Macroemulsion Copolymerization of Vinyl

Ind. Eng. Chem. Res. 2000, 39, 2855-2865

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Batch and Semibatch Mini/Macroemulsion Copolymerization of Vinyl Acetate and Comonomers X. Q. Wu and F. J. Schork* School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100

Three monomer systems, vinyl acetate/butyl acrylate [VAc/BA], vinyl acetate/dioctyl maleate [VAc/DOM] and vinyl acetate/n-methylol acrylamide [VAc/NMA], with large differences in reactivity ratios and water solubilities were selected to carry out the macroemulsion and miniemulsion copolymerizations in batch and semibatch processes. Smaller particle size and greater particle number resulted from the macroemulsion copolymerization in both of batch and semibatch processes. In batch runs, the rate of copolymerization was faster in the macroemulsion polymerization than in the miniemulsion process. The copolymer composition versus conversion curves in the batch and semibatch processes were very different. The difference could be attributed to differences in reactivity ratios and in the ability of various monomers to diffuse across the aqueous phase. It is concluded that the miniemulsion copolymerization in batch or semibatch operations can compensate to some extent, for the poor monomer transport of highly water-insoluble monomers in corresponding macroemulsion polymerizations. Introduction The properties of copolymer latexes primarily depend on the copolymer composition, polymer morphology, and colloidal characteristics of polymer particles. Controlling copolymer composition has long been of prime interest in polymer reaction engineering. Because of possible differences in reactivity ratios, the copolymer composition distribution may be broad, and only the overall (average) copolymer composition need be at the ratio of monomer feeds. Mayo and Lewis1 studied the kinetics of copolymerization and developed an equation to describe the relationship between the molar concentrations at the site of propagation and reactivity ratio of monomers for the homogeneous copolymerization, such as bulk or solution polymerization. However, in the conventional emulsion polymerization (here called macroemulsion polymerization), the site of propagation is in monomer-swollen polymer particles. The monomers are supplied by monomer transport from the monomer droplets, across the aqueous phase and into the polymer particles (polymerization sites). In this case, the composition of copolymer depends on not only the reactivity ratios but also the water solubility of monomers. Schuller2 and Guillot3 reported the effect of the water solubility of monomers on the composition of copolymer in emulsion polymerization. Schuller accounted for this phenomenon by adjusting the reactivity ratios based on monomer solubility in the aqueous phase. In Schuller’s equation, it is assumed that the particles are saturated with each monomer, and the monomer partition coefficient and the ratio of monomer to water are used to define the “effective” reactivity ratios. However, if one of the monomers is extremely water insoluble, it cannot readily saturate the monomer-swollen particles by passing through the aqueous phase, and Schuller’s equation cannot be used. Samer4 proposed a pseudo-partition coefficient to deal with this case. * To whom correspondence should be addressed.

In the past two decades, a large amount of work was focused on the batch,5-15 semibatch,15-19 and continuous emulsion copolymerization.20,21 For slightly watersoluble monomers, the composition drift occurring during the course of emulsion polymerization was satisfactorily described in terms of reactivity ratios and monomer partitioning.7,9,14 Semibatch operation provides a method for controlling copolymer composition. The monomer starved or unstarved condition caused by choosing the feed strategy of monomers will lead to different monomer concentrations in monomer-swollen particles and aqueous phase, thus instantaneous copolymer composition. As can be expected, a sufficiently low monomer addition rate will lead to a rate of polymerization controlled by the feedrate,15 and to a homogeneous copolymer.16,17,19 To produce homogeneous copolymers in a relatively short time, optimal monomer addition profiles can be determined based on monomer partitioning data and known reactivity ratios.17,19 Some studies indicate that a lower molecular weight and more homogeneous copolymer can be made from semibatch emulsion polymerization relative to the batch run.15,22 In addition, continuous emulsion copolymerization has been studied, and a steady-state model has been developed for emulsion copolymerization in a continuous stirred tank reactor (CSTR).20,21 Miniemulsion polymerization is quite different from macroemulsion polymerization in some aspects. In miniemulsion polymerization, the creation of very small monomer droplets (with a combination of cosurfactant and high shear), and the subsequent large interfacial area of droplets leads to the adsorption of surfactant molecules on the monomer-water interface and the elimination of micelles from the system. The large interfacial area is capable of capturing water-borne free radicals. Thus, the locus of nucleation is predominantly in monomer droplets, and the monomer for propagation is provided from within the polymerizing particles. This mechanism tends produce copolymer compositions closer to those predicted from the copolymerization equation since monomer transport is a less significant factor.

