Free Nitroxide Research Articles

Nitroxide Radical-Containing Redox Nanoparticles Protect Neuroblastoma SH-SY5Y Cells against 6-Hydroxydopamine Toxicity

Parkinson's disease (PD) patients can benefit from antioxidant supplementation, and new efficient antioxidants are needed. The aim of this study was to evaluate the protective effect of selected nitroxide-containing redox nanoparticles (NRNPs) in a cellular model of PD. Antioxidant properties of NRNPs were studied in cell-free systems by protection of dihydrorhodamine 123 against oxidation by 3-morpholino-sydnonimine and protection of fluorescein against bleaching by 2,2-azobis(2-amidinopropane) hydrochloride and sodium hypochlorite. Model blood-brain barrier penetration was studied using hCMEC/D3 cells. Human neuroblastoma SH-SY5Y cells, exposed to 6-hydroxydopamine (6-OHDA), were used as an in vitro model of PD. Cells were preexposed to NRNPs or free nitroxides (TEMPO or 4-amino-TEMPO) for 2 h and treated with 6-OHDA for 1 h and 24 h. The reactive oxygen species (ROS) level was estimated with dihydroethidine 123 and Fluorimetric Mitochondrial Superoxide Activity Assay Kit. Glutathione level (GSH) was measured with ortho-phtalaldehyde, ATP by luminometry, changes in mitochondrial membrane potential with JC-1, and mitochondrial mass with 10-Nonyl-Acridine Orange. NRNP1, TEMPO, and 4-amino-TEMPO (25-150 μM) protected SH-SY5Y cells from 6-OHDA-induced viability loss; the protection was much higher for NRNP1 than for free nitroxides. NRNP1 were better antioxidants in vitro and permeated better the model BBB than free nitroxides. Exposure to 6-OHDA decreased the GSH level after 1 h and increased it considerably after 24 h (apparently a compensatory overresponse); NRNPs and free nitroxides prevented this increase. NRNP1 and free nitroxides prevented the decrease in ATP level after 1 h and increased it after 24 h. 6-OHDA increased the intracellular ROS level and mitochondrial superoxide level. Studied antioxidants mostly decreased ROS and superoxide levels. 6-OHDA decreased the mitochondrial potential and mitochondrial mass; both effects were prevented by NRNP1 and nitroxides. These results suggest that the mitochondria are the main site of 6-OHDA-induced cellular damage and demonstrate a protective effect of NRNP1 in a cellular model of PD.

Ethylene Glycol Dicyclopentenyl (Meth)Acrylate Homo and Block Copolymers via Nitroxide Mediated Polymerization.

Nitroxide-mediated polymerization (NMP), (homo and block copolymerization with styrene (S) and butyl methacrylate/S) of ethylene glycol dicyclopentenyl ether (meth)acrylates (EGDEA and EGDEMA) was studied using BlocBuilder alkoxyamines. EGDEA homopolymerization was not well-controlled, independent of temperature (90–120 °C), or additional free nitroxide (0–10 mol%) used. Number average molecular weights (Mn) achieved for poly(EGDEA) were 4.0–9.5 kg mol−1 and were accompanied by high dispersity (Ð = Mw/Mn = 1.62–2.09). Re-initiation and chain extension of the poly(EGDEA) chains with styrene (S) indicated some block copolymer formation, but a high fraction of chains were terminated irreversibly. EGDEA-stat-S statistical copolymerizations with a low mol fraction S in initial feed, fS,0 = 0.05, were slightly better controlled compared to poly(EGDEA) homopolymerizations (Ð was reduced to 1.44 compared to 1.62 at similar conditions). EGDEMA, in contrast, was successfully polymerized using a small fraction of S (fS,0 ~ 10 mol%) to high conversion (72%) to form well-defined EGDEMA-rich random copolymer (molar composition = FEGDEMA = 0.87) of Mn = 14.3 kg mol−1 and Ð = 1.38. EGDEMA-rich compositions were also polymerized with the unimolecular succinimidyl ester form of BlocBuilder initiator, NHS-BlocBuilder with similar results, although Ðs were higher ~1.6. Chain extensions resulted in monomodal shifts to higher molecular weights, indicating good chain end fidelity.

