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Seeded Emulsion Polymerization of Butyl Acrylate Using a Redox Initiator System: Kinetics and Mechanism Zhen-guo Liu,†,‡ Ye Han,‡ Chao Zhou,‡ Ming-yao Zhang,‡ Wei-ming Li,‡ Hui-xuan Zhang,*,†,‡ Feng-qi Liu,§ and Wan-Jun Liu| Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Engineering Research Center of Synthetic Resin and Special Fiber, Ministry of Education, Changchun UniVersity of Technology, Changchun, 130012, China, Department of Chemistry, Jilin UniVersity, Changchun 130023, China, and Michigan State UniVersity, Michigan 48824
Seeded emulsion polymerization of n-butyl acrylate (n-BA) was initiated by a redox initiator system: cumene hydroperoxide/ferrous sulfate hexahydrate/ethylene-diaminetetraacetic acid monosodium salt/sodium formaldehyde sulfoxylate (CHP-Fe2+-EDTA-SFS). Final poly(n-butylacrylate) (PBA) particles with narrow size distribution were obtained. The kinetics and mechanism of seeded emulsion polymerization initiated by the redox initiator system were investigated. Special care was focused on effects of component concentration of the redox initiator system on the seeded emulsion polymerization kinetics. Particle growth and pH variation during the polymerization were also studied. It was found that the polymerization rate and the overall conversion increased with increasing CHP and SFS concentration. Interestingly, the optimal molar ratio of CHP/SFS was 1.10/3.81 rather than being equimolar in our work. In addition, an optimal molar ratio of EDTA to ferrous ions (2.19 × 10-5 mol/7.12 × 10-6 mol) was found. The polymerization rate increased first and then decreased with increases in ferrous ion concentration. The pH value in the system decreased as the polymerization proceeded. A limiting conversion phenomenon of conversion not being able to exceed 20% was found, and an explanation to the limiting conversion was proposed on the basis of the views of the practical cumulative formation constant log KMY and reaction rate comparison. When another activator solution (EDTA/FES/SFS (mol) 2.19 × 10-5/7.12 × 10-6/1.91 × 10-3) was added, the limiting conversion was removed and the final conversion reached 97%. Introduction Emulsion polymerization is a versatile technique, widely used in industry to produce latexes for a large variety of applications including paints, coatings, adhesives, and binders in the textile and paper industries.1 Particularly, seeded emulsion polymerization has been studied widely2-6 owing to its specific characteristics, such as being a most straightforward way to produce good colloidal stability and producing latex particles of well-defined and predetermined size. Researches on various stages of the seeded emulsion polymerization process were conducted, either to better understand the influence of the key parameters on the overall kinetics or as a means to achieve specific features in the final latex. The kinetics and structure properties of the seeded semibatch emulsion polymerization of n-BA were investigated by Plessis.7 Maxwell et al.8 studied seeded emulsion polymerizations of butyl acrylate at 50 °C as a function of the persulfate concentration with the aid of a dilatometer. Rate coefficients for entry, exit, termination, and propagation were obtained with data fitting techniques in their study. Redox-initiating systems consisting of a reducing agent and an oxidizing agent have been successfully used in emulsion polymerization.9-11 Traditionally, cumene hydroperoxide was used as the oxidizing agent in these systems. The reactions between peroxides and ferric ions were investigated by * To whom correspondence should be addressed. E-mail: hy@ mail.ccut.edu.cn;
[email protected]. Tel.: +86-431-85716465. Fax: +86-431-85716465. † Chinese Academy of Sciences. ‡ Changchun University of Technology. § Jilin University. | Michigan State University.
