Thiocarbonylthio End Groups Research Articles

Controlled switching thiocarbonylthio end-groups enables interconvertible radical and cationic single-unit monomer insertions and RAFT polymerizations

To emulate the ordered arrangement of monomer units found in natural macromolecules, single-unit monomer insertion (SUMI) have emerged as a potent technique for synthesizing sequence-controlled vinyl polymers. Specifically, numerous applications necessitate vinyl polymers encompassing both radically and cationically polymerizable monomers, posing a formidable challenge due to the distinct thiocarbonylthio end-groups required for efficient control over radical and cationic SUMIs. Herein, we present a breakthrough in the form of interconvertible radical and cationic SUMIs achieved through the manipulation of thiocarbonylthio end-groups. The transition from a trithiocarbonate (for radical SUMI) to a dithiocarbamate (for cationic SUMI) is successfully accomplished via a radical-promoted reaction with bis(thiocarbonyl) disulfide. Conversely, the reverse transformation utilizes the reaction between dithiocarbamate and bistrithiocarbonate disulfide under a cationic mechanism. Employing this strategy, we demonstrate a series of synthetic examples featuring discrete oligomers containing acrylate, maleimide, vinyl ether, and styrene, compositions unattainable through the SUMI of a single mechanism alone.

Photoassisted Radical Depolymerization.

Controlled radical polymerization techniques enable the synthesis of polymers with predetermined molecular weights, narrow molecular weight distributions, and controlled architectures. Moreover, these polymerization approaches have been routinely shown to result in retained end-group functionality that can be reactivated to continue polymerization. However, reactivation of these end groups under conditions that instead promote depropagation is a viable route to initiate depolymerization and potentially enable closed-loop recycling from polymer to monomer. In this report, we investigate light as a trigger for thermal depolymerization of polymers prepared by reversible-addition-fragmentation chain-transfer (RAFT) polymerization. We study the role of irradiation wavelength by targeting the n → π* and π → π* electronic transitions of the thiocarbonylthio end-groups of RAFT-generated polymers to enhance depolymerization via terminal bond homolysis. Specifically, we explore depolymerization of polymers with trithiocarbonate, dithiocarbamate, and p-substituted dithiobenzoate end groups with the purpose of increasing depolymerization efficiency with light. As the wavelength decreases from the visible range to the UV range, the rate of depolymerization is dramatically increased. This method of photoassisted depolymerization allows up to 87% depolymerization efficiency within 1 h, results that may further the advancement of recyclable materials and life-cycle circularity.

Controlled Arrangement of Gold Nanoparticles on Planar Surfaces via Constrained Dewetting of Surface-Grafted RAFT Polymer.

Linear and four-arm star polystyrene samples prepared by RAFT polymerization were grafted to gold surfaces directly via their thiocarbonylthio-end groups. Nanoscale polymer patterns were subsequently formed via constrained dewetting. The patterned polymer films then served as a template for the precise arrangement of gold nanoparticles in a monolayer with a well-defined and regular structure. Using star polymers as a linker between the planar gold surface and the particles, the structural stability of the arranged particles can be further enhanced. The surface-bound nanocomposite films made of polymer and nanoparticles can also reversibly switch their nanostructures by simple wetting or dewetting treatment.

Tetrazole functional copolymers: Facile access to well-defined Rhenium(I)-Polymeric luminescent materials

Well-defined random copolymers containing tetrazole functional groups were prepared by a combination of reversible addition-fragmentation chain transfer (RAFT) radical homopolymerisation of pentafluorophenyl acrylate (PFPA), end-group modification and post-polymerisation modification of the activated ester side-groups with primary amines. PolyPFPA with a size exclusion chromatography (SEC) measured Mn of 10,800 and dispersity, Ð, of 1.05 was prepared with 2-cyano-2-propyl benzodithioate as the RAFT chain transfer agent and 2,2′-azobisisobutyronitrile (AIBN) as the source of primary radicals in 1,4-dioxane. Prior to modification of the pentafluorophenyl (PFP) ester side-groups, the thiocarbonylthio end-group was removed via a radical-mediated desulfurisation employing two equivalents of AIBN. Complete removal of the end-group was successful as judged by UV–vis spectroscopy with no effect on the molecular weight distribution of the parent homopolymer based on SEC analysis. The pendent PFP ester groups were subsequently modified in a sequential manner with n-butylamine and 5-aminotetrazole yielding the corresponding random amide species quantitatively, as judged by 19F NMR and FTIR spectroscopies, and of varying molar composition as determined by 1H NMR spectroscopy. The reaction of the tetrazole functional copolymers with Re(CO)3(phen)Br (phen = 1,10-phenanthroline) yielded the corresponding rhenium-polymer hybrids via coordination through the pendent tetrazole functional group. Successful, and quantitative coordination was confirmed by FTIR spectroscopy and detailed photophysical studies. In the case of the latter, the absorption profiles of all hybrid materials possessed broad bands centred around ca. 370 nm associated with metal-to-ligand charge transfer (MLCT) transitions; excitation as this wavelength results in a broad emission centred at 606 nm for all hybrid species and is due to emission from the triplet MLCT excited states.

