Di-tert-butyl Nitroxide Research Articles

Effect of charge on the rotation of prolate nitroxide spin probes in room-temperature ionic liquids

We have studied the rotational diffusion of two prolate nitroxide probes, the doubly negatively charged peroxylamine disulfonate (Frémy's salt − FS) and neutral di-tert-butyl nitroxide (DTBN), in a series of 1-alkyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquids (RTILs) having alkyl chain lengths from two to eight carbons using electron paramagnetic resonance (EPR) spectroscopy. Though the size and shape of the probes are reasonably similar, they behave differently due to the charge difference. The rotation of FS is anisotropic, and the rotational anisotropy increases with the alkyl chain length of the cation, while the rotation of DTBN is isotropic. The hyperfine coupling constant of DTBN decreases as a function of the alkyl chain length and is proportional to the relative permittivity of ionic liquids. On the other hand, the hyperfine coupling constant of FS increases with increasing chain length. These behaviors indicate the location of each probe in RTILs. FS is likely located in the polar region near the network of charged imidazolium ions. DTBN molecules are predominately distributed in the nonpolar domains.

PH-responsive aggregation states of chiral polymerizable amphiphiles from L-tyrosine and L-phenyl alanine in water.

Sodium salts of maleamic acid derivatives of lauryl ester of tyrosine (MTNa) and phenyl alanine (MPNa) in water exhibited strong pH-responsive behaviors of viscosity and specific conductivity that originate from the concentration and pH dependence of their aggregation states. The aggregates were characterized by a novel spin-probe-partitioning electron paramagnetic resonance (SPPEPR) method and dynamic light scattering (DLS). Results of high-precision fitting of the second-harmonic EPR spectra of the small spin probe di-tert-butyl nitroxide (DTBN) in these aggregates together with viscosity, conductivity, and DLS showed that, at pH ~ 7.54, MTNa formed micelles and MPNa vesicles and MTNa exhibited a pH-induced micelle to vesicle transition as pH was lowered toward 6. MTNa, at pH ~ 7.54, formed small micelles at low concentrations that transformed to long worm-like micelles for concentrations ≥ 0.05 M, accompanied by a 30-fold increase in solution viscosity. The hydrodynamic radii from DLS confirmed the presence of small micellar aggregates of radius ~ 2 nm in MTNa at pH ~ 7.54 at the lower concentrations, with coexisting micelles (~2 nm) and vesicles (~50 nm) at pH near 6.5, vesicles (radii ~ 70 nm) at pH near 6, and large vesicles (85 nm) in MPNa at pH ~ 7.60. Both MTNa and MPNa precipitated upon reduction of pH below 6 and below 7, respectively. The rate of transfer of DTBN between the aqueous phase and the aggregate was calculated from the high-field Lorentzian linewidths of the electron paramagnetic resonance (EPR) spectra. The activation energy for the transfer determined from the temperature dependence of the rate of transfer is 12.7 kJ/mol for MTNa vesicles (pH ~ 6) and 20.6 ± 1.3 kJ/mol for MPNa (pH ~ 7.60). The pH-induced transformations were reversible.

Macroscopic characterization of bilayer membranes composed of triglyceride and phosphatidylcholine investigated using high-pressure ESR spin probe technique

The effect of high static pressure on the microscopic properties of triglyceride (HCO-10) and phosphatidylcholine (DLPC) membranes was investigated by ESR (electron spin resonance). The external pressure was varied to ascertain the distribution equilibrium constant between the aqueous and vesicle phases, depending on the molecular sizes of various nitroxide probes relative to that of the interstitial space of the HCO-10 and DLPC membranes. The distribution equilibrium of any probe for the HCO-10 membrane was shifted to the aqueous phase side with increasing the external pressure. However, the distribution equilibrium of di-tert-butyl nitroxide (DTBN) for the DLPC membrane was shifted to the vesicle phase side with increasing external pressure. Furthermore, information on the rotational motion of the nitroxide probes in the HCO-10 and DLPC vesicle was obtained from the anisotropic ESR signals. Rotational correlation times (τR) of the nitroxide probes in the vesicle phases increased with increasing external pressure. The magnitude of the activation volume (ΔV‡) obtained using τR in both vesicles is TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)>DTBN>DMDEN (2,2-dimethyl-5,5-diethylpyrroline N-oxide), which might be attributed to the interaction between the probes and the membrane chains.