Kolthoff.12-16 Warson pointed out that the hydroperoxides with the lowest solubility in water were the most effective in grafting reactions.17 Lamb et al. studied emulsion polymerization initiated by a redox initiator from a view of radical entry mechanisms.18 They proposed a new model adapted for the entry mechanism of hydrophobic radicals. Kohut et al. assessed several redox initiator systems for the batch and semibatch emulsion polymerization of n-BA19 mimicking industrial conditions. They reported that for systems using persulfates, the ammonium persulfate (APS)/TMEDA system provided the lower induction period and higher conversion, and, for the systems with hydroperoxide oxidants, tert-butyl hydroperoxide (TBHP)/FF7, TBHP/SFS, and H2O2/FF7 were the best alternatives. Capek et al. studied the effect of initiator type and reaction temperature on the kinetics of polymerization.20,21 It was shown that the average molecular weights decreased with increasing temperature and increased with conversion, but they did not vary with the initiator concentration. Daniels et al. studied effects of polymerization temperature, type, and concentration of initiator and initiator addition mode on kinetics of polymerization using hydroperoxide redox initiators to prepare ABS latexes.22 It was shown that the continuous addition of cummene or t-butyl hydroperoxides in monomer solution gave a slower initial polymerization rate than the incremental addition in the batch polymerizations due to lower monomer concentration. Kinetics of the mini-emulsion polymerization of styrene was studied using the redox initiator system CHP/Fe2+/EDTA/SFS.23 The same initiator system was used to synthesize graft copolymers from natural rubber.24 Effects of CHP amount used in the secondary polymerization, polymerization temperature, emulsifier, chain-transfer agent, and monomer-to-rubber ratio on the grafting level were investigated. Prince et al. studied effects of
10.1021/ie901359z 2010 American Chemical Society Published on Web 07/13/2010
Ind. Eng. Chem. Res., Vol. 49, No. 16, 2010 Table 1. Formulation Used to Prepare the PBA Seed and its Properties
Table 2. Formulation of the Seeded Semicontinuous Emulsion Polymerization of BA with Redox Initiator Systems
ingredient
initial charge (g)
monomer pre-emulsion (g)
ingredient
n-BA DI water SDS K2S2O8/H2O
20 260 0.4 0.4/20
100 40 0.2 0.5/40
seed latex (g)a n-BA(g) DI Water (g) SDS (g) CHPb activator solutionb
PBA seed properties a
110.8 0.005 1.8 95 24
effective particle size (nm) polydispersitya (PID) coefficient of variation (COV, %)b conversion (%) solids content (wt %) a Dynamic light scattering (Brookhaven 90plus). deviation/number average particle diameter.
b
15 40 150 0.12 variable variable
a The solids content is 24 wt %, seen in Table 1. further details.
Experimental Section Materials. n-Butyl acrylate used for the polymerization was commercially available from Jilin Chemical, China. It was washed three times first with 10 wt % sodium hydroxide solutions followed by washing with deionized (DI) water until the wash water was neutral. Then, it was dried by CaCl2. Sodium dodecylsulfate (SDS) was used as received. A redox initiator system CHP-Fe2+-EDTA-SFS was used as initiator. Cumene hydroperoxide (CHP, Merck Co.), ethylene-diaminetetraacetic acid monosodium salt (EDTA, Ishisu Pharm. Co. Ltd.), ferrous sulfate hyxahydrate (FES, Ishisu Pharm. Co. Ltd.), and sodium formaldehyde sulfoxylate (SFS, Katayama Chemical) were used without further purification. A stock iron solution containing 1 mg/mL of iron was prepared from FES. Diethyl hydroxylamine (DEHA, Aldrich) was used as inhibitor to stop further reaction. Deionized (DI) water was used in the polymerization. Polymerization. All reactions were seeded under nitrogen atmosphere. The PBA seed was prepared batchwise in advance at 75 °C following the recipe shown in Table 1. The table also shows the properties of the seed. The polymerization reactor was a 1 L four-neck glass reactor, equipped with a reflux condenser, a sampling device, a nitrogen inlet, and a two-bladed anchor-type impeller coated with PTFE rotating at 400 rpm The polymerization procedure was as follows: the DI water, a part of SDS, BA, and a fraction of KPS solution were charged into the reactor. After replacing gas with nitrogen, the monomer preemulsion was fed dropwise to the reactor. A feeding time of 2 h was used. Subsequently, the system was allowed to react for more 12 h to decompose the residual initiator (KPS) completely. Then, the seeded emulsion polymerizations were carried out at 65 °C in the same reactor and at the same rotating speed as above. Table 2 shows the recipe used in the seeded emulsion polymerizations. In these experiments, the reactor was charged with the seed (dp ) 110.8 nm and solids content ) 24 wt %), SDS, DI water, and n-BA. After the gas was replaced
b
See Table 3 for
Table 3. The Main Recipe of Redox Initiator System in Seeded Emulsion Polymerization
COV ) standard
chelating agents in sulfoxylate polymerization.25 It was pointed out that EDTA was capable of sustaining a sulfoxylate polymerization reaction activated by lower ferrous ion levels, and the suitable molar ratio of EDTA and Fe2+was 1:1. The main focus of this work was thus kinetic study and mechanism of seeded emulsion polymerization of butyl acrylate using a redox initiator system. In this work, the redox initiator system (CHP/Fe2+/EDTA/SFS) was used to initiate the seeded emulsion polymerization of butyl acrylate. Effects of each initiator components on kinetics in seeded emulsion polymerization were conducted. Moreover, particle growth and pH variation in the system were investigated.
