A kinetic study of the emulsion copolymerization of N,N′-methylenebis(acrylamide) and an unsaturated polyester

The emulsion copolymerization of N,N′-methylenebis(acrylamide) (MBA) and an unsaturated polyester (UP) initiated by potassium peroxodisulfate was kinetically investigated at 50°C by conventional gravimetric and dilatometric methods. The rate of polymerization, the size of latex particles and the number of polymer particles were determined as a function of MBA concentration. The rate of MBA polymerization was found to be proportional to the 0,8th (at 0–15% conversion) and 0,9th (at 20–40% conversion) order with respect to [MBA] at 0,0231 mol · dm^?3 of UP. The values of the reaction order on the monomer concentration are discussed in terms of homogeneous and emulsion polymerization and crosslinking effects. The rate of UP polymerization in the emulsion copolymerization of MBA and UP does not depend on the total monomer concentration. The specific rate of MBA (or UP) polymerization increases with increasing monomer concentration and reaches a maximum at a certain concentration of monomer. The size of polymer particles decreases and the number of particles increases with increasing UP fraction. The stability of polymer particles increases with increasing UP fraction in the monomer feed. The ratio (k_p/k_t^0,5)₀ of the relative rate constants for propagation k_p and termination k_t calculated for the MBA or UP polymerizations at zero conversion increases with increasing MBA concentration. The growth of the polymer particles proceeds via polymerization in particles and by interparticle crosslinking reactions.     

Thermally initiated emulsion polymerization of 1,4-divinylbenzene

The rate of polymerization and the size of the latex particles formed in the thermally initiated emulsion polymerization of 1,4-divinylbenzene (DVB) was studied as a function of emulsifier concentration, monomer-to-water ratio and reaction temperature. Furthermore, kinetic data of the thermal polymerization in solution have been determined. Contrary to normal emulsion polymerization the thermal polymerization of DVB occurs preferentially in the monomer droplets at the rate of a bulk polymerization, whereas latex particles are formed only from the monomer solubilized in micelles at a rate similar to a polymerization in solution. This results in an apparent decrease of the polymerization rate and an increase in size of the latex particles with increasing emulsifier concentration. Crosslinking in polymerization of DVB is discussed as the ultimate reason for the observed features and a mechanism of polymerization based on these conclusions is presented.     

Free-radical polymerization up to high conversion. A general kinetic treatment

The dependence of propagation and termination rate coefficients on conversion in free-radical polymerization is described by a purely kinetic model which considers contributions of segmental diffusion, translational diffusion, and reaction diffusion to termination and which also takes diffusion-control of the propagation step into account. The kinetic scheme provides a satisfactory representation and interpretation of the termination rate coefficient measured in an extended range of conversion for ethylene, butyl acrylate, and methyl methacrylate polymerization. Reaction diffusion is of primary importance for termination at moderate and high conversion, which leads to fairly simple expressions for kinetic parameters even at conversions where the solution viscosity exceeds the pure monomer viscosity by several orders of magnitude.     

Thermodynamics of swelling of latex particles with two monomers

The partitioning of two monomers between the latex particle, monomer droplet and aqueous phases of an emulsion polymer latex are measured at saturation swelling of the latex particle phase (corresponding to intervals I and II of an emulsion polymerization). The monomer and polymer types are varied systematically and the experiments are performed at two temperatures. The results of these experiments correspond well to a simplified thermodynamic theory of the saturation swelling of an emulsion polymer with two monomers, in which it is realized that the fraction of one monomer is equivalent in the latex particle and monomer droplet phases. Further, it is shown that Henry's law holds for monomers, both in the absence and in the presence of swollen latex particles. A simple empirical relationship is developed whereby the concentration of two monomers at any ratio can be calculated from the individual saturation concentration of the two monomers in the latex of interest.     

Emulsion Polymerization of Styrene Stabilized with an Amphiphilic PEG‐Containing Graft Copolymer

The amphiphilic graft copolymer comprising monomeric units of the macromonomer poly(ethylene glycol) acrylate (mPEG acrylate), stearyl methacrylate, and 2‐hydroxyethyl methacrylate was synthesized and characterized. The effectiveness of this PEG‐containing graft copolymer in stabilizing styrene emulsion polymerization was evaluated. Latex particles are nucleated by the entry of free radicals into the polymeric micelles and monomer droplets only serve as a reservoir to supply the continuously growing latex particles with monomer. The polymerization rate increased with increasing concentration of the graft copolymer due to the increased number of reaction loci (i. e., latex particles). The total amount of scraps produced during the polymerization was greatly reduced when the graft copolymer concentration increased. However, the amount of coagulum was still quite high for the polymerization with the highest level of the graft copolymer (30×critical micelle concentration (CMC)). The poor colloidal stability of the polymerization system is attributed to the retarded molecular diffusion of the graft copolymer from the monomer droplet surface to the growing latex particle surface. Thus, latex particles with a relatively low surface PEG density tend to coagulate with one another. Supporting evidence for this postulation obtained from three independent sets of polymerizations is presented in this work.     

