Aqueous-phase Termination Research Articles

Catalytic chain transfer polymerization for molecular weight control in microemulsion polymerization

Catalytic chain transfer using low-spin cobalt(II) complexes is applied in the microemulsion polymerization of methyl and n-butyl methacrylate. In the presence of a catalytic chain transfer agent the rate of polymerization decreases with an increasing cobalt(II) catalyst loading. The course of the polymerization can be captured by the Morgan-Kaler model extended with radical desorption and aqueous phase termination. Proper molecular weight control (i.e. a molecular weight distribution with a dispersity of ∼2) is achieved, however, proves to be strongly dependent on the balance of cobalt(II) molecules and propagating radicals. The presence of a cobalt(II) catalyst enhances the likelihood of homogeneous nucleation, which decreases the colloidal stability in the methyl methacrylate microemulsion polymerizations. The n-butyl methacrylate microemulsion polymerizations on the other hand, display good colloidal stability and a remarkable increase in the nucleation efficiency.

Mechanism of Radical Entry in Electrosterically Stabilized Emulsion Polymerization Systems

The kinetics and mechanism for the process of entering latex particles from the aqueous phase (radical entry) in electrosterically stabilized emulsion polymerization particles were studied. Polystyrene particles stabilized with differing lengths of poly(acrylic acid) chains bound to the surface were synthesized using RAFT-controlled radical polymerization techniques to ensure that the hydrophilic block was of low polydispersity; these latexes had been studied previously to examine the exit rate coefficient (k) in such systems via gamma-relaxation experiments. Particles stabilized with poly(acrylic acid) chains were shown to have a significantly lower average number of per particle (n) (and hence reaction rate) than that of equivalent systems stabilized with a conventional surfactant when used in seeded chemically initiated dilatometry experiments with styrene. Assuming that these emulsion systems obey second-order loss kinetics (i.e., an exited radical will re-enter another particle to either propagate or terminate, a phenomenon well accepted for styrene emulsion systems), the calculated entry rate coefficient (F) was significantly lower than that predicted by the control by aqueous phase growth mechanism for radical entry. Excellent agreement with the accepted entry model was found to occur when radical loss was assumed to be a first-order process with respect to (n) over bar (i.e., an exited radical is terminated in the aqueous phase) or if significant amounts of aqueous phase termination is introduced into the evolution equations of initiator-derived oligomers. These results may be explained by rapid transfer of radical activity to a poly(acrylic acid) chain on the surface, forming midchain radicals that increase the likelihood of termination events in the poly(acrylic acid) surface layer.

Synthesis and cross-linking of polyisoprene latexes

Seeded and ab initio emulsion polymerizations of isoprene using redox initiation systems were investigated and suitable reaction conditions determined to prepare polyisoprene latexes with minimal cross-linking. Polymerizations initiated with the potassium persulfate/sodium bisulfite (KPS/SBS) redox couple had a significant inhibition period and low yield. Polymerizations initiated with the tert-butyl hydroperoxide/tetraethylene-pentamine redox couple showed reasonable yields and no apparent inhibition. It is postulated that the lipophilic nature of the t-butyl group plays a favorable role in the entry of hydroperoxide-initiated oligomeric radicals, while persulfate-initiated radicals are more likely to undergo aqueous phase termination before entry. The cross-linking reaction by benzoyl peroxide (BPO) at 70 °C was investigated using this lightly cross-linked polyisoprene latex. 1H NMR and gel permeation chromatography results were consistent with a reaction mechanism in which the radicals formed by the decomposition of BPO react exclusively with polyisoprene to abstract a hydrogen atom, and the resulting radicals react by termination to form cross-links. No loss of double bonds was found, suggesting that radical formation is overwhelmingly achieved by hydrogen abstraction and cross-linking occurs by termination between two radicals. Cross-linking was accompanied by chain scission, which was observed only at the beginning of the reaction. At low weight-fractions of polymer, the rate of cross-linking was dependent on the concentrations of BPO and abstractable hydrogens in a manner consistent with the postulated mechanism.