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activator solution runno. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
CHP (mol) 1.10 1.10 1.10 1.10 1.10 1.10 8.28 5.52 4.42 3.31 2.21 1.10 1.10 1.10 1.10 1.10 1.10 5.52 5.52 5.52 5.52 4.42 4.42
× × × × × × × × × × × × × × × × × × × × × × ×
-3
10 10-3 10-3 10-3 10-3 10-3 10-4 10-4 10-4 10-4 10-5 10-3 10-3 10-3 10-3 10-3 10-3 10-4 10-4 10-4 10-4 10-4 10-4
EDTA (mol) 1.10 1.10 1.10 1.10 1.10 1.10 2.19 2.19 2.19 2.19 2.19 0.00 2.19 4.38 2.19 4.38 2.19 5.48 1.10 2.19 3.29 2.19 4.38
-4
× × × × × × × × × × ×
10 10-4 10-4 10-4 10-4 10-4 10-5 10-5 10-5 10-5 10-5
× × × × × × × × × × ×
10-5 10-5 10-4 10-4 10-3 10-6 10-5 10-5 10-5 10-5 10-5
FES (mol) 3.56 3.56 3.56 3.56 3.56 3.56 7.12 7.12 7.12 7.12 7.12 7.12 7.12 7.12 7.12 7.12 7.12 1.78 3.56 7.12 1.07 7.12 1.42
× × × × × × × × × × × × × × × × × × × × × × ×
-6
10 10-6 10-6 10-6 10-6 10-6 10-6 10-6 10-6 10-6 10-6 10-6 10-6 10-6 10-6 10-6 10-6 10-6 10-6 10-6 10-5 10-6 10-5
SFS (mol) 6.36 9.54 1.91 2.54 2.86 3.81 1.91 1.91 1.91 1.91 1.91 1.91 1.91 1.91 1.91 1.91 1.91 1.27 1.27 1.27 1.27 6.36 6.36
× × × × × × × × × × × × × × × × × × × × × × ×
10-4 10-4 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-4 10-4
with N2, CHP and activator solution were added. The amounts of CHP and activator solution were varied in order to investigate effects of component concentration of the redox initiator system, which was summarized in Table 3. Activator solutions were prepared by mixing varied amount of iron solution with EDTA to give the desired chelant-iron molar ratio, adding certain amount of SFS and diluting to 20 g with DI water. Since CHP initiates polymerization, the time of addition was taken as zero time. The reaction was carried out at 65 °C for 3 h to obtain the aim polymer. Characterization Kinetic Study. Duplicate samples were withdrawn from the reactor during the polymerization at appropriate intervals to determine the overall conversion gravimetrically. The samples were poured into aluminum foil dishes, and 0.1 g of DEHA (inhibitor) was added to stop polymerization quickly. The homogeneous polymers were dried to constant weight in a vacuum oven at 100 °C to remove residual monomer and solvent. Plots of conversion versus time were used as the main criteria for comparing the efficiencies of the redox system. Particle Size Analysis. Particle size of the latex was measured by dynamic light scattering spectroscopy, DLS (Brookhaven 90 Plus Particle Size Analyzer). Samples were diluted to suitable concentrations (less than 1 × 10-3 wt %) that could be safely assumed to not contain monomer in the polymer particles. Each sample was analyzed five times with a
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Figure 1. The size distribution and morphology for (a) the seed particles and (b) the final particles of PBA prepared by run 23.
run of 180 s at 90° angle under 25 °C, and only samples containing baseline error below 1% were considered for calculations. The latex morphology was observed using a JEOL 2000 transmission electron microscope with an accelerating voltage of 200 kV. All samples were prepared by dispersing diluted latex on a 230 mesh copper grid coated with a thin layer of Formvar. After about 5 min, the grid was placed in the 1%-1.5% phosphotungstic acid (PTA) solution for 3-5 min at room temperature followed by drying in a dust-free environment before observation. The pH value of the PTA solution plays a key role, and the pH value is about 2.0 in our work so as to keep PTA mainly in the PBA particles. pH Value Analysis. The pH value was measured during the polymerization at appropriate intervals with a precision digital pH meter (PHS-2C) at room temperature.
