Particle formation and coagulation in the seeded semibatch emulsion polymerization of butyl acrylate

Particle formation and coagulation in the seeded semibatch emulsion polymerization of butyl acrylate were studied under monomer‐starved conditions. To investigate the importance of the kinetics of the water phase in the nucleation process, the monomer feed rate was used as a variable to alter the monomer concentration in the aqueous phase. The emulsifier concentration in the feed was employed to alter the particle stability. Particle formation and coagulation were discussed in terms of critical surface coverage ratios. Particle coagulation occurred if the particle surface coverage dropped below θ_cr1 = 0.25 ± 0.05. The secondary nucleation occurred above a critical surface coverage of θ_cr2 = 0.55 ± 0.05. The number of particles remained approximately constant if the particle surface coverage was within θ_cr1 = 0.25 < θ < θ_cr2 = 0.55. This surface coverage band is equivalent to the surface tension band of 42.50 ± 5.0 dyne/cm that is required to avoid particle formation and coagulation in the course of polymerization. The kinetics of the water phase was shown to play an important role during homogeneous and micellar nucleations. For any fixed emulsifier concentration in the feed and above θ_cr2, the number of secondary particles increased with monomer concentration in the aqueous phase. Moreover, the presence of micelles in the reaction vessel is not the only perquisite for micellar nucleation to occur, a sufficient amount of monomer should be present in the aqueous phase to enhance the radical capture by partially monomer‐swollen micelles. The rate of polymerization increased with the surfactant concentration in the aqueous phase. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3612–3630, 2000     

Semibatch emulsion polymerization of methyl methacrylate using different polyurethane particles

Aqueous acrylic‐polyurethane (AC–PU) hybrid emulsions were prepared by semibatch emulsion polymerization of methyl methacrylate (MMA) in the presence of four polyurethane (PU) dispersions. The PU dispersions were synthesized with isophorone diisocyanate (IPDI), 1000 and 2000 molecular weight (MW) poly(neopentyl) adipate, 1000 MW polytetramethyleneetherglycol, butanediol (BD), and dimethylol propionic acid (DMPA). MMA was added in the monomer emulsion feed. We studied the effect of the use of different PU seed particles on the rate of polymerization, the particle size and distribution, the number of particles, and the average number of radicals per particle. The PU rigidity was controlled by varying the polyol chemical structure, the polyol MW (M_n), and by adding BD. The monomer feed rate was varied to study its influence on the process. It was observed that the PU particles that had been prepared with a higher MW polyol swelled better with MMA before the monomer‐starved conditions occurred. There seemed to be no significant discrepancies between the series with different PU seeds in the monomer‐starved conditions. The overall conversion depended on the monomer addition rate, and the polymerization rate acquired a constant value that was comparable to the value of the monomer addition rate. The instantaneous conversion increased slightly. The average particle size increased, and the total particle number in the reactor was constant and similar to the number of PU particles in the initial charge. The average number of radicals per particle increased. The differences between the system with a constant particle number and average number of radicals per particle and the system with a fixed radical concentration are discussed. The semibatch emulsion polymerization of MMA in the presence of PU particles studied was better compared to the system with a fixed radical concentration. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 844–858, 2005     

Emulsifier-free emulsion copolymerization of styrene and butyl acrylate. I. Kinetic studies in the absence of surfactant

Soapless emulsion copolymerization of styrene (S) and n-butyl acrylate (BuA) has been investigated using two types of initiator and different comonomer feed mixtures. When using K₂S₂O₈ as initiator, the particle size and size distribution of the final latexes (500 nm and 1.003, respectively) is not significantly affected by the comonomer feed composition, whereas the molecular weight and surface characteristics were found to sharply change at high butyl acrylate content. Based on the most probable particle nucleation mechanism and type of chain termination in the monomer swollen particles, a tentative explanation of these results has been proposed. Replacing persulfate by a carboxylic initiator (4-4′-azobiscyanopentanoic acid) results in the formation of stable particles as α observed with the persulfate, provided the aqueous phase pH is fixed in between 6 and 7. Results on the initiator residue location as a function of the conversion point out that the particle flocculation mechanism is strongly significant in the preparation of such latexes.     

