The pressure dependence of the termination rate constant kt for the free radical polymerization of monomers such as styrene is a function of polymer chain length, chain stiffness, and monomer viscosity, all of which influence the rate of segmental diffusion of an active radical chain end out of the coiled polymer chain to a position in which it can react with a proximate radical. Although kt is not sensitive to changes in chain length, the large increase in molecular weight is responsible for a significant reduction in kt at high pressures. For most of the common vinyl polymers, which exhibit some degree of chain stiffness, kt is inversely proportional to a fractional power of the monomer viscosity because it depends in part on the resistance of chain segments to movement and in part on the influence of viscosity in controlling diffusion of the chain ends. The fractional exponent appears to increase with pressure and this is interpreted as evidence that the polymer chains become more flexible in a more viscous solvent. Because the fractional exponent is higher for more flexible chains, the value of the activation volume for chain termination is an indication of the degree of flexibility of the polymer chains, provided that the monomer is a good solvent for the polymer and that chain transfer is negligible. 相似文献
We have studied the kinetics and mechanism of particle nucleation in the emulsion polymerization of vinyl pivalate (VPi) under
a wide variety of conditions. Quantitative comparisons between the theoretical and experimental estimations of the average
number of radicals per polymer particle, as a function of the amounts of surfactant were performed. The relationship between and the parameter αw, the ratio of radical production in the aqueous phase to termination per particle, can be explained by assuming that chain-transferred
monomer radicals escape from the particle. We studied the influence of the chain transfer agents (CTA), namely, n-dibutyl disulfide, t-dibutyl disulfide and l-cysteine in the emulsion polymerization. The addition of a CTA had a strong influence on the resulting degree of polymerization.
The experimental results can be accounted on the basis of a kinetic analysis of the chain transfer reaction, assuming an increase
of the rate of escape of chain-transferred radicals from the polymer particle. 相似文献
Isothermal emulsion polymerization at 60°C of styrene in a batch reactor were studied by using sodium lauryl sulfate as surfactant and potassium persulfate as initiator source. The concentrations of surfactant and initiator were varied during the runs. The polymerization evolution was followed as samples were taken at regular intervals. These emulsion samples were analyzed for monomer conversion, rate of polymerization, as well as for the size and the size distribution of the particles. The molecular weight and molecular weight distribution were obtained by gel permeation chromatography. Our study showed that fresh nucleation takes place even at high conversion, causing a continuous shifting toward broadening of particle size distribution. Contrary to the theory of Smith and Ewart, which assumes a constant number of particles during interval II of the polymerization reaction, our digital simulation of the reaction presents better experimental results with a variable number of particles, and indicates that the Hui–Hamielec model for termination constant kt as function of conversion is not applicable under our working conditions. 相似文献
The solid-state postpolymerization of slowly crystallized methacrylic acid was studied at 0°C with 60Co γ-radiation as the initiator. The yield, molecular weight, molecular weight distribution, and stereosequencing of the polymer product were determined as a function of polymerization time. The narrow molecular weight distribution and the linear dependence of molecular weight on polymer yield were attributed to a polymerization mechanism characterized by both independent chain propagation and essentially no termination step. The overall polymerization rate was substantially faster than that reported previously for shock-crystallized monomer, a result which was attributed to termination by the occlusion of propagating radicals at defects in the shock-crystallized monomer. Although largely atactic, the polymer synthesized in the solid state contained a secondary kind of stereosequencing; the meso triad probability was highest at the end of the chain, where propagation had initiated and decreased continuously with chain growth. The gradient in stereosequencing along the chains was attributed to defects that were introduced into the monomer crystals by the growing polymer chains. 相似文献