Figure 2. Schematic diagram of initiation process in the polymer particle.
Results and Discussion The Particles Obtained by the Seeded Emulsion Polymerization. It is known that the homogeneous nucleation in the aqueous phase can be effectively avoided in seeded emulsion polymerization. The distribution and morphology of PBA particles are shown in Figure 1. The particle size distribution of the final PBA particles (conversion, 96%) obtained by DLS was appreciably larger than the seed latex, which was consistent with TEM photography. The final effective particle size of the latex was about 262 nm (COV 5.6%) prepared by run 23, which was enlarged from the seed of 110.8 nm. However, the final particle size was a little larger than the theoretical particle size (251.2 nm) calculated by eq I. It might be caused by experimental errors and monomer swelling in the particles.
(
D ) Dseed
WmC +1 Sseed · Wseed
)
1/3
(I)
where D is the final particle size, Dseed is the particle size of
Figure 3. Overall conversions vs time curves for different SFS concentrations with CHP/EDTA/FES (mol) of 1.10 × 10-3/1.10 × 10-4/3.56 × 10-6 (runs 1-6).
the seed latex, Wm is the weight of added monomer, Sseed is the solids content of the seed latex, C is the eventual conversion of the monomer, and Wseed is the weight of the seed latex. The Mechanism of the Redox System and the Initiation Process. The redox initiator system in this study contained hydrophobic oxidant (cumene hydroperoxide, CHP), hydrophilic
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reducing agent (ferrous ion, Fe , and sodium formaldehyde sulfoxylate, SFS), and chelating agent (ethylenediaminetetraacetic acid, EDTA). The reaction behaviors of seeded emulsion polymerization in this study were as follows: C6H5C(CH3)2OOH + [Fe(H2O)7]2+ f C6H5C(CH3)2O · + [Fe(H2O)7]3+ + OH- (1)
2[Fe(H2O)7]3+ + HOCH2SO2Na + 2OH- f 2[Fe(H2O)7]2+ + HOCH2SO3Na + H2O (4) HOCH2SO3Na f CH2O + NaHSO3
(5)
2+ HSO3 f H + SO3
(6)
2C6H5C(CH3)2OOH + HOCH2SO2Na f 2C6H5C(CH3)2O · + HOCH2SO3Na + H2O (7) C6H5C(CH3)2OOH + [Fe(H2O)7]3+ f [Fe(H2O)7]2+ + C6H5C(CH3)2OO · + H+ (8) C6H5C(CH3)2O · + [Fe(H2O)7]2+ f C6H5C(CH3)2O- + [Fe(H2O)7]3+ (9) In the redox initiator system, the hydrophobic radicals (RO•) are obtained by the redox reaction between CHP and Fe2+ following reaction 1. EDTA and ferrous ions would become the EDTA-Fe2+ complex as reaction 2, which could dominate the dissociated ferrous ions, and the Fe2+ would be regenerated by the reducing agent SFS as reaction 4, which could sustain polymerization and minimize the amount of ferrous ions used. Reactions 3 and 5-9 were other reactions in the system. An oxidation-reduction cycle in emulsion polymerization was investigated by Wall and Swoboda.26 In their work, sodium stearate was used as the emulsifier, which not only promoted emulsification but also could render the iron salts oil soluble. If the iron were not solubilized in the oil phase to a sufficient extent, the redox cycle would break down. Additionally it was pointed out that if the quaternary ammonium salts were substituted for sodium stearate without other changes in the recipe, the redox system failed. However, it was reported by Wang23 that the redox system CHP/SFS/EDTA/FES was carried out at the interface between the aqueous phase and the oil phase by diffusion. In our work, SDS was used as the emulsifier, but it was less effective than sodium stearate in rendering the iron salts oil soluble. Therefore the oil soluble iron salts might be
