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Evaluation of heterophase polymerizations by means of reaction calorimetry
Colloids and Surfaces A: Physiochemical and Engineering Aspects 153 Ž1999. 143]151
Evaluation of heterophase polymerizations by means of reaction calorimetry Klaus Tauer a,U , Harmin Muller ¨ a , Carsten Schellenberg a , Lutz Rosengartenb a
Max-Planck-Institut fur Kantstraße 55, D-14513 Teltow-Seehof, Germany ¨ Kolloid und Grenzflachenforschung, ¨ b PolymerLatex GmbH& Co. KG, D-41538 Dormagen, Germany
Abstract Reaction calorimetry is a powerful tool for systematic investigations of heterophase polymerizations. The heat flow]time or heat flow]conversion profiles clearly reflect any changes of the recipe components. Results of batch heterophase polymerizations are presented proving the dependence of the reaction rate profiles on the water solubility of the monomers, on the presence of a chain transfer agent, on the type and concentration of the stabilizer and the initiator, respectively, and on the polymerization temperature. A complete mechanistic interpretation of this data collection is nowadays still impossible. Q 1999 Elsevier Science B.V. All rights reserved. Keywords: Reaction calorimetry; Emulsion polymerization; Stabilizer; Initiator; Chain transfer agent
1. Introduction Two main advantages of heterophase polymerizations are, firstly, the ease of removing the polymerization heat through the continuous phase of low viscosity and, secondly, the possibility, in the case of feeding procedures, to polymerize most of the time at a maximum polymerization rate w1x. The complete exploitation of these advantages requires knowledge of the polymeriza-
U
Corresponding author. Tel.: q49 3328 46258; fax: q49 3328 46255. E-mail address: [email protected] ŽK. Tauer.
tion kinetics up to very high conversions, i.e. the change of the rate of polymerization during the whole polymerization period, the so called reaction rate profile ŽRRP.. The overall rate of polymerization of a heterophase polymerization is given by Eq. Ž1.. r p s k p ? N ? CM ? n
Ž1.
Eq. Ž1. describes with the assumptions that after the nucleation period Žinterval I. the particle concentration, N Žmol cmy3 ., as well as the mean number of radical per particle, n, and the monomer concentration in the particle, CM Žmol cmy3 . are almost constant a constant polymeriza-
0927-7757r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 Ž 9 8 . 0 0 4 3 6 - 1
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K. Tauer et al. r Colloids Surfaces A: Physiochem. Eng. Aspects 153 (1999) 143]151
tion rate during interval II as long as a free monomer phase is present Žcf. Fig. 1.. During interval I the rate of polymerization increases due to increasing values of N, CM and n. In contrast, during interval III Žmonomer starvation. the rate of polymerization decreases due to a decreasing value of CM . As the decrease in CM leads to an increasing viscosity inside the polymer particles a Norrish w2x or Trommsdorff w3x or gel effect frequently occurs leading to a temporary increase in r p during interval III. Fig. 1 sketches the time development of the rate of polymerization, the conversion, and the latex surface tension, respectively, for a batch styrene emulsion polymerization with a weak gel effect at an emulsifier concentration above the critical micelle concentration ŽCMC. according to the classical assumptions of Harkins w4x and SmithrEwart w5x. Such a behaviour is indeed sometimes observed as it was shown by Gerrens w6x. But it is also known that for more water soluble monomers the observed RRPs strongly deviate from that classical picture w6x. For example, the RRP of a methyl methacrylate ŽMMA. emulsion polymerization is characterized by the absence of a period with a constant r p and by a very strong gel effect w6x. The reaction calorimetry offers nowadays the unique possibility of an on-line monitoring of RRPs even in combination with an on-line infrared or turbidimetric analysis with a high accuracy and reaction volumes of up to several litres. It is a powerful tool for basic experiments on a laboratory scale, as well as experiments on a
Fig. 1. Schematic drawing of generalised features of an emulsion polymerization Ž r p , rate of polymerization; X, monomer conversion; s, latex surface tension..
