Emulsion polymerization of acrylonitrile. Part I. Role and effect of emulsifiers in the emulsion polymerization of acrylonitrile

The role in and effects on the emulsion polymerization of acrylonitrile (AN) of three different groups of emulsifiers, i.e., low molecular emulsfiers, well-known water-soluble polymers, and new water-soluble polymers containing a sulfonate group have been investigated by a dilatometry and electron microscopy. The major part of this paper concentrates on the study of the relation between the properties of the third group of emulsifiers and emulsion polymerization characteristics of AN such as rate, degree of polymerization, diameter and number of particles, and the degree of dispersion, by adding copolymers of AN and sodium p-styrenesulfonate (SSS) having various compositions. In the emulsion polymerization of AN, the hydrophobic portion of the emulsifier seems to act as a kind of nucleus around which polymer molecules precipitate and particle formation may occur, and the hydrophilic portion stabilizes the polymer particles thus formed. As the number of particles and the degree of dispersion increases, the total surface of the particles increases, which may raise the overall rate of polymerization due mainly to an increased polymerization on the surface of the polymer particles. The well-known emulsifiers may be classified by the properties and ratio of the nucleus portion and the stabilizing portion. The unusual effect of emulsifiers on the degree of polymerization may be explained by a chain-transfer mechanism.     

Mechanism of styrene emulsion polymerization during interval II

A systematic study of the kinetics of styrene emulsion polymerization in the postnucleation stage by the way of seed particle growth of monodisperse latices was undertaken, in which the colloidally important parameters were varied: R_p was independent (within limits) of (a) ionic strength, (b) pH, (c) initiator concentration (potassium persulfate), and (d) surfactant (sodium dodecyl sulfate) concentration; R_pp was independent (within limits) of (a) seed particle number concentration N, (b) oil:water phase ratio, and (c) monomer:polymer ratio; R_p was directly proportional to seed-particle surface area. The viscosity average molecular weight of the polymer formed during interval II, M_v(i–j), was approximately constant and increased linearly with N. Log M_v(i–j) was inversely proportional to reaction temperature; M_v(i–j) was inversely proportional to initiator concentration. The overall activation energy of polymerization E′_p was equal to the activation energy of propagation E_p during interval II. The value of k_p at 60°C was 615 dm³ mol^?1 s^?1. Trace of oxygen seems to affect the average number of radicals per particle ī during interval II polymerization.     

Emulsion polymerization of vinyl acetate. II

The emulsion polymerization of vinyl acetate was investigated at low ionic strengths and has quite unusual kinetics. The rate of polymerization is dependent on the initiator concentration to the first power and independent of soap concentration. In seeded polymerizations, the rate of polymerization depends on initiator to the 0.8 power, particle concentration to the 0.2 power, and monomer volume to 0.35 power. In all cases the rate of polymerization is almost independent of monomer concentration in the particles until 85–90% conversion. These results were rationalized by the following mechanism: (a) polymerization initiates in the aqueous phase because of the solubility of the monomer and is stabilized there by adsorption of ionic soap on the growing polymer molecule; (b) the growing polymer is swept up by a particle at a degree of polymerization (under our conditions) of about 50–200. Growth continues in the particle. This sweep-up is activation-controlled as both particle and polymer are charged. (c) Chain transfer to the acetyl group of monomer gives a new small radical which cyclizes to the water-soluble butyrolactonyl radical, and reinitiates polymerization in the aqueous phase; (d) the main termination step is reaction of an uncharged butyrolactonyl radical with a growing aqueous polymer radical. A secondary reaction at low ionic strength is sweep-up of an aqueous radical by a particle containing a radical. At high ionic strength, this is the major termination step. The unusual kinetic steps are justified by data from the literature. They are combined with the usual mechanisms operating for vinyl acetate polymerization and kinetic equations are derived and integrated. The integral equations were compared with the experimental data and shown to match it almost completely over the whole range of experimental variables.     