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negligible, and the main mechanism was that CHP and ferrous iron reacted mainly at the particle interface. As shown in Figure 2, the ferrous ion was tied up for the most part as EDTA-FES in the aqueous phase, and a little part of ferrous ion tended to diffuse to the interface, where CHP could react with the ferrous ion to generate the hydrophobic radical (RO•) as shown in eq 1. In the polymer particle, the hydrophobic radical initiated n-butyl acrylate and the polymerization started. Ferric ion would diffuse from the particle interface to the aqueous phase where ferric ion was reduced by SFS, and a redox cycle would form. The Effect of SFS Concentration. The role of SFS in the reaction is to reduce ferric ion to ferrous ion according to reaction 4, which can then keep the redox cycle going on. The net reaction of the whole reaction cycle is the reduction of CHP into free radicals and acid ions by SFS with ferrous ions serving as the effective intermediary. Hence, the SFS concentration has the strongest influence on the polymerization kinetics. Moreover, it could effectively avoid reaction 8 between CHP and Fe3+ and reaction 9 between RO• and Fe2+. If the SFS concentration is not sufficient in the redox system, a large number of Fe3+ ions would replace Fe2+ to complex with EDTA as reaction 3 since the cumulative formation constant of Fe3+ (log KFe3+Y ) 24.23 at T ) 25 °C and ionic strengths ≈ 0) is larger than that of Fe2+ (log KFe2+Y ) 14.33 at T ) 25 °C and ionic strengths ≈ 0)27 in theory. As polymerization proceeds, there would no ferrous ion available. In other words, the polymerization would have to stop and the overall conversion would be very low if the SFS concentration is not sufficient in the redox system. Figure 3 shows the overall conversions versus time curves for various SFS concentrations. It could be seen that the polymerization rate significantly increased with increasing SFS concentration. It is commonly thought that the oxidant and reductant should be equimolar.19 Interestingly, the final conversion was not able to exceed 20% when the molar ratio of CHP/ SFS was 1.10/0.954 (almost equi-molar) in our work. However, when the molar ratio of CHP/SFS was 1.10/3.81, an almost full conversion could be achieved. It can be concluded that the optimal molar ratio of CHP/SFS is not equi-molar but that an excess of SFS (above 3 times) is needed in this work. In addition, pH value variations were investigated when the SFS was 2.54 × 10-3 mol in this series. It was found that the pH value decreased with the polymerization as shown in Figure 4. It was ascribed to sodium bisulfite, by which the H+ concentration was increased in the system. Reasons could be concluded from equations as follows: C6H5C(CH3)2OOH + [Fe(H2O)7]2+ f C6H5C(CH3)2O · + [Fe(H2O)7]3+ + OH2[Fe(H2O)7]3+ + HOCH2SO2Na + 2OH- f 2[Fe(H2O)7]2+ + HOCH2SO3Na + H2O HOCH2SO3Na S CH2O + NaHSO3 -2 + HSO3 S H + SO3
In our study, a “limiting conversion” phenomenon that the overall conversion was not able to exceed 20% in a given time (120 min) was found in run 22, shown in Figure 5. To identify whether or not the “limiting conversion” was due to the insufficiency of SFS, 1.27 × 10-6 mol SFS was added and the time was labeled as point A. It was found that the conversion
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Figure 4. Overall conversions, pH value vs time curves with CHP/EDTA/ FES/SFS (mol) of 1.10 × 10-3/1.10 × 10-4/3.56 × 10-6/2.54 × 10-3 (run 4).
Figure 5. Overall conversions, particle size vs time curves, with CHP/ EDTA/FES/SFS (mol) of 4.24 × 10-4/2.19 × 10-5/7.12 × 10-6/6.36 × 10-4 (run 22). SFS (1.27 × 10-3 mol) was added at point A; EDTA/FES/ SFS (mol) 2.19 × 10-5/7.12 × 10-6/1.91 × 10-3 was added at point B.
