Determination of the activation rate constants of alkyl halide initiators for atom transfer radical polymerization

The second-order activation rate constants k_A for low-molar-mass alkyl halides catalyzed by cuprous halide complexes Cu(I)X/2L (X = Cl or Br; L = 4,4′-diheptyl-2,2′-bipyridine) were determined by the nitroxide capping method along with ¹H NMR. The k_A for 1-phenylethyl bromide, a typical initiator for atom transfer radical polymerization (ATRP), with the Cu(I)Br complex was found to be close to the known value of the k_A for a polystyryl bromide, being large enough for the initiation to be completed at an early stage of polymerization. It was also found that k_A strongly depends on the kind of halogen and the steric factor of the alkyl halide in question.     

Determination of equilibrium constants for atom transfer radical polymerization

Atom transfer radical polymerization (ATRP) equilibrium constants (K(ATRP)) were determined using modified Fischer's equations for the persistent radical effect. The original Fischer's equations could be used only for low conversion of Cu(I) to X-Cu(II) and consequently for relatively low values of K(ATRP). At higher conversion to X-Cu(II) (>10%) and for larger values of K(ATRP) (>10(-)(7)), modified equations that take into account the changes in catalyst and initiator concentrations should be used. The validity of new equations was confirmed by detailed kinetic simulations. UV-vis spectrometric and GC measurements were used to follow the evolution of X-Cu(II) species and the initiator concentration, respectively, and to successfully determine values of K(ATRP) for several catalysts and alkyl halides. The effect of structure on reactivities of ATRP components is presented.     

On the accuracy of the hydroperoxide method of determining absolute rate constants for hydrogen atom abstraction by the tert-butylperoxy radical

Measurements of rates of oxygen absorption and steady-state peroxy radical concentrations for the autoxidation of tetralin in the presence of tert-butyl hydroperoxide have shown that the rate constant for reaction of the tert-butylperoxy radical with tetralin at 60°C is approximately 11.0 M^?1 s^?1. This rate constant is about a factor of 4 larger than the value recently reported by Niki, Okayasu, and Kamiya for this reaction. The present work emphasizes that great care should be taken when the hydroperoxide method is used to estimate cross-propagation rate constants for a substrate as reactive as tetralin at a temperature as high as 60°C.     

Use of stable radicals for the determination of rate constants of elementary processes

A direct method is proposed for determining the concentration of active centers by introducing the stable radical 2, 2, 6, 6-tetramethyl-4-hydroxypiperidine-1-oxyl at the commencement, and during the course of, the polymerization process of tetrafluoroethylene. On the basis of the data obtained, the rate constants of the elementary processes are calculated.     

A new method for the determination of thermodynamic equilibrium constants in separation processes

A new approach to the determination of thermodynamic equilibrium constants in ion-exchange and solvent extraction processes is described. A plot of the logarithm of the distributing species activity in the aqueous phase as a function of its concentration in the non-aqueous phase has an inflection point. The ordinate of the inflection point gives directly the logarithm of the thermodynamic equilibrium constant of the reaction considered. This was made possible by finding that the ratio between n power of the free extractant molecule (or free site) activity and the activity of the distributing species-extractant complex is unity as this point.     

Absolute rate constants for some hydrogen atom abstraction reactions by a primary fluoroalkyl radical in water

A combination of laser flash photolysis and competitive kinetic methods have been used to measure the absolute bimolecular rate constants for hydrogen atom abstraction in water from a variety of organic substrates including alcohols, ethers, and carboxylic acids by the perfluoroalkyl radical, *CF(2)CF(2)OCF(2)CF(2)SO(3)(-) Na(+). Comparison, where possible, of these rate constants with those previously measured for analogous reactions in the non-polar organic solvent, 1,3-bis(trifluoromethyl)benzene (J. Am. Chem. Soc, 1999, 121, 7335) show that the alcohols react 2-5 times more rapidly in the water solvent and that the ethers react at the same rate in both solvents. A transition state for hydrogen abstraction that is more reminiscent of an "intimate ion pair" than a "solvent separated ion pair" is invoked to explain these modest solvent effects.     