Emulsion polymerization of butyl methacrylate initiated by 2,2′-azoisobutyronitrile, 3. On the applicability of the modified Smith-Ewart model

The applicability of the modified Smith-Ewart model to the butyl methacrylate emulsion polymerization in interval II, initiated by both 2,2′-azoisobutyronitrile (AIBN) and ammonium persulfate at 60°C, was tested. The equilibrium monomer concentration in the latex particles in interval II was found to be constant and independent of the emulsifier concentration and the initiator type used. The rate of initiation increases with increasing emulsifier concentration for the polymerizations initiated with a water-soluble initiator. In the systems with the oil-soluble initiator, the rate of initiation is independent of the emulsifier concentration. The rate of polymerization increase with increasing emulsifier concentration for both initiator systems. In the system with the oil-soluble initiator lower values of the polymerization rate and higher values of the viscosity-average molecular masses of the polymer were observed. The number of particles, the radical concentration within polymer particles and the calculated rate of polymerization increase, while the mean particle radius decreases with increasing concentration of the emulsifier for both systems. The average-number of radicals per particle and the ratio of the rate constants for propagation and termination was found to be independent of the emulsifier concentration and the initiator type. A good agreement was observed between the estimated and observed particle radii for both initiator systems. The Smith-Ewart model was found to be applicable for the emulsion polymerization of butyl methacrylate initiated by both ammonium persulfate and AIBN.     

Emulsion polymerization of butyl methacrylate initiated by 2,2′-azoisobutyronitrile, 1. Kinetics and mechanism

The effect of 2,2′-azoisobutyronitrile (AIBN) on the kinetics and mechanism of emulsion polymerization of butyl methacrylate was studied in the presence of anionic emulsifier disodium dodecylphenoxybenzene disulfonate (Dowfax® 2A1) at 60°C. The ratio between the proportion of the polymerization in monomer droplets and that of the polymerization in the aqueous phase was determined for the overall initial rate of butyl methacrylate polymerization in the region of the increasing polymerization rate (interval I). Using the model of polymerization in discrete particles, the portion of the polymerization in monomer droplets with a diameter of 100 nm in the overall polymerization rate is 24,4%; the portion of the polymerization in the water phase is only 0,022% for a concentration of Dowfax® 2A1 of 5 · 10^?2 mol · dm^?3, and 60,4% and 0,054% for a Dowfax® 2A1 concentration of 1 · 10^?2 mol · dm^?3. The exponent of the emulsifier concentration in the equation for the polymerization rate is 0,56 for interval I and 0,36 for interval II; the exponent for the concentration of AIBN over the conversion range between 0 and 30% is 0,34. For the proposed reaction mechanism it is assumed that 2-cyanoisopropyl radicals, generated from AIBN in the water phase, are responsible for the initiation of polymerization in micelles swollen by monomer and in polymer/monomer particles. Polymer/monomer particles are formed also by co-precipitation of oligomer radicals, which in turn are formed by polymerization of monomer molecules present in the water phase. Polymerization within monomer droplets has no significant influence on the course of emulsion polymerization.     

Emulsion copolymerization of acrylonitrile and butyl acrylate, 7. Effect of concentration and type of emulsifier on polymerization kinetics

The emulsion copolymerization of acrylonitrile and butyl acrylate initiated by potassium peroxodisulfate (K₂S₂O₈) in the presence of an anionic, cationic, or nonionic emulsifier has been kinetically investigated at 60°C under batch conditions by gas chromatography (GLC) and gravimetric methods. It is found convenient to define three stages of emulsion polymerization. In the range of low conversion (interval 1) are generated particles and the rate of polymerization increases with increasing polymer particle concentration. In interval 2 (ca. from 15 to 70% conversion) the number of polymer particles and the rate of polymerization (maximum) are constant. At very high conversion (above ca. 70% conversion) the monomer concentration within polymer particles decreases abruptly and the polymerization rate slows down (interval 3). The rate of polymerization in interval 2 was found to be proportional to the 0,41, 0,48, and ?0,42 power of the emulsifier concentration for the anionic, nonionic, and cationic emulsifier, respectively. The dependence of the final particle number on the emulsifier concentration deviates from simple micellar theory predictions. The exponent of the emulsifier concentration decreases with increasing emulsifier concentration (for the anionic and nonionic emulsifiers). These results were explained by a coagulative nucleation mechanism. The unexpected polymerization behaviour in the presence of the cationic emulsifier was explained by the formation of a product (due to interaction of the initiator with the emulsifier).     