Simple and Highly Sensitive Measurement Method for Detection of Glass Transition Temperatures of Polymers: Application of ESR Power Saturation Phenomenon with Conventional Spin-Probe Technique

A combination of the microwave power saturation (MPS) method of electron spin resonance (ESR) and spin probing is proposed as a simple and practical technique for detecting the glass transition temperatures, T(g), of polymers with high sensitivity. Effects of the spin-probe size and concentration on the T(g) value of polystyrene (PS) determined by MPS, T(g,ESR), were first evaluated. Spin-probed PS with four types of nitroxides, namely, di-tert-butyl nitroxide (DBN), 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), 4-benzoyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl (BZONO), and 4',4'-dimethyl-spiro(5α-cholestane-3,2'-oxazolidin)-3'-yloxy free radical (CHOL), having molecular weights of 144, 156, 276, and 473, respectively, and spin-labeled PS with TEMPO were prepared. The T(g,ESR) values for the spin-probed PS with DBN, TEMPO, BZONO, and CHOL and spin-labeled PS were determined to 360, 363, 374, 374, and 375 K, respectively, within experimental uncertainties of 2 K, whereas the glass transition temperature determined by DSC, T(g,DSC), was 375 K for all samples. A significant decrease in T(g,ESR) for small spin probes was shown to be due to decoupling between the mobilities of small spin probes and PS segments. Concerning the concentration, a decrease in the saturation factor, S, induced by shortening of the spin-spin relaxation time was observed for the spin-probed PS with CHOL when the concentration of CHOL was more than 1.0 wt %. Furthermore, T(g,ESR) decreased slightly with increasing weight fraction of CHOL because of the "plasticizer effect" of CHOL. However, the T(g,ESR) and T(g,DSC) values corresponded for each concentration. Thus, large spin probes, such as CHOL and BZONO, are appropriate for the determination of T(g,ESR) values; the concentration of the spin probes does not affect the T(g,ESR) value unless the overall T(g) value is reduced by blending of excess spin probes. Finally, measurements of T(g,ESR) in PS/silica composites containing more than 95 wt % silica are shown as an application example of the present method. T(g,ESR) was clearly determined even for the PS/silica composites with 98 wt % silica; a decrease in T(g,ESR) with increasing silica content was observed.

Electron Spin Resonance Study of Nitroxide Radical Adsorption at Cupric Ions in the Metal−Organic Framework Compound Cu3(btc)2

The Cu(II) pairs in the paddle-wheel building blocks of the metal−organic framework compound Cu3(btc)2 give rise to an antiferromagnetic spin state with an electron spin resonance (ESR)-silent S = 0 ground state. However, the adsorption of di-tert-butyl nitroxide (DTBN) radicals leads to the formation of an unusual nitroxide ESR spectrum and later, upon thermal treatment of the samples, to distinct paramagnetic Cu(II) centers, whose ESR signals can be observed at temperatures below 70 K. Various scenarios for the suppression of the antiferromagnetic coupling of the Cu(II) ions in the paddle-wheel units by interaction with the nitroxide and the subsequent formation of these S = 1/2 copper centers are discussed.

Molecular Dynamics and Partitioning of Di‐tert‐butyl Nitroxide in Stratum Corneum Membranes: Effect of Terpenes

In this work, we have used electron paramagnetic resonance (EPR) spectroscopy of the small spin label di-tert-butyl nitroxide (DTBN), which partitions the aqueous and hydrocarbon phases, to study the interaction of the terpenes alpha-terpineol, 1,8-cineole, L(-)-carvone and (+)-limonene with the uppermost skin layer, the stratum corneum, and the membrane models of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The EPR spectra indicated that the terpenes increase both the partition coefficient and the rotational correlation time of the spin labels in the stratum corneum membranes, whereas similar effects were observed in the DMPC and DPPC bilayers only at temperatures below the liquid-crystalline phase. The EPR parameter associated to probe polarity inside the membranes showed thermotropically induced changes, suggesting relocations of spin probe, which were dependent on the membrane phases. While the DMPC and DPPC bilayers showed abrupt changes in the partitioning and rotational correlation time parameters in the phase transitions, the SC membranes were characterized by slight changes in the total range of measured temperatures, presenting the greatest changes or membranes reorganizations in the temperature range of approximately 50 to approximately 74 degrees C. The results suggest that terpenes act as spacers, weakening the hydrogen-bonded network at the polar interface and thus fluidizing the stratum corneum lipids.