Kinetics and mechanism of emulsifier-free emulsion polymerization. III. Styrene/nonionic comonomer (2-hydroxyethyl methacrylate) system

The emulsifier-free emulsion polymerizations of styrene in the presence of about 0.33–2.7% (relative to styrene) of the water soluble comonomer, 2-hydroxyethyl methacrylate (HEMA), and of the initiator, potassium persulfate (KPS), were carried out. It was found that KPS plays a predominant role in the particle nucleation process, since the number density of polymer particles (N_p) was dependent on the 0.97-power of [KPS]. The nucleation ability of HEMA was weak, since N_p was dependent only on the 0.17-power of [HEMA]. The particle nucleation stage ceased quite early before 1% conversion, leading to nearly monodispersed polymer particles. The nucleation is suggested to be via the homogeneous nucleation mechanism. The particles grow via the core-shell structure mechanism (shell region polymerization), since the particle size is rather large—from 1500 to 6000 Å. The amount of HEMA can affect the shell thickness and physical properties of the shell, such as the monomer swelling capacity and monomer diffusion rate.     

Mechanistic studies on particle nucleation in the batch emulsion polymerisation of<Emphasis Type="Italic"> n</Emphasis>-butyl acrylate containing multifunctional monomers

This paper reports the mechanistic details concerning the synthesis of crosslinked poly(n-butyl acrylate) dispersions intended to be used as seeds in the preparation of core-shell emulsions. The influence of crosslinking comonomers and the amount and type of surfactants on the kinetics, particle nucleation, particle size and particle size distribution in the batch emulsion polymerisation of n-butyl acrylate (BA) is explored. In the case of EGDA (ethylene glycol diacrylate) crosslinker the particle number decreased with increasing crosslink density, whereas the opposite trend was observed in the case of m-diisopropenylbenzene (m-DIPB) in the presence and absence of the surfactant sodium dodecyl sulfate (SDS). The observed behaviour is mainly attributed to the variation in the aqueous phase kinetics caused by the water solubility of the comonomer, which influences the formation rate of precursor particles during the nucleation stage. Only for the less water soluble crosslinker, DIPB, could the increase of particle number be explained within the Smith–Ewart theory by assuming prolonged nucleation due to reduced swelling of growing particles with monomer as a result of the crosslinking reaction.Abbreviations EGDA ethylene glycol diacrylate - m-DIPB meta-diisopropenylbenzene - SDS sodium dodecyl sulfate - PBA poly(n-butyl acrylate) - AFFF asymmetric field flow fractionation - MALLS multiangle laser light scattering - CMC critical micelle concentration     

Chitosan Encapsulation of Poly(n-butyl acrylate-co-methyl methacrylate) Particles

Poly(n-butyl acrylate-co-methyl methacrylate) particles encapsulated with chitosan were synthesized at 1:1 monomer ratio using potassium persulfate as an initiator. Effect of concentration and molecular weight of chitosan on particle encapsulation process was investigated. Chitosan concentration was varied from 0.5%, 1%, and 1.5% w/v, while molecular weights were 120, 370, and 850 kDa, respectively. The results showed that the particle diameter of the copolymer particles with chitosan encapsulation was increased when the concentration and molecular weight of chitosan were increased. The average particle size and zeta potential were obtained in a range of 144–625 nm and +27 – +47 mV, respectively. TEM micrographs suggested a presence of the chitosan encapsulation layer on the particles.     

Emulsion polymerization of styrene. I. Review of experimental data and digital simulation

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 k_t as function of conversion is not applicable under our working conditions.     