Radiation-induced emulsion polymerization of ethylene. III. Emulsion polymerization with ammonium perfluorooctanoate as the emulsifier

Radiation-induced emulsion polymerization of ethylene with ammonium perfluoro-octanoate as an emulsifier was studied in order to elucidate the effect of the number of polymer particles. Owing to the stable structure of the emulsifier from a radical attack, no C? F bond was detected in the polyethylene as expected. The polyethylene produced was mostly gel containing a small amount of low molecular weight polyethylene. This may be attributable to chain transfer to the polyethylene. The effects of dose rate and of concentration of the emulsifier were determined without considering the chain-transfer reaction to the emulsifier. By considering the escape of the radical which is produced by chain transfer to the monomer from the polymer particle to the aqueous phase at the steady state, the following equation is derived: The experimental results could be explained by this equation, and the apparent rate constants were obtained.     

Vinyl polymerization by cobaltic ions in aqueous solution. Part II. Polymerization of acrylonitrile and methyl acrylate

Polymerization of methyl acrylate in HClO₄ and HNO₃ was studied in the temperature range 10–15°C. The kinetics of the polymerization were found to be very simple, involving initiation and termination by cobaltic ions. Kinetic studies on polymerization of acrylonitrile in HClO₄ and HNO₃ revealed that water oxidation, and monomer oxidation were side reactions as in the case of methyl methacrylate. Experimental evidence favored the simultaneous initiation by Co³⁺ and CoOH²⁺ species. In H₂SO₄, certain unusual features were encountered. At low [Co³⁺], linear termination as well as termination by mutual combination occurred. Another interesting aspect was that CoSO₄⁺ initiated at low [Co³⁺]. This was unlike the case of other monomers in H₂SO₄. The rates of polymerization and rates of cobaltic ion disappearance were measured with respect to changes in [M], [Co³⁺], [H⁺], temperature, etc. The various rate constants were evaluated.     

Topochemical aspects of elementary reactions of the emulsion polymerization of acrylonitrile

A combination of statistic and kinetic methods of analysis is used to collect quantitative data on the kinetics of elementary reactions of growth and termination of kinetic chains in each phase of a heterophase system as well as on interfacial mass exchange at the quasistationary stage of radiation-induced emulsion polymerization of acrylonitrile. It is found that, in the system, a steady-state concentration of polymer—monomer particles is attained, and these particles are able to increase their dimensions with an increase in monomer conversion. Activation of particles and chain termination on particles are due to the entrapment of macroradicals from water. As a result of a gradual increase in the dimensions of polymer-monomer particles and a decrease in the monomer concentration in the aqueous phase, the adsorption layer of acrylonitrile becomes so thin that, at the final stage, monomolecular layers lose integrity and the conditions of quasi-stationarity are violated.     

Pre-irradiation induced emulsion graft polymerization of acrylonitrile onto polyethylene nonwoven fabric

Acrylonitrile has been widely used in the modification of polymers by graft polymerization. In the present work, pre-irradiation induced emulsion graft polymerization method is used to introduce acrylonitrile onto PE nonwoven fabric instead of the traditional reaction in organic solvents system. The degree of grafting (DG) is measured by gravimetric method and the kinetics of the graft polymerization is studied. The existence of the graft chains is proven by Fourier transform infrared spectroscopy (FT-IR) analysis. Thermal stability of the grafted polymer is measured by Thermogravimetric analysis (TGA).     

Photoinitiation. II. Kinetics of the acrylonitrile polymerization photoinitiated by aromatic hydrocarbons

