Critically Evaluated Rate Coefficients for Free‐Radical Polymerization, 4

Propagation rate coefficients, k_p, which have been previously reported by several groups for free‐radical bulk polymerizations of cyclohexyl methacrylate (CHMA), glycidyl methacrylate (GMA), benzyl methacrylate (BzMA), and isobornyl methacrylate (iBoMA) are critically evaluated. All data were determined by the combination of pulsed‐laser polymerization (PLP) and subsequent polymer analysis by size‐exclusion chromatography (SEC). This so‐called PLP‐SEC technique has been recommended as the method of choice for the determination of k_p by the IUPAC Working Party on Modeling of Polymerisation Kinetics and Processes. The present data fulfill consistency criteria and the agreement among the data from different laboratories is remarkable. The values for CHMA, GMA, and BzMA are therefore recommended as constituting benchmark data sets for each monomer. The data for iBoMA are also considered reliable, but since SEC calibration was established only by a single group, the data are not considered as a benchmark data set. All k_p data for each monomer are best fitted by the following Arrhenius relations: CHMA: , GMA: , BzMA: , iBoMA: . Rather remarkably, for the methacrylates under investigation, the k_p values are all very similar. Thus, all data can be fitted well by a single Arrhenius relation resulting in a pre‐exponential factor of 4.24 × 10⁶ L · mol^?1 · s^?1 and an activation energy of 21.9 kJ · mol^?1. All activation parameters refer to bulk polymerizations at ambient pressure and temperatures below 100 °C. Joint confidence intervals are also provided, enabling values and uncertainties for k_p to be estimated at any temperature.

95% joint confidence intervals for Arrhenius parameters A and E_A for cyclohexyl (CHMA), glycidyl (GMA), benzyl (BzMA), and isobornyl (iBoMA) methacrylate; for details see text.     

Low Conversion 4‐Acetoxystyrene Free‐Radical Polymerization Kinetics Determined by Pulsed‐Laser and Thermal Polymerization

Summary: The free‐radical polymerization kinetics of 4‐acetoxystyrene (4‐AcOS) is studied over a wide temperature range. Pulsed‐laser polymerization, in combination with dual detector size‐exclusion chromatography, is used to measure k_p, the propagation rate coefficient, between 20 and 110 °C. Values are roughly 50% higher than those of styrene, while the activation energy of 28.7 kJ · mol⁻¹ is lower than that of styrene by 3–4 kJ · mol⁻¹. With known k_p, conversion and molecular weight data from 4‐AcOS thermal polymerizations conducted at 100, 140, and 170 °C are used to estimate termination and thermal initiation kinetics. The behavior is similar to that previously observed for styrene, with an activation energy of 90.4 kJ · mol⁻¹ estimated for the third‐order thermal initiation mechanism.

Joint confidence (95%) ellipsoids for the frequency factor A and the activation energy E_a from non‐linear fitting of k_p data for 4‐AcOS (black) and styrene (grey).     

A Suitable Photoinitiator for Pulsed Laser‐Induced Free‐Radical Polymerization

Detailed kinetic studies into free‐radical polymerization via pulsed laser experiments ideally require photoinitiators which almost instantaneously dissociate into primary free‐radical fragments that rapidly add to monomer molecules and thus induce macromolecular growth. 2‐Methyl‐4′‐(methylthio)‐2‐morpholinopropiophenone (MMMP) is shown to be such a suitable photoinitiator. Measurement of monomer conversion induced by a single laser pulse, within the so‐called single‐pulse pulsed laser polymerization (SP–PLP) experiment, provides direct information about the chain‐length dependence of the termination rate coefficient if MMMP is used as the photoinitiator.

Relative monomer concentration vs time trace of a methyl acrylate homopolymerization at 40 °C and 2 000 bar where MMMP was used as the initiator. The primary radical concentration from MMMP photo‐decomposition increases from curve (a) to (b) to (c) by the ratios 1:2.2:5.7.     