It is assumed that the propagating polymeric radicals have no diffusive mobility in the very highly viscous medium within latex particles. Chain growth takes place on a lattice when the polymeric radical reacts with a monomer present on a lattice site adjacent to that occupied by the reactive chain end. For termination, two radical chain ends must be positioned on adjacent lattice sites at the same instant. The kt/kp ratios calculated with this model are either similar to or somewhat lower than the values determined in emulsion polymerization experiments. A minimum value of kt/kp can be calculated with the aid of the rate equation of Part III by assuming that only “living” polymer is produced during emulsion polymerization. This value of kt/kp is significantly lower than that calculated by the lattice model. Since the value corresponding to the lattice model gives the slowest practically achievable termination rate, it is concluded from these calculations that emulsion polymerization cannot be carried out under conditions in which chain termination is completely suppressed. 相似文献
Self‐consistent hybrid MC/PRISM method is presented for calculating properties of polyelectrolytes in semidilute and more concentrated regimes in a poor solvent. The static structure and conformational behavior of salt‐free polyelectrolyte solutions composed of semiflexible polyions and monovalent counterions are studied using the approach which combines the traditional Monte‐Carlo (MC) simulation with the numerical solution of the polymer integral PRISM equation. The MC technique is applied to generate the configurations of a single chain molecule and obtain the averaged intrapolymer correlation function. The PRISM equation is then numerically solved for a given monomer density to obtain the various correlation functions and the medium‐induced intrapolymer potential. This is used in a single chain MC simulation, where the polymer sites interact via the bare Coulomb potential together with the short range attractive potential and a self‐consistently determined medium‐induced potential. The monomer‐monomer pair correlation functions and static structure factors are calculated for a large variety of parameters. Conformational properties such as the radius of gyration and visual images are obtained as a function of attractive short‐range interaction, monomer density, Bjerrum length, and chain stiffness. The MC/PRISM study predicts that there is a range of hydrophobicity and monomer density for which polyion chains can form the toroidal structure in a poor solvent. Nonmonotonic dependence of the chain size on monomer density is predicted over the entire range of parameters. Polyion structure factor peak position as a function of density is described. Two concentration regimes in which the polyion structure factors exhibit physically different peaks were found. Over the entire concentration regime considered polyelectrolyte chains undergo strong compression with Rg ∝ lequation/tex2gif-stack-1.gif.
Conformation of a polyion chain for lB = 2, ε = 0.18 at ρ* = 0.2 and α = 10°. 相似文献
The low-temperature polymerization of methyl methacrylate initiated with butyllithium–diethylzinc has been studied in toluene and in toluene–tetrahydrofuran and toluene–dioxane mixtures in various proportions. The polymerization process is typically anionic; it is characterized by a very rapid initiation reaction, and the absence of termination and chain transfer reactions, the molecular weight increasing proportionally with the degree of conversion. With toluene as a solvent, the polymer chains are associated, as is shown by viscometric measurements; moreover the polymers produced are highly polydisperse (Mv/Mn = 5.4). The kinetics are very complicated and vary with the range of the catalyst and monomer concentrations. In pure toluene in the presence of the organometallic complex, butyllithium–diethylzinc, the monomer addition is more stereospecific than when butyllithium alone is used as catalyst. By adding tetrahydrofuran to the reaction mixture, the polymer chain association disappears; concomitantly the stereochemical structure of the polymer changes from an isotactic to a mainly syndiotactic configuration. In toluene–tetrahydrofuran mixtures containing from 1 to 10 vol.-% tetrahydrofuran, the kinetics of polymerization can easily be interpreted by assuming the presence of two propagating reactive species which are in equilibrium with each other: the ion pair and the THF-solvated ion pair. The energy of activation of propagation for the free ion pair is equal to 7.5 kcal./mole; for the solvated ion pair a value of 5.5 kcal./mole was found, including the solvation enthalpy of the organometal with tetrahydrofuran. The existence of any relation between the reactivity of the propagating species and the tactic incorporation of the monomeric units has been discussed. The polymerization in mixtures of toluene–dioxane is intermediate between that in pure toluene and that in toluene–HF mixtures; the reaction mechanism however cannot be interpreted with the usual kinetic scheme. The experimental data concerning the rate dependence on catalyst and monomer concentrations are briefly summarized. 相似文献