The kinetics of acrylonitrile polymerization photoinitiated by aromatic hydrocarbons have been studied. For the acrylonitrile polymerization photoinitiated by naphthalene the rate of polymerization depends on the square root of incident light intensity, on the square root of naphthalene concentration, and on the 1.5 power of acrylonitrile concentration. In the system acrylonitrile-1-methoxynaphthalene the rate of acrylonitrile polymerization depends on the first power of acrylonitrile concentration. The monoradical character of this polymerization process has been established. For the interpretation of experimental results a reaction mechanism involving the formation of the exciplex between the first singlet or triplet of aromatic hydrocarbon and acrylonitrile in the ground state as a precursor of polymerization reactions is suggested. The photoinitiating efficiency of various aromatic hydrocarbons in acrylonitrile polymerization increases in the order: fluoranthene (zero efficiency) ? pyrene < phenanthrene, fluorene ≈ 2-methoxynaphthalene ≈ biphenyl < anthracene < 2-methylnaphthalene < 1-methoxynaphthalene < 2,3,6-trimethylnaphthalene < 2,3-dimethylnaphthalene ≈ naphthalene < 1-methylnaphthalene < 2,6-dimethylnaphthalene < p-terphenyl < acenaphthene, provided that the systems absorb the same amount of the incident light. The explanation of this result ensues from the study of the effect of concentration on the rate of polymerization and from the quenching of hydrocarbon fluorescence by acrylonitrile. The photoinitiating efficiency of a given aromatic hydrocarbon is mainly determined by the value of the rate constant k_q for the formation of exciplex as well as the self-quenching efficiency of aromatic hydrocarbon. By using the literature data for the lifetime of fluorescence τ the values of k_q were calculated from the Stern-Volmer equation expressing the quenching of hydrocarbon fluorescence by acrylonitrile. The order of aromatic hydrocarbons according to increasing values of k_q is as follows: pyrene < phenanthrene < anthracene ≈ naphthalene < 2-methylnaphthalene ≈ 1-methylnaphthalene ≈ 2,3-dimethylnaphthalene < 2,6-dimethylnaphthalene < acenaphthene < p-terphenyl < 1-methoxynaphthalene. The study of the concentration effect reflecting the self-quenching of aromatic hydrocarbons during polymerization has given the following sequence for decreasing self-quenching efficiency of aromatic hydrocarbons: 2-methoxynaphthalene ≈ pyrene > anthracene > 1-methoxynaphthalene > fluorene > 2,6-dimethylnaphthalene, phenanthrene, acenaphthene > 2,3,6-trimethylnaphthalene > 2,3-dimethylnaphthalene > 1-methylnaphthalene > naphthalene. It has been shown that the photoinitiating efficiency of a given aromatic hydrocarbon in the polymerization of acrylonitrile can be roughly predicted from the position of that aromatic hydrocarbon in the above-mentioned sequences.     

Kinetics of polymerization of N-tert-butylacrylamide. Part II

The effect of ferric chloride on the kinetics of the radical polymerization of N-tert-butylacrylamide has been investigated in methanol solution at 25°C, with the use of 4,4′-dicyano-4,4′-azodipentanoic acid as initiator. A shrinkage factor of 0.193 mmole polymerized for 1 mm contraction in a capillary of 1 mm diameter has been obtained from density measurements. In the absence of ferric chloride, rates of polymerization were found to be proportional to the concentration of monomer and to the square root of the initiator concentration. With ferric chloride present, the rate of polymerization becomes directly proportional to the initiator concentration and inversely proportional to the concentration of ferric salt. From measurements of the rates of production of ferrous iron, the specific rate constant of the initiation reaction has been found to be (1.8 ± 0.4) × 10^?6sec^?1 at 25°C, compared with a value of 7.63 × 10^?8 sec^?1 calculated from the kinetic data obtained with no ferric salt present. The value of the ratio k_p/k₄. where k_p is the propagation coefficient and k₄ is the velocity coefficient for termination by ferric chloride, has been calculated to be 6.0 × 10^?4 at 25°C, which is considerably smaller than the value found for the ferric chloride-terminated polymerization of acrylamide in water. This markedly lower value of k_p/k₄ has been attributed principally to the steric effect of the tert-butyl group on the magnitude of k_p.     