Fast and Complete Neutralization of Thiocarbonylthio Compounds Using Trialkylborane and Oxygen: Application to Their Removal from RAFT-Synthesized Polymers.

A rapid and efficient method to remove thiocarbonylthio end groups from polymers prepared by reversible addition-fragmentation chain transfer (RAFT) is described. The elimination process is obtained in less than 1 min by treating the solution of RAFT-synthesized polymers with 5 equiv of trialkylborane (TAB) in the presence of oxygen under an ambient temperature. The versatility of this method was checked on the most relevant families of thiocarbonylthio chain transfer agents (CTA), including dithioesters, trithiocarbonates, dithiocarbamates, and xanthates, carried by the corresponding RAFT-synthesized polymers. UV, NMR, and MALDI-TOF MS characterization results all confirm the complete removal of their terminal CTA groups.

Effect of Scandium Triflate on the RAFT Copolymerization of Methyl Acrylate and Vinyl Acetate Controlled by an Acid/Base “Switchable” Chain Transfer Agent

Modulation of the activity of an acid/base switchable dithiocarbamate RAFT agent, cyanomethyl (4-fluorophenyl)(pyridin-4-yl)carbamodithioate, with the Lewis acid scandium triflate (Sc(OTf)3) was investigated to examine the ability to deliver improved control over RAFT copolymerizations involving both more-activated and less-activated monomers—specifically the copolymerization of methyl acrylate (MA) and vinyl acetate (VAc). The introduction of either 0.5 or 1 mol equiv of Sc(OTf)3, with respect to RAFT agent, into a RAFT copolymerization of MA and VAc provides substantially improved control resulting in significantly reduced molar mass dispersities (Đ) (∼1.1–1.3) than achieved in its absence (Đ ∼ 1.3–1.4). Furthermore, similar introduction of Sc(OTf)3 into MA homopolymerization mediated by the same RAFT agent also delivered polymers of very low Đ (∼1.15). Sc(OTf)3 was also found to lower the rate of polymerization and alter the copolymerization reactivity ratios for MA and VAc. Increasing the Lewis acid concentration provides enhanced incorporation of the less active monomer, VAc, into the copolymers ([Sc(OTf)3]/[RAFT] = 0, rMA = 4.04, rVAc = 0.032; [Sc(OTf)3]/[RAFT] = 0.5, rMA = 3.08, rVAc = 0.17; [Sc(OTf)3]/[RAFT] = 1, rMA = 2.68, rVAc = 0.62). Carbon nuclear magnetic resonance (13C NMR) and differential scanning calorimetry (DSC) analysis of preparative samples confirm the enhanced VAc incorporation with increased levels of Sc(OTf)3. Importantly the inclusion of Sc(OTf)3 does not deleteriously affect the thiocarbonylthio end-groups of the RAFT polymers, with high end-group fidelity being observed in all copolymerizations.

RAFT polymer cross-coupling with boronic acids.

The ability to modify the thiocarbonylthio end-groups of RAFT polymers is important for applications where an inert or highly functionalised material is required. Here we report a copper promoted cross-coupling reaction between RAFT polymer end-groups and aryl boronic acids. This method gives high conversion to the modified polymers, and is compatible with a wide variety of functional molecules.