Living Radical Polymerization of Styrene Derivatives Containing Trialkylmetal Groups

Fourteenth group oganometallic styrene derivatives were synthesised by nitroxide-mediated controlled radical polymerization using di-tert butyl nitroxide (A-T) as initiator. This is the first time that nitroxide-controlled radical polymerization has been successfully adapted for the synthesis of new polystyrenes bearing organometallic species with controlled molecular weights and narrow polydispersities. Monomer reactivity ratios were determined in controlled nitroxide-mediated radical copolymerization between styrene and substituted styrene. All experiments permitted the synthesis of new organometallic polymers that will be used for the development of a polymer capsule for Inertial Confinement Fusion Experiments.

Condensation process of alcohol molecules on mesoporous silica MCM-41 and SBA-15 and fumed silica: a spin-probe ESR study

A few alcoholic solutions of di-tert-butyl nitroxide (DTBN), a spin probe, at a high concentration were condensed on several silica materials, such as MCM-41, two types of SBA-15, and fumed silica, at various amounts in vacuum. At a very low solution dose the electron spin resonance (ESR) spectrum is that of an immobilized nitroxide radical. With increasing solution dose, the spectrum is gradually sharpened and a well-separated three-line spectrum is observed at the dose that is estimated to fill the surface with a monomolecular layer. Thus, the DTBN molecule can make rapid tumbling motion on this solvent layer. With a further increase in the solution dose the ESR spectrum is modified in different ways from system to system: the line width increases approximately linearly with respect to the solution dose for the SBA-15 and fumed silica systems, but it remains almost constant for the MCM-41 system until the solution dose exceeds the total volume of a nanochannel. The line width increase with respect to the solution dose is small for the SBA-15 system but large for the fumed silica system. These results have been interpreted geometrically with the structures of these silica materials and a condensation model for the alcohols on these surfaces. In relation to the present results, a model of the collective molecular flow of the alcohol solutions through the nanochannel of MCM-41 is given.

A mechanistic investigation of di- tert-butyl nitroxide using scanning electrochemical microscopy (SECM)

Cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM) have been used to evaluate the electron-transfer mechanism of di- tert-butyl nitroxide (DTBN) in acetonitrile. The oxidation of DTBN is coupled to a rapid, irreversible chemical follow-up step that is difficult to characterize quantitatively with CV due to distortion of the voltammograms by solution resistance and mixed radial–linear diffusion within the scan rate region of interest. Collection efficiencies from the tip generation substrate collection (TGSC) mode of SECM were used to determine a rate constant of 21 s −1 for the follow-up reaction. Collection efficiency versus distance plots obtained at 5 and 50 mM DTBN concentration are identical, confirming the first-order nature of the chemical reaction. Numerical simulation of linear scan voltammograms obtained at different tip/substrate distances provides a heterogeneous electron-transfer rate constant of 0.85 cm s −1.

Calorimetric and spin-label ESR studies of PEG:2000-DPPE containing DPPC/lyso-PPC mixtures

The thermotropic behavior of dipalmitoylphosphatidylcholine (DPPC) multibilayers containing up to 10 mol% of lyso-palmitoylphosphatidylcholine (lyso-PPC) with and without low content of poly(ethylene glycol:2000)-grafted dipalmitoylphosphatidylethanolamine (PEG:2000-DPPE) has been studied by high sensitivity differential scanning calorimetry (DSC) and electron spin resonance (ESR) using the spin probe di-tert-butyl-nitroxide (DTBN). The three lipids, dispersed in buffer at appropriate concentrations, form thermosensitive liposomes used as site-specific drug-delivery systems. Without polymer–lipids, the DPPC main transition temperature is downshifted of 1.2–1.3 °C at the highest lyso-PPC content. The molar enthalpy and the cooperative unit of the DPPC main transition first decrease rapidly, then more slowly and finally slightly increase with lyso-PPC content. Moreover, in the mixed dispersions, the membrane fluidity increases at any temperature. The addition up to 5 mol% of PEG:2000-DPPE to DPPC/10 mol% lyso-PPC mixtures does not affect neither the thermotropic phase behavior nor the transition cooperativity and the fluidity of the dispersions.