Gel permeation chromatographic determination of activation rate constants in nitroxide-controlled free radical polymerization, 2. Analysis of evolution of polydispersities

The dissociation rate constant k_d of the PS-TEMPO adduct, where PS is polystyrene and TEMPO is 2,2,6,6-tetramethylpiperidin-1-oxyl, was determined by the gel permeation chromatographic tracing of the evolution of polydispersities at an early stage of styrene polymerization with a model PS-TEMPO adduct as initiator. The results agree with those obtained by the direct method in Part 1.     

Understanding atom transfer radical polymerization: effect of ligand and initiator structures on the equilibrium constants

Equilibrium constants in Cu-based atom transfer radical polymerization (ATRP) were determined for a wide range of ligands and initiators in acetonitrile at 22 degrees C. The ATRP equilibrium constants obtained vary over 7 orders of magnitude and strongly depend on the ligand and initiator structures. The activities of the Cu(I)/ligand complexes are highest for tetradentate ligands, lower for tridentate ligands, and lowest for bidentate ligands. Complexes with tripodal and bridged ligands (Me6TREN and bridged cyclam) tend to be more active than those with the corresponding linear ligands. The equilibrium constants are largest for tertiary alkyl halides and smallest for primary alkyl halides. The activities of alkyl bromides are several times larger than those of the analogous alkyl chlorides. The equilibrium constants are largest for the nitrile derivatives, followed by those for the benzyl derivatives and the corresponding esters. Other equilibrium constants that are not readily measurable were extrapolated from the values for the reference ligands and initiators. Excellent correlations of the equilibrium constants with the Cu(II/I) redox potentials and the carbon-halogen bond dissociation energies were observed.     

Differential method for the determination of the order of reaction and the rate constants

A method is proposed whereby the orders and rate constants for processes obeying the rate law ?dA/dt = kAⁿ may be determined. The method is illustrated in two ways. First, simulated data for processes of various orders are treated, and the treatment is shown to be capable of reproducing orders and rate constants to a high degree of accuracy. The factors affecting the accuracy with which n and k can be determined are considered. These are inaccuracy in the determination of concentration values, irregularity of the time intevals between concentration determinations, and the length of those time intervals. It is shown that if concentrations are determined at times that are close together, the effect of the other two factors is small, but if the time intervals are made longer, the errors due to the other two factors affect the calculated values of n and k much more seriously. Second, the method was applied to two homogeneous reactions, of which one was first-order and one was second order, and three heterogeneous reactions, of which one was found by the original workers to be first order, one to be zero order, and one to vary between zero and first order, depending on the initial pressure. The present method gives results in agreement with these conclusions and reproduces the rate constants to within ±5% in all cases.     

Gel permeation chromatographic determination of activation rate constants in nitroxide-controlled free radical polymerization, 1. Direct analysis by peak resolution

The dissociation rate constant k_d related to the homolytic cleavage of the C ON bond formed between a polystyrene (PS) and 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) is determined by adopting the gel permeation chromatography peak-resolution method to the styrene polymerization with a PS-TEMPO adduct as a probe and the radical initiator tert-butyl hydroperoxide. The result was given by the Arrhenius equation, k_d = A exp(−E/(RT)) with A = 3.0 × 10¹³ s⁻¹ and E = 124 kJ · mol⁻¹.     

Determination of rate constants for radical telomerization chain growth and transfer from the molecular-mass distribution of oligomer

The ratio of rate constant for growth and transfer X(n), as a function of the chain length n has been found from the measured molecular-mass distributions of the products of tetrafluoroethylene telomerization in acetone, ethyl acetate, chloroform, and carbon tetrachloride. For all these telogens, the function increases by a factor of 1.5–2.5 in the range of n from 2 to 5, is almost constant for n of 6 to 10, and increases by a factor of 7–10 in the range of n from 12 to 20. This behavior of the function X(n) has been explained in terms of the model of diffusion-controlled propagation and kinetic chain transfer. The model takes into account the change in the diffusion nature of oligomers in the form of rigid rods with an increase in their length. A sharp increase in X(n) occurs when the oligomers that accumulate in the environment of growing macroradical sterically restrict the withdrawal of the forming oligomer to the bulk by an effective solid angle, which decreases with the increasing oligomer length and becomes minimal in the region of formation of colloidal particles.     

