures.

Modeling of Miniemulsion Polymerization of Styrene with Macro-RAFT Agents to Theoretically Compare Slow Fragmentation, Ideal Exchange and Cross-Termination Cases

A 5-dimensional Smith-Ewart based model is developed to understand differences for reversible addition-fragmentation chain transfer (RAFT) miniemulsion polymerization with theoretical agents mimicking cases of slow fragmentation, cross-termination, and ideal exchange while accounting for chain length and monomer conversion dependencies due to diffusional limitations. The focus is on styrene as a monomer, a water soluble initiator, and a macro-RAFT agent to avoid exit/entry of the RAFT leaving group radical. It is shown that with a too low RAFT fragmentation rate coefficient it is generally not afforded to consider zero-one kinetics (for the related intermediate radical type) and that with significant RAFT cross-termination the dead polymer product is dominantly originating from the RAFT intermediate radical. To allow the identification of the nature of the RAFT retardation it is recommended to experimentally investigate in the future the impact of the average particle size (dp) on both the monomer conversion profile and the average polymer properties for a sufficiently broad dp range, ideally including the bulk limit. With decreasing particle size both a slow RAFT fragmentation and a fast RAFT cross-termination result in a stronger segregation and thus rate acceleration. The particle size dependency is different, allowing further differentiation based on the variation of the dispersity and end-group functionality. Significant RAFT cross-termination is specifically associated with a strong dispersity increase at higher average particle sizes. Only with an ideal exchange it is afforded in the modeling to avoid the explicit calculation of the RAFT intermediate concentration evolution.

Upper Critical Solution Temperature (UCST) Behavior of Polystyrene-Based Polyampholytes in Aqueous Solution

Strong polyampholytes comprising cationic vinylbenzyl trimethylammonium chloride (VBTAC) bearing a pendant quaternary ammonium group and anionic sodium p-styrenesulfonate (NaSS) bearing a pendant sulfonate group were prepared via reversible addition-fragmentation chain-transfer polymerization. The resultant polymers are labelled P(VBTAC/NaSS)n, where n indicates the degree of polymerization (n = 20 or 97). The percentage VBTAC content in P(VBTAC/NaSS)n is always about 50 mol%, as revealed by ¹H NMR measurements, meaning that P(VBTAC/NaSS)n is a close to stoichiometrically charge-neutralized polymer. Although P(VBTAC/NaSS)n cannot dissolve in pure water at room temperature, the addition of NaCl or heating solubilizes the polymers. Furthermore, P(VBTAC/NaSS)n exhibits upper critical solution temperature (UCST) behavior in aqueous NaCl solutions. The UCST is shifted to higher temperatures by increasing the polymer concentration and molecular weight, and by decreasing the NaCl concentration. The UCST behavior was measured ranging the polymer concentrations from 0.5 to 5.0 g/L.

RAFT-mediated polymerisation of dialkylaminoethyl methacrylates in tert-butanol

Dialkylaminoethyl methacrylates were polymerised by RAFT in tert-butanol to make macro-chain transfer agents for subsequent grafting onto various substrates.

Temperature-responsive copolymers without compositional drift by RAFT copolymerization of 2-(acryloyloxy)ethyl trimethylammonium chloride and 2-(diethylamino)ethyl acrylate

Thermoresponsive copolymers based on AEtMACl and protonated DEAEA feature RAFT copolymerization kinetics with both apparent reactivity ratios of about 1.

Degradable vinyl copolymers through thiocarbonyl addition-ring-opening (TARO) polymerization

The radical copolymerization of the thionolactone dibenzo[c,e]oxepane-5-thione with acrylates, acrylonitrile, and N,N-dimethylacrylamide afforded copolymers containing a controllable amount of backbone thioesters which could be selectively cleaved. The process is compatible with RAFT polymerization and promising for the development of advanced degradable polymers.

Mapping Dithiobenzoate-Mediated RAFT Polymerization Products via Online Microreactor/Mass Spectrometry Monitoring

2-cyano-2-propyl dithiobenzoates (CPDB)-mediated reversible addition-fragmentation chain transfer (RAFT) polymerization was monitored by online flow microreactor/mass spectrometry. This enabled the reactions to be followed in a time-resolved manner, closely resolving product patterns in the reaction mixtures at any point in time. RAFT polymerization was investigated for low RAFT to monomer ratios, enabling the monitoring of the early stages of a typical RAFT polymerization. The expected transition from pre- to the RAFT main equilibrium is observed. However, very high abundancies for cross-termination products were also identified, both in the pre- and main equilibrium stage. This is a somewhat surprising result as such products have always been expected, but to date have not been observed in the majority of studies. Product isolation and NMR analysis revealed that cross-termination occurs in the para position of the benzoate ring and becomes fully irreversible via re-aromatization of the ring in a H-shift reaction. The present data suggest a pronounced chain-length dependence of the cross-termination reaction, which would explain why the products are seen here, but not in other studies.