10.1021/ie990861k CCC: $19.00 © 2000 American Chemical Society Published on Web 06/30/2000

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Samer carried out miniemulsion copolymerization with a extremely water-insoluble comonomer.4 The results indicated an increase in the fraction of this comonomer incorporated in the initial stages of reaction in comparison with macroemulsion copolymerization. A different kinetic behavior between miniemulsion and macroemulsion batch copolymerization of vinyl acetate and butyl acrylate monomer system was shown in the experimental work of Delgado et al.23 In another paper,24 they developed a mathematical model to describe the monomer transport between monomer droplets, the aqueous phase, and monomer-swollen polymer particles in macro- and miniemulsion polymerization process. The simulation analysis revealed a different distribution of monomer between phases for macro- and miniemulsion polymerization processes. Other researchers have reported on the features of batch miniemulsion copolymerization.25-29 Recently, Samer et al.30 published work on continuous miniemulsion copolymerization. It was found that the fraction of the extremely water-insoluble comonomer incorporated into the copolymer is significantly less in continuous than in batch polymerization. This was attributed to the large driving force for diffusion of the more water-soluble monomer from the droplets to the monomer-swollen particles. Semibatch operation is a common mode for the production of polymer latexes. However, there has been little or no work published on semibatch miniemulsion copolymerization. This is the focus of this paper. Three very different monomer systems, vinyl acetate/butyl acrylate (VAc/BA), vinyl acetate/dioctyl maleate (VAc/ DOM), and vinyl acetate/N-methylol acrylamide (VAc/ NMA), were studied. For the VAc/BA system, the reactivity ratio of BA is much higher than that of VAc, but the difference between water solubilities of VA and BA is less significant compared with the VA/DOM system. In the VAc/DOM system, the reactivity ratios are close, but the water solubilities are very different. Finally, in the VAc/NMA system, NMA is very watersoluble and has a much higher reactivity ratio than VAc. Published data on the copolymerization of the two latter monomer systems is scarce. For these three monomer systems, the copolymer composition, monomer conversion, molecular weight, and particle size were investigated for batch macroemulsion and miniemulsion copolymerization, and for semibatch miniemulsion copolymerizations where the semibatched monomer was fed as a macroemulsion or a miniemulsion. Experimental Section Reagents. Reagent grade vinyl acetate (VAc) was supplied by Aldrich Co. and was distilled under vacuum to remove the inhibitor. Butyl acrylate (BA) and dioctyl maleate (DOM), supplied by Aldrich Co. and MonomerPolymer/Dajac Labs, Inc., respectively were washed with 10% sodium hydroxide solution to remove inhibitors and then washed with deionized water to remove the residual base. The washed monomers were dried over calcium chloride. All the purified monomers were stored at -2 °C until used. N-Methylol acrylamide (NMA, 48 wt % in water, Aldrich), potassium persulfate (KPS, 99% Aldrich), sodium lauryl sulfate (SLS, 98% Aldrich), hexadecane (HD, 99% Aldrich), sodium bicarbonate (99.5% Fisher), and hydroquinone (98% Fisher) were used as supplied. The water used was deionized. Polymerization. For all batch operations, the cosurfactant (HD) was dissolved in the monomer mixture.

Table 1. Recipes and Polymerization Conditions experimental codea

monomer ratio (wt)b VAc BA DOM NMA

D1, BM1 D2, BM2 D3, BM3 D4, BM4

80 60 95 80

20

BE1 BE2 BE3 BE4

80 60 95 80

20

feed conditionc monomer mixture

initiator solution

40 5 20 40 5 20

SM1 SM2 SM3 SM4 SM5

80 60 95 80 80

20

SE1 SE2 SE3 SE4 SE5

80 60 95 80 80

20

40 5 20 20

40 5 20 20

miniemulsion feedrate (mL/min) 1.00 1.00 1.00 0.60 0.30 macroemulsion feedrate (mL/min) 1.00 1.00 1.00 0.60 0.30

feedrate (mL/min) 0.14 0.14 0.14 ∼0.07 ∼0.035

0.14 0.14 0.14 ∼0.07 ∼0.035

(It should be noted that DOM and similar dialkyl maleates could exhibit cosurfactant properties on theirs own, but for the sake of consistency, HD was added to the DOM polymerizations.) A part of total recipe water (20%) was taken for the preparation of initiator solution. The emulsifier SLS and sodium bicarbonate (sodium bicarbonate used for VAc/NMA monomer system only) were dissolved in the rest of water. Agitation was used to create a preemulsion. The preemulsion was pumped through a sonication cell equipped with a Fisher 300W Sonic Dismembrator to form a miniemulsion. Then the miniemulsion was pumped into a 250-mL reactor, which was equipped with nitrogen purging tube, condenser, temperature controller, and stirrer. (In macroemulsion polymerization, the preemulsion did not undergo the sonication process.) The reaction material in the reactor was heated to 55 °C during 20 min under nitrogen purge. The initiator solution was then injected into the reactor to start the polymerization. At intervals, samples of 5-6 g were removed from the reactor with a syringe for gravimetric conversion analysis. Samples were injected into vials containing 0.5 wt % hydroquinone solution to quench the polymerization. The semibatch miniemulsion polymerization involved two stages, i.e., miniemulsion batch stage and semibatch stage. The miniemulsion batch stage was performed as above. When the monomer conversion was estimated to be about 80%, the feeding stage was started by pumping monomer emulsion (miniemulsion or macroemulsion, which was continuously formed while feeding) and initiator solution at set flow rates simultaneously into the reactor. The composition of the monomer feed was identical to the composition of the monomer in the batch stage. Samples of 5-6 g were removed at some time intervals from reactor with a syringe for gravimetric conversion analysis. Samples were injected into vials containing 0.5 wt % hydroquinone solution to quench the polymerization. The reactor system was continuously purged with nitrogen during polymerization. The recipes and polymerization conditions for different monomer systems and operation modes are listed in Table 1. Shelf Life, Droplet, and Particle Size. About 30 mL of fresh miniemulsion was placed in a glass vial with

Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000 2857

Figure 1. NMR spectrum for VAc/BA System.

a cap. The vial was observed periodically, and the time until a visible cream line appearing was taken as the shelf life. Monomer droplet and polymer particle sizes were measured by light scattering with a Protein Solution LSR-TC instrument. As soon as the emulsion was sonicated, a miniemulsion sample was taken and tested. In the measurement of monomer droplet size, the miniemulsion was diluted 100:1 in a monomer-saturated solution with 0.2 wt % SLS in water. The sample was then put into a quartz curette for analysis. To measure polymer particle size, if the sample was of low monomer conversion, the latex was diluted in a 0.2 wt % SLS aqueous solution. The diluted solution was heated at 50 °C for 4 h to drive off the residual volatile monomer. The treated particle suspension was placed in a plastic curette for analysis. A possible error may exist for the sample of low conversion due to the residual DOM. Samples of relatively high conversion were diluted in DI-water directly for measuring. Molecular Weight. A Waters gel permeation chromatography (GPC) system was used to measure the molecular weight of the synthesized polymers. The GPC system included a Waters 510 HPLC pump, Waters 410 differential refractometer, and three columns (300 × 7.8 mm, in series, gel pore sizes of 103, 104, and 105 Å). The TriSec GPC Software (Viscotek) was used to calculate the molecular weight via conventional calibration based on polystyrene standards. The samples were prepared by dissolving the dried latex in tetrahydrofuran (THF) at a concentration of 4 mg/mL THF. The GPC was operated at a temperature of 30 °C and a mobile-phase flow rate of (inhibitor-free HPLC-grade THF) of 1.0 mL/ min. Composition of Copolymer. Copolymer composition was determined by 1H NMR analysis. The 1H NMR spectra of samples were recorded at 24 °C with Bruker AMX 400 system and Xwinnmr software. Deuterated chloroform (CDCl3) was used as reference and solvent for preparing samples of VAc/BA and VAc/DOM, and deuterated dimethyl sulfoxide (DMSO-d6) for VAc/NMA. To dissolve the sample of VAc/NMA polymer in DMSOd6, the sample was heated. Three typical spectra for the polymers of VAc/BA, VAc/DOM, and VAc/NMA were shown in Figures 1-3.

In Figure 1, the reference peak of CDCl3 (internal standard) is at 7.2 ppm. The resonance peak around 4.8 ppm corresponds to the chemical shift of the -CH-Ogroup in the vinyl acetate unit. The peak at 4.0 ppm is the resonance of the -O-CH2 group in the butyl acrylate unit. In addition, the peaks at 2.0 ppm are related to the resonance of the -CH3. The copolymer composition was calculated from the relative intensities (integral value) of the peaks at 4.8 and 4.0 ppm; thus the percentage of vinyl acetate was calculated by using the following equation:

VAc% )

2S4.8 × 100 2S4.8 + S4.0

(1)

where S4.8 and S4.0 are integral areas of the peaks at 4.8 and 4.0 ppm, respectively. For the polymer of VAc/DOM, -CH-O- and -OCH2 groups were still used to characterize the involved VAc and DOM units. In Figure 2, the peaks of these two groups are similar to the ones in Figure 1. The chemical shifts for the two groups are around 4.8 and 3.8 ppm, respectively. Interestingly, in some spectra, one can find a group of left-shoulder peaks overlapped with the peak around 3.8 ppm. This peak group was caused by the group -O-CH2 in DOM. In addition, the peak caused by sCHdCHs in DOM can also be found at 6.2 ppm. This indicates that there is some residual DOM in the dried polymer and the conversion measured with gravimetric method should be corrected for this. In this case, eq 2 was used to calculate the percentage of vinyl acetate, and eq 3 was used to calibrate the monomer conversion for VAc/DOM system:

VAc% )

4S4.8 × 100 4S4.8 + S4.0 - 2S6.2 X ) XgrP

P)

4S4.8 × 86 + (S4.0 - 2S6.2) × 340 4S4.8 × 86 + S4.0 × 340

(2) (3) (4)

where S4.8, S4.0, and S6.2 are areas of the peaks at 4.8, 4.0, and 6.2 ppm respectively, 86 and 340 are the

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Figure 2. NMR spectrum for VAc/DOM System.

Figure 3. NMR spectrum for VAc/NMA System.

molecular weights of VAc and DOM, and Xgr refers to the conversion obtained by the gravimetric method. P is a mass proportion of polymer in dried material for gravimetric analysis. In Figure 3, the resonance of the -CH-O- group in VAc is the peak around 5.5 ppm, and the small peaks at 4.4 ppm were caused by the resonance of the -NCH2-O- group. So the percentage of VAc was calculated with the following equation:

VAc% )

2S5.5 × 100 2S5.5 + S4.4

(5)

where S5.5 and S4.4 are integral areas of the peaks at 5.5 and 4.4 ppm, respectively. Results and Discussion Shelf Life, Droplet Size, Particle Size, and Number. The shelf life and droplet sizes of miniemulsions for selected recipes were shown in Table 2. The miniemulsions prepared were very stable. With these miniemulsions, the batch polymerization experiments were carried out. The changes of particle size versus monomer conversion were plotted in Figure 4. Compared with the systems of VAc/BA and VAc/DOM, the VAc/NMA sys-