Statistical copolymers of N-vinylpyrrolidone and benzyl methacrylate via RAFT: Monomer reactivity ratios, thermal properties and kinetics of thermal decomposition

ABSTRACTStatistical copolymers of N-vinylpyrrolidone (NVP) with benzyl methacrylate (BzMA) were prepared by Reversible Addition-Fragmentation chain Transfer Polymerization (RAFT), employing three different xanthates as RAFT agents: [(O-ethylxanthyl)methyl]benzene, [1-(O-ethylxanthyl)ethyl]benzene and O-ethyl S-(phthalimidylmethyl) xanthate. The reactivity ratios were estimated using the Fineman-Ross, inverted Fineman-Ross, Kelen-Tüdos and extended Kelen-Tüdos linear graphical methods, as well as the computer program COPOINT. Structural parameters of the copolymers were obtained by calculating the dyad sequence fractions and the mean sequence length. All the methods are in agreement that the probability for the incorporation of BzMA units is much greater than for that of NVP units. The glass-transition temperature (Tg) values of the copolymers were measured by Differential Scanning Calorimetry and examined by means of several theoretical equations, allowing for the prediction of these Tg values. Subsequently, a systematic and detailed investigation of the thermal degradation of these copolymers compared with that of the respective homopolymers, was carried out by Thermogravimetric Analysis, within the framework of the Ozawa-Flynn-Wall and Kissinger methodologies. As expected, the thermal stability of the copolymers is influenced by the structure of the substituents of the thiocarbonylthio end groups due to the RAFT agents.

Statistical copolymers of N‐vinylpyrrolidone and 2‐(dimethylamino)ethyl methacrylate via RAFT: Monomer reactivity ratios, thermal properties, and kinetics of thermal decomposition

ABSTRACTStatistical copolymers of N‐vinylpyrrolidone (NVP) with 2‐(dimethylamino)ethyl methacrylate (DMAEMA) were prepared by Reversible Addition‐Fragmentation chain Transfer Polymerization (RAFT), employing three different RAFT agents: [(O‐ethylxanthyl)methyl]benzene, [1‐(O‐ethylxanthyl)ethyl]benzene, and O‐ethyl S‐(phthalimidylmethyl) xanthate. The reactivity ratios were estimated using the Fineman‐Ross, inverted Fineman‐Ross, Kelen‐Tüdos, and extended Kelen‐Tüdos graphical methods, along with the computer program COPOINT. Structural parameters of the copolymers were obtained by calculating the dyad sequence fractions and the mean sequence length. All the methods indicate that the DMAEMA reactivity ratio is much greater than the one of NVP, thus, the statistical copolymers are in fact pseudo‐diblocks. The glass‐transition temperature (Tg) values of the copolymers were measured by Differential Scanning Calorimetry. Furthermore, a systematic and detailed investigation has been done, on the thermal degradation of the copolymers compared with the respective homopolymers, by Thermogravimetric Analysis, within the framework of the Ozawa‐Flynn‐Wall and Kissinger methodologies. Apparently, the thermal stability of the copolymers is influenced by both monomers and by the structure of the thiocarbonylthio end groups due to the RAFT agents. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 3776–3787

Catalyst-Free Photoinduced End-Group Removal of Thiocarbonylthio Functionality.

An initiator- and catalyst-free method for polymer end-group modification has been designed. Under long-wave ultraviolet irradiation, polymers with thiocarbonylthio end groups undergo photolytic cleavage to reveal an active macroradical capable of irreversible termination with a suitable hydrogen source. This straightforward method was successfully demonstrated by the removal of a range of end groups that commonly result from reversible addition-fragmentation chain transfer or photoiniferter polymerizations, including trithiocarbonate, dithiobenzoate, xanthate, and dithiocarbamate mediating agents. This strategy proved efficient for polymers derived from acrylamido, acrylic, methacrylic, styrenic, and vinylpyrrolidone monomers.

Surface-enhanced Raman encoded polymer stabilized gold nanoparticles: Demonstration of potential for use in bioassays

The preparation of biotinylated, self-assembled polymer stabilized gold nanoparticle hybrids encoded with a SERS active compound is described. The polymers used for nanoparticle stabilization are carefully designed for this purpose and are synthesized by the RAFT polymerization process, as the thiocarbonylthio end group provides a functional handle for anchoring the polymers to the gold surface. Functionalised biotin moieties are attached to the hybrid nanoparticles via Cu-catalyzed azide-alkyne cycloaddition. Binding of the biotinylated hybrid nanoparticles to streptavidin was confirmed by nanoparticle detection and identification by the SERS spectrum of the surface-bound SERS active compound, quinolinethiol. This investigation includes the requisites that constitute a bioassay, demonstrating the potential of polymer-coated hybrid nanoparticles for this purpose.