Termination Kinetics of 1‐Vinylpyrrolidin‐2‐one Radical Polymerization in Aqueous Solution

Termination kinetics of 1‐vinylpyrrolidin‐2‐one radical polymerization in aqueous solution has been studied at 40 °C between 20 and 100 wt.‐% VP. The <k_t>/k_p values from laser single‐pulse experiments with microsecond time‐resolved NIR detection of monomer conversion, in conjunction with k_p from literature, yield chain‐length‐averaged termination rate coefficients, <k_t>. Because of better signal‐to‐noise quality, experiments were carried out at 2 000 bar, but also at 1 500, 1 000, and 500 bar, thus allowing for estimates of <k_t> at ambient pressure. The dependence of <k_t> on monomer conversion indicates initial control by segmental diffusion followed by translational diffusion and finally reaction diffusion control. To assist the kinetic studies, viscosities of VP–water mixtures at ambient pressure have been determined.

     

Kinetics and Modeling of the Radical Polymerization of Trimethylaminoethyl methacrylate chloride in Aqueous Solution

For the radical polymerization of ionized trimethylaminoethyl methacrylate chloride (TMAEMA) in aqueous solution, two strategies to determine the propagation rate coefficient (k_p) are proposed for systems where the pulsed‐laser polymerization–size‐exclusion chromatography (PLP–SEC) method fails. This problem occurs with some fully ionized or sterically highly hindered monomers, where termination may become too slow. As TMAEMA is a borderline case with k_p being accessible by PLP–SEC and from single‐pulse–pulsed‐laser polymerization with electron paramagnetic resonance (SP–PLP–EPR) spectroscopy, studies into this monomer allow for judging the quality of the suggested alternative approaches of k_p measurement and serve for consistency checks of the previously published k_p and termination rate coefficient (k_t) data. Within both approaches, k_p/〈k_t〉 ^0.5 is measured via chemically initiated polymerization, with 〈k_t〉 referring to chain‐length‐averaged termination. The k_p/〈k_t〉 ^0.5 data are combined either with k_p/〈k_t〉 values from highly time‐resolved near‐infrared detection of monomer conversion induced by a single laser pulse (SP–PLP–NIR) or with Predici modeling on the basis of known chain‐length‐dependent termination kinetics. As coupled rate coefficients are measured, the obtained k_p data also provide 〈k_t〉 for a particular chain‐length distribution. The differences between propagation and termination rates of nonionized and fully ionized monomers are discussed.

     

Critically Evaluated Rate Coefficients for Free‐Radical Polymerization, 5,

Summary: Propagation rate coefficients, k_p, for free‐radical polymerization of butyl acrylate (BA) previously reported by several groups are critically evaluated. All data were determined by the combination of pulsed‐laser polymerization (PLP) and subsequent polymer analysis by size exclusion (SEC) chromatography. The PLP‐SEC technique has been recommended as the method of choice for the determination of k_p by the IUPAC Working Party on Modeling of Polymerization Kinetics and Processes. Application of the technique to acrylates has proven to be very difficult and, along with other experimental evidence, has led to the conclusion that acrylate chain‐growth kinetics are complicated by intramolecular transfer (backbiting) events to form a mid‐chain radical structure of lower reactivity. These mechanisms have a significant effect on acrylate polymerization rate even at low temperatures, and have limited the PLP‐SEC determination of k_p of chain‐end radicals to low temperatures (<20 °C) using high pulse repetition rates. Nonetheless, the values for BA from six different laboratories, determined at ambient pressure in the temperature range of ?65 to 20 °C mostly for bulk monomer with few data in solution, fulfill consistency criteria and show excellent agreement, and are therefore combined together into a benchmark data set. The data are fitted well by an Arrhenius relation resulting in a pre‐exponential factor of 2.21 × 10⁷ L · mol^?1 · s^?1 and an activation energy of 17.9 kJ · mol^?1. It must be emphasized that these PLP‐determined k_p values are for monomer addition to a chain‐end radical and that, even at low temperatures, it is necessary to consider the presence of two radical structures that have very different reactivity. Studies for other alkyl acrylates do not provide sufficient results to construct benchmark data sets, but indicate that the family behavior previously documented for alkyl methacrylates also holds true within the alkyl acrylate family of monomers.

Arrhenius plot of propagation rate coefficients, k_p, for BA as measured by PLP‐SEC.     