Table 2. Shelf Life and Miniemulsion Droplet Size experiment code

shelf life (day)

droplet diameter (nm)

polydispersity index

droplet number (×10-13/mL)

D1 D2 D3 D4

>10 >10 ∼10 >10

119.4 198.3 171.7 161.0

1.07 1.14 1.13 1.10

33.66 7.35 11.32 13.73

tem appears to produce a much larger polymer particles and a broader size distribution. The polydispersity index was around 1.40 in comparison with 1.06 for VAc/BA and 1.18 for VAc/DOM. In addition, the miniemulsion droplet size of VAc/NMA system was also much less than the particle size. This can be attributed to the high polarity of the polymer of VAc/NMA. The polymer particles in the latex of VAc/NMA system are easy to be flocculated. This flocculation effect leads to a large average particle size and broad size distribution. In fact, the polymer latex of VAc/NMA is not stable. It was found that even when an extra 0.05 mol/L of SLS solution was added to stabilize the latex, the flocculant still appeared in several hours. It may be that SLS is not a good surfactant for the VAc/NMA system. Figure 5 shows the particle size changes with conversion for selected batch macroemulsion copolymerization runs. Figure 6 gives the dependence of the particle


Figure 7. Dependence of particle size on relative conversion for VAc/DOM(60/40) system. Figure 4. Changes of particle size versus conversion in batch miniemulsion copolymerization.

Figure 8. Dependence of particle size on relative conversion for VAc/DOM(80/20) with 0.6 mL/min feedrate. Figure 5. Particle size changes versus conversion for batch macroemulsion copolymerization of VAc/BA and VAc/DOM.

Figure 9. Dependence of particle size on relative conversion for VAc/DOM(80/20) with 0.3 mL/min feedrate.

Figure 6. Particle number changes versus conversion for batch macro- and miniemulsion copolymerization of VAc/BA and VAc/ DOM.

number on conversion for batch macroemulsion and miniemulsion copolymerizations. In comparison with macroemulsion copolymerization, it is clear that miniemulsion copolymerization produced fewer, larger particles. The particle number continued to increase throughout the polymerization for all cases. The explanation of this phenomenon can be considered separately for macroemulsion and miniemulsion systems. In the macroemulsion system, due to the high water solubility of VAc, the existence of the homogeneous nucleation is possible.31 This nucleation can occur until the conversion reaches a high level. Whereas, in miniemulsion copolymerization, only a fraction of droplets can be nucleated and converted to polymer particles. Here, the percentage of nucleation (particle number in the final latex divided by droplet number in the miniemulsion solution) can be estimated as 41% for BM1, 61% for BM2 and 56% for BM4. In other words, there is a certain fraction of droplets left unnucleated, which serve as monomer

reservoirs. The process of monomer transport from these reservoirs to polymerizing particles is similar to the mass transfer in the macroemulsion process, so that homogeneous nucleation is also possible in this case. On the other hand, these unnucleated droplets are still capable of being nucleated later in the polymerization if they absorb a free radical. New free radicals are generated continuously throughout the polymerization. (The rate constant of KPS decomposition is approximately 0.04-0.1 h-1.32) This too, will cause a rise in the number of particles. The dependence of polymer particle size and particle number on the relative conversion, Xr (defined as the weight of polymer in the reactor divided by the total weight of monomer to be added by the end of the polymerization) for the VAc/DOM system in the three modes of operation are shown in Figures 7-9 and Figures 10-12, respectively. A certain difference in particle sizes and particle numbers between macroemulsion and miniemulsion feed semibatch runs can be seen. The fact that the particle sizes in macroemulsion feed runs were smaller can be attributed to the new particles formed by micellar nucleation during the feeding stage. (Micelles are present in the macroemulsion semibatch feed.) The particle size decreased with a decrease in feedrate for both macro- and miniemulsion

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Figure 10. Dependence of particle number on relative conversion for VAc/DOM(60/40) system. Figure 14. Conversion versus time for batch macro- and miniemulsion copolymerization of VAc/DOM system.

Figure 11. Dependence of particle number on relative conversion for VAc/DOM(80/20) with 0.6 mL/min feedrate. Figure 15. Instantaneous conversion versus time for VAc/BA system with 1.0 mL/min feedrate.

Figure 12. Dependence of particle number on relative conversion for VAc/DOM(80/20) with 0.3 mL/min feedrate.

Figure 13. Conversion versus time for batch macro- and miniemulsion copolymerization of VAc/BA system.

feeds. The data in Figure 9 indicate that the particle sizes in semibatch runs were smaller than in batch miniemulsion runs when the feedrate of monomer emulsion was sufficiently low. Monomer Conversion. The conversion-time curves for batch runs of the VAc/BA and VAc/DOM systems are shown in Figures 13 and 14. For both systems, it can be seen that the rate for macroemulsion copolymerization was faster than in the miniemulsion process.

