The effects of polymerization conditions (the types and concentrations of initiator and solvent, the TEMPO-to-initiator molar ratio, and temperature) on the kinetics and mechanism of pseudoliving radical polymerization of 4-vinylpyridine mediated by the nitroxide TEMPO are studied. The key quantitative parameters of the process are calculated: namely, the rate constant of reinitiation, the rate constant of inhibition, and the product of the propagation-rate constant and the pseudoliving equilibrium constant. The optimum conditions for the controlled synthesis of poly(4-vinylpyridine) mediated by TEMPO are ascertained. 相似文献
The kinetics and mechanism of the pseudoliving free-radical polymerization of styrene are studied for the first time under the conditions of reversible inhibition by TEMPO nitroxides. An abnormal decline in the reduced rate of polymerization, which is inconsistent with a decrease in the concentration of the monomer, and an increase in the steady-state concentration of the free nitroxide are discovered. The main quantitative characteristics of the pseudoliving process are determined, namely, the rate constants of reinitiation and reversible recombination, and the constant of pseudoliving equilibrium between dormant and growing chains. It is shown that the polarity of a medium and the concentration of the monomer determine the character of polymerization in a solution: With an increase in the polarity of the solvent, the pseudoliving equilibrium constant increases, the reduced rate of styrene polymerization decreases, and the molecularmass-distribution of the polymer formed at initial conversions narrows. The smaller the concentration of styrene in the reaction system, the more pronounced the above differences associated with the solvent nature. 相似文献
The gradient copolymers of styrene and tert-butyl acrylate are synthesized by pseudoliving free-radical polymerization in the presence of TEMPO. Despite the inability of tert-butyl acrylate to undergo polymerization in the presence of the nitroxide TEMPO, the introduction of styrene makes it possible to perform the process under the controlled reversible-inhibition regime. The introduction of an additional high-temperature initiator, cumene hydroperoxide, increases the yield of the copolymer, while the pseudoliving mechanism of the process is preserved. This phenomenon is confirmed by the facts that the concentration of nitroxide remains almost invariable during polymerization and that the molecular mass of the polymerization product increases with conversion. Variations in the composition of the copolymer and its molecular mass during polymerization are evidence that the gradient copolymers are formed. 相似文献
The main kinetic and thermodynamic parameters of the pseudoliving radical polymerization of styrene mediated by 4-linoleamido-2,2,6,6-tetramethyl-1-piperidinyloxy have been studied. It has been shown that the introduction of the said substituent into nitroxide leads to a marked reduction in the rate constant of reinitiation that is compensated for by the simultaneous reduction in the rate constant of reversible termination. As a result, the rate of pseudoliving polymerization, the rate of molecular mass growth, and the polydispersity of the polymer appear to be practically the same for processes mediated by both unsubstituted and substituted nitroxides. 相似文献
The kinetics of three-dimensional radical copolymerization of methyl methacrylate and divinyl sulfide in the presence of the iniferter N,N′-bis(vinyloxyethyl)thiuram disulfide has been studied. The living chain mechanism of the process performed in the presence of the iniferter is verified by precision isothermal calorimetry. Conditions are found for more efficient kinetic parameters of the pseudoliving three-dimensional radical copolymerization of methyl methacrylate and divinyl sulfide carried out in the presence of the iniferter-AIBN initiating system. General kinetic features and differences in the pseudoliving polymerization of methyl methacrylate and its copolymerization with divinyl sulfide are ascertained. 相似文献
It is shown through a numerical experiment that, in the presence of extra (spontaneous) initiation, the equilibrium constant and the ratio between the rates of quadratic and reversible terminations cannot affect the rate of pseudoliving radical polymerization. The kinetics of the process in the steady-state region is determined only by the ratio between the rates of additional initiation and quadratic chain termination. Discrepancies between experimental data and theory must be explained with allowance for side reactions that can occur in these complex systems rather than via re-examination of the reliably verified mechanism of pseudo-living radical polymerization. 相似文献
A new approach is proposed for determination of quantitative kinetic parameters of pseudoliving radical polymerization proceeding in the mode of reversible termination, which include the reversible-termination rate constant, the length of the elementary step (number of growth events beginning from chain initiation to termination), and the number of steps that compose the chain during its growth. The method is based on the linearization of the chain length distribution of macromolecules and constitutes further development of the previously known approach to the determination of the chain transfer constant in nonliving polymerization. The applicability of the method was theoretically substantiated and experimentally verified using, as an example, some systems with reversible inhibition by nitroxides. 相似文献
2,2,6,6-Tetramethylpiperidine-N-oxyl (TEMPO)-mediated free radical polymerization of 4-vinylpyridine is found to proceed in a “controlled” manner. A linear increase of molecular weight along with an increase in conversion occurs at varying temperatures. Polymerization of styrene with a poly(4-vinylpyridine) block as macromer results in block copolymers with narrow polydispersity. The polymers are characterized by different size exclusion chromatography methods and converted to cationic polyelectrolytes as well as to polysulfo- and polycarbobetaines. 相似文献