Terpolymérisation acrylonitrile — styrène — acrylate de méthyle, 1. Etude cinétique de la terpolymérisation en émulsion,en réacteur fermé

Binary and ternary copolymerizations of the acrylonitrile (A) / styrene (S) / methyl acrylate (M) system were carried out by emulsion polymerization, using sodium lauryl sulfate as emulsifier and potassium persulfate as initiator. Kinetic studies of the reaction mixture by gas chromatography and simulation calculation of the copolymer composition versus conversion show that the classical theory of copolymerization (propagation rate by Markov chain 1^st order) can be applied to the emulsion polymerization of this system, even at high conversion level. It was found that, when a conversion limit is obtained, the polymerization can be started again owing to a new addition of the more rapidly consumed monomer (styrene, in this case); it appears that both polymerization process and conversion effect do not involve appreciable variations of the polymer radical reactivity.     

Emulsion copolymerization of vinyl chloride with butyl acrylate

The emulsion copolymerization of vinyl chloride and butyl acrylate initiated by ammonium peroxodisulfate at 60°C in the presence of an anionic emulsifier was investigated. Random copolymers of butyl acrylate and vinyl chloride covering several sets of monomer compositions were synthesized by emulsion copolymerization conducted to both low and high conversions. The rate of butyl acrylate polymerization at medium conversion was found to be proportional to the 0,5 and 1,0 power of the butyl acrylate concentration for the system with and without vinyl chloride. The overall rate of poymerization was found to decrease with increasing vinyl chloride concentration in the monomer feed. The particle size increases with increasing vinyl chloride monomer concentration in the monomer feed and conversion. The decrease in molecular weight of butyl acrylate/vinyl chloride copolymer is accompanied with the incease of vinyl chloride concentration in the monomer feed and of conversion. The results are discussed in terms of radical desorption, particle nucleation and the dependence of the equilibrium monomer concentration on the reactor pressure.     

Emulsion polymerization of butadiene, 1. The effects of initiator and emulsifier concentration

The kinetics of the emulsion polymerization of butadiene was investigated using Dresinate 214 as emulsifier and three dissociative initiators, viz. potassium peroxodisulfate, 4,4′-azobis-(4-cyanopentanoic acid) and 2,2′-azoisobutyronitrile. All experiments were conducted in the presence of a thiol as chain transfer agent, as usual in diene-polymerizations. The polymerization rate in interval II, R_pol, was found to be highly insensitive to changes in the initiator concentration (R_pol ∝ [I]^0,08). Primary radicals are generated in large abundance in interval I as compared with the final particle number, indicating that the initiator efficiency with regard to particle nucleation is very low. The development of particle number as a function of conversion at several emulsifier concentrations shows that limited coagulation is occurring in the present system. R_pol depends on the emulsifier concentration with an exponent of 0,61, while the final particle number after cessation of coagulation depends on the emulsifier concentration to the 1,6th power. As a consequence the average number of radicals per particle must be a function of particle size, because the monomer concentration in the latex particles is approximately constant in interval II. A certain analogy in behaviour between the emulsion polymerization of various polar monomers, kinetically dominated by radical desorption, and the emulsion polymerization of butadiene, suggests that similar events determine the kinetic course in the present system.     

Partial swelling of latex particles with monomers

Two methods are described for experimentally determining the concentrations of monomer in both the aqueous phase and the latex particle phase during partial swelling of latex particles, and therefore also during intervall III of an emulsion polymerization. The ratio of the monomer concentrations in the aqueous phase, both below and at saturation, can be related to the volume fraction of polymer in the latex particles via the Vanzo equation. Comparison of theory and experiments for the methyl acrylate and poly(methyl acrylate-co-styrene) system shows that the monomer partitioning is insensitive to temperature, latex particle radius, polymer composition, polymer molecular weight and polymer cross-linking. Thermodynamic treatment of these and previously published partitioning results shows, at higher volume fractions of polymer, that the conformational entropy of mixing of monomer and polymer is the significant term determining the degree of partial latex particle swelling by monomer. Theoretical predictions of experimental results are quite insensitive to values of the Flory-Huggins interaction parameter and to the latex particle-water interfacial tension. A simple model is developed for the estimation of monomer partitioning which requires only the saturation monomer concentrations in the particle and aqueous phases.     