Surface Modification of Gd Nanoparticles with pH-Responsive Block Copolymers for Use As Smart MRI Contrast Agents.

Despite recent advances in the understanding of fundamental cancer biology, cancer remains the second most common cause of death in the United States. One of the primary factors indicative of high cancer morbidity and mortality and aggressive cancer phenotypes is tumors with a low extracellular pH (pHe). Thus, the ability to measure tumor pHe in vivo using noninvasive and accurate techniques that also provide high spatiotemporal resolution has become increasingly important and is of great interest to researchers and clinicians. In an effort to develop a pH-responsive magnetic resonance imaging (MRI) contrast agent (CA) that has the potential to be used to measure tumor pHe, well-defined pH-responsive polymers, synthesized via reversible addition-fragmentation chain transfer polymerization, were attached to the surface of gadolinium-based nanoparticles (GdNPs) via a "grafting to" method after reduction of the thiocarbonylthio end groups. The successful modification of the GdNPs was verified by transmission electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis and dynamic light scattering. The performance of the pH-responsive polymer modified GdNPs was then evaluated for potential use as smart MRI CAs via monitoring the relaxivity changes with changing environmental pH. The results suggested that the pH-responsive polymers can be used to effectively modify the GdNPs surface to prepare a smart contrast agent for MRI.

Transformation of RAFT Polymer End Groups into Nitric Oxide Donor Moieties: En Route to Biochemically Active Nanostructures.

Polymers with a terminal S-nitrosothiol moiety were synthesized by modifying the thiocarbonylthio end group formed by reversible addition-fragmentation chain transfer polymerization. Specifically, benzodithioate-terminated poly[oligo(ethylene glycol) methyl ether methacrylate] (POEGMA) was first synthesized by polymerizing OEGMA in the presence of 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid. Sequential treatment with hydrazine hydrate and a stoichiometric amount of nitrous acid resulted in the formation of S-nitrosothiol-terminated polymers. A similar approach was applied to block copolymers of POEGMA incorporating a domain of poly[(N,N-diisopropylamino)ethyl methacrylate], thus, enabling the preparation of pH responsive nitric oxide (NO)-releasing micelles. The micelles possessed substantially modified S-nitrosothiol loss kinetics compared to the hydrophilic homopolymer analogue. Moreover, thiol-triggered degradation of the S-nitrosothiol was significantly slower when the S-nitrosothiol was embedded in a micellar structure. These results demonstrate that it is possible to incorporate nitric oxide donor moieties directly onto a polymer chain end, enabling simple synthesis of biochemically active nanostructures.

Sequential flow process for the controlled polymerisation and thermolysis of RAFT-synthesised polymers

The RAFT (Reversible Addition-Fragmentation Chain Transfer) process greatly enhances the control over radical polymerisations, while leaving behind a thiocarbonylthio end-group. Thermolysis presents a convenient and efficient way of removing the thiocarbonylthio end-group from RAFT polymers, without the use of additional reagents. This paper describes a simple two-step flow process for the synthesis of RAFT polymers followed by the subsequent removal of the RAFT end-group via thermolysis, without the need for isolating intermediates. A range of different polymers based on styrene, acrylates, methacrylates and acrylamides were synthesised with different RAFT agents and successfully tested for thermolysis at temperatures between 220 and 250 °C in a stainless steel tube flow reactor, resulting in complete removal of thiocarbonylthio end-groups.

Zero-valent metal catalyzed removal of thiocarbonylthio end groups from RAFT-made polystyrene: producing controllable bimodal molecular weight distribution

This work presents a facile method for removing thiocarbonylthio end groups from RAFT-made polystyrene (PS) under the catalysis of Cu(0) or Fe(0). In Cu(0)-catalyzed cases, 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) was able to promote the removal of thiocarbonylthio end groups with the production of a distinct bimodal molecular weight distribution (MWD). The chain end structure of the resultant polymer was carefully identified by the matrix-assisted laser desorption ionization time-of-flight mass spectrum. However, in the Fe(0)-catalyzed system, TEMPO and ligands were not needed for the removal of thiocarbonylthio end groups with a more efficient removal. This work provides an alternative approach for removing thiocarbonylthio end groups of RAFT-made PS as well as producing thiocarbonylthio-free PS with controllable bimodal MWD.