Unseeded semibatch emulsion polymerization of butyl acrylate: Bimodal particle size distribution

Unseeded semibatch emulsion polymerization of butyl acrylate (BA) using sodium lauryl sulfate as emulsifier and potassium persulfate as initiator was carried out at the conditions where secondary nucleation was probable. This was achieved by using no emulsifier in the initial reactor charge. The effects of changes in monomer emulsion feed rate, initiator concentration and distribution, emulsifier concentration in the feed, and temperature on the evolution of particle size averages and distribution were investigated. Bimodal particle size distributions (PSD) were obtained for most of the latexes. Inhibition effects were found to be important in the development of PSD. Primary particle formation occurred through micellar nucleation, whereas secondary nucleation probably occurred through homogenous nucleation. The polydispersity index (PDI) of the latexes increased with the decreasing monomer emulsion feed rate. The application of a larger amount of initiator to the reactor charge or using a higher temperature, reduced the formation of secondary particles and resulted in a formation of an unimodal PSD. The overall steady‐state rate of polymerization was found to approach the rate of monomer addition (R_p ≈ R_a ), if the emulsifier concentration in the aqueous phase was appreciable. This is different from the correlation 1/R_p = 1/K + 1/R_a obtained for the BA semibatch process with neat monomer feed. This suggests that different rate expressions can be used for BA semibatch emulsion polymerization at different conditions. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 528–545, 2000     

Semicontinuous heterophase polymerization under monomer starved conditions to prepare nanoparticles with narrow size distribution

The semicontinuous polymerization of methyl methacrylate (MMA) in heterogeneous medium under monomer‐starved conditions is reported here. The effect of monomer addition rate on kinetics, particle size, particle number, and PMMA average molar masses are reported. This process permits the synthesis of high‐solid content latexes containing nano‐sized particles (<40 nm) with narrow particle size distributions [(D_w/D_n) < 1.1]. Moreover, the molar masses (M_n ≈ 0.3–1.2 × 10⁶ g/mol) are much lower than those expected by chain transfer to monomer, which is the typical termination mechanism in 0–1 emulsion and microemulsion reactions. Both particle size and average molar masses decrease as the rate of monomer addition is diminished. Possible explanations for this process are provided. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1463–1473, 2007     

Miniemulsion copolymerization of vinyl acetate and butyl acrylate. IV. Kinetics of the copolymerization

The copolymerization kinetics of conventional emulsions and miniemulsions of 50:50 and 25:75 molar ratios vinyl acetate–butyl acrylate monomer mixtures were studied using sodium hexadecyl sulfate as surfactant. Hexadecane was the cosurfactant used in the preparation of the miniemulsions, and ammonium persulfate was the initiator used in the polymerizations. The rate of polymerization showed four regions which extended to different conversions depending on the type of emulsion used (conventional or miniemulsion). The rate of polymerization for the miniemulsion process was always slower than for the conventional process. The dependence of the rate on the initiator concentration was higher for the miniemulsion process. The number of particles nucleated in the miniemulsion copolymerization process was lower than in the conventional emulsion copolymerization process. The initiator and surfactant concentration dependence of the number of particles were 0.8 and 0.25 for the miniemulsion copolymerization process and 0.0 and 0.68 for the conventional emulsion copolymerization process respectively. These effects were attributed to the different particle nucleation mechanism operating in each process.     

Hydrophilic magnetic polymer latexes. 2. Encapsulation of adsorbed iron oxide nanoparticles

The encapsulation of seed polymer particles coated by anionic iron oxide nanoparticles has been investigated using N-isopropylacrylamide as a main monomer, N,N-methylene bisacrylamide as a crosslinking agent, itaconic acid as a functional monomer and potassium persulfate as an anionic initiator. The magnetic latexes obtained have been characterized with regard to particle size, iron oxide content and electrophoretic mobility. All these properties have been examined by varying several polymerization parameters: reaction medium, monomer(s) and crosslinking agent concentrations, nature of seed latexes and type of polymerization (batch versus shot process). The magnetic content in the polymer microspheres strongly depends on the polymerization procedure (i.e., encapsulation process) and varies between 6 and 23 wt%, and monodisperse magnetic polymer particles were obtained. Received: 28 December 1999 Accepted in revised form: 15 June 1999     

Anionic graft polymerization of ethylene oxide on starch. II. Structure of the graft polymers