Combining precision spin-probe partitioning with time-resolved fluorescence quenching to study micelles: Application to micelles of pure lysomyristoylphosphatidylcholine (LMPC) and LMPC mixed with sodium dodecyl sulfate

Micelles of lysomyristoylphosphatidylcholine (LMPC) and mixed micelles of LMPC with anionic detergent sodium dodecyl sulfate (SDS) have been characterized by spin-probe-partitioning electron paramagnetic resonance (SPPEPR) and time-resolved fluorescence quenching (TRFQ) experiments. SPPEPR is a novel new method to study structure and dynamics in lipid assemblies successfully applied here for the first time to micelles. Several improvements to the computer program used to analyze SPPEPR spectra have been incorporated that increase the precision in the extracted parameters considerably from which micelle properties such as effective water concentration and microviscosity may be estimated. In addition, with this increased precision, it is shown that it is feasible to study the rate of transfer of a small spin probe between micelles and the surrounding aqueous phase by SPPEPR. The rate of transfer of the spin probe di- tert-butyl nitroxide (DTBN) and the activation energy of the transfer process in LMPC and LMPC-SDS micelles have been determined with high precision. The rate of transfer increases with temperature and SDS molar fraction in mixed micelles, while it remains constant with LMPC concentration in pure LMPC micelles. The activation energy of DTBN transfer in pure lysophospholipid micelles does not change with LMPC concentration while it decreases with the increasing molar fraction of SDS in mixed LMPC-SDS micelles. Both this decrease in activation energy and the increase in the rate of transfer are rationalized in terms of an increasing micelle surface area per molecule (decreasing compactness) as SDS molecules are added. This decreasing compactness as a function of SDS content is confirmed by TRFQ measurements showing an aggregation number that decreases from 122 molecules for pure LMPC micelles to 80 molecules for pure SDS micelles. The same increase in surface area per molecule is predicted to increase the effective water concentration in the polar shell of the micelles. This increase in hydration with SDS molar fraction is confirmed by measuring the effective water concentration in the polar shell of the micelles from the hyperfine spacing of DTBN. This work demonstrates the potential to design mixed lysophospholipid surfactant micelles with variable physicochemical properties. Well-defined micellar substrates, in terms of their physicochemical properties, may improve the studies of protein structure and enzyme kinetics.

One-electron redox reaction of di- tert-butyl nitroxide at platinum electrode in acetonitrile

The electrochemical oxidation of di- tert-butyl nitroxide (DTBN) at a platinum electrode in acetonitrile was examined. The cyclic voltammogram indicated an irreversible response during a normal time scale measurement, whereas chemically reversible voltammograms were obtained during a shorter time using a micro disk electrode with relatively fast sweep rates. The apparent formal redox potential and heterogeneous electron transfer rate constant were estimated to be 0.218 V (versus Fc +|Fc) and 0.035 ± 0.015 cm s −1 from the digital simulation analysis.

Precision parameters from spin-probe studies of membranes using a partitioning technique. Application to two model membrane vesicles

A new version of the ESR spin probe partitioning method is developed and applied to the study of hydration properties of dimyristoyl-phosphatidylglycerol (DMPG) and dimyristoyl-phosphatidylcholine (DMPC) vesicles as functions of salt concentration and temperature above the lipid phase transition. The small spin probe di- tert-butyl nitroxide (DTBN) is used in order to achieve motionally narrowed Electron Spin Resonance (ESR) spectra which may be analyzed with high precision. The new method relies on the use of the second harmonic display of the ESR spectrum followed by spectral line fitting. Spectral fitting yields precise ESR parameters giving detailed information on the surroundings of the spin probe in both phospholipid and aqueous phases. The nitrogen hyperfine coupling constant of DTBN arising from those probes occupying the vesicles is used to study the hydration of the vesicle surface. The hydration properties of the negatively charged vesicle surface of DMPG vesicles are affected by the addition of salt at all temperatures. In contrast, the hydration of DMPC vesicles does not change with salt concentration at the low temperatures. However, at higher temperatures the hydration properties of DMPC vesicle are affected by salt which is interpreted to be due to the faster motion of the phospholipid molecules. The partitioning of the spin probe increases with salt concentration for both DMPG and DMPC vesicles, while water penetration decreases simultaneously. The spin probe in the phospholipid bilayer exhibits anisotropic motion and the extent of the anisotropy is increased at the higher salt concentrations.