Fast conversion of terminal thiocarbonylthio groups of RAFT polymers to “clickable” thiol groups via versatile sodium azide

A facile and fast way of converting thiocarbonylthio end groups of RAFT polymers to thiol groups was demonstrated.

Synthesis and Characterization of Dye-Labeled Poly(methacrylic acid) Grafted Silica Nanoparticles

The synthesis of dye-labeled poly(methacrylic acid) (PMAA) grafted silica nanoparticles was studied. Surface-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization of tert-butylmethacrylate (tBuMA) was conducted on dye-labeled CPDB coated silica nanoparticles followed by sequential removal of the thiocarbonylthio end groups and the tert-butyl moieties. Additionally, as a more straightforward strategy, direct polymerization of methacrylic acid on silica nanoparticles with a diameter size as small as 15 nm was conducted via the RAFT polymerization technique. A variety of PMAA brushes with different lengths and densities were prepared on nanoparticle surfaces via surface-initiated RAFT polymerization with excellent control and surface grafting densities as high as 0.65 chains/nm2. The grafted PMAA was methylated by trimethylsilyldiazomethane to conduct organic phase GPC characterization. The dye-labeled PMAA grafted nanoparticles provide a platform to bind biomolecules and to track the movement of the nanoparticles in biological systems.

Zero-valent metal catalyzed radical-induced adjustable removal/modification of thiocarbonylthio end groups of RAFT polymer at ambient temperature

This work describes a facile removal/modification of thiocarbonylthio end groups of RAFT-made polymers by the utilization of zero-valent copper (Cu(0)) powder/2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) under mild conditions, especially for poly(methyl methacrylate) (PMMA). At [PMMA]0/[Cu(0)]0/[ligand]0/[TEMPO]0 = 1/5/5/1.5, the molecular weights (MWs) gradually increased to twofold that of the precursor, due to the polymeric radical–polymeric radical cross coupling. While the MW remained unchanged at [PMMA]0/[Cu(0)]0/[ligand]0/[TEMPO]0 = 1/5/5/5, generating vinyl group terminated telechelic polymers via a polymeric radical–TEMPO disproportionation process. The utilization of Cu(0) wire can also remove the thiocarbonylthio end group effectively. Zero-valent iron (Fe(0)) (powder or wire) can be applied to remove/modify the thiocarbonylthio end group instead of Cu(0)/ligand with comparable results. This work provided an alternative and promising approach for an adjustable end group removal/modification of RAFT polymers, and would eventually strengthen and facilitate the potential large-scale application of RAFT-related polymers.

Multi-Block Polyurethanes via RAFT End-Group Switching and Their Characterization by Advanced Hyphenated Techniques

The detailed characterization of poly(styrene)-b-poly(tetrahydrofuran) (pS-b-pTHF) multiblock copolymers (17800 g mol–1 ≤ Mn ≤ 46800 g mol–1) generated via urethane linkages is presented. The synthesis of the block copolymers is enabled via a mechanistic switch of the thiocarbonyl thio end group of a poly(styrene) to dihydroxyl terminated polymers that subsequently react with a diisocyanate terminated polytetrahydrofuran based prepolymer to form multiblock copolymer structures. The characterization of the multiblock copolymers and their substructures includes size exclusion chromatography (SEC), liquid chromatography at critical conditions (LCCC), nuclear magnetic resonance (NMR), and infrared (IR) spectroscopy as well as matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. To obtain even further details of the polymer size and its composition, SEC with triple detection as well as newly developed SEC coupled online to IR spectroscopy was carried out. The quantification of the average blo...

From NMP to RAFT and Thiol‐Ene Chemistry by In Situ Functionalization of Nitroxide Chain Ends

A straightforward, novel strategy based on the in situ functionalization of polymers prepared by nitroxide-mediated polymerization (NMP), for the use as an extension toward block copolymers and post-polymerization modifications, has been investigated. The nitroxide end group is exchanged for a thiocarbonylthio end group by a rapid transfer reaction with bis(thiobenzoyl) disulfide to generate in situ reversible addition-fragmentation chain transfer (RAFT) macroinitiators. Moreover, not only have these macroinitiators been used in chain extension and block copolymerization experiments by the RAFT process but also a thiol-terminated polymer is synthesized by aminolysis of the RAFT end group and subsequently reacted with dodecyl vinyl ether by thiol-ene chemistry.