Starch–poly(ethylene oxide) graft polymers were prepared in DMSO at various monomer and starch alkoxide concentrations. Complimentary and varied information on the structure of the graft polymers was obtained from NMR and periodic acid oxidation of the polymers. From the NMR spectra of the graft polymers in pyridine containing a trace of HCl, which causes shifting of the resonance of the internal ? CH₂O? protons from the terminal ? CH₂OH protons, the polyethylene oxide content, the DP _n of the grafted side chains, and the efficiency of the alkoxides were calculated. With increase of the alkoxide concentration there was a small decrease in ? DP _n, and in the efficiency of the alkoxides in initiating graft polymerization. With increase of monomer concentration, there was only a small increase in ? DP _n but a large increase in the efficiency, indicating the existence of transfer reactions between the growing anions and the free hydroxyl groups on the starch. The results of he periodic acid oxidation showed that with increase of alkoxide concentration there was no significant change in the per cent oxidation of the graft polymers, but with increase of monomer, there was an increase in the participation of the secondary hydroxyl groups in initiation. This supports the NMR evidence for the existence of transfer reactions leading to ? DP _n values much lower than those calculated from [monomer]/[catalyst] ratios.     

Kinetics and mechanism of emulsifier-free emulsion polymerization. II. Styrene/water soluble comonomer (sodium methallyl sulfonate) system

The emulsifier-free emulsion polymerizations of styrene in the presence of about 1 wt% (related to styrene) of the water soluble comonomer, sodium methallyl sulfonate (NaMS), which has short hydrophobic group and strong hydrophilic ionic group, and of the initiator, potassium persulfate, are carried out. Under constant ionic strength, the number density of polymer particles (N_p) is found to depend on 0.5-power of the initiator concentration and shows a minimum in the comonomer concentration plot. Under constant concentration of monomer, comonomer and initiator, N_p is found to depend on ?1.1-power of the ionic strength. In the earlier period, the presence of styrene oligomer having MW about 1000 and water soluble poly(NaMS) or copolymer with high NaMS content suggests a micellar nucleation mechanism, by which the styrene oligomer behaves as emulsifier and the poly(NaMS) can either stabilize or destabilize the existing particles, depending on its concentration in the aqueous phase. The particle size is rather uniform having an uniformity very close to 1 (ca. 1.001) throughout the entire process. It is much larger than that of the conventional emulsion polymerization or emulsifier-free emulsion polymerization with the other comonomers by about 3 to 4 times in diameter or 27 to 64 times in volume, leading to that the average radical number in the particle could be much greater than 0.5. The (conversion)^2/3 versus time plot is found to be linear from 6 to 50% conversion. During this period, for the conversion from 10 to 40% the polymerization rate increases twice but the particle volume increases four-fold. In addition, MWD shows bimodal (excluding the styrene oligomer peak in the earlier period) during the growth period. But the lower MW peak shifts to higher MW and become larger, while the higher MW peak decreases, and finally the MWD becomes single mode after 58.6% conversion. These results suggest a “gradient polymerization” or “transition stage to core-shell structure” in the earlier stage of particle growth and a “shell part polymerization” in the later stage.     

Microemulsion polymerization of styrene

Initiation of polymerization in styrene oil-in-water microemulsions by water-soluble potassium persulfate of oil-soluble 2,2′-azobis-(2-methyl butyronitrile) at 70°C gave stable latexes which were bluish and less translucent than the original microemulsions. The effects of initiator concentration, polymerization temperature, and monomer concentration on the kinetics, particle size distributions, and molecular weight distributions were investigated. The kinetics of polymerization were measured by dilatometry. In all cases, the polymerization rate shows only two intervals, which increased to a maximum and then decreased. There was no apparent constant rate period and no gel effect. A longer nucleation period was found for polymerizations initiated by potassium persulfate as compared to 2,2′-azobis-(2-methyl butyronitrile). The small latex particle size (20–30 nm) and high polymer molecular weight (1–2 × 10⁶) implies that each latex particle consists of two or three polystyrene molecules. The maximum polymerization rate and number of particles varied with the 0.47 and 0.40 powers of potassium persulfate concentration, and the 0.39 and 0.38 powers of 2,2′-azobis-(2-methyl butyronitrile) concentration, respectively. This is consistent with the 0.4 power predicted by Smith–Ewart Case 2 kinetics. Microemulsion polymerizations of styrene–toluene mixtures at the same oil-water phase ratio gave lower polymerization rates and lower molecular weights, but the same latex particle size as with styrene alone. A mechanism is proposed, which comprised initiation and polymerization in the microemulsion droplets, by comparing the kinetics of microemulsion polymerization with conventional emulsion and miniemulsion polymerization systems.     

