Reversible crosslinking of polymers bearing pendant or terminal thiol groups prepared by nitroxide-mediated radical polymerization

Monomers or N-alkoxyamine initiators containing protected thiol groups are utilized to prepare polymers via nitroxide-mediated radical polymerization. Following thiol deprotection, the macromolecular properties of these polymers are manipulated, by adjusting the redox conditions to either form or cleave disulfide bonds, or irreversibly cap free thiols by the rapid addition to a maleimide Michael acceptor. Formation of disulfide bonds under dilute conditions results in intramolecular disulfide formation, resulting in internal polymer collapse. Alternatively, disulfide formation under high concentration results in intermolecular crosslinking of polymers to form networked macromolecular assemblies.© 2013 Elsevier Ltd. All rights reserved

Urushiol Detection using a Profluorescent Nitroxide

A method to visually detect minute amounts of urushiol, the toxic catechol from poison oak, poison ivy, and poison sumac, has been developed utilizing the reaction of a profluorescent nitroxide with the B-n-butylcatecholboronate ester formed in situ from urushiol and B-n-butylboronic acid. The resulting N-alkoxyamine is strongly fluorescent upon illumination with a fluorescent lamp, allowing the location of the toxic urushiol contamination to be visualized. This methodology constitutes the groundwork for the future development of a spray to detect urushiol to avoid contact dermatitis, as well as to detect catecholamines for biomedical applications

Phthalate plasticizers covalently linked to PVC \u3ci\u3evia\u3c/i\u3e copper-free or copper catalyzed axide-alkyne cycloadditions

Plasticization of PVC was carried out by covalently linking phthalate derivatives via copper-free (thermal) or copper catalyzed azide-alkyne cycloadditions. Di(2-ethylhexyl) phthalate derivatives (DEHP-ether and DEHP-ester) were synthesized and appended to PVC at two different densities. The glass transition temperatures of the modified PVC decreased with increasing content of plasticizer. PVC-DEHP-ether gave lower glass transition temperatures than PVC-DEHP-ester, reflecting the enhanced flexibility of the ether versus ester linker

Degradation of Polyvinyl Chloride by Sequential Dehydrochlorination and Olefin Metathesis

Polyvinyl chloride (PVC) is a problematic waste plastic with limited options for recycling or upcycling. Herein, we demonstrate preliminary results in breaking down the long carbon chains of PVC into oligomers and small organic molecules. First, treatment with a substoichiometric amount of alkali base effects elimination of HCl to form a salt and creates regions of conjugated carbon-carbon double bonds, as determined by 1 H NMR and UV-Vis spectroscopy. Olefin cross metathesis with an added partner alkene then cleaves carbon-carbon double bonds of the polymer backbone. Addition of allyl alcohol to the dehydrochlorination step introduces allyloxy groups by substitution of allylic chlorides. Subsequent metathesis of the pendant allyloxy groups provides a reactive terminal alkene to promote insertion of the metathesis catalyst onto the olefins in the all-carbon backbone. The products obtained are a mixture of PVC oligomers with greatly reduced molecular weights and a small-molecule diene corresponding to the substituents of the added alkene, as evidenced by 1 H and DOSY NMR and GPC. This mild procedure provides a proof of concept towards harvesting carbon resources from PVC waste

Relative Rates of Metal-Free Azide-Alkyne Cycloadditions: Tunability over 3 Orders of Magnitude.

The thermal (3 + 2) dipolar azide-alkyne cycloaddition, proceeding without copper or strained alkynes, is an underutilized ligation with potential applications in materials, bioorganic, and synthetic chemistry. Herein, we investigate the effects of alkyne substitution on the rate of this reaction, both experimentally and computationally. Electron-withdrawing groups accelerate the reaction, providing a range of relative rates from 1.0 to 2100 between the slowest and fastest alkynes studied. Unexpectedly, aryl groups conjugated to the alkyne significantly retard the reaction rate. In contrast, a sulfonyl, ester-substituted alkyne is reactive enough that it couples with an azide at room temperature in a few hours. This reactivity scale should provide a guide to those who wish to use this ligation under mild conditions

Distance-Dependent Fluorescence Quenching and Binding of CdSe Quantum Dots by Functionalized Nitroxide Radicals

Quantum dot (QD) fluorescence is effectively quenched at low concentration by nitroxides bearing amine or carboxylic acid ligands. The association constants and fluorescence quenching of CdSe QDs with these derivatized nitroxides have been examined using electron paramagnetic resonance (EPR) and fluorescence spectroscopy. The EPR spectra in the non-protic solvent toluene are extremely sensitive to intermolecular and intramolecular hydrogen bonding of the functionalized nitroxides. Fluorescence measurements show that quenching of QD luminescence is nonlinear, with a strong dependence on the distance between the radical and the QD. The quenched fluorescence is restored when the surface-bound nitroxides are converted to hydroxylamines by mild reducing agents, or trapped by carbon radicals to form alkoxyamines. EPR studies indicate that photoreduction of the nitroxide occurs in toluene solution upon photoexcitation at 365 nm. However, photolysis in benzene solution gives no photoreduction, suggesting that photoreduction in toluene is independent of the quenching mechanism. The fluorescence quenching of QDs by nitroxide binding is a reversible process

Synthesis of fluorinated alkoxyamines and alkoxyamine-initiated nitroxide-mediated precipitation polymerizations of styrene in supercritical carbon dioxide

TIPNO (2,2,5-trimethyl-4-phenyl-3-azahexane-3-nitroxide)-alkoxyamine was found to give reasonably controlled/living nitroxide-mediated (NMP) precipitation polymerizations of styrene in supercritical carbon dioxide (scCO(2)). In contrast under the same conditions, the analogous SG1 (N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl)nitroxide)-alkoxyamine gave higher rates of polymerization and inferior controlled/living character. The circumvention of the requirement for excess free (nitroxide](0) allowed the study of nitroxide partitioning effects in scCO(2) for three newly synthesized fluorinated alkoxyamines. Two alkoxyamines dissociated into scCO(2)-philic fluorinated TIPNO-nitroxide derivatives, while another contains a similar sized fluorinated "foot". Despite the increased steric bulk about the N-O bond for the novel fluorinated alkoxyamines, all polymerizations proceeded at a similar rate and level of control to the TIPNO system in solution (toluene). PREDICI simulations for the styrene/TIPNO system are used to support extensive partitioning effects observed in scCO(2) for the fluorinated alkoxyamines.Irish Research Council (formerly IRCSET) IUPAC Transnational Call in Polymer Chemistry to F.Aldabbagh. National Science Foundation (NSF CHE-1057927, USA) to R. Braslau.peer-reviewe