In tetrahydrofuran, with Na+ as counter-ion, the anionic polymerization of acrolein involves numerous transfer reactions to monomer and to polymer; on the other hand, termination of growing chains does not occur. The use of initiators, like carbanions or oxanions, does not affect the polymerization rate. The kinetic order of the reaction is unity for monomer and unity for initiator; these results indicate that the living ends are not associated at the studied concentrations of initiator. Without stating precisely the mechanism of the transfer reactions, we have proposed a kinetic scheme.In tetrahydrofuran, with Na+ as counter-ion, the anionic polymerization of acrolein involves numerous transfer reactions to monomer and to polymer; on the other hand, termination of growing chains does not occur. The use of initiators, like carbanions or oxanions, does not affect the polymerization rate. The kinetic order of the reaction is unity for monomer and unity for initiator; these results indicate that the living ends are not associated at the studied concentrations of initiator. Without stating precisely the mechanism of the transfer reactions, we have proposed a kinetic scheme. 相似文献
A novel process to produce homo‐ and copolymers by RAFT polymerization in emulsion is presented. It is known that RAFT‐controlled radical polymerization can be conducted in emulsion polymerization without disturbing the radical segregation characteristic of this process, thus leading to polymerization rates identical to those encountered in the corresponding nonliving systems. However, RAFT agents are often characterized by very low water solubility and, therefore, they diffuse very slowly from the monomer droplets, where they are initially solubilized, to the reaction loci, i.e., the polymer particles. Accordingly, when used in emulsion polymerization, they are practically excluded from the reaction. In this work, we show that cyclodextrins, well‐known for their ability to form water‐soluble complexes with hydrophobic molecules, facilitate the transport across the H2O phase of the RAFT agent to the polymer particles. Accordingly, chains grow through the entire process in a controlled way. This leads to the production of low‐polydispersity polymers with well‐defined structure and end functionalities as well as to the possibility of synthesizing block copolymers by a radical mechanism. 相似文献
Particle nucleation in the polymerization of styrene microemulsions was found to take place throughout the polymerization as indicated by measurements of the particle number as a function of conversion. A mechanism based on the nucleation in the microemulsion droplets was proposed to explain the experimental findings although homogeneous nucleation and coagulation during polymerization were not completely ruled out. A thermodynamic model was developed to simulate the partitioning of monomer in the different phases during polymerization. The model predicts that the oil cores of the microemulsion droplets were depleted early in the polymerization (4% conversion). Due to the high monomer/polymer swelling ratio of the polymer particles, most of the monomer resides in the polymer particles during polymerization. The termination of chain growth inside the polymer particles was attributed to the chain transfer reaction to monomer. The low n? (less than 0.5) of the microemulsion system was attributed to the fast exit of monomeric radicals. 相似文献
The first synthesis of high-molecular-weight poly(N,N-diallyl-N-methylamine) by thermal (at 30 and 50°C) and photoinduced (at 21 °C) radical polymerization ofN,N-diallyl-N-methylamine (DAMA) in aqueous solution in the presence of an equimolar amount of trifluoroacetic acid (TFA) and by polymerization
of the newly synthesized equimolar DAMA·TFA salt is reported. Data of1H NMR spectroscopy indicate that the molecules of the monomer under chosen conditions are in the protonated form. This leads
to a decrease in the contribution of the reaction of degradative chain transfer to the monomer and its transformation into
effective chain transfer to the monomer. A bimolecular chain termination mechanism was estabilished and the possibility of
controlling the polymerization rate and the molecular weight of the polymer was demonstrated.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 430–436, March. 2000. 相似文献