Nitroxide-mediated polymerization of methacrylates in the presence of 4-vinyl pyridine as controlling comonomer

The controlled polymerization of methacrylic monomers by nitroxide-mediated polymerization (NMP) still represents a challenge in polymer science. This issue can be circumvented by the addition of a small amount of a comonomer known to exhibit a controlled character in NMP, the most representative example being styrene. The purpose of this work is to explore the use of 4-vinyl pyridine (4VP) as controlling comonomer, while conferring at the same time some functionality to the polymer chains. The NMP of methyl methacrylate (MMA), poly(ethylene glycol) methyl ether methacrylate (PEOMA950, Mn = 950 g mol−1) and methacrylic acid (MAA) using a small amount of 4VP under mild experimental conditions (<90 °C, atmospheric pressure), was investigated in this work. The copolymerization of MMA or PEOMA950 with 10 mol% 4VP, mediated by the use of the BlocBuilder® alkoxyamine and a small amount of free nitroxide SG1, exhibited all the features of a controlled system in agreement with the favored incorporation of 4VP at the chain ends, leading to an efficient deactivation of the propagating radicals by the nitroxide SG1. In contrast, the polymerization of MAA was uncontrolled in the same conditions likely due to acid/base interaction between MAA and 4VP, affecting the reactivity of the functional comonomer. Consistently, the copolymerization of MAA with MMA also led to a progressive loss of control as the MAA content in the feed was increased. Among all the polymers synthesized, the P(MMA72-co-4VP10)-SG1 macroalkoyamine was then successfully used to reinitiate the copolymerization of n-butyl methacrylate (BMA) and 4VP both in solution and via dispersion polymerization in a mixture of ethanol and water. Finally, raspberry-like polymer/silica composite particles were prepared through nitroxide-mediated dispersion polymerization performed in the presence of silica nanoparticles, taking benefit of the strong acid-base interaction between 4VP and silica.

Nitroxide‐mediated polymerization of adamantyl‐functional methacrylates for 193 nm photoresists

Three different methacrylic monomers (γ‐butyrolactone methacrylate (GBLMA), 3‐hydroxyl‐1‐adamantyl methacrylate (HAMA), and 2‐methyl‐2‐adamantyl methacrylate (MAMA)), that are most often terpolymerized for 193 nm photoresists, were studied individually by nitroxide mediated polymerization (NMP) with succinimidyl ester terminated BlocBuilder (NHS‐BlocBuilder) using a small fraction ∼ 0.05 mol/mol (5 mol%) in the initial mixture of 2,3,4,5,6 pentafluorostyrene (PFS) controlling co‐monomer. The copolymerizations were done in 0.35 g/g (35 wt%) dioxane solution and were studied as functions of temperature (75 versus 90 °C) and added free SG1 nitroxide (none versus SG1:NHS‐BlocBuilder molar ratio ≈ 0.15) with the objective of minimizing the dispersity (). MAMA/PFS copolymerizations were not strongly influenced by temperature or addition of free nitroxide, with relatively linear number average molecular weight versus conversion plots and = 1.28–1.55. Further, MAMA/PFS polymerization kinetics was notably slower compared to GBLMA or HAMA copolymerizations with PFS. For GBLMA/PFS and HAMA/PFS copolymerizations, was reduced at the lower temperature of 75 °C but added SG1 did not play a strong role. For GBLMA/PFS, was reduced from 1.70–1.71 at 90 °C to 1.42–1.50 at 75 °C and for HAMA/PFS, was reduced from 1.36–1.47 at 90 °C to 1.22–1.31 at 75 °C.