semi-technical scale for product optimizations. However, it is astonishing that with respect to the economic importance of heterophase polymerizations only a relatively small number of papers have been published concerning systematic investigations of heterophase polymerizations with reaction calorimetry w7]14x. A systematic calorimetric investigation of the styrene emulsion polymerization with sodium dodecyl sulfate ŽSDS. as stabilizer and potassium persulfate ŽKPS. as initiator revealed that the classical constant rate period Žinterval II. must not necessarily occur w11x. Instead, the authors concluded that the end of the nucleation period Žinterval I. and the start of the monomer starvation Žinterval III. take place in the same conversion range between 36% and 40%. The aim of this contribution is to report on the beginning of systematic investigations into the dependence of RRPs of heterophase polymerizations on changes of the particular recipe components, i.e. the kind of monomer, kind of initiator, kind of stabilizer, kind of chain transfer agent ŽCTA. and the temperature, respectively. It is mainly still a report on experimental data, as a detailed mechanistic discussion is, alas, not possible at the moment. However, the acquisition of this kind of data will be useful for a better understanding of heterophase polymerizations. 2. Experimental section 2.1. Materials Styrene Ž STY . , M M A, 2,29-azobisiso butyronitrile ŽAIBN., SDS, dodecyl mercaptan, carbon tetrabromide, 1,3-diisopropenylbenzene ŽDIPB. all from Sigma-Aldrich, 2-ethyl hexyl methacrylate Ž2EHMA., butyl methacrylate ŽBMA. both from Huls ¨ AG, polyŽethylene glycol. ŽPEG. of different molecular weight, cerium ammonium nitrate ŽCAN., sodium hydrogen carbonate, and nitric acid Ž70%., all from FLUKA were used all except the monomers as received. The polystyrene-b-polyŽethylene glycol. stabilizer with a molecular weight of the STY block and the PEG block of 1000 g moly1 and 3000 g moly1 , respectively, SE1030, was a gift of the Th. Gold-
K. Tauer et al. r Colloids Surfaces A: Physiochem. Eng. Aspects 153 (1999) 143]151
schmidt AG, Essen, Germany. The polyŽethylethylene.-b-polyŽstyrenesulfonate . ionic block copolymer stabilizer with a degree of sulfonation of the polystyrene block of 52% Ž1-H52. was synthesized as described elsewhere w15x. The synthesis of symmetrical polyŽethylene glycol.-azo-initiators ŽPEGA where the number indicates the molecular weight of the PEG chains. was carried out according to w16x. De-ionized water was taken from a REWA HQ 5 system Ž18 MV cmy1 . and degassed prior to use. STY, MMA and BMA were distilled under reduced pressure to remove inhibitors and 2EHMA was purified by passing it through a basic activated Al 2 O 3 column. The monomers were stored in a refrigerator. Prior to use, the monomers were checked for oligomer formation by instilling a drop into an excess of methanol. Only oligomer-free monomers were used. 2.2. Emulsion polymerization Emulsion polymerizations were carried out batchwise in a reaction calorimeter RM2-S from ChemiSens ŽLund, Sweden. with a volume of 100 ml equipped with a stainless steel stirrer and a heating facility through the reactor bottom. The standard procedure was as follows: the water, the monomer, the stabilizer, in the case of the cerium ionrPEG redox system also 0.33 g of HNO3 and 3.5 g of PEG with a molecular weight of 10 000 g moly1 , and if necessary the chain transfer agent or the cross-linker were added to the reactor and heated up to reaction temperature. After the thermal equilibrium was reached the reactor was calibrated with an input of 3 W of electric power, and after re-equilibration the reaction was started by injecting the initiator dissolved in water. In several experiments we observed an induction period, which is reproducible with a standard deviation of 5%. The reaction was finished until no further heat development was detected by the calorimeter. 2.3. Latex and polymer characterization The particle size of the final latexes was analysed either by dynamic light scattering ŽDLS. with
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a Nicomp particle sizer ŽModel 370, PSS Santa Barbara, USA. at a fixed scattering angle of 908 or by enumeration of transmission electron microscopy ŽTEM. pictures. The polymer molecular weights were calculated from gel permeation chromatography ŽGPC. data. The GPC analysis was performed on a P1000 pump with a UV1000 detector Ž l s 260 nm. both from Thermo Separation Products with 5-mm 8 = ˚ from 300 mm SDV columns with 10 6 , 10 5, 10 3 A Polymer Standard Service in THF with a flow rate of 1 ml miny1 at 308C. The molecular weights were calculated with a calibration relative to PS standards. 3. Results 3.1. Kind of monomer As it has been known for a long time that the water solubility of the monomers has a strong influence on emulsion polymerization kinetics w4,6x, one might also expect differences in the corresponding RRPs. The results depicted in Fig. 2 show that this is really the case. BMA and STY differ slightly in water solubility, whereas STY has a slightly higher water solubility than BMA w17,18x. Curve 1 of Fig. 2a and curve 2 of Fig. 2b represent RRPs for almost the same polymerization conditions. The different water solubility is very likely the reason for the faster rate during interval I in the case of BMA compared to STY, as a lower water solubility lead to an earlier nucleation and subsequently to a higher polymerization rate compared to a not yet nucleated system. Furthermore, the results in Fig. 2 prove both the influence of the polymerization temperature and of the surfactant concentration on the resulting rate profiles. Curve 1 of Fig. 2b corresponds to a SDS concentration well above the CMC, whereas curve 4 corresponds to an SDS concentration well below the CMC. A direct comparison with the data published in Delarosa et al. w11x is not possible as the initiators are different. Fig. 2 describes the case of a non-ionic watersoluble initiator leading to oligomeric radicals whereas the data in Delarosa et al. w11x were obtained with KPS leading to low molecular
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K. Tauer et al. r Colloids Surfaces A: Physiochem. Eng. Aspects 153 (1999) 143]151
Fig. 2. RRPs of BMA Ža. and STY Žb. emulsion polymerization recipes: 80 g of water, 20 g of monomers, 400 mg PEGA200; Ža. 200 mg of SDS, varying temperature; Žb. 828C, varying amount of SDS.