133Cesium HYSCORE Investigation of the Di-tert-butyl Nitroxide−Cs+ Adsorption Complex in CsNaY Zeolite

133Cs hyperfine sublevel correlation (HYSCORE) spectroscopy has been employed to characterize the structure of adsorption complexes of di-tert-butyl nitroxide (DTBN) with cesium cations in zeolite CsNaY. The experimental 133Cs HYSCORE data proved the direct coordination of the adsorbed DTBN molecules to the Cs+ ions and revealed nonambiguously the existence of two different types of adsorption complexes. Evaluation of the orientation selective 133Cs HYSCORE spectra provided the 133Cs hyperfine coupling tensors and thus information about the geometrical structure of those complexes. For one type of adsorption complexes, a complex geometry was obtained where the Cs+ ion is located within the molecular mirror plane of the DTBN radical with an oxygen−cesium cation bond length of 0.25 nm. For this, the isotropic Cs hyperfine coupling was negative. The second Cs+−DTBN complex is characterized by a bent structure. With an isotropic 133Cs hyperfine coupling of 9 MHz, the unpaired electron spin density in this Cs+...

Reactivity of sulfur centered nucleophiles in photoinduced reactions with 1-bromonaphthalene

The reactivity of S 2- (1), - SCN (2) and - SCNH(NH2) (3) in photoinduced substitution reactions with 1-bromonaphthalene (5) in DMSO was studied. The photostimulated reaction of ion 1 with 5 renders after quenching with MeI 1-(methylthio)naphthalene (16%), bis (1-naphthyl) sulfide (17.5%) and naphthalene (42%). The lack of reaction in the dark and the inhibition of the photoinduced one by the presence of di-tert-butylnitroxide (DTBN) and p-dinitrobenzene (pDNB) are evidence of a radical chain mechanism for these substitution reactions. On the other hand, ion 2 was unreactive towards 5, both as an electron donor under photostimulation and as a nucleophile. Finally the thiourea anion (3) affords after a photoinduced reaction with 5 and quenching with MeI, 1-methylthionaphthalene (50%), bis (1-naphthyl) sulfide (22%) and naphthalene (9%). This reaction does not occur in the dark and is inhibited in the presence of DTBN. By using hydrogen abstraction from DMSO as competitive reaction, the absolute rate constants for the addition of ions 1, 3 and benzene thiolate ions (4) to 1-naphthyl radicals have been determined to be 0.5 x 10 9 M -1 s -1 , 1.0 x 10 9 M -1 s -1 and 5.1 x 10 9 M -1 s -1 respectively.

Fluorescent quenching of the 2-naphthoxide anion by aliphatic and aromatic halides. Mechanism and consequences of electron transfer reactions.

The fluorescent excited state of the 2-naphthoxide ion (1) is quenched by aliphatic and aromatic halides according to an electron-transfer mechanism, with generation of the corresponding alkyl and aryl radicals by a concerted or consecutive C-X bond fragmentation reaction. Whereas bromo- and iodobenzene follow a concerted ET mechanism (C-X, BDE control), 1-bromonaphthalene exhibits a stepwise process (pi LUMO control). The photoinduced reaction of anion 1 with 1-iodoadamantane (2) in DMSO affords substitution products on C3, C6, and C8, 1-adamantanol, 1-adamantyl 2-naphthyl ether, and adamantane (3.2, 13.2, 12.2, 2.8, 2.5, and 14.1% yields, respectively). A complex mixture is also observed in the photochemical reaction of neopentyl iodide (3) with anion 1, which renders substitution on C1, C3, C6, C8, and 2-naphthyl neopentyl ether (8.1, 1.3, 19.1, 31.1, and 2.8% yields, respectively). The absence of reaction in the dark and the inhibition of the photoinduced reaction by the presence of the radical traps di-tert-butylnitroxide (DTBN) and 1,4-cyclohexadiene are evidence of a radical chain mechanism for these substitutions. On the other hand, only coupling at C1 is achieved by the photostimulated reaction of anion 1 with iodobenzene (5), to afford 41.9% of 1-phenyl-2-naphthol and 5.4% of disubstitution product. The regiochemistry of these reactions can be ascribed to steric hindrance and activation parameters.