curves increased insignificantly. When activator solution (EDTA/ FES/SFS (mol) 2.19 × 10-5/7.12 × 10-6/1.91 × 10-3) was added at point B, the overall conversion increased obviously and the “limited conversion” was removed. One possible explanation is that before the point B, all ferrous ions have translated into ferric ions, which would chelate with EDTA so adequately that SFS could hardly regenerate them to ferrous ions, and this could be illustrated from variations of the practical cumulative formation constant log KMY′. Owing to the effect of pH value, the practical cumulative formation constant log KMY′ is log KMY′ ) log KMY - log RY(H)
(II)
where log KMY′, log KMY, and log RY(H) are the practical cumulative formation constant, the cumulative formation constant, and the effect constant of the pH value, respectively. After 120 min, at point A, the pH value and log RY(H) was 3.4 and 9.70, respectively. According to eq II, log KFe3+Y′ and log KFe2+Y′ could be calculated, which were 14.53 and 4.63, respectively. These data indicated that the EDTA-Fe2+ complexes were very unstable while the EDTA-Fe3+ complexes were stable in the absence of SFS. Under this condition, all EDTA-Fe2+complexes would have translated into EDTA-Fe3+complexes; there would be few dissociated Fe3+
Figure 6. Overall conversions vs time curves for different CHP concentrations with EDTA/FES/SFS (mol) of 2.19 × 10-5/7.12 × 10-6/1.91 × 10-3 (runs 7-11).
ions and the redox cycle would break down. Consequently, additional SFS added in point A had an insignificant effect on the overall conversion. But the limiting conversion could be removed by adding activator solution, which was due to additional EDTA-Fe2+complex added. As discussed above, it could be concluded that the reaction between SFS and Fe3+ was faster than the chelating reaction between Fe3+ and EDTA. Immediately Fe3+ produced by the reaction 1, it would react with SFS so that Fe3+ could not chelate with EDTA. Once EDTA-Fe3+complex formed, no obvious effect on conversion could be seen with the increment of SFS. In other words, the initial SFS concentration played a key role in the redox system. The particles growth of the seeded emulsion polymerization in run 22 was also investigated. As shown in Figure 5, it was found that the particles size increased slowly from 110.8 nm in 120 min and did not increase obviously though additional SFS was added at point A (120 min). However, when activator solution was added at point B (165 min), the particles size increased rapidly in accordance with the results of conversion, and the conclusion described above was illustrated. The Effect of CHP Concentration. The combination in alkaline medium of an iron salt with a reducing sugar was first found to be highly effective in initiating polymerization in redox systems with benzoyl peroxide as oxidizing agent.28 The reaction between ferrous ions and peroxides, particularly hydrogen peroxide, is of considerable importance in many fields and has been studied from several points of view.29-31 The effect of the CHP concentration on the polymerization kinetics was studied from runs 7-11. As shown in Figure 6, the higher the concentration of CHP is, the faster the polymerization rate is and the higher overall conversion would be with fixed EDTA/FES/SFS concentration. When the concentration of CHP was not enough in run 11, the polymerization died off with a low final conversion (about 85%). This is due to rapid reduction of a portion of the CHP by the subreducing agent SFS as in reaction 7, so that the amount of CHP remaining was insufficient to maintain polymerization up to a high conversion. The Effect of EDTA Concentration. Apparently, for the system studied, EDTA maintains the ferrous ion at the concentration resulting in optimum radical generation rate by holding a large part of the ferrous ions in the water phase in the form of the stable water-soluble EDTA-Fe2+ complex as eq 2 shows. Moreover, the EDTA-Fe2+ complex could avoid Fe2+ oxida-
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Figure 7. Overall conversions vs time curves for different EDTA concentrations with CHP/FES/SFS (mol) of 1.10 × 10-3/7.12 × 10-6/1.91 × 10-3 (runs 12-17).
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TBHP and SFS without the presence of an added metal catalyst (Fe2+), and they reported that low and irreproducible polymerization rates were obtained without the metal “catalysts”.17 In other words, a smooth and rapid polymerization at reduced temperature can be found if ferrous ion is present. The effect of ferrous ion concentration was also conducted by Kohut et al.19 In their work, lower conversion was found for lower ferrous ion concentrations. The effect of ferrous ions on the polymerization was studied in detail in runs 18-21. In this series, the molar ratio of EDTA/ FES was fixed at 2.19 × 10-5/7.12 × 10-6 and the concentration of ferrous ion was varied to avoid the effect of the cheating manner between ferrous ion and EDTA. It was found that the polymerization rate was increased noticeably by increasing the ferrous ion amounts. But when the amount of ferrous ions was above 2.19 × 10-5 mol, the polymerization rate and final conversion were not noticeably increased as shown in Figure 8. Conclusion
Figure 8. Overall conversions vs time curves for different ferrous ion concentrations with fixed EDTA/ferrous ions ratio with CHP/SFS (mol) of 5.52 × 10-4/1.27 × 10-3 (runs 18-21).