The initial rate of carbon monoxide evolution in the reaction of Octacarbonyl dicobalt with 2,2,6,6-tetramethylpiperidin-1-oxyl free radical (TEMPO) at 15°C in n-octane solution leading to the 16e complex (TEMPO)Co(CO)2 was found to be first order with respect to the TEMPO concentration, 0.5 order with respect to the Co2(CO)8 concentration, and negative 0.5 order with respect to the CO concentration. Scavenging ·Co(CO)4 and ·Co(CO)3 by the free radical TEMPO in the rate-determining steps are in accord with the kinetic observation. The observed rate constant is kobs = 6.4 × 10–5 s–1. 相似文献
The preparation of poly-4-vinylpyridine (poly-4VP) by electrochemical polymerization of 4-vinylpyridine (4VP) in pyridine containing sodium tetraphenylboron (NaBPh4) is described. Information on the influence of monomer concentration, current density, polymerization rate, molecular weight, and electrochemical efficiency is presented. The polymerizations were performed under conditions of constant electrolysis current. Polymer formed in the cathodic compartment only, where a red-orange solution developed after about 15 min of electrolysis time. The optical absorption spectra of these colored solutions were studied. Cyclic voltammograms of 4VP in pyridine and NaBPh4 are also reported, and the influence of the scan rate upon peak current is described. The results indicate that the polymerization was anionic and nonterminating. The characteristics of the electrochemical polymerization of 4VP in pyridine are compared with those of the same monomer in liquid NH3. In the former case, the catholyte was homogeneous, and polymer growth occurred in the liquid phase, while in the latter growth took place in a heterogeneous environment. Kinetic consequences of these physical differences are pointed out. Suggestions for the mechanism of this electrochemical initiation are advanced. 相似文献
Monolayer films of a water-soluble surfactant, 4-octaneamido-2,2,6,6-tetramethyl-1-piperidinyloxy (C8-TEMPO) were investigated at the air/water interface. An electrochemical, horizontal touch method was developed to measure the equilibrium surface concentrations (gamma) of C8TEMPO. The dependence of gamma on the solution concentration followed a Langmuir isotherm and yielded the partition constant K = (2.3+/-0.2) x 10(4) M(-1). These results were verified by surface tension measurements and Brewster angle microscopy. Within experimental error, the same K values were obtained. The lateral diffusion constants vs surface concentration of this molecule were measured by 2D voltammetry. In these experiments, the component of the oxidation current due to C8TEMPO in the bulk of the solution was subtracted from the total measured current to obtain the component due to the lateral surface diffusion. In the ange of mean molecular areas from 120 to 400 A2/molecule, the lateral diffusion constant of C8TEMPO increased from 1.0 x 10(-6) to 1.0 x 10(-5) cm2/s. The latter value is about 2.5 times larger than the C8TEMPO diffusion constant in bulk water. Comparison of the lateral mobilities of C8TEMPO and two longer alkane chain, water-insoluble homologues, C14TEMPO and C18TEMPO, showed no statistically significant differences. 相似文献
The present paper evaluates largely unstudied kinetic aspects of styrene polymerization with a (relatively new) acyloxyamine over several temperature levels and contrasts these features with regular styrene polymerization and styrene polymerization with TEMPO. These comparisons show that the system behaves rather like regular thermal polymerization of styrene at temperatures between 120–180°C. However, at higher temperatures (> 180°C), acyloxyamine has an initiator-like contribution, giving the rate of polymerization an extra boost while decreasing molecular weights. This is further corroborated by mathematical modeling for both conversion and molecular weight averages. 相似文献
With the use of the competitive inhibition method and ESR spectroscopy, it is shown that, in pseudoliving radical polymerization, the rate constant of decomposition of PS and poly(acrylic acid) adducts with nitroxide radicals significantly increases as the polarity of a medium increases. This phenomenon opens a new approach to the activation of chains in nitroxide-mediated polymerization. 相似文献
A mathematical model has been developed to describe the interfacial mass transfer of TEMPO in a nitroxide‐mediated miniemulsion polymerization (NMMP) system in the absence of chemical reactions. The model is used to examine how the diffusivity of TEMPO in the aqueous and organic droplet phases, the average droplet diameter and the nitroxide partition coefficient influences the time required for the nitroxide to reach phase equilibrium under non‐steady state conditions. Our model predicts that phase equilibrium is achieved quickly (< 1 × 10−4 s) in NMMP systems under typical polymerization conditions and even at high monomer conversions when there is significant resistance to molecular diffusion. The characteristic time for reversible radical deactivation by TEMPO was found to be more than ten times greater than the predicted equilibration times, indicating that phase equilibrium will be achieved before TEMPO has an opportunity to react with active polymer radicals. However, significantly longer equilibration times are predicted, when average droplet diameters are as large as those typically found in emulsion and suspension polymerization systems, indicating that the aqueous and organic phase concentrations of nitroxide may not always be at phase equilibrium during polymerization in these systems.