PLP‐SEC Study into the Free‐Radical Propagation Rate Coefficients of Partially and Fully Ionized Acrylic Acid in Aqueous Solution

Summary: Propagation rate coefficients, k_p, for acrylic acid (AA) polymerization at 6 °C in aqueous solution were measured via pulsed laser polymerization (PLP) with the degree of ionization, α, varied over the entire range between 0 and 1. These measurements were carried out in conjunction with aqueous‐phase size‐exclusion chromatography (SEC). Strictly speaking, the reported k_p's are “apparent” propagation rate coefficients deduced from the PLP‐SEC data under the assumption that the local monomer concentration at the radical site is identical to overall monomer concentration. At an AA concentration of 0.69 mol · L^?1, the apparent k_p decreases from 111 000 L · mol^?1 · s^?1 at α = 0 to 13 000 L · mol^?1 · s^?1 at α = 1.0. The significant lowering of k_p with higher α is attributed to the repulsion between both monomer molecules and macroradicals becoming negatively charged. Addition of up to 10 mol‐% (with respect to AA) sodium hydroxide to the fully ionized aqueous AA solution leads to an enhancement of k_p up to 57 000 L · mol^?1 · s^?1.

Dependence of apparent k_p values on the degree of ionization of acrylic acid (a) and on pH (b) for aqueous polymerizations of acrylic acid.     

Termination Kinetics of Dibutyl Itaconate Free‐Radical Polymerization Studied via the SP–PLP–ESR Technique

Summary: The termination kinetics of dibutyl itaconate (DBI) bulk polymerization was studied via SP–PLP–ESR single pulse–pulsed laser polymerization with time‐resolved detection of free‐radical concentration by electron‐spin resonance, at temperatures between 0 and 60 °C. As is characteristic of PLP experiments, termination rate coefficients, k_t(i,i), are measured for radicals of (almost) identical chain length (CL) i. CL‐averaged 〈k_t〉, for chain lengths up to 200 monomer units, and also kequation/tex2gif-stack-1.gif referring to termination of very small‐size radicals are directly deduced from measured DBI radical concentration vs time traces. At 45 °C, 〈k_t〉 is (3.4 ± 0.6) · 10⁵ L · mol^?1 · s^?1 and kequation/tex2gif-stack-2.gif is (7.2 ± 1.0) · 10⁵ L · mol^?1 · s^?1. Both rate coefficients are independent of monomer conversion up to the highest experimental conversion of 18%. The associated activation energies are E_A(〈k_t〉) = 23.0 ± 3.2 kJ · mol^?1 and E_A(kequation/tex2gif-stack-3.gif) = 27.6 ± 2.8 kJ · mol^?1, respectively. “Model‐dependent” and “model‐free” analyses of radical concentration vs time profiles indicate a pronounced CL dependence of k_t(i,i) for DBI radicals of moderate size, 5 < i < 50. The lowering of k_t(i,i) with CL corresponds to an exponent α close to 0.5 in a power‐law expression k_t(i,i) = kequation/tex2gif-stack-4.gif · i^?a. At higher chain lengths, the variation of k_t(i,i) with CL becomes weaker and may be represented by an α value of 0.16 or even below. These results are consistent with models according to which α varies to a larger extent at low CL and to a smaller extent at high CL with the crossover region between the two ranges being located somewhere around i = 100.

Conversion‐dependence of 〈k_t〉 and kequation/tex2gif-stack-5.gif from laser‐induced photopolymerization of DBI.     

Propagation Kinetics of the Radical Polymerization of Methylated Acrylamides in Aqueous Solution

Propagation rate coefficients, k_p, of the radical polymerization of N,N‐dimethylacrylamide (N,N‐dimethylprop‐2‐enamide), N‐methylmethacrylamide (N,2‐dimethylprop‐2‐enamide), and methacrylamide (2‐methylprop‐2‐enamide) in aqueous solution are measured via pulsed‐laser polymerization in conjunction with size‐exclusion chromatography within wide ranges of monomer concentration, temperature, and pressure. As observed for aqueous solutions of other monomers, k_p decreases significantly towards higher monomer concentration. For methacrylamide, a higher activation energy and a larger volume of activation are found as compared with N,N‐dimethylacrylamide. These two activation parameters are almost identical for N‐methylmethacrylamide and for N,N‐dimethylacrylamide.