Batch emulsion copolymerization of 3‐O‐methacryloyl‐1,2:5,6‐di‐O‐isopropylidene‐α‐D‐glucofuranose and butyl acrylate: Synthesis and properties of the sugar latices

To obtain new polymer latices based on sugar derivative, batch emulsion copolymerizations of 3‐O‐methacryloyl‐1,2:5,6‐di‐O‐isopropylidene‐α‐D ‐glucofuranose (3‐MDG) and n‐butyl acrylate (BA) were carried out at 70 °C, with potassium persulfate as the initiator. 3‐MDG polymerizes faster than BA because of its higher reactivity ratio, r_(3‐MDG) = 1.94 versus r_(BA) = 0.54. The effect of the initial monomer composition on the polymerization rate and the thermal properties of the end copolymers was investigated. The overall rate of polymerization increases by enhancing the sugar content in the initial monomer composition. The glass‐transition temperature is linearly related to the sugar content in the copolymer. The influence of the type of surfactant showed that the particle size increases by changing from ionic to nonionic surfactant. Furthermore, the effect of the added acrylic acid (AA) on the rheological properties suggests that the sugar latices exhibit different non‐Newtonian flows depending on the pH of the latex and on the AA concentration on the particle surface. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 788–803, 2003     

Polymerization of acrylamide initiated by the persulfate–thiosulfate redox couple

Conversion–time data were obtained for the polymerization of acrylamide initiated by the redox couple persulfate–thiosulfate by using a dilatometer. A plot of initial rate as a function of thiosulfate concentration shows a well-defined maximum and three distinct regions of behavior. In each region the shape of the conversion–time curves demonstrates the differences in apparent order with respect to monomer arising from changes in initiator concentration during an individual run. A reaction mechanism is proposed to explain the results, and a limiting form of the rate expression is derived for each of the three regions. The ranges of concentration studied are: persulfate, 9.5 × 10^?4?4.7 × 10^?2M; thiosulfate, 2 × 10^?5?2 × 10^?2M; initial monomer, 0.05–1.0M; and temperature, 30–50°C. Within these ranges the initial rate shows a halforder dependence on persulfate and a first-order dependence on initial monomer concentration.     

Role of grafting in the emulsion polymerization of vinyl acetate with poly(vinyl alcohol) as an emulsifier. I. Effect of the degree of blockiness on the kinetics and mechanism of grafting

The role of grafting in particle nucleation during the emulsion polymerization of vinyl acetate with partially hydrolyzed poly(vinyl alcohol) (PVA) as an emulsifier and potassium persulfate as an initiator was investigated. The polymerizations were carried out in batch with a low solids (10%) recipe. An automated reaction calorimeter (Mettler RC1) was used for the direct monitoring of the kinetics of emulsion polymerizations with three medium molecular weight PVAs differing in their degrees of blockiness (Poval 217EE > 217E > 217). Smith–Ewart case 1 kinetics (average number of free radicals per particle < 0.5) were followed in all cases, and no constant rate in interval II was observed. Contrary to what was expected, a nonlinear relationship was observed between the rate of polymerization (R_p) and the number of particles (N_p). At R_{p max,} N_p (217E) > N_p (217EE) > N_p (217), and the final N_p was independent of the degree of blockiness of PVA. The particle size distributions were broad (particle diameter = 20–100 nm) and bimodal. On the basis of these data, we concluded that particle nucleation was continuous and was accompanied by extensive limited aggregation during the particle growth stages. The evolution of the amounts of grafted PVA and poly(vinyl acetate) (PVAc) were determined in polymerizations employing the two PVAs differing the most in blockiness (Poval 217EE and 217). The grafted PVAc followed similar profiles, increasing with conversion, particularly near the end of the two reactions. The amounts of grafted PVAc were about the same in the final latexes (37–39%). In contrast, the grafting of PVA was nearly complete by the time monomer droplets had disappeared in each reaction (25% conversion). However, the extent of grafting differed significantly, with the blockier PVA having about one‐third the grafting of the more random PVA (～10% vs ～30%). In these low solids recipes, grafting appeared to be primarily a solution event, occurring predominantly in the aqueous phase and not at the particle/water interface, as was previously speculated. The PVAc grafts grew until the molecules became water‐insoluble and precipitated, forming polymer particles. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3633–3654, 2001     