tion to ferric hydroxide (Fe (OH)3) precipitate.23 If EDTA were not added, too many of the ferrous ions would diffuse to the interface with the result that the peroxide would decompose too rapidly for sustaining polymerization; therefore, a large stock of ferrous ions would be needed and a smooth polymerization would not be obtained. Consequently, the effect of EDTA concentration on polymerization rate is worthy of investigation. The effect of EDTA concentration on the polymerization was shown in Figure 7 (from run 12 to run 17). It was found that the initial polymerization rate quickened sharply, and the conversion could reach 90% in the first 10 min when there was no EDTA in the redox system. In run 13 where the EDTA was 2.19 × 10-5 mol, the initial polymerization was accelerated and the conversion could reach 90% in 5 min. Further with increasing the EDTA concentration, the initial polymerizing rate decreased. This indicates that the overall conversion was not affected by the EDTA concentration greatly. In other words, the concentration of EDTA does not play a key role for the redox system to work, although the redox pair works better when the molar ratio of EDTA to FES was 2.19 × 10-5/7.12 × 10-6. The Effect of Ferrous Ion Concentration. The uses of metal catalysts in redox systems have been reported extensively.32-37 Lamb et al. observed a variety of experimental conditions using
The seeded emulsion polymerization of n-butyl acrylate initiated by a redox initiator system at 65 °C was investigated. Compared with the PBA seed, less monodispersion PBA particles were synthesized. The oil soluble ion salts might be negligible, and the main initiation mechanism was that CHP and ferrous ions reacted mainly at the particle interface in this work. The net reaction of the whole reaction cycle was the reduction of CHP into free radicals and acid ions by SFS with iron serving as the effective intermediary. The optimal molar ratio of CHP/SFS is not equimolar, instead an excess of SFS (above 3 times) is needed. The polymerization rate increased with increasing the concentrations of CHP and SFS. It increased first and then decreased with EDTA. Further addition of EDTA does not improve the performance of the redox system. There is an optimal molar ratio of EDTA and FES (2.19 × 10-5/7.12 × 10-6), which is necessary to reach the maximum conversion. However, when the amount of ferrous ions was above 2.19 × 10-5 mol, there is no further improvement in the polymerization rate or final conversions. In addition, pH value in the system decreased with polymerization. A phenomenon of “the limiting conversion” was found too, which was removed with using another activator solution. It can be concluded that the initial SFS concentration played a key role in the redox system for controlling the polymerization rate and conversions, and no significant effect on conversion could be seen with the increment addition of SFS. Literature Cited (1) Urban, D.; Distler, D. In Polymer Dispersions and their Industrial Applications; Urban, D, Takamura, K, Eds.; John Wiley & Sons: New York, 2002; Chapter 1, p 1. (2) Valter, C.; Cinzia, D. V.; Giacomo, G.; Simone, G. Role of Anionic and Nonionic Surfactants on the Control of Particle Size and Latex Colloidal Stability in the Seeded Emulsion Polymerization of Butyl Methacrylate. J. Appl. Polym. Sci. 2006, 102, 3083. (3) He, W. D.; Pan, C. Y. Influence of Reaction between Second Monomer and Vinyl Group of Seed Polysiloxane on Seeded Emulsion Polymerization. J. Appl. Polym. Sci. 2001, 80, 2752. (4) Xu, Y. S.; Yuan, C. D.; Wang, Y. J.; Cao, T. Y.; Cao, P. Mechanism and Grafting Reactions in Seeded Emulsion Polymerization with Emulsified Monomer Feeding. J. Appl. Polym. Sci. 1999, 72, 1495. (5) Verdurmen, E. M.; Dohmen, E. H.; Verstegen, J. M.; Maxwell, I. A.; German, A. L.; Gilbert, R. G. Seeded Emulsion Polymerization of Butadiene. 1. The Propagation Rate Coefficient. Macromolecules. 1993, 26, 268.
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ReceiVed for reView August 29, 2009 ReVised manuscript receiVed June 15, 2010 Accepted June 21, 2010 IE901359Z