Nitroxide delivery system for Nrf2 activation and skin protection

Cyclic nitroxides are a large group of compounds composed of diverse stable radicals also known as synthetic antioxidants. Although nitroxides are valuable for use in several skin conditions, in in vivo conditions they have several drawbacks, such as nonspecific dispersion in normal tissue, preferential renal clearance and rapid reduction of the nitroxide to the corresponding hydroxylamine. However, these drawbacks can be easily addressed by encapsulating the nitroxides within microemulsions. This approach would allow nitroxide activity and therefore their valuable effects (e.g. activation of the Keap1-Nrf2-EpRE pathway) to continue. In this work, nitroxides were encapsulated in a microemulsion composed of biocompatible ingredients. The nanometric size and shape of the vehicle microemulsion and nitroxide microemulsion displayed high similarity, indicating that the stability of the microemulsions was preserved. Our studies demonstrated that nitroxide microemulsions were more potent inducers of the Keap1-Nrf2-EpRE pathway than the free nitroxides, causing the activation of phase II enzymes. Moreover, microemulsions containing nitroxides significantly reduced UVB-induced cytotoxicity in the skin. Understanding the mechanism of this improved activity may expand the usage of many other Nrf2 modulating molecules in encapsulated form, as a skin protection strategy against oxidative stress-related conditions.

Reevaluation of the Formation and Reactivity of Midchain Radicals in Nitroxide-Mediated Polymerization of Acrylic Monomers

In radical polymerization of acrylic monomers, intramolecular transfer to polymer, and the reactions of the resulting midchain radical, can have a strong impact on both the kinetics and microstructure of the resulting polymer. It has previously been stated that, in nitroxide-mediated polymerization, a substantial proportion of midchain radicals reacts with free nitroxide to form a capped species and that this can lead to a reduction in branch points, in the resulting polymer. In this article, we show that, contrary to previous evidence, the nitroxide capped midchain species cannot be observed in nitroxide-mediated polymerization of butyl acrylate. In addition, we show that, in nitroxide-mediated polymerization, lower than expected concentrations of products arising from transfer to polymer are observed.

Permethyl-β-Cyclodextrin Spin-Labeled with Nitronyl Nitroxide: Synthesis and EPR Study

A method for synthesis of new covalently linked spin-labeled cyclodextrin (CD) via the attachment of nitronyl nitroxide 2-(4-hydroxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl to permethylated β-cyclodextrin is described. Electron spin echo envelope modulation (ESEEM) studies demonstrate that the new spin-labeled CD exhibits dynamic equilibrium between conformations with radical fragment capping the cavity of CD and radical fragment located outside the cavity. In solution, nitronyl nitroxide attached to CD retains its sensitivity to nitric oxide (NO), as reaction with NO leads to formation of iminonitroxide fragment evidenced by continuous-wave (CW) electron paramagnetic resonance (EPR). At the same time, CW EPR study of the reaction with ascorbic acid shows that the described binding of nitronyl nitroxide to CD does not provide higher stability of radical towards the reduction, and the corresponding rate constants are close to those obtained for free nitronyl nitroxide. Plausible explanations of these observations are discussed.

In vivo Overhauser-enhanced MRI of proteolytic activity.

There is an increasing interest in developing novel imaging strategies for sensing proteolytic activities in intact organisms in vivo. Overhauser-enhanced MRI (OMRI) offers the possibility to reveal the proteolysis of nitroxide-labeled macromolecules thanks to a sharp decrease of the rotational correlation time of the nitroxide moiety upon cleavage. In this paper, this concept is illustrated in vivo at 0.2 T using nitroxide-labeled elastin orally administered in mice. In vitro, this elastin derivative was OMRI-visible and gave rise to high Overhauser enhancements (19-fold at 18 mm nitroxide) upon proteolysis by pancreatic porcine elastase. In vivo three-dimensional OMRI detection of proteolysis was carried out. A keyhole fully balanced steady-state free precession sequence was used, which allowed 3D OMRI acquisition within 20 s at 0.125 mm(3) resolution. About 30 min after mouse gavage, proteolysis was detected in the duodenum, where Overhauser enhancements were 7.2 ± 2.4 (n = 7) and was not observed in the stomach. Conversely, orally administered free nitroxides or pre-digested nitroxide-labeled elastin were detected in the mouse's stomach by OMRI. Combined with specific molecular probes, this Overhauser-enhanced MRI technique can be used to evaluate unregulated proteolytic activities in various models of experimental diseases and for drug testing.