The rate of polymerization depends on many factors, such as particle number, free radical number in polymerizing particles, monomer transport from monomer reservoirs to polymerizing sites, etc. For similar recipes, although higher radical numbers can be accommodated by miniemulsion particles, owing to their larger size,33 a larger number of particles are produced in the macroemulsion. As mentioned in the former section, there may still be a quite fraction of droplets unnucleated in miniemulsion system. These unnucleated droplets will serve as monomer reservoirs. But the cosurfactant used in the miniemulsion system can retard the monomer diffusion from droplet to polymerizing sites, so the ability of monomer transport from these monomer reservoirs is much poorer than that in the macroemulsion process. Therefore, the overall feature of reaction rate in the macroemulsion polymerization is always faster than in the miniemulsion process. The amount of DOM in the recipe affects the reaction rate significantly for both of macroemulsion and miniemulsion cases. It was found that increasing the concentration of DOM in the recipe resulted in a reduction of the total polymerization rate. Donescu et al.34 investigated the emulsion polymerization of the vinyl acetate/ dibutyl maleate system, and reported similar results. Figures 15-19 show the instantaneous conversion (defined as the weight of polymer in the reactor divided by the weight of polymer plus monomer in the reactor) versus time curves for all monomer systems under the three modes of operation. To keep the monomer conversion at a constant level, the feedrate of semicontinuous operation must be a nonlinear relation versus time. In this work, the feed flow rates were kept constant, so the conversions of all semibatch runs drifted over the feeding period. At a feedrate of 1.0 mL/min for the three monomer systems, the conversion decreased after the


Figure 16. Instantaneous conversion versus time for VAc/DOM (60/40) system with 1.0 mL/min feedrate. Figure 20. Cumulative composition versus conversion in batch macro/miniemulsion copolymerization of VAc/BA system.

Figure 17. Instantaneous conversion versus time for VAc/NMA system with 1.0 mL/min feedrate.

Figure 21. Cumulative composition versus conversion in batch macro/miniemulsion copolymerization for VAc/DOM (60/40) system.

Figure 18. Instantaneous conversion versus time for VAc/DOM (80/20) system with 0.6 mL/min feedrate.

Figure 22. Cumulative composition versus conversion in batch macro/miniemulsion copolymerization for VAc/DOM (80/20) system.

Figure 19. Instantaneous conversion versus time for VAc/DOM(80/20) system with 0.3 mL/min feedrate.

feeding stage started (The solid marks correspond to the initial point of feeding stage.). This indicates that the reaction ran under monomer-unstarved condition, whereas if the feedrate was set at a low level (0.6 or 0.3 mL/min), for the system of VAc/DOM (80/20), the conversion increased first, across a maximum, then decreased slowly. This indicates that with these feedrates, the polymerization was run under the monomerstarved condition at the beginning polymerization rate. After the apex was reached, the instantaneous conversion decreased gradually to zero, indicating that the

polymerization ran under a slightly monomer-unstarved condition. Copolymer Composition. The copolymer composition was analyzed with proton NMR, and the relations of cumulative polymer composition versus instantaneous conversion for batch runs are plotted in Figures 20-23. One will note that in the initial region of all batch runs except batch macroemulsion runs of the VAc/ DOM system, the percentage of comonomers, BA, DOM, or NMA, is obviously higher than the molar percentage in the recipe. The explanation of this phenomenon is that the reactivity ratio of VAc is lower than the ratio of others. On the basis of the Q-e value,35 the reactivity ratios (based on homogeneous copolymerization) for the three monomer systems were estimated and are listed in Table 3. With increasing conversion, the percentage of comonomer decreases, approaching the molar per-

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1 K2φ r1′ ) r1 1 1+ K1φ 1+

1 1+ K1φ r2′ ) r2 1 1+ K2φ Figure 23. Cumulative composition versus conversion in batch miniemulsion copolymerization of VAc/NMA system. Table 3. Reactivity Ratios for the Three Monomer Systems monomer system(1/1)

r1

r2

r1/r2

VAc/BA VAc/DOM VAc/NMA

0.0392 0.1953 0.0555

4.9513 0.9448 9.0408

0.0079 0.2067 0.0061

centage in the recipe. In these figures, the dashed curves give the composition predicted by the integration of Mayo-Lewis equation:

d[M1] d[M2]

)

[M1]

r1[M1] + [M2]

[M2] [M2] [M1] + r2[M2]

(6)

where [Mi] is the monomer molar concentration at the polymerizing site. The reactivity ratios, r1 and r2, are defined as follows:

r1 ) kp11/kp12 and r2 ) kp22/kp21

(7)

kpii and kpij are the homopropagation rate constant and crosspropagation rate constant, respectively. In Figure 20, the measured copolymer composition of VAc/BA copolymer in the course of batch miniemulsion polymerization is in good agreement with the ideal case (i.e., Mayo-Lewis equation). For batch macroemulsion copolymerization, there is a significant deviation, especially at low conversion. Delgado et al.28 attributed this effect to the small amount of water existing in the monomer-swollen polymer particles. Because of the high water solubility of VAc, the particles then contain a higher level of VAc, and so BA incorporation is suppressed. This effect is not seen in the miniemulsion, perhaps because the presence of the cosurfactant suppresses water absorption into the polymerizing particle. A significant deviation of the composition of the VAc/ DOM copolymer from the ideal case can be seen in Figures 21 and 22. In both cases, the miniemulsion copolymerization more closely follows the integrated Mayo-Lewis equation. This is believed to be due to the reduced effect of DOM transport in miniemulsion polymerization. (DOM is extremely water-insoluble.) Since not all miniemulsion droplets are nucleated, an effect of DOM transport remains, but it is less significant than in the macroemulsion case. According to the method proposed by Samer,4 for extremely water-insoluble monomer, a pseudo-partition coefficient, κ, was used to replace the monomer partition coefficient, K, in Schuller’s equation2, shown as below:

(8)

where Ki ) [Mi]p/[Mi]a and φ ) organic volume/aqueous volume. [Mi]p and [Mi]a are for the monomer concentration in the polymerizing particle and aqueous phase, respectively. Here, the partition coefficient of VAc, K1, and pseudopartition coefficient of DOM, κ2, can be assumed as K1 ) ∞ and κ2 ) 1. The Mayo-Lewis equation, with the pseudo-reactivity ratios of Schuller, as modified by Samer to account for mass transfer resistance are also shown in Figures 21 and 22. An evident improvement in the prediction of composition of copolymer for the macroemulsion process is demonstrated. However, the experimental line is still below the predicted data. A small amount of viscous coagulum sticking on the wall of reactor could be found in the experiments; the amount for the miniemulsion process was less than the macroemulsion process. The composition of this coagulum was about 32% of DOM, more than five times of the recipe (VAc/DOM, 80/20 w/w) molar proportion, 5.95%. The formation of coagulum is another confirmation for the influence of monomer transport on the copolymerization. The mechanism for the formation of coagulum can be described here. With the polymerization going, the difference in mass transfer resistance between VAc and DOM leads to a concentration process of DOM within the monomer droplets. Because of the high viscosity of the DOM-concentrated droplets, it is easy for these droplets to coalesce with other droplets or polymerizing particles. With the former, the DOMconcentrated droplets become larger and easier to coalesce with others. With the latter, the polymerizing site is subjected to a DOM-rich environment, producing a DOM-rich polymer. As will be seen in the following section, the higher the DOM concentration, the lower the molecular weight will be. Therefore, viscous polymer with relatively low molecular weight is formed, sticking together or on the wall of reactor. In Figure 23, the experimental copolymer compositions for the batch macro- and miniemulsion copolymerization of VAc/NMA are plotted along with the integrated Mayo-Lewis equation. In this case, the macroemulsion composition closely follows Schuller’s equation, while the miniemulsion more closely follows the MayoLewis equation. With the increase of conversion, the trends of all lines are approximately identical. These effects can be interpreted by taking account of the great difference between the reactivity ratios of VAc and NMA, r1 and r2, here, r1/r2 ) 0.0061. If the partition coefficient is considered for the extremely water-soluble NMA, K1 ) ∞ and K2 ) 1 may be taken. For macroemulsion copolymerization, therefore, on the basis of eq 8, r1′ ) 0.1872, r2′ ) 2.685, and r1′/r2′ ) 0.0697. The disparity in the reactivity ratios is amplified by the relative water solubilities, such that the content of VAc in the copolymer at low conversion is increased. However, since the VAc was consumed more quickly than


Figure 24. Cumulative composition versus relative conversion during feeding period for VAc/BA and VAc/NMA systems.

Figure 25. Cumulative composition versus relative conversion during feeding period for VAc/DOM system.

in the case of ideal copolymerization, the ratio of r1′/r2′ would decrease with the conversion. Therefore, copolymer composition curve for the macroemulsion approaches the ideal curve at high conversion. The fact that the miniemulsion polymerization follows the MayoLewis equation (no effect of monomer solubility) rather than the Schuller equation (accounting for monomer solubilities) seems to indicate that the presence of the cosurfactant shifts the equilibrium monomer concentration in the particles in the direction of increased NMA. There is no particular justification for this. The semibatch miniemulsion copolymerization of each monomer system was carried out with both macro- and miniemulsion feeds. The cumulative composition of copolymer involves the results of two stages, i.e., miniemulsion batch stage and the feeding stage. Here, an attempt has been made to remove the effect of the batch stage. First, two terms, the cumulative molar fraction of monomer incorporated, Ff, and relative conversion, Xf, in feeding period, are defined as

Table 4. Molecular Weight and Distribution of Polymers in Final Latexes

Ff2 )

F′f2/Mw2 (1 - F′f2)/Mw1 + F′f2/Mw2

F′f2 )

F′tXtr - F′BXBr

X′f2 )

Xer - XBr Xtr - XBr Xer - XBr

(9)

(10)