Hybrid atom transfer radical polymerization system for balanced polymerization rate and polymer molecular weight control

A hybrid polymerization system that combines the fast reaction kinetics of conventional free radical polymerization and the control of molecular weight and distribution afforded by ATRP has been developed. High‐free radical initiator concentrations in the range of 0.1–0.2 M were used in combination with a low concentration of ATRP catalyst. Conversions higher than 90% were achieved with ATRP catalyst concentrations of less than 20 ppm within 2 h for the hybrid ATRP system as compared with ATRPs where achieving such conversions would take up to 24 h. These reaction conditions lead to living polymerizations where polymer molecular weight increases linearly with monomer conversion. As in living polymerization and despite the fast rates and low ATRP catalyst concentrations, the polydispersity of the produced polymer remained below 1.30. Chain extension experiments from a synthesized macroinitiator were successful, which demonstrate the living characteristics of the hybrid ATRP process. Catalyst concentrations as low as 16 ppm were found to effectively mediate the growth of over 100 polymer chains per catalytic center, whereas at the same time negating the need for post polymerization purification given the low‐catalyst concentration. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2294–2301, 2010     

Evidence for chain transfer in the atom transfer radical polymerization of butyl acrylate

Poly(butyl acrylate) (PBuA) of high molecular weight was synthesized by atom transfer radical polymerization (ATRP) in ethyl acetate. Whereas for low molecular weight polymers, a linear increase of the number‐average molecular weight, M_n, versus conversion and narrow molecular weight distributions indicate the suppression of side reactions, a downward curvature in the plot of M_n versus conversion was observed for high molecular weights (M_n > 50 000). This effect is explained by chain transfer reactions, leading to branched polymers. GPC measurements with a viscosity detector give evidence for the branched structure of high molecular weight polymers obtained in ATRP. In addition, transfer to solvent or monomer is likely to occur.     

Effect of impurities and the initiation and transfer rate constants on the statistical character of anionic polymers

The simultaneous effect of impurities, hite rate of initiation, and chain transfer through the monomer on the size distribution in anionic polymers is investigated. It is assumed that monomer is added to the system containing monofunctional initiators in small doses 80 that there is no accumulation of the monomer or the impurities at any stage of the reaction. Firstly, by using the continuum approach, expressions for the size distribution and the averages are derived for the case when initiation is infinitely fast and chain transfer is absent. These results are compared with those obtained by Oroho and Wenger by a more rigorous analysis, and the adequacy of the simpler results is established. The complexities arising from the finite rate of initiation and chain transfer are considered subsequently. Finally, we describe a procedure for the determination of the initiation and transfer rate constants in terms of propagation rate constant and the fraction of impurities in the monomer supply.     

Polystyrene/mesoporous diatomite composites by in situ simultaneous reverse and normal initiation technique for atom transfer radical polymerization

Mesoporous diatomite platelets were employed to synthesize different polystyrene/diatomite composites by in situ polymerization of styrene via simultaneous reverse and normal techniques of atom transfer radical polymerization. Furrier transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, scanning and transmission electron microscopy, gas and size exclusion chromatography were used to examine characteristics of polymer and composite. Addition of 3 wt% pristine mesoporous diatomite leads to increase of conversion from 79 to 93%, while control over molecular weight characteristics become worse.     

Quantitative rate constants for radical reactions in the nanopores of cotton

The understanding of radical reactions in nanostructured materials is important for developing new synthetic procedures and controlling degradation reactions. To develop this area, an easy method for measuring quantitative rate constants of some radical reactions in nanostructures is required. A simple method for measuring the rate constant of dye bleaching, kdye, by organic radicals in such materials is introduced, involving the measurement of microsecond bleaching kinetics by diffuse reflectance spectroscopy, following laser flash creation of the radicals. Using wet and dry cotton as model substrates, we obtained kdye of 2-hydroxy-2-propyl and 1-hydroxy-1-cyclohexyl radicals with reactive red 3 and reactive orange 4 and compared them to solution-phase values. Surprisingly, the reactions in cotton follow simple liquid-phase kinetics and are diffusion-controlled. A cage effect in cotton is also found.