Electron paramagnetic resonance characterization of drug‐binding to monoacylglycerol lipase using the dynamic signature of a protein spin‐label (779.3)

In recent years, monoacylglycerol lipase (MGL) has been an important pharmaceutical target because of its relationship to pathways that have been implicated in cancer proliferation. The purpose of this study is to find an efficient and streamlined way of characterizing drug‐binding to MGL using electron paramagnetic resonance (EPR). The free nitroxide radical methyl thiosulfonyl spin label (MTSL) was used to label the single cysteine mutant S208C on a domain of MGL that is thought to constitute a “lid” covering the substrate binding pocket of MGL. EPR spectra shows reproducible line shape changes reflecting a significant reduction in the mobility of the lid when MGL binds reversible inhibitors. The results are consistent with NMR data that reveal an equilibrium between the putative “open” and “closed” positions of the lid as a function of temperature or pH. These findings establish a useful assay for screening potential cancer drugs that bind in a specific way to MGL that can be carried out more rapidly and with significantly smaller amounts of protein than other assays for the dynamic motion of the drug target.Grant Funding Source: Supported by grants DA09158, DA00493, DA03801 and DA07312

Control of spin–spin exchange interactions in polynitroxides through inclusion within γ-cyclodextrin

Supramolecular interactions of polynitroxides with γ-cyclodextrin (γ-CD) were used to control intramolecular electron spin–spin exchange couplings in these stable radicals. In the absence of γ-CD, the binitroxide T2 and trinitroxide T3 showed strong spin–spin coupling as evidenced by the high exchange coupling constant of J = 120 MHz for T2 in its EPR spectra. Host/guest complexes are formed upon addition of γ-CD to the polynitroxides where the nitroxide moiety interacts with the γ-CD cavity, which causes suppression of spin exchange. By variation of the host/guest ratio, the nature of a trinitroxide spin system can be tuned from a three-spin to a two-spin to a one-spin system. In addition to the host/guest ratio, the spin exchange coupling can also be tuned by temperature, which shifts the equilibrium between CD-encapsulated nitroxides and free nitroxides.

Thermo-responsive, UV-active poly(phenyl acrylate)-b-poly(diethyl acrylamide) block copolymers

The homopolyinerization of phenyl acrylate (PA) was investigated for the first time by nitroxide mediated polymerization (NMP) with the succinimidyl form of the SG-1-based unimolecular initiator 24N-tert-butyl-2,2-(dimethylpropy1)-aminooxy]propionic acid (BlocBuilder MA). The control of PPA hoinopolymerization was improved by the use of 15 mot% additional free nitroxide SG1 atert-butyl[1-(diethoxyphospholy1)-2,2-dimethylpropyl]amino]oxidanyl) and dispersities, M-w/M-n of around 1.2 were achieved. A PPA homopolyiner was then successfully chain-extended with diethyl acrylainide (DEAAin) to form a block copolyiner of PPA-b-PDEAAm where the PDEAAm segment is therino-responsive, while the PPA block is potentially UV-active. The thermo-responsive behavior of the block copolymer in 0.5 we/) aqueous solution was studied by UV-Vis spectrometry and dynamic light scattering (DLS), indicating cloud point temperatures of 26-30 degrees C, close to that reported for PDEAAm homopolymers.

Rate enhanced nitroxide-mediated miniemulsion polymerization: effect of nitroxide water solubility

Nitroxide-mediated radical polymerization of styrene in miniemulsion based on in situ surfactant generation of potassium oleate has been conducted at 130 °C for three nitroxides of vastly different water solubilities; 4-stearoyl-TEMPO, TEMPO and 4-hydroxy-TEMPO. These miniemulsions contain high levels of surfactant (120 wt% relative to styrene) and have particle diameters of <20 nm, and as such compartmentalization effects are important. The TEMPO-based system proceeded with extraordinarily high polymerization rate (initial rate ∼30 times higher than in the corresponding bulk system), much faster than ever before achieved, while control/livingness remained satisfactory. The results for the other two nitroxides were consistent with the propensity (or lack thereof) of the free nitroxide to exit to the aqueous phase – the 4-stearoyl-TEMPO system (negligible nitroxide exit) exhibited a polymerization rate similar to the corresponding bulk system with good control/livingness, whereas the 4-hydroxy-TEMPO system polymerized much faster than the corresponding bulk system, accompanied by loss of control. Consumption of free nitroxide caused by the presence of oleic acid may also influence these polymerizations.