Part II. Pyrolysis of a tetrafluoroethylene–acrylonitrile copolymer

The pyrolysis of copolymers obtained by radiation-induced grafting of acrylonitrile onto polytetrafluoroethylene has been studied. Infrared spectroscopy shows disappearance of ? C?N and appearance ? C?N? absorptions, indicative of the conjugation of the nitrile groups in the polyacrylonitrile segments. Aromatization is suggested by the appearance of aromatic C? H absorption and by the disappearance of aliphatic C? H absorption in the infrared spectra, as well as by the decrease of the C/H ratio and the constancy of the N content during exposure to 280°C. shown by elementary analysis. Melting and remelting was studied by differential thermal analysis with respect to temperature, extent, and shape of peak area. An increase in the melting temperature proportional to the content of acrylonitrile in the copolymer was explained in terms of entropy effects. The melting peak obtained after grafting appears as a doublet. The latter disappears after annealing for 14 hr. at 300°C. and does not reappear on crystallization or on remelting. In addition, the areas obtained on first and repeated meltings shows a decrease which is proportional to the content of grafted polyacrylonitrile. These results are taken as an indication of an exothermic reaction involving crosslinking. The rate of disappearance of the ? C?N absorption reaches a maximum at the melting temperature. It seems that the exothermic reaction occurring in the crystalline region is identical with “propagation crosslinking” suggested by Grassie and Hay. It is concluded that an interesting new type of polymeric material has been found.     

Microgel formation in emulsion copolymerization: II. Seeded polymerization

Microgel formation in seeded emulsion copolymerization of methyl methacrylate and ethylene glycol dimethacrylate is investigated both experimentally and theoretically. By introducing seed latex, the network structure development can be changed significantly. Even when the crosslinking density development takes a similar pattern as the crosslinking copolymerization in homogeneous media, the molecular weight development shows both types of behavior that is characteristic of emulsion polymerization without seed latex and of homogeneous polymerization, depending on the primary polymer chain length and the mole fraction of the divinyl monomer used. Once the microgels are formed, the weight-average molecular weight increases just linearly with conversion due to a very small locus of polymerization. The present investigation reveals important characteristics of gelation phenomena in a limited space. © 1996 John Wiley & Sons, Inc.     

Effect of interfacial rheology on model emulsion coalescence II. Emulsion coalescence

In Part I, surface pressure isotherms were measured for model interfaces between a dispersed water phase and a continuous phase of asphaltenes, toluene, and heptane. Here, the coalescence rate of model emulsions prepared from the same components is determined from measured drop size distributions at 23 degrees C. A correlation is found between the initial coalescence rate and the interfacial compressibility. It is shown that the change in coalescence rate as the emulsion ages and coalesces can be predicted from surface pressure isotherm data also obtained at 23 degrees C. The stability of the emulsions was further assessed in terms of free water resolved after a treatment of heating at 60 degrees C and centrifugation. The emulsions were aged up to 24 h prior to treatment. The free water resolution appears to correlate to the "capacity for coalescence" of the interfacial film; that is, to the product of the initial film compressibility and (1-CR), where CR is the film ratio at which the film crumples.     

Kinetics and mechanism of acrylonitrile polymerization by p-toluenesulfinic acid. II. Polymerization mechanism

The retardation of acrylonitrile (AN) polymerization by p-toluenesulfinic acid (TSA) in the presence of relatively strong acids has been further investigated. Conductance measurements supported the hypothesis that an ionic complex, presumably RSO₂H₂⁺, is obtained by a reaction of the sulfinic acid with a proton. It is postulated that this complex is a chain transfer agent for the observed retardation. On the basis of this assumption, a kinetic scheme was developed involving additional termination steps by the complex. The scheme accounts for the maximum in initial rate observed on increasing the concentration of added sulfonic acid at a constant TSA concentration. It also provides an explanation for the elimination of the autoacceleration in the bulk polymerization of AN when strong acids are added. The orders derived from the kinetic equations are in good agreement with the orders evaluated from the kinetic experiments.     

Molecular weight distribution in emulsion polymerization. II. The copolymer case

A model for evaluating instantaneous degree of polymerization distribution and the chain composition distribution of copolymers produced in emulsion is developed. The approach adopted is based on the mathematics of Markov processes and represents an extension of the one developed for homopolymers in Part I. As in the homopolymer case, the main aspect of the theoretical treatment is the definition of the proper one step transition probability matrix through the so called subprocess-main process procedure. The model accounts for monomolecular and bimolecular termination (both by combination and disproportionation) and, in principle, it can be applied to any number of reacting monomer species as well as to any number of active chains per particle. However, only the 0–1–2 and 0–1–2–3 emulsion copolymerization systems are discussed in detail. In the case of the chain composition distribution, the model allows the calculation of its moments only, through the method of the Generating Function associated with the probability density function. The expression obtained for the instantaneous probability density functions, as well as for the corresponding cumulative distributions, are all in explicit form and involve only algebraic operations among matrices. Efficient numerical procedure for their application are reported in the Appendix. Illustrative calculations are reported for a 0–1–2–3 copolymerization system, simulating the copolymer styrene–methylmethacrylate. The effect of the various termination mechanisms on the distribution of degrees of polymerization and on the first two moments of the chain composition distribution is discussed in detail. Finally, the three dimensional overall distribution function of both chain length and composition is shown under the assumption of Gaussian type chain composition distribution.     