     

Termination Kinetics of tert‐Butyl Methacrylate and of n‐Butyl Methacrylate Free‐Radical Bulk Homopolymerizations

Summary: Termination kinetics in tert‐butyl methacrylate (tert‐BMA) and n‐butyl methacrylate (n‐BMA) bulk homopolymerizations has been studied via the single pulse‐pulsed laser polymerization‐near infrared (SP‐PLP‐NIR) method between 40 and 80 °C at pressures from 500 to 2 250 bar. Toward increasing monomer conversion, the chain‐length averaged termination rate coefficient, 〈k_t〉, for both monomers exhibits the methacrylate‐specific sequence of an initial plateau region, assigned to control by segmental diffusion, followed by a steep decrease of 〈k_t〉 at intermediate conversion, which is assigned to translational diffusion control, and a weaker decrease of 〈k_t〉, associated with reaction‐diffusion control, at still higher degrees of monomer conversion. Despite this similarity, the two isomeric monomers clearly differ in absolute size of 〈k_t〉 and in the monomer concentration ranges where the transitions between the different types of diffusion control occur. The differences are assigned to effects of chain mobility which is hindered to a larger extent in tert‐BMA than in n‐BMA. As a consequence, the 〈k_t〉 behavior of tert‐BMA at 80 °C is close to the one of n‐BMA at 40 °C. Investigations into the chain‐length dependence of k_t, in particular into k_t(i,i), the rate coefficient for termination of two radicals of identical size, support the evidence on the different types of diffusion control that operate as a function of monomer conversion. In the initial conversion range, the power‐law exponent which characterizes the chain‐length dependence of larger (entangled) radicals, is found for both monomers to be close to the theoretical value of α = 0.16.

Dependence of log(〈k_t〉/k_p) on monomer conversion, X, for n‐BMA and tert‐BMA bulk homopolymerizations at 2 000 bar and 70 °C. Circles and triangles represent independent data sets obtained from separate experiments.     

A Combined Computational and Experimental Study on the Free‐Radical Copolymerization of Styrene and Hydroxyethyl Acrylate

Bulk free‐radical copolymerization of styrene and 2‐hydroxyethyl acrylate (HEA) is investigated experimentally at 50 °C using pulsed‐laser polymerization and computationally using ab initio simulations. Arrhenius parameters for HEA chain‐end homopropagation are A = 1.72 × 10⁷ L mol^?1 s^?1 and E_a = 16.8 kJ mol^?1, based on experiments between 20 and 60 °C. Copolymer composition data are well fitted by the terminal model with reactivity ratios r_ST = 0.44 ± 0.03 and r_HEA = 0.18 ± 0.04, but the variation in the propagation rate coefficient with monomer composition is underpredicted. Results are compared with computational predictions assuming the terminal as well as the penultimate unit effect (PUE) model. Intramolecular H‐bonding is shown to have a significant influence on PUE calculations. Discrepancies between computational predictions and experiment are attributed to the influence of intermolecular H‐bonding.     

The Influence of Hydrogen Bonding on the Propagation Rate Coefficient in Free‐Radical Polymerizations of Hydroxypropyl Methacrylate

The propagation rate coefficient k_p was determined for hydroxypropyl methacrylate by applying pulsed laser initiated polymerizations and subsequent analysis of the polymer by size‐exclusion chromatography. k_p data were derived for polymerizations in bulk and in several solvents: toluene, tetrahydrofuran (THF), benzyl alcohol, and supercritical CO₂. With the exception of THF, no solvent influence on k_p was observed. For polymerizations in THF k_p values 40% below the corresponding bulk data were obtained. In addition, the activation energy of k_p for polymerizations in THF is higher than for the other systems. The results are explained by a complexation of the OH group contained in the ester group with THF. As a consequence, H bonds between OH groups and carbonyl O atoms, which occur in the other systems, are not formed in the presence of THF. This explanation is supported by Raman spectra, which show that association of carbonyl groups does not occur for systems containing THF, whereas for all other systems the occurrence of two peaks at 1 703 cm^?1 and 1 720 cm^?1 is indicative of the vibrations of two different – associated vs. not associated – types of carbonyl groups. Based on the change in activation energy it is suggested that a true kinetic solvent effect occurs.

Temperature dependence of k_p for HPMA polymerizations in bulk and in solution of THF. The literature data for bulk polymerizations are taken from ref. 22 . Open symbols refer to ν_rep = 10 Hz and filled symbols to ν_rep = 25 Hz.     