Polymers of carbonic acid. XXVII. Macrocyclic polymerization of trimethylene carbonate

2,2-Dibutyl-2-stanna-1,3-dioxepane (DSDOP) was used as cyclic initiator for the polymerization of trimethylene carbonate (TMC). The polymerizations were either conducted in concentrated chlorobenzene solution at 50 and 80°C or in bulk at 60 and 120°C. With monomer/initiator ratios ≤100 the conversion was complete within 2 h at 80°C and within 12 h at 50°C. Variation of the reaction time revealed that the rapid polymerization is followed by a relatively rapid (backbiting) degradation even at 80°C. The polymerizations in bulk at 60°C were somewhat slower than those at 80°C in solution, but the influence of degradation reactions was less pronounced. With optimized reaction time the number average molecular weight (M_n) roughly parallels the monomer/initiator ratio and M_n's up to 100,000 were obtained. In contrast to a classical living polymerization broader polydispersities (1.5–1.7) were found. In the case of 5,5-dimethyltrimethylene carbonate rapid degradation and chain transfer reactions prevented the formation of high molecular weight polymers. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2179–2189, 1999     

Direct syntheses of a series of cucurbit[n]uril-anchored polyacrylamides

Using the one-pot, direct strategy reported by Su and co-workers, we have synthesised a series of cucurbit[n]urils (Q[n], n = 5–8) and alkyl-substituted cucurbit[6]urils (SQ[6]s) anchored on polymers. Acrylamide, as a typical monomer, was used to synthesise a series of Q[n]s (n = 5–8) and SQ[6]-anchored polyacrylamides (PAMs) using a persulfate salt as initiator and oxidant. The Q[n]s (n = 5–8) and SQ[6]-anchored PAM samples have been characterised by ¹H NMR, ¹H NMR titrations of probe guests, Fourier-transform infrared and thermogravimetric analyser. The results confirmed that PAM chains had been successfully grafted on the back of the Q[n]s (n = 5–8) and SQ[6]s through an in situ radical polymerisation approach. It was further confirmed that the hydrophobic cavities of the Q[n]s on the polymers were still freely accessible. This synthetic approach may be extended to a variety of Q[n]s that are difficult to functionalise.     

Kinetics of successive seeding of monodisperse polystyrene latexes. I. Initiation via potassium persulfate

The polymerization kinetics of monodisperse polystyrene latexes prepared via successive seeding were studied in the region between Smith-Ewart Case 2 (n? = 1/2) and Case 3 (n? ? 1). Potassium persulfate was used as the initiator. The kinetics were measured in a piston/cylinder dilatometer designed for microgravity experiments. A recipe formulation method was developed by which a constant emulsifier (Aerosol-MA) surface coverage was maintained throughout a sequence, beginning with a 0.19 μm polystyrene seed. Monodisperse latexes up to 1 μm in size were prepared using 0.5 mM K₂S₂O₈ with a 4% emulsifier surface coverage. The polymerizations were commenced in Interval III, the particles being swollen with twice their weight in monomer. The kinetics were characterized by the autoacceleration of the gel effect with the overall polymerization rate decreasing with increasing particle size (decreasing N_p). The Case 2 to Case 3 kinetic transition was experienced in the first seeding step, however, independence of the rate on the number of particles was not evident even at high values of n? (n? > 10). This was attributed to a dependency of the free radical capture efficiency on the particle size (constant solids). Corroborating indirect evidence was supplied through surface charge analysis and detailed examination of the polymerization kinetics.