Particle growth in butadiene emulsion polymerization, 1. The use of Fremy salt as aqueous radical scavenger

The kinetics of the seeded emulsion polymerization of butadiene in combination with variable Fremy salt (potassium nitrosodisulfonate) concentrations was investigated in order to monitor non-steady state kinetics necessary for obtaining information on entry and exit of radicals from latex particles. Fremy salt is used because it is an entirely water-soluble stable radical that in principle can scavenge water-soluble radicals produced by a water-soluble initiator like peroxodisulfate, but allegedly will not interfere with radicals within latex particles. Adequate buffering of the pH is a prerequisite for stable Fremy salt concentrations in time. Emulsion polymerizations in the presence of Fremy salt showed that not all radicals in the aqueous phase of a swollen polybutadiene latex are scavenged. The use of Fremy salt to monitor non-steady state kinetics in the seeded emulsion polymerization of butadiene in the presence of tertiary dodecyl mercaptan yields no satisfying results. The use of Fremy salt seems to be restricted to systems where the rate of polymerization is strongly influenced by variation in initiator concentration, i.e. the styrene system or the butadiene system in the absence of tertiary dodecyl mercaptan.     

On the kinetics of emulsion copolymerization of methyl methacrylate and ethyl acrylate in the presence of hydrocarbons

The emulsion copolymerization of methyl methacrylate with ethyl acrylate initiated by ammonium peroxodisulfate in the presence of a blend of anionic and non-ionic emulsifiers was investigated at 60°C. The polymerization under batch conditions was conducted to both low and high conversion. The kinetics of the emulsion copolymerization was studied in both the absence and the presence of diluents such as toluene and octane. Unexpectedly both diluents strongly reduced the rate of copolymerization. This reduction increased with increasing diluent concentration. The rate of polymerization was found to increase with increasing conversion up to high conversion. The average particle size increased very slightly with increasing concentration of the diluent. The average number of radicals per particle was found to increase with increasing conversion and to decrease with increasing diluent concentration. The decrease of the radical concentration with diluent concentration was attributed to the desorption of monomer radicals from a polymer particle and to the increase of the termination rate. It is proposed that the hydrophilicity of the monomeric radicals is the driving power of the desorption process.     

Partitioned polymerization, 4. Seeded emulsion polymerization of methyl methacrylate

The shape of the conversion curves and the rates of the emulsion polymerization in batch of methyl methacrylate (MMA) in the presence of seed particles of poly(butyl acrylate) (PBA) depend on the mass ratio of methyl methacrylate/poly(butyl acrylate) MMA/PBA and on the presence or absence of 2,2,6,6-tetramethyl-4-octadecanoyloxypiperidinyl-1-oxyl (STMPO). Irrespective of the presence and/or absence of STMPO in the reaction system, the diameter of a polumer particle experimentally found and calculated on the assumption that no new crop of polymer particles is formed, agrees well for systems with “low” (0,632) mass ratio of MMA/PBA. For systems with “high” (3,160) mass ratio of MMA/PBA, the calculated diameter is higher than that experimentally measured. This indicates that for “low” values of MMA/PBA, all poly(methyl methacrylate) formed was only used for building up shells of poly(butyl acrylate) particles. For “high” values of MMA/PBA, about 37 mass-% (in the absence of STMPO) or 57 mass-% (in the presence of STMPO) of poly(methyl methacrylate) formed was used for generating a crop of poly(methyl methacrylate) particles. Retardation of the polymerization rate in the presence of STMPO is more pronounced for systems characterized by a “low” value of the mass ratio MMA/PBA. Only short inhibition periods (up to five minutes) were observed for both “low” and “high” MMA/PBA ratios. It is shown that for the “low” mass ratio of MMA/PBA the “inner” monomer transport (from PBA particles swollen with MMA) to the locus of propagation is decisive. For systems with a “hugh” mass ratio MMA/PBA, the “outer” monomer transport from monomer droplets and micelles swollen by monomer to the locus of propagation is of primary importance. To explain the experimental results a reaction mechanism for the seeded emulsion polymerization is proposed.     