PH‐ and temperature‐sensitive statistical copolymers poly[2‐(dimethylamino)ethyl methacrylate‐stat‐2‐vinylpyridine] with Functional succinimidyl‐ester chain ends synthesized by nitroxide‐mediated polymerization

AbstractStatistical copolymerizations of 2‐(dimethylamino)ethyl methacrylate (DMAEMA) with 2‐vinylpyridine (2VP) with 80 to 99 mol % DMAEMA in the feed utilizing a succinimidyl ester‐terminated alkoxyamine unimolecular initiator (NHS‐BlocBuilder) at 80 °C in bulk were performed. The effectiveness of 2VP as a controlling comonomer is demonstrated by linear increases in number‐average molecular weight versus conversion, relatively low PDI (1.5–1.6 with up to 98% DMAEMA) and successful chain extensions with 2VP. Additional free nitroxide does not significantly improve control for the DMAEMA/2VP copolymerizations. The succinimidyl ester on the initiator permits coupling to amine‐terminated poly(propylene glycol) (PPG), yielding an effective macroinitiator for synthesizing a doubly thermo‐responsive block copolymer of PPG‐block‐P(DMAEMA/2VP). A detailed study of the thermo‐ and pH‐sensitivities of the statistical and block copolymers is also presented. The cloud point temperature of the statistical copolymers is fine tuned from 14 to 75 °C by varying polymer composition and pH. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012.

Versatility of a succinimidyl‐ester functional alkoxyamine for controlling acrylonitrile copolymerizations

AbstractStyrene/acrylonitrile (S/AN) and tert‐butyl methacrylate/acrylonitrile (tBMA/AN) copolymers were synthesized in a controlled manner (low polydispersity $ {{\overline M _w } / {\overline M _n }} $ with linear growth of number average molecular weight $ \overline M _n $ vs. conversion X) by nitroxide mediated polymerization (NMP) with a succinimidyl ester (NHS) terminated form of BlocBuilder unimolecular initiator (NHS‐BlocBuilder) in dioxane solution. No additional free nitroxide (SG1) was required to control the tBMA‐rich copolymerizations with NHS‐BlocBuilder, a feature previously required for methacrylate polymerizations with BlocBuilder initiators. Copolymers from S/AN mixtures (AN molar initial fractions fAN,0 = 0.13–0.86, T = 115°C) had $ {{\overline M _w } / {\overline M _n }} $ = 1.14–1.26 and linear $ \overline M _n $ versus conversion X up to X ≈ 0.6. tBMA/AN copolymers (fAN,0 = 0.10–0.81, T = 90°C) possessed slightly broader molecular weight distributions ( $ {{\overline M _w } / {\overline M _n }} $ = 1.23–1.50), particularly as the initial composition became richer in tBMA, but still exhibited linear plots of $ \overline M _n $ versus conversion X up to X ≈ 0.6. A S/AN/tBMA terpolymerization (fAN,0 = 0.50, fS,0 = 0.40) was also conducted at 90°C and revealed excellent control with $ \overline M _n $ = 13.6 kg/mol, $ {{\overline M _w } / {\overline M _n }} $ = 1.19, and linear $ \overline M _n $ versus conversion X up to X = 0.54. Incorporation of AN and tBMA in the final copolymer (molar composition FAN = 0.47, FtBMA = 0.11) was similar to the initial composition and represents initial designs to make tailored, acid functional AN copolymers by NMP for barrier materials. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Nitroxide Radicals@US‐Tubes: New Spin Labels for Biomedical Applications

AbstractThe encapsulation of nitroxide radicals within ultrashort (ca. 50 nm) single‐walled carbon nanotubes (US‐tubes) is achieved. Tempo‐ and Iodo‐Tempo@US‐tubes are characterized by thermogravimetric analysis (TGA), X‐ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Electron paramagnetic resonance (EPR) spectra display characteristic signals due to the detection of the spin probes within the US‐tubes. Longitudinal proton relaxivities (r1) of both nitroxide@US‐tubes samples are 7 to 13 times greater than the free nitroxide radicals in solution, giving relaxivities comparable to the clinical contrast agent (CA) Magnevist. In addition, transverse proton relaxivities (r2) show unprecedented proton relaxation enhancement in comparison to any other reported nitroxide radical‐based system or the clinically approved T2 CA, Resovist, under the same conditions. T2‐weighted magnetic resonance imaging (MRI) phantom images show that the encapsulation of nitroxide radicals within the US‐tubes produces good contrast enhancement due to their high r2 relaxivities. The nitroxide radicals@US‐tube agents are a new promising class of spin probes for MRI and electronic paramagnetic resonance imaging (EPRI) labeling, tracking, and diagnosis.