Particle nucleation in emulsion polymerization. II. Nucleation in emulsifier-free systems investigated by seed polymerization

Experiments with seeded polymerization in emulsifier-free systems were carried out with styrene in order to test the theory of particle nucleation presented in the first article in this series. The effect of amount, size, and surface charge density of the seed particles on the formation of new particles was investigated. An expression for the capture rate of oligomeric radicals from the water phase was evaluated in which the rate of capture was considered to be governed by the absorption of oligomers with chain length one less than the critical chain length for precipitation of the oligomer. Coagulation of primary particles was also included in the expression for the number of new particles obtained in the system. Limited coagulation of primary particles with already formed particles and with seed particles was found to play an important role in determining the final number of new particles found at the end of the runs.     

Modification of multiwall carbon nanotubes via soap‐free emulsion polymerization of acrylonitrile

A novel method for the synthesis of polyacrylonitrile (PAN)‐coated multiwall carbon nanotubes (MWCNTs) via a simple soap‐free emulsion polymerization is presented for the first time. The polymerization was initiated with conventional anionic ammonium persulfate (APS) at 65 °C. The modification of PAN on MWCNT surfaces was confirmed by Fourier‐transform infrared (FT‐IR) spectroscopy, X‐ray photoelectron spectra (XPS), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), and Raman spectroscopy. It is found that all the surfaces of the MWCNTs were coated by PAN chains, and the PAN coating thickness could be controlled by simply adjusting the polymerization time. The obtained PAN‐coated MWCNTs could be well dispersed in water. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2057–2062, 2010     

Effects of mercaptides on anionic polymerization. II. Polymerization of acrylonitrile initiated by mercaptides

The polymerization of acrylonitrile by the several alkali metal mercaptides was investigated. The initiation reaction was found spectroscopically and by the sodium fusion method to proceed in a Michael-like form similar to a cyanoethylation reaction. From the results of the copolymerization of acrylonitrile and methyl methacrylate by the mercaptides, it was found that the mercaptides behaved as the anionic initiators. The effects of the counterions on the rate of polymerization were found to increase with increasing the electropositivites in the order of Li < Na < K. A negative overall activation energy was obtained, ?2.2 kcal/mole, in the temperature region of ?30°C to +20°C. The catalytic reactivities of the mercaptides were smaller than those of the corresponding oxygen analogs, except in the case of the phenyl substituent. Only in the case of the phenyl substituent, thiophenoxide was found to be much more effective than phenoxide, phenyl-SNa ? phenyl-ONa. The catalytic reactivities of the various sodium mercaptides were also determined to follow not only the basicities of the nucleophiles, but also the polarizabilities as follows: tert-butyl-SNa ≈ n-dodecyl-SNa > phenyl-SNa > n-octyl-SNa > isopropyl-SNa > n-propyl-SNa > ethyl-SNa. The enhanced reactivity of thiphenoxide in spite of the low pK_a value was attributed to the higher polarizability based on the low α effect.     

Polymerization in nonuniform latex particles. II. Kinetics of two-phase emulsion polymerization

A new theory, based on the concept of nonuniform distribution of free radicals in polymerizing latex particles, has been developed for the kinetics of two-phase emulsion polymerization reactions. This theory also takes into account the diffusion controlled termination and propagation reactions to describe the gel effect and limiting conversion. The kinetic model permits prediction of the distribution of free radicals in the two polymer phases and rate of polymerization as a function of reaction conditions. Experimental data for polystyrene/polymethyl methacrylate and polymethyl methacrylate/polystyrene (postformed polymer/preformed polymer) in the literature have been used to assess the proposed idea of nonuniform distribution of free radicals in the latex particle.