A Kinetic Investigation of Seeded Emulsion Polymerization of Styrene Using Reversible Addition−Fragmentation Chain Transfer (RAFT) Agents with a Low Transfer Constant

The mechanisms were investigated for the seeded emulsion polymerization of styrene mediated by xanthate-based reversible addition-fragmentation chain transfer (RAFT) agents with low transfer constants (ca. 0.7). gamma-Radiolysis relaxation measurements were used to determine the exit rate coefficient directly; this increased with the amount of RAFT, consistent with the chain-transfer characteristics of RAFT and the standard transfer/diffusion mechanism for exit. Combining the exit data with measurements of the steady-state rate with chemical initiation showed that the entry rate coefficients decreased with the amount of RAFT, while with gamma initiation the entry rate coefficients were unaffected, unless a very high RAFT concentration was used. This behavior is inconsistent with conventional assumptions about RAFT and the aqueous-phase propagation model for entry (which predicts the entry rate coefficient successfully for ordinary emulsion polymerization systems). It is postulated that the RAFT agents used in this work are surface active, which is not unexpected given the canonical forms of these RAFT agents. The dramatic decrease in entry rate coefficient for the chemically initiated system can be taken into account within the framework of the standard aqueous-phase model for entry if it is assumed that the surface activity gives an increased concentration of RAFT near the surface, which leads to increased transfer to radicals which desorb instead of entering, and whose subsequent reentry is frustrated by this event recurring, until they eventually undergo aqueous-phase termination.

Free-Radical Polymerization of Styrene in Emulsion Using a Reversible Addition−Fragmentation Chain Transfer Agent with a Low Transfer Constant: Effect on Rate, Particle Size, and Molecular Weight

The ab initio emulsion polymerizations of styrene in the presence of RAFT agent (1-(O-ethylxanthyl)ethylbenzene; 1) with a low chain transfer constant to styrene (C-tr similar to 0.8) were carried out using conventional surfactant (sodium dodecyl sulfate, SDS) and initiator (sodium persulfate, SPS). The influence of varying the concentrations of SDS, SPS, and 1 on the polymerization rate, particle size distribution, and molecular weight distribution were studied. It was found that with an increased concentration of SDS both the average-number particle size decreased and the particle size distribution became narrower, prescribed to be due to a greater number of micelles that are nucleated during interval I. A similar result was also found when the concentrations of 1 increased. It was postulated that R-., produced from the fragmentation of 1, exited particles and reentered micelles, thus creating more particles. At high initiator concentrations, reentry of R-. should not play a dominant role due to the high amount of aqueous phase termination. The rate was also influenced by the concentrations of SDS, SPS, and 1. An increase in the concentration of 1 at a low initiator concentration and at constant SDS concentration resulted in significant retardation in rate, which is prescribed to be due to exit and reentry to terminate radicals already growing in the particles. The rate has been shown to increase by increasing the concentration of SDS. For all polymerizations, the number-average molecular weight ((M) over bar (n)) is approximately twice as high compared to theoretical calculations. R tentative explanation put forth is that the RAFT agent could be surface active, and therefore the local concentration surrounding the growing radical chains in the particles will be less than the global concentration. Block copolymers of poly(styrene-co-butyl acrylate)-acetoacetoxyethyl methacrylate were then synthesized in a second stage polymerization under strarved feed conditions to give a core-shell morphology with a very low gel content (similar to3%), in which 76.5% of the polystyrene dormant species were converted to blocks.

Effect of temperature on styrene emulsion polymerization in the presence of sodium dodecyl sulfate. II

The batch emulsion polymerization kinetics of styrene initiated by a water-soluble peroxodisulfate at different temperatures in the presence of sodium dodecyl sulfate was investigated. The curves of the polymerization rate versus conversion show two distinct nonstationary-rate intervals and a shoulder occurring at a high conversion, whereas the stationary-rate interval is very short. The nonstationary-state polymerization is discussed in terms of the long-term particle-nucleation period, the additional formation of radicals by thermal initiation, the depressed monomer-droplet degradation, the elimination of charged radicals through aqueous-phase termination, the relatively narrow particle-size distribution and constant polydispersity index throughout the reaction, and a mixed mode of continuous particle nucleation. The maximum rate of polymerization (or the number of polymer particles nucleated) is proportional to the rate of initiation to the 0.27 power, which indicates lower nucleation efficiency as compared to classical emulsion polymerization. The low activation energy of polymerization is attributed to the small barrier for the entering radicals. The overall activation energy was controlled by the initiation and propagation steps. The high ratio of the absorption rate of radicals by latex particles to the formation rate of radicals in water can be attributed to the efficient entry of uncharged radicals and the additional formation of radicals by thermally induced initiation. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1477–1486, 2000

Compartmentalized polymerization with oil-soluble initiators. Kinetic effect of single radical formation

The stationary state distribution of radicals in compartmentalized systems initiated by oil-soluble initiators have been calculated for various cases of single radical formation as well as a simultaneous generation of single radicals and pairs of radicals in the particles. The effect ofa contribution from radicals produced by initiator dissolved in the aqueous phase has been considered. Desorption and reabsorption of radicals, aqueous phase termination, total rate of radical formation and the water-solubility of the initiator are quantified in terms of dimensionless parameters. The calculations predict that single radicals generated in the particles are kinetically indistinguishable from radicals produced in the aqueous phase over a wide range of variation of the parameters. It is shown that if the rate of generation of single radicals constitutes only about 10 per cent of the overall rate of radical formation in the particles, the former radicals account for the major part of the rate of polymerization. The mechanisms previously proposed to account for the similar kinetic behaviour observed with water-soluble and oil-soluble initiators are discussed. It is concluded that the present calculations support the view that this similarity is mainly due to radicals produced by the water-soluble fraction of the initiator.