Chain Transfer to 2‐Mercaptoethanol in Methacrylic Acid Polymerization in Aqueous Solution

The chain‐transfer constant, C_S = k_tr/k_p, of 2‐mercaptoethanol (ME) for methacrylic acid (MAA) polymerization in aqueous solution has been measured at MAA concentrations between 5 and 30 wt% to be 0.12 ± 0.01 at 50 °C. Analysis has been carried out via both the Mayo and the chain‐length distribution (CLD) methods. No change of C_S with monomer concentration is observed. The chain‐transfer rate coefficient, k_tr, thus exhibits the same strong dependence on monomer concentration as the propagation rate coefficient, k_p.     

Propagation Kinetics of Binary Acrylate–Methacrylate Free‐Radical Bulk Copolymerizations

Rate coefficients of propagation, k_p,copo, for MA–MMA, MA–DMA, DA–MMA, and DA–DMA bulk copolymerizations have been determined via PLP–SEC. The k_p,copo values are analyzed in terms of the terminal and penultimate unit effect models. Alkyl acrylate and alkyl methacrylate reactivity ratios, as obtained from copolymer composition, are not significantly dependent on the size of the alkyl ester moiety, whereas k_p,copo clearly depends on the size of the alkyl group. For MA–DMA, an almost perfect simultaneous fit of copolymer composition and k_p,copo is obtained via the terminal model. For DA–MMA, even the penultimate unit effect model affords no adequate simultaneous fit. The observed effects are assigned to hindrance of internal rotational motion in the transition state due to intermolecular and intramolecular mobility terms.

     

Determining Initiator Efficiency in Radical Polymerization by Electrospray‐Ionization Mass Spectrometry

A method for measuring initiator efficiency in radical polymerization using ESI‐MS is presented. The method is based on the evaluation of relative MS peak intensities of polymer that has been initiated by a mixture of initiators, of which one serves as an internal reference. The method is quickly and easily performed and was found to be reproducible and robust. In polymerization of methyl methacrylate in solution, the efficiency of BTMHP at 80 °C was determined to be f_BTMHP = 0.57 ± 0.08, and the efficiency of DBPO at 100 °C was measured to be f_DBPO = 0.89 ± 0.08.

     

Critically Evaluated Termination Rate Coefficients for Free‐Radical Polymerization, 1. The Current Situation

This is the first publication of an IUPAC‐sponsored Task Group on “Critically evaluated termination rate coefficients for free‐radical polymerization.” The paper summarizes the current situation with regard to the reliability of values of termination rate coefficients k_t. It begins by illustrating the stark reality that there is large and unacceptable scatter in literature values of k_t, and it is pointed out that some reasons for this are relatively easily remedied. However, the major reason for this situation is the inherent complexity of the phenomenon of termination in free‐radical polymerization. It is our impression that this complexity is only incompletely grasped by many workers in the field, and a consequence of this tendency to oversimplify is that misunderstanding of and disagreement about termination are rampant. Therefore this paper presents a full discussion of the intricacies of k_t: sections deal with diffusion control, conversion dependence, chain‐length dependence, steady state and non‐steady state measurements, activation energies and activation volumes, combination and disproportionation, and theories. All the presented concepts are developed from first principles, and only rigorous, fully‐documented experimental results and theoretical investigations are cited as evidence. For this reason it can be said that this paper summarizes all that we, as a cross‐section of workers in the field, agree on about termination in free‐radical polymerization. Our discussion naturally leads to a series of recommendations regarding measurement of k_t and reaching a more satisfactory understanding of this very important rate coefficient.

Variation of termination rate coefficient k_t with inverse absolute temperature T^?1 for bulk polymerization of methyl methacrylate at ambient pressure.^[6] The plot contains all tabulated values^[6] (including those categorized as “recalculated”) except ones from polymerizations noted as involving solvent or above‐ambient pressures.     