Influence of droplet phase TEMPO diffusivity, DTEMPO,drop, on the predicted organic phase concentration of TEMPO. 相似文献
Recently we reported an experimental and theoretical (simulation) investigation on the mechanism of the induction period and the initial polymerization stages in the nitroxide mediated autopolymerization of styrene. In this paper we extend some of the results presented there and perform preliminary induction period experiments for the study of the mechanism and kinetics of the spontaneous copolymerization of styrene (S) and maleic anhydride (MA) in the presence of TEMPO and 4-OH-TEMPO. With even small amounts of MA (2% wt) the induction period is dramatically reduced by a factor of about 20 in comparison with the nitroxide-mediated styrene autopolymerization at 120 °C. Our results suggest that the initiation mechanism involves a first step of reaction between S and MA. We speculate that this reaction is a Diels-Alder cycloaddition followed by hydrogen abstraction through a monomer or TEMPO assisted homolysis to form a radical pair (monomer case) or a single radical (TEMPO case), which either initiates polymerization or is trapped by TEMPO depending on the conditions. Hall and Padias have studied similar electron donor-acceptor co-monomer pairs and favor the formation of a tetramethylene diradical as the initiating species for spontaneous copolymerization. In any case, the rate-limiting step would be the initial reaction of S and MA. These induction experiments allow us to obtain an initial estimate of the order of magnitude for the kinetic constant of the rate-limiting step, as 10−6 Lmol−1s−1. 相似文献
The mechanism of 2,2'6,6'-tetramethylpiperidin- N-oxyl (TEMPO)-mediated oxidation of alcohols to aldehydes and ketones in ionic liquids has been investigated using cyclic voltammetry and rotating disk electrode voltammetry. It is shown that the presence of bases (B) and their conjugate acids (BH (+)), as well as their p K as, strongly influences the rate of reaction. Data indicated that the first step in the oxidation is the formation of the alcoholate species via acid-base equlibrium with B. The alcoholate subsequently reacts with the active form of TEMPO (T (+), i.e., the one-electron oxidized form) forming an intermediate that further reacts with T (+) and B returning TEMPO catalytically, BH (+), and the carbonyl product. A kinetic model incorporating this pre-equilibrium step has been derived, which accounts for the experimentally observed reaction kinetics. Overall, the rate of reaction is controlled by the equilibrium constant for the pre-equilibrium step; as such, strong bases are required for more kinetically efficient transformations using this redox catalyst. 相似文献
Alkoxyamines and persistent nitroxides are important regulators of nitroxide-mediated radical polymerization (NMP). Since the polymerization time decreases with the increasing equilibrium constant K (k(d)/k(c)), i.e., the increasing rate constant k(d) of the homolysis of the C-ON bond between the polymer chain and the nitroxide moiety, the factors influencing the cleavage rate constants are of considerable interest. SG1-based alkoxyamines have turned out to be the most potent alkoxyamine family to use for NMP of various monomers. Therefore, it is of high interest to determine the factors which make SG1 derivatives better regulators than TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) derivatives. Contrary to what we had observed with TEMPO derivatives, we observed two relationships for the plot E(a) vs BDE(C-H), one for the nonpolar released alkyl radicals (E(a) (kJ/mol) = -133.0 + 0.72BDE) and the other one for the polar released alkyl radicals (E(a) (kJ/mol) = -137.0 + 0.69BDE). However, for both families (SG1 and TEMPO derivatives), the rate constants k(d) of the C-ON bond homolysis were correlated to the cleavage temperature T(c) (log(k(d)(s(-)(1))) = 1.51 - 0.058T(c)). Such correlations should help to design new alkoxyamines to use as regulators and to improve the tuning of NMP experiments. 相似文献