The chain‐length distributions (CLDs) of polymers prepared by rotating‐sector (RS) techniques under pseudostationary conditions were simulated for the case of chain‐length dependent termination and analysed for their suitability of determining the rate constant of chain propagation kp from the positions of their points of inflection. The tendency to underestimate kp is a little more pronounced than in pulsed‐laser polymerization (PLP) but, interestingly, the situation improves in the presence of chain‐length dependent termination. The estimates also were found to be more precise a) for smaller rates of initiation, b) for higher order points of inflection, c) if termination is by combination, d) if the role played by the shorter one of the two chains becomes less dominant. Taken in all, the determination of kp from the points of inflection in the CLD of RS‐prepared polymers may well compete with the more famous PLP method, especially if some care is taken with respect to the choice of experimental conditions. 相似文献
In this paper, the basic principle and a Monte Carlo method are described for numerically simulating the chain-length distribution in radical polymerization with transfer reaction to monomer. The agreement between the simulated and analytical results shows that our algorithm is suitable for systems with transfer reaction. With the simulation algorithm, we confirm that transfer reaction has a similar effect as disproportionation on the molecular weight distribution in radical polymerization with continuous initiation. In the pulsed laser (PL) initiated radical polymerization with transfer reaction, the ‘waves’ on the chain-length distribution profile become weaker as the ratio of transfer reaction rate constant, ktr, to the propagation rate constant, kp, is increased in the case with either combination-type or disproportionation-type termination. Moreover, it seems that the combination termination has a broadening effect on the waves. Therefore, kp can also be determined by precisely locating the inflection point Lo on the chain-length distribution profile for radical polymerization with transfer reaction, unless ktr is large enough to smear out the waves on the chain-length distribution. 相似文献
The kinetics of polymerization of acrylamide and methacrylamide, photoinitiated by azidopentamminecobalt(III) chloride in homogeneous aqueous acid medium was studied systematically. Monochromatic wavelengths 365, 405, and 435 mμ were employed for irradiation. Polymerization proceeded without any induction period, and the reaction was followed by measurements of rate of monomer disappearance (bromometrically), rate of complex disappearance (spectrophotometrically), and the chain lengths of the polymer formed (viscometrically). The dependences of the rate of polymerization on variables like light intensity, light absorption fraction by the complex, wavelength, monomer concentration, hydrogen ion concentration, nature of the acid used (HClO4, HNO3, and H2SO4), etc., were studied. The rate of polymerization of acrylamide depended on the unit power of monomer concentration and on the square root of light absorption fraction kε and light intensity I. The rate of methacrylamide polymerization was proportional to the unit power of monomer concentration and fractional powers of 0.25 and 0.30 of kε and I, respectively. A kinetic reaction scheme is proposed and discussed in the light of the experimental results, and it has been concluded that (1) the primary photochemical act is an electron transfer reaction from the azide ion to Co(III) in the complex, (2) initiation of polymerization is by azide radical, (3) termination is by mutual destruction of polymer radicals. 相似文献
Chain transfer constants to monomer have been measured by an emulsion copolymerization technique at 44°C. The monomer transfer constant (ratio of transfer to propagation rate constants) is 1.9 × 10?5 for styrene polymerization and 0.4 × 10?5 for the methyl methacrylate reaction. Cross-transfer reactions are important in this system; the sum of the cross-transfer constants is 5.8 × 10?5. Reactivity ratios measured in emulsion were r1 (styrene) = 0.44, r2 = 0.46. Those in bulk polymerizations were r1 = 0.45, r2 = 0.48. These sets of values are not significantly different. Monomer feed compcsition in the polymerizing particles is the same as in the monomer droplets in emulsion copolymerization, despite the higher water solubility of methyl methacrylate. The equilibrium monomer concentration in the particles in interval-2 emulsion polymerization was constant and independent of monomer feed composition for feeds containing 0.25–1.0 mole fraction styrene. Radical concentration is estimated to go through a minimum with increasing methyl methacrylate content in the feed. Rates of copolymerization can be calculated a priori when the concentrations of monomers in the polymer particles are known. 相似文献