Solvent dependence of the di- tert-butyl nitroxide (DTBN) hyperfine tensors: an experimental and computational study

The di- tert-butyl nitroxide (DTBN) hyperfine tensors are computed in a reasonable time frame using the UB1LYP method and Barone's electron paramagnetic resonance EPR-III basis sets. The effects of six solvents on the 14N hyperfine splittings are calculated using the polarizable continuum method and reproduce the experimental trends. Further improvement in the accuracy is obtained when one extra water molecule is made to interact with the NO moiety. All the DTBN atoms are not magnetically equivalent since the molecule has C 1 symmetry. This must be taken into consideration when simulating the EPR line shapes of randomly oriented DTBN molecules isolated in an argon matrix.

Electron pair acceptor properties of alkali cations in zeolite Y: an electron spin resonance study of adsorbed di-tert-butyl nitroxide

Di-tert-butyl nitroxide (DTBN) was adsorbed at several alkali metal ion exchanged Y zeolites to investigate the electron pair acceptor properties of the monovalent zeolite cations Li +, Na +, K +, and Cs +. This probe molecule forms adsorption complexes with the Lewis acidity possessing zeolite cations and thereby the spin density distribution at the nitroxide group of the DTBN molecule changes. This can directly be detected by electron spin resonance spectroscopy via the nitrogen hyperfine coupling constants of the probe molecule. A linear dependence of the nitrogen spin density at the π-orbital of the nitroxide group on the electronegativity of the zeolite cations was found. The Q values which relate the isotropic hyperfine structure coupling constant to the π-electron spin densities were determined. In comparison with these results, the nitrogen hyperfine coupling data of DTBN sorbed on the protonated form of the Y zeolite, possessing OH groups acting as Brøsnstedt acid sites, cannot be explained within this simple electronegativity model.

Nitroxide-controlled free radical polymerization of a sugar-carrying acryloyl monomer

Controlled free radical polymerization of a sugar-carrying acrylate, 3-O-acryloyl-1,2: 5,6-di-O-isopropylidene-a-D-glucofuranoside (AIpGlc), was achieved in p-xylene at 100°C by using a di-tert-butyl nitroxide (DBN)-based alkoxyamine as an initiator and dicumyl peroxide (DCP) as an accelerator. The polymerization gave low-polydispersity (1.2 < M w /M n < 1.6) polymers with predicted molecular weights. The same approach with a DBN-capped polystyrene (PS-DBN) as an initiator afforded block copolymers of the type PS-b-PAIpGlc. The acidolysis of the homopolymers and block copolymers gave well-defined glucose-carrying polymers PAGlc and PS-b-PAGlc, respectively. These amphiphilic PS-b-PAGlc block copolymers were observed to exhibit microdomain surface morphologies that differ for different copolymer compositions. This success opens up a new, simple route to the synthesis of well-defined sugar-carrying polymers of various architectures.

Kinetic Study on Nitroxide-Mediated Free Radical Polymerization of tert-Butyl Acrylate

The kinetics of nitroxide-mediated free radical polymerization of tert-butyl acrylate (tBA) was studied. It was demonstrated that the polymerization rate can be increased by addition of a proper amount of a radical initiator, dicumyl peroxide (DCP), without causing any appreciable broadening of the polydispersity. The use of di-tert-butyl nitroxide (DBN) allowed the controlled polymerization of tBA to proceed at a lower temperature than the polymerization controlled by a TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxy) derivative, which was conducted by Georges et al. [Macromolecules 1996, 29, 8993]. The pseudo-first-order activation rate constant kact of the poly(tBA)−DBN adduct was also determined by the GPC curve-resolution method to be 1.0 × 10-3 s-1 at 120 °C, which indicates that the adduct experiences activation−deactivation cycles frequently enough to afford a low polydispersity. Nevertheless, the polydispersity markedly increased and the rate of polymerization decreased at later stages of polymerizat...