(11)

where F′ is the mass composition of copolymer, Mw is the molecular weight of the monomer, and for the subscripts, B stands for the end of the batch stage, e for the end of the feeding stage, f for the feeding period, t for the time of the feeding period, and 1 and 2 for VAc and comonomer, respectively. The relationships between Ff and Xf for the feeding period were plotted in Figures 24 and 25. In the figures, the dashed lines are the mole fraction of comonomer in the feed stream. For the VAc/NMA system, the experimental copolymer composition is below but close to the corresponding feed composition for both macroemulsion and miniemulsion feeds. Thus, a relatively homogeneous copolymer composition was formed for both feed modes. This result can be expected since, as previously discussed, the monomer in the feed quickly equilibrates with the monomer in the polymer particles due to the high water solubility of both monomers. Experimental compositions are below the feed line due to the significant water solubility of NMA. In the VAc/BA system, the composi-

experiment code BM1 BE1 SM1 SME1

Mn *0.001

Mw *0.001

VAc/BA System 387.7 284.1 147.6 152.6

Pd inx

2,380.0 2,438.0 1,762.0 1,881.0

6.14 8.58 11.94 12.33

BM2 BE2 SM2 SE2

VAc/DOM (60/40) System 129.7 722.3 178.4 991.0 80.5 495.8 111.8 719.0

5.57 5.55 6.16 6.43

BM4 BE4 SM4 SE4 SM5 SE5

VAc/DOM (80/20) System 186.4 1,316.0 236.7 1,477.0 116.0 829.8 118.3 923.9 120.5 856.0 131.8 952.2

7.06 6.24 7.15 7.81 7.10 7.22

tion trend for the miniemulsion-feed run is closer to the dashed line than that of the macroemulsion-feed run, and both trend lines are above the feed composition. It would seem that in the miniemulsion-feed run, the new monomer droplets in the feed are being nucleated and are polymerizing under semistarved conditions, at approximately the feed composition, whereas, in the macroemulsion feed, original (seed) particles containing cosurfactant have a higher affinity for BA, causing an enrichment at low conversion. For the VAc/DOM system, the two copolymer composition curves (macro-feed and mini-feed) are on opposite sides of the feed line. It would seem, as before, that the macroemulsion-feed system suffers from significant mass transfer resistance, resulting in polymer which is poor in DOM at low conversions, whereas the miniemulsion-feed gives a copolymer which is enriched in the more hydrophobic monomer. The relatively low composition of DOM at the initial feeding period for the miniemulsion-feed runs can be attributed to propagation within the seeds particles formed during the batch stage; beyond the initial feeding period, propagation in the new nucleation loci became more and more important. In all cases, to close the mass balance, the composition at full conversion must equal the feed composition. Molecular Weight Distribution. The molecular weight averages and polydispersity of polymers in the final latexes were measured and listed in Table 4. As would be expected, the polydispersity indexes of polymers produced in semibatch runs are higher than in batch runs. However, the average molecular weights for the batch runs are larger than ones for the semibatch

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runs. It can also be found that the content of DOM in the recipe plays a negative role on the molecular weight; high DOM content corresponds to the low molecular weight of the copolymer. The reason for the high molecular weight of VAc/BA copolymers may be attributed to the very low termination rate constant of BA. In addition, the average molecular weights in macroemulsion feed runs are slightly higher than those in miniemulsion-feed runs, perhaps indicating a lower number of radials per particle in the macroemulsionfeed runs. Conclusions Miniemulsion and macroemulsion copolymerizations of vinyl acetate and comonomers with extremely different physical and kinetic properties (BA, NMA, and DOM) were investigated in batch and semibatch systems. The results in polymer particle size and number, monomer conversion, composition, and molecular weight of copolymers indicated that there exists an obvious diversity between macroemulsion and miniemulsion copolyerization. In all cases, the particle size was smaller and the particle number was higher in macroemulsion copolymerization than in miniemulsion copolymerization. For the systems VAc/BA and VAc/DOM, the particle number increase with increasing conversion throughout the reaction for both batch macroemulsion and miniemulsion runs. This indicates that nucleation of new particles takes place via homogeneous nucleation throughout these reactions. For the batch runs, the rate of polymerization of the macroemulsion polymerization runs was faster than that of miniemulsion. Investigation of the copolymer composition demonstrated the important effect of monomer transport on the copolymerization. The droplets in the macroemulsion act as monomer reservoirs. In this system, the effect of monomer transport will be predominant when an extremely water-insoluble comonomer, such as DOM, is used. In contrast with the macroemulsion system, the miniemulsion system tends to more closely follow the integrated Mayo-Lewis equation, indicating less influence of mass transfer. Likewise, for the semibatch operation, the influence of monomer is seen in the differences between macroand miniemulsion feeds. For extremely water-insoluble monomers, the miniemulsion-feed mode lessens the departure of the copolymer composition from the feed composition during semistarved semibatch polymerization. Results of the GPC analysis indicated that the polymers with lower molecular weight and broader distribution were formed in the semibatch process in contrast with the batch run. Acknowledgment The financial support of Air Products and Chemicals, Inc. is gratefully acknowledged. Literature Cited (1) Mayo, V. F.; Lewis, F. M. Copolymerization I. A Basis for Comparing the Behavior of Monomers in Copolymerization; The Copolymerization of Styrene and Methyl Methacrylate. J. Am. Chem. Soc. 1944, 66, 1594.