Characterization of Styrene - Carboxylic Acid Monomer Emulsion Copolymerization

ABSTRACTA kinetic analysis of styrene emulsion copolymerization with acid monomers is presented. The effects of acid monomer addition on styrene emulsion polymerization kinetics were examined under two conditions. First, the system was analyzed under conditions in which radical desorption from particles is significant and aqueous phase termination effects are negligible. Second, reaction conditions were adjusted to provide significant aqueous phase termination effects with negligible radical desorption. Comparison of the results for these two situations provides insight into the influence of acid monomer on the mechanisms of styrene emulsion polymerization and also yields estimates of related kinetic parameters. Radical desorption from particles appears to be a mechanism that is considerably enhanced by acid monomer addition. The kinetic parameters reported in this paper for batch reactions were used, without further adjustment, to successfully simulate runs in a continuous stirred-tank reaction system; t...

New approach for the estimation of kinetic parameters in emulsion polymerization systems. II. Homopolymerization under conditions where n̄ > 0.5

AbstractA new approach for the estimation of kinetic parameters in emulsion polymerization systems in which the average number of radicals per particle exceeds 0.5 is presented. The approach uses the time evolution of the conversion in chemically initiated systems and is based on a model that includes fundamental parameters such as the propagation rate constant, kp, the termination rate constant in the polymer particles, kt, the rate coefficient for initiator decomposition, kI, and the entry, ka, and exit, kd, rate coefficients. It was found that kp, kt, kI, ka, and, under some circumstances, kd can be accurately estimated provided that termination in the aqueous phase is significant. When the extent of the aqueous phase termination is negligible, only kp, kt, and kI can be estimated. The effect of both the experimental noise level and the run‐to‐run irreproducibility on the accuracy of the estimates was studied. In addition, it was found that significant inaccuracies resulted from the poor determination of the exact time when polymerization begins. A method to circumvent this problem was proposed.

On the main locus of radical formation in emulsion polymerization initiated by oil‐soluble initiators

AbstractThe effect of the monomer/water ratio on the rate of polymerization per polymer particle in both seeded emulsion polymerizations and miniemulsion polymerizations was used in an attempt to elucidate the main locus of radical formation in emulsion polymerization initiated by an oil‐soluble initiator (AIBN). It was found that, for the rest of conditions constant, the polymerization rate per polymer particle increased when the monomer/water ratio increased, namely when the amount of initiator dissolved in the aqueous phase per polymer particle decreased. This is an evidence against a dominant aqueous phase formation of radicals. On the other hand, these results are consistent with a mechanism in which the radicals are mainly produced in the oil‐phase with significant aqueous phase termination.

Entry of free radicals into latex particles in emulsion polymerization

A model, together with a comparison with previously published experimental data, is presented for initiator efficiency in seeded styrene emulsion polymerization systems in the absence of secondary particle formation. The data had shown that a number of previous models are inapplicable, viz., those assuming that the rate-determining step for free-radical entry into a particle is either diffusional capture, surfactant displacement, or colloidal entry. The data support the supposition that the rate-determining step for free-radical capture by latex particles is aqueous-phase propagation to a critical degree of polymerization, whereupon capture (irreversible adsorption) of the resulting oligomeric free radical by a particle is essentially instantaneous. Mutual aqueous-phase termination of smaller species also occurs. When account is taken of the fact that the rate coefficients for (a) the first aqueous-phase propagation step and (b) aqueous-phase termination are both in the diffusion limit, this model is in qualitative and quantitative accord with the experimental dependences of the entry rate coefficient on the concentrations of initiator, of surfactant, of aqueous-phase monomer, and of latex particles as well as on particle size and on ionic strength. For styrene emulsion polymerization initiated by persulfate, the critical oligomer size for entry was found to be dimeric.