Routes to carboxylic acid functional acrylonitrile copolymers via N‐tert‐butyl‐N‐(1‐diethylphosphono‐2,2‐dimethylpropyl) free nitroxide based nitroxide‐mediated polymerization

AbstractStyrene (S)/acrylonitrile [AN; initial acrylonitrile molar feed compositions (fAN,0's) = 0.10–0.86) and tert‐butyl methacrylate (tBMA)/AN (fAN,0 = 0.10–0.80) copolymers were synthesized at 90°C in 50 wt % 1,4‐dioxane solutions with a unimolecular initiator, N‐(2‐methylpropyl)‐N‐(1‐diethylphosphono‐2,2‐dimethylpropyl)‐O‐(2‐carboxylprop‐2‐yl) hydroxylamine [BlocBuilder (BB)]. In the tBMA/AN copolymerizations, 8.0–8.5 mol % N‐tert‐butyl‐N‐(1‐diethylphosphono‐2,2‐dimethylpropyl) free nitroxide relative to BB was added. The S/AN copolymers exhibited narrow, monomodal molecular weight distributions (MWDs) with low polydispersities [weight‐average molecular weight (Mw)/number‐average molecular weight (Mn) = 1.14–1.26], and the Mn versus monomer conversion (X) plots were relatively linear (Mn = 18.1 kg/mol, X ≈ 0.7); this suggested that pseudo‐living behavior was approached. AN proved to be an effective controlling comonomer for tBMA because the tBMA/AN copolymers exhibited narrow monomodal MWDs with Mw/Mn = 1.17–1.50 and relatively linear Mn versus X plots to reasonably high X values (Mn = 15.6 kg/mol, X ≈ 0.6). The AN and S monomer reactivity ratios were rAN = 0.07 ± 0.01 and rS = 0.27 ± 0.02 (Fineman–Ross) and rAN = 0.10 ± 0.01 and rS = 0.28 ± 0.02 (Kelen–Tüdos), respectively; these values were in good agreement with conventional free‐radical polymerization. Error‐in‐variables model (EVM) analysis indicated that the use of cumulative composition S/AN data was more effective than typical approaches using low‐X data with the Mayo–Lewis model. The AN and tBMA reactivity ratios [rAN = 0.07 ± 0.01 and rtBMA = 1.24 ± 0.20 (Fineman–Ross) and rAN = 0.14 ± 0.01 and rtBMA = 0.89 ± 0.19 (Kelen–Tüdos)] were similar to those reported for related alkyl methacrylate/AN conventional radical copolymerizations. EVM analysis suggested significant experimental error was associated with the tBMA/AN system, and this warrants further investigation. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Fluorescent, Thermoresponsive Oligo(ethylene glycol) Methacrylate/9-(4-Vinylbenzyl)-9H-carbazole Copolymers Designed with Multiple LCSTs via Nitroxide Mediated Controlled Radical Polymerization

9-(4-Vinylbenzyl)-9H-carbazole (VBK) was used as the “controlling” comonomer for nitroxide mediated polymerization with 10 mol % SG1 free nitroxide relative to BlocBuilder initiator at 80 °C of oligo(ethylene glycol) methyl ether methacrylate (8–9 ethylene glycol (EG) units) (OEGMA8–9), 2-(2-methoxyethoxy)ethyl methacrylate (MEO2MA) and for an OEGMA8–9/MEO2MA-mixed feed. The synthesis of MEO2MA/VBK and OEGMA8–9/VBK copolymers and MEO2MA/OEGMA8–9/VBK terpolymers exhibited linear increases in number-average molecular weight (M̅n) versus conversion X, up until X = 0.6, with final copolymers characterized by relatively narrow, monomodal molecular weight distributions (M̅w/M̅n < 1.4, in most cases). A series of MEO2MA/OEGMA8–9/VBK terpolymers were synthesized and by varying the OEGMA8–9:MEO2MA feed ratios, the terpolymers exhibited tunable lower critical solution temperatures in water (28 °C < LCSTs < 81 °C). MEO2MA/OEGMA8–9/VBK terpolymers were deemed sufficiently pseudo-“living” to reinitiate a second batch of MEO2MA/OEGMA8–9/VBK, with few apparent dead chains, as indicated by the monomodal shift in the GPC chromatograms. The resulting MEO2MA/OEGMA8–9/VBK block copolymers were designed so that each block exhibited a distinct LCST, which was confirmed by UV–vis and dynamic light scattering. In addition to controlling the terpolymerization, the VBK units imparted thermo-responsive fluorescence into the final copolymers.