Emulsion polymerization of butadiene, 3. Kinetic effects of stirring conditions and monomer/water ratio

The stirring speed n influences the emulsion polymerization of butadiene, (1) by reducing the effective emulsifier concentration [E]_eff available for particle nucleation and stabilization at high n, and (2) by limiting diffusion of monomer to the latex particles at low n. The large density difference between butadiene and water promotes the breaking up of droplets at high n, while the same condition constitutes a large driving force for (partial) phase separation at low n. Increasing the monomer/water ratio at constant [E] decreases [E]_eff, and thus the final particle number. At monomer volume fractions >0,6 mixed emulsions are likely to be formed initially, reducing [E]_eff even further. In the presence of mixed emulsions, polymerization in the monomer phase may no longer be neglected, giving rise to a complex kinetic behaviour.     

Polymerization of methyl acrylate in aqueous cationic soap solutions

The polymerization of methyl acrylate in aqueous cationic soap solutions has been examined. The rate of polymerization was found to pass through a minimum with increasing soap concentration. The inhibition period increases considerably with increase in soap concentration. The charge on the latex particles was changed from negative to positive. The latexes of low charge were quite unstable and these systems showed the slowest rate of polymerization. Either the polymerization rate constant in solution is considerably higher than in bulk, or the decomposition of peroxydisulphate in the presence of monomer is increased. Transfer reactions play an important rǒle in the polymerization.     

Spontaneous polymerization of amphiphilic vinyl monomers, 1. Spontaneous polymerization of sodium dodecyl 2-hydroxy-3-methacryloypropyl phosphate

Sodium dodecyl 2-hydroxy-3-methacryloyloxypropyl phosphate as an amphiphilic vinyl monomer was found to polymerize spontaneously without initiator in water. The polymerization takes place at a monomer concentration higher than the critical micelle concentration via a free-radical mechanism, and complete conversion is achieved even at a very low initial monomer concentration (3 mmol/L) and a rather low temperature (35°C). In benzene, capable of forming inverse micelles, the polymerization also occurs spontaneously. In methanol, which gives an isotropic solution, the spontaneous polymerization proceeds only at high concentration (near 300 mmol/L). The ¹H spin-spin relaxation time (T^*₂) of the monomer changes discontinuously in a range of monomer concentration from 250 to 300 mmol/L in CD₃OD, suggesting that some kind of monomer mutual interaction takes place in methanol too. Thus, it can be concluded that the micellar aggregation monomer molecules triggers the polymerization.     

Emulsion polymerization of styrene with sodium hexadecyl sulphate/hexadecanol mixtures as emulsifiers. Initiation in monomer droplets

Part 1 describes the emulsification of styrene with mixtures of sodium hexadecyl sulphate (SHS), and hexadecanol (HD). Addition of HD led to a much better emulsification than with the anionic emulsifier alone. The effect of the fatty alcohol depended strongly upon the method of preparation of the emulsion. To obtain a fine dispersion of styrene it was necessary to treat the anionic emulsifier and the fatty alcohol with water at an elevated temperature prior to the addition of styrene. In this way emulsions with droplet sizes in the range of 0,5–1 μm could be prepared with relatively small amounts of fatty alcohol and with moderate stirring. The finely dispersed monomer emulsions produced by this method were capable of adsorbing the major part of the anionic emulsifier. Part 2 describes emulsion polymerization experiments with mixtures of SHS and HD. With emulsifier systems providing the most effective emulsification, the monomer droplets became the main loci for particle initiation and polymerization. Evidence for this hypothesis was provided by the bimodal dispersion of the latex particles, the major part being particles with a size distribution similar to that of the monomer droplet emulsion. The results of the kinetic investigations provided additional arguments for a mechanism involving initiation and polymerization in monomer droplets.     

Emulsion copolymerization of methyl methacrylate with ethyl acrylate, 1. Effect of the initiator concentration on the polymerization behaviour

The emulsion copolymerization of methyl methacrylate with ethyl acrylate, initiated by ammonium persulfate in the presence of both anionic and nonionic emulsifiers, was kinetically investigated at 60°C by the conventional gravimetric method. In the range of low conversions the rate of polymerization was found to be proportional to the 0,5 power of the initiator concentration. Over the range of medium conversions the rate of polymerization is approximately proportional to the 0,35 power of the initiator concentration and the number-average molecular weight is inversely proportional to the 0,4 power of the initiator concentration. The molecular weight increases up to 30% conversion and then it remains constant up to the total monomer consumption. It was found that the mean particle diameter decreases while the number of particles per volume unit, the conductivity of final latices, the coagulate yield and the number of radicals per particle increase with increasing concentration of the initiator.