(2) Schuller, H. Copolymerization. in Emulsion. In Polymer Reaction Engineering; Reichert, K., Geisler, W., Eds.; WileyVCH: Frankfort, Germany, 1986; p 137. (3) Guillot, J. Computer Simulation Of Emulsion Processes For Monomers Of Different Water Solubility In Polymer Reaction Engineering; Reichert, K., Geisler, W., Eds.; Wiley-VCH: Frankfort, Germany, 1986; p 147. (4) Samer, C. J. Polymer-Stabilized Miniemulsion Polymerization in a Continuous Stirred-Tank Reactor. Ph.D. Thesis, Georgia Institute of Technology, Atlanta, Georgia, 1998. (5) Hagiopol, C.; Deleanu, T. H.; Geogescu, M.; Deaconescu, I. Formation of New Particles in Seeded Emulsion Copolymerization. Acta Polym. 1985, 36, 540. (6) Capek, I. Emulsion Copolymerization of Acrylonitrile and Butyl Acrylate. Acta Polym. 1986, 37, 195. (7) Capek, I.; Mlynarova, M.; Barton, J. Emulsion Copolymerization of Acrylonitrile and Butyl Acrylate. VIII. Effect of the Emulsifier Type on the Copolymer Composition. Acta Polym. 1988, 39, 142. (8) Capek, I. Emulsion Copolymerization of Methyl Methacrylate and Ethyl Acrylate. III. Kinetic Studies. Acta Polym. 1988, 39, 221. (9) Djekhaba, S.; Graillat, C.; Guillot, J. Emulsion Copolymerization of Ethyl Acrylate With Styrene. Eur. Polym. J. 1988, 24, 109. (10) Kong, X. Z.; Pichot, C.; Guillot, J. Kinetics of Emulsion Copolymerization of Vinyl Acetate With Butyl Acrylate. Eur. Polym. J. 1988, 24, 485. (11) Van Doremaele, G. H. J.; Van Herk, J A. M.; Ammerdorffer, L.; German, A. L. Emulsion Copolymerization of Hydrophobic With Hydrophilic Monomers Leading to Products With Anomalous Chemical Composition Distributions. Polym. Commun. 1988, 29, 299. (12) Forcada, J.; Asua, J. M. Modeling of Unseeded Emulsion Copolymerization of Styrene and Methyl Methacrylate. J. Polym. Sci. A. Polym. Chem. 1990, 28, 987. (13) Forcada, J.; Asua, J. M. Emulsion Copolymerization of Styrene and Methyl Methacrylate. II. Molecular Weights. J. Polym. Sci. A. Polym. Chem. 1991, 29, 1231. (14) Van Doremaele, G. H. J.; Geerts, F. H. J. M.; Schoonbrood, H. A. S.; Kurja, H. A. S.; German, A. L. Effect of Composition Drift on Emulsion Copolymerization Rate. Polymer 1992, 33, 1914. (15) Lee, K. C.; El-Aasser, M. S.; Vanderhoff, J. W. Batch and Semicontinuous Emulsion Copolymerization of Vinylidene Chloride and Butyl Methacrylate. I. Kinetics in VDC-BMA Emulsion Polymerization and Surface and Colloidal. J. Appl. Polym. Sci. 1992, 45, 2207. (16) Urquiola, B.; Arzamendi, G.; Leiza, J. R.; Zamora, A.; Asua, J. M.; Delgado, J.; El-Aasser, M. S.; Vanderhoff, J. W. Semicontinuous Seeded Emulsion Copolymerization of Vinyl Acetate and Methyl Acrylate. J. Polym. Sci. A. Polym. Chem. 1991, 29, 169. (17) Van Doremaele, G. H. J.; Schoonbrood, H. A. S.; Kurja, H. A. S.; German, A. L. Copolymer Composition by Means of Semicontinuous Emulsion Copolymerization. J. Appl. Polym. Sci. 1992, 45, 957. (18) Scott, P. J.; Penlidis, A.; Rempel, G. L. Ethylene-Vinyl Acetate Semi-Batch Emulsion Copolymerization: Use of Factorial Experiments for Improved Process Understanding. J. Polym. Sci. A. Polym. Chem. 1993, 31, 2205. (19) Schoonbrood, H. A. S.; Thijissen, H. A.; Brouns, H. M. G.; Peters, M.; German, A. L. Semicontinuous Emulsion Copolymerization to Obtain Styrene-Methyl Acrylate Copolymers With Predetermined Chemical Composition Distributions. J. Appl. Polym. Sci. 1993, 49, 2029. (20) Mead, R. N.; Poehlein, G. W. Emulsion Copolymerization of Styrene-Methyl Acrylate and Styrene-Acrylonitrile in Continuous Stirred Tank Reactors. I. Ind. Eng. Chem. Res. 1988, 27, 2283. (21) Mead, R. N.; Poehlein, G. W. Emulsion Copolymerization of Styrene-Methyl Acrylate and Styrene-Acrylonitrile in Continuous Stirred Tank Reactors. II. Aqueous-Phase Polymerization and Radical Capture. Ind. Eng. Chem. Res. 1989, 28, 51. (22) Pichot, C.; Llauro, M. F.; Pham, Q. T. Microstructure of Vinyl Acetate-Butyl Acrylate Copolymers Studied by 13C NMR Spectroscopy: Influence of Emulsion Polymerization Process. J. Polym. Sci. A. Polym. Chem. 1981, 19, 2619.

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Received for review November 29, 1999 Revised manuscript received April 20, 2000 Accepted April 28, 2000 IE990861K

Macroemulsion Copolymerization of Vinyl

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