Synthesis by nitroxide-mediated aqueous dispersion polymerization, characterization, and physical core-crosslinking of pH- and thermoresponsive dynamic diblock copolymer micelles

Diblock copolymers consisting of a poly(acrylic acid) (PAA) segment and a LCST-type poly(N,N-diethylacrylamide) (PDEAAm) block were obtained by nitroxide-mediated polymerization in aqueous dispersion using a water-soluble macroalkoxyamine. The influence of several parameters on the polymerization (temperature, initial free nitroxide or macroalkoxyamine concentrations, and solids content) was evaluated in terms of kinetics, macromolecular control, and colloidal features. As determined by dynamic light scattering (DLS), stable dispersions of monodisperse particles could be obtained for solids content as high as 39 wt% without the need for any additional surfactant via a polymerization-induced self-assembly mechanism. Rendered possible by the use of a controlled/living polymerization process, the effective semi-batch incorporation of hydrophobic units (styrene) in the growing chains during the polymerization allowed the formation of physically crosslinked nanogels. The pH and temperature sensitivity were proved by means of DLS and high-sensitivity differential scanning calorimetry (HSDSC) measurements. Due to the formation of aggregates observed by size-exclusion chromatography in N,N-dimethylformamide, accurate molar masses could not be determined directly but deconvoluted hydrodynamic volume distributions suggested a good control of the polymerization.

Interaction between Encapsulated Excited Organic Molecules and Free Nitroxides: Communication Across a Molecular Wall

Communication between two molecules, one confined and excited (triplet or singlet) and one free and paramagnetic, has been explored through quenching of fluorescence and/or phosphorescence by nitroxides as paramagnetic radical species. Quenching of excited states by nitroxides has been investigated in solution, and the mechanism is speculated to involve charge transfer and/or exchange processes, both of which require close orbital interaction between excited molecule and quencher. We show in this report that such a quenching, which involves electron-electron spin communication, can occur even when there is a molecular wall between the two. The excited state molecule is confined within an organic capsule made up of two molecules of a deep cavity cavitand, octa acid, that exists in the anionic form in basic aqueous solution. The nitroxide is kept free in aqueous solution. (1)H NMR and EPR experiments were carried out to ascertain the location of the two molecules. The distance between the excited molecule and the paramagnetic quencher was manipulated by the use of cationic, anionic, and neutral nitroxide and also by selectively including the cationic nitroxide within the cavity of cucurbituril. Results presented here highlight the role of the lifetime of the encounter complex in electron-electron spin communication when the direct orbital overlap between the two molecules is prevented by the intermediary wall.

Nitroxide-Mediated Living/Controlled Radical Polymerization of N,N-Diethylacrylamide

Well-defined poly(N,N-diethylacrylamide)s were synthesized for the first time by nitroxide-mediated polymerization. The polymerizations were first performed in toluene solution, using BlocBuilder, a tertiary alkoxyamine based on the nitroxide SG1 (N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide), as an initiator without initial free nitroxide. High monomer conversions were reached, up to completion and the number-average molar masses increased linearly with conversion. However the polydispersity indices (PDI) remained quite high, despite a constant diminution with increasing conversion until a value of 1.9 at high conversion. This could be corrected by the addition of free SG1 at the onset of the reaction, leading to PDI of 1.2 at 93% conversion in the best case. Various alkoxyamine concentrations were used to target different molar masses. The presence of SG1 functionality at the ω-end of the polymer chains was evidenced by size-exclusion chromatography coupled to electospray ionizatio...