Particle formation in emulsifier-free aqueous-phase polymerization of styrene

Particle nucleation in emulsifier-free emulsion polymerization of styrene using potassium persulfate as initiator is thought to involve an in situ micellization mechanism. The particle nucleation period can be divided into two stages. Stage 1 is short (less than 10 min) and large amounts of oligomer micellar particle nuclei is produced during this stage, which consist mainly of oligomers of molecular weight ca. 1000 and are stable due to surface charge generated by the initiator end groups. As these particle nuclei capture free radicals from the aqueous phase and produce internal high molecular weight polymers, the surface charge density decreases dramatically. The particle nuclei then lose their stability and begin to undergo coagulation, which marks the beginning of Stage 2. The critical chain length for in situ micellization, n∗, decreases during Stage 1 due to the increase of oligomer concentration through significant aqueous-phase termination of radicals. Particle size increases during Stage 2 through coagulation and polymerization. Radical capture by particles begins to be more significant than the aqueous-phase termination during Stage 2. The concentration of oligomer stops increasing and n∗ becomes constant. Particle number decreases toward a constant value with time in Stage 2 because of coagulation. A model has been developed and used to simulate particle nucleation during the transient period (both Stage 1 and Stage 2) described above. The criterion used to identify the beginning of Stage 2 was arbitrarily selected as that point when the ratio of the rate of radical capture by the particles to the rate of aqueous-phase termination becomes 2.

EMULSION POLYMERIZATION IN A SEED-FED CONTINUOUS STIRRED-TANK REACTOR

ABSTRACT A numerical method has been developed to predict the particle size distribution (PSD) of the product latex from a steady-state polydisperse-seeded continuous reactor. Simulations have been carried out for the emulsion polymerization of vinyl chloride based on the experimental conditions reported by Berens(l). The simulation results can be reasonably well fitted to the PSD data published by Berens. The radical desorption constant, kd, for Berens’ vinyl chloride emulsion polymerization can be estimated by fitting the simulated PSD to experimental measurements. The simulation work presented in this article demonstrates that the combination of mathematical modeling and PSD measurements can be a useful tool in studying radical transport rates and aqueous phase termination phenomena. The simulation results also indicate that the continuum diffusion theory for free radical absorption by the particles leads to a better PSD fit than a model based on equal diffusion rates per unit area.

Styrene emulsion polymerization. The effects of initiator charge

The results are reported of a series of kinetic studies of the seeded emulsion polymerization of styrene, designed to investigate the effect of different initiator species on the rate coefficient for the entry of free radicals into the latex particles. The initiators employed were persulphate anions (which give negatively charge free-radical species), Fe2+/H2O2(giving rise to neutral free-radical species) and 2,2′-azobis-(2-amidinopropane) hydrochloride (generating positively charged free-radical species). Entry rate coefficients were determined from the observed rates of approach to the steady state, making due allowance for the processes of free-radical exit, re-entry and aqueous-phase heterotermination. It was found that the efficiency of capture of aqueous-phase free radicals by the latex particles was relatively low (the rate coefficient for entry being proportional to [initiator]0.5–0.9), at least for charged initiator species; the dependence on initiating species concentration for the Fe2+/H2O2 system could not be directly determined. The dependence of the entry rate coefficient (pA) on particle number density (Nc) and initiator concentration ([I]) was fitted by a mechanism incorporating the conversion of initiating free-radical species into oligomeric free radicals, mutual aqueous-phase termination of these oligomers and entry of these oligomers into latex particles. This gives an expression for pA(Nc, [I]) containing only two rate parameters. It was found that data over wide ranges of [I] and Nc could be accurately fitted with this expression. It was thus found that the charge on the initiating species has only a weak effect on entry and mutual termination of oligomers, indicating that both processes are dominated by the hydrophobic tail of the species involved.

Radical capture efficiencies in emulsion polymerization

AbstractA theoretical procedure is developed that allows the importance of bimolecular termination in the aqueous phase of an emulsion polymerization to be determined. This shows that with sparinglysoluble and slowly propagating monomers like styrene, significant termination occurs in the aqueous phase at high initiator concentrations, as found experimentally. With more water soluble monomers, aqueous phase termination is likely to be small.

Improved methods for solving the smith — ewart equations in the steady state

An exact, steady-state solution to the Smith-Ewart equations for emulsion polymerization kinetics (1) was first obtained by Stockmayer (2) and later modified by O’Toole (3). Recent experimental studies of the emulsion polymerization of styrene (4-6) indicate that exit (desorption) of free radicals from the particles can play a significant role in determining the rate of polymerization and hence the properties of the latex formed (e.g., the particle size distribution). In addition reentry and/or aqueous phase termination of these exited particles may also occur. Unfortunately, the Stockmayer-O’Toole solution is difficult to evaluate computationally if k, the exit rate coefficient, is zero. Indeed, it requires the computation of Bessel functions of noninteger order, which usually demands solution of the appropriate differential equations. Moreover, the method is inapplicable if reentry and/or aqueous phase termination of exited free radicals occurs.