Chain‐Length‐Dependent Termination in Radical Polymerization of Acrylates

The technique of SPPLP EPR, which is single‐pulse pulsed‐laser polymerization (SPPLP) in conjunction with electron paramagnetic resonance (EPR) spectroscopy, is used to carry out a detailed investigation of secondary (chain‐end) radical termination of acrylates. Measurements are performed on methyl acrylate, n‐butyl acrylate, and dodecyl acrylate in bulk and in toluene solution at ?40 °C. The reason for the low temperature is to avoid formation of mid‐chain radicals (MCRs), a complicating factor that has imparted ambiguity to the results of previous studies of this nature. Consistent with these previous studies, composite‐model behavior for chain‐length‐dependent termination (CLDT) rate coefficients, , is found in this work. However, lower and more reasonable values of α_s, the exponent for variation of at short chain lengths, are found in the present study. Most likely this is because of the absence of MCRs, thereby validating the methodology of this work. Family‐type termination behavior is observed, with the following average parameter values adequately describing all results, regardless of acrylate or the presence of toluene: α_s = 0.79, α_l = 0.21 (long chains) and i_c ≈ 30 (crossover chain length). All indications are that these values carry over to termination of acrylate chain‐end radicals at higher, more practical temperatures. Further, these values largely make sense in terms of what is understood about the physical meaning of the parameters. Variation of the rate coefficient for termination between monomeric radicals, , is found to be well described by the simple Smoluchowski and Stokes–Einstein equations. This allows easy prediction of for different alkyl acrylates, solvent, and temperature. Through all this the unrivalled power of SPPLP EPR for measuring and understanding (chain‐length‐dependent) termination rate coefficients shines through.

     

Gel Formation and Molecular Characteristics of Poly(N‐isopropylacrylamide) Prepared by Free‐Radical Redox Polymerization in Aqueous Solution

The molecular characteristics of poly(N‐isopropylacrylamide) (PNIPA), prepared by free‐radical polymerization using an aqueous redox initiator and reaction conditions comparable to those used in the synthesis of nanocomposite gels, were investigated by altering the monomer concentration ([NIPA]) and the polymerization temperature (T_p) across the transition temperature (LCST). When T_p<LCST, there is a critical [NIPA] (=n*) above which PNIPA partially forms gels in the absence of a chemical crosslinker, and the gel fraction increases with increasing [NIPA] and decreasing T_p. In the range of n<n*, the molecular weight of soluble PNIPA correlated well with [NIPA]. When T_p>LCST, gels were not formed regardless of [NIPA]. The structure and mechanism of formation of self‐crosslinked PNIPA gels are discussed.

     

Xanthate‐Mediated Controlled Radical Polymerization of N‐Vinylcarbazole

Summary: Well‐defined poly(N‐vinylcarbazole) [poly(NVC)] was synthesized by macromolecular design via interchange of the xanthates (MADIX)/reversible addition‐fragmentation chain transfer (RAFT) polymerization. The homopolymers with controlled molecular weights ( = 3 000–48 000) and low polydispersities indices ( = 1.15–1.20) were obtained by the polymerization of NVC with AIBN in the presence of O‐ethyl‐S‐(1‐phenylethyl) dithiocarbonate as a xanthate‐type chain transfer agent (CTA). Good control of the polymerization was confirmed by the linear first‐order kinetic plot, the molecular weight controlled by the monomer/CTA molar ratio, linear increase in the molecular weight with the conversion, and the ability to extend the chains by the second addition of the monomer.

Radical polymerization of NVC in the presence of CTA and plot of number‐average molecular weight (circles) and polydispersity (squares) as a function of conversion.     

Stable Free Radical Polymerization Kinetics of Alkyl Acrylate Monomers Using in situ FTIR Spectroscopy: Influence of Hydroxyl‐Containing Monomers and Additives

Summary: The homopolymerization and copolymerization of various alkyl acrylate monomers was studied under stable free radical polymerization (SFRP) conditions using in situ FTIR spectroscopy to monitor polymerization kinetics. The IR absorbance corresponding to the C? H deformation of the monomer (968 cm^?1) was measured to determine monomer conversion in real‐time fashion. The monomer disappearance profiles were subsequently converted to pseudo‐first order kinetic plots. Altering the alkyl ester chain length and configuration did not reveal a significant trend in the resulting polymerization kinetics. However, addition of 2‐hydroxyethyl acrylate (HEA) to a polymerization of n‐butyl acrylate (nBA) substantially accelerated the rate of total monomer conversion, increasing the observed rate constant almost two times. ¹H NMR spectroscopy also showed that the resulting HEA/nBA copolymers were enriched with the HEA monomer. Moreover, a similar but enhanced effect was also observed upon the addition of small amounts of dodecanol to an n‐butyl acrylate homopolymerization resulting in more than a doubling of the observed rate constant.

Resonance forms associated with the DEPN nitroxide and stabilization resulting from hydrogen bonding.