weight sulfate ion radicals. However, the results of both investigations agree in the point that the SDS concentration has a strong influence on the rate profiles. 3.2. Initiator influence The importance of the kind of initiator for emulsion polymerization, as compared to that of the emulsifier, is still underestimated today. To prove the initiator influence, emulsion polymerizations of BMA were carried out also with KPS instead of PEGA200. The use of an ionic and low molecular weight initiator changes the RRP significantly Žcf. Figs. 3 and 4.. The most ostentatious and most interesting change occurs at low conversions during interval I. If KPS is used, a distinct rate maximum at conversions lower than 10% is observed Žcf. Fig. 3.. It is straightforward to conclude that this maximum is linked with the particle nucleation. Such a distinct maximum in the polymerization rate in connection with a maximum in the particle concentration was also observed during the emulsion polymerization of vinyl chloride with SDS and KPS w19x. In contrast, the non-ionic PEGA200 initiator does not lead to a distinct rate maximum during interval I. The results depicted in Figs. 3 and 4 reveal an initiator influence on the particle nucleation. It is not possible to decide whether the chemical structure
of the nucleating species ŽPEG vs. sulfate end group. or the increase in the ionic strength is responsible for the measured differences of the RRPs. Figs. 3 and 4 represent two sets of polymerizations with varying rates of initiation adjusted either by changing the initiator concentration or by changing the polymerization temperature. It is interesting to note that in the case of KPS as initiator the final particle size clearly increases with decreasing rate of initiation, whereas in the case of PEGA200 as initiator at temperatures above 708C the final particle size remains almost unchanged Žcf. Table 1.. These differences are in agreement with earlier published results on the temperature dependence of the particle concentration for different initiator]emulsifier systems in STY emulsion polymerization w20x. Table 1 Dependence of the final particle size on the rate of initiation for a BMA emulsion polymerization with different initiators Polymerization condition
Di Žnm.a
KPS s 50 mg; T s 708C KPS s 100 mg; T s 708C KPS s 200 mg; T s 708C
109.2 91.9 78.9
PEGA200 s 400 mg; T s 608C PEGA200 s 400 mg; T s 708C PEGA200 s 400 mg; T s 808C
108.1 91.2 91.4
a
Di intensity weighted diameter ŽDLS..
K. Tauer et al. r Colloids Surfaces A: Physiochem. Eng. Aspects 153 (1999) 143]151
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Fig. 3. Influence of the KPS concentration on the RRPs of a BMA emulsion polymerization recipe: 80 g of water, 20 g of BMA, 200 mg of SDS, 708C; Ža. heat flow]time plot; Žb. heat flow]conversion plot.
3.3. Stabilizer influence The kind and concentration of the stabilizer used in a heterophase polymerization determines the particle size and hence the particle concentration. According to Eq. Ž1., one might expect a strong influence of the chemical structure of the stabilizer on the RRPs. It is to note that n strongly depends on both the particle size and the particle concentration. Fig. 5 compares the influence of SDS with that of a polymeric stabilizer Ž1-H52.. The polymeric stabilizer 1-H52 has, up to concentrations of approx. 5 wt.% relative to monomer, almost no influence on the RRP Žcf. Fig. 5b., whereas the SDS concentration has a strong influence on the resulting rate profiles Žcf. Fig. 5a..
This behaviour can be attributed to differences in the particle size stabilizer concentration dependence for both systems Žcf. Table 2.. In the case of SDS the particle size decreases with increasing stabilizer concentration, whereas for 1-H52 the DLS data suggest an unusual stabilizer concentration dependence w15x. However, for 1-H52 latexes the TEM data reveal that at stabilizer concentrations between 1 and relative to 10 wt.% relative to monomer the hard core particle diameters are almost constant. The difference between TEM and DLS data Žcf. Table 2. is attributed to the chemical structure of the polymeric stabilizer that allows an interparticular adsorption at higher concentrations, leading to a limited aggregation and consequently to higher DLS values w15x. In
Fig. 4. Influence of the polymerization temperature on the RRPs of a BMA emulsion polymerization recipe: 80 g of water, 20 g of BMA, 200 mg of SDS, 400 mg of PEGA200; Ža. heat flow]time plot; Žb. heat flow]conversion plot.
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Table 2 Dependence of average particle diameters on stabilizer concentration Stabilizer Ž%. a
Di Žnm. b
DN Žnm. c
SDS 0.2 SDS 0.7 SDS 1.0 SDS 1.7
227.0 99.7 71.7 62.6
] ] 68.2 ]
1-H52 0.4 1-H52 1.0 1-H52 5.1
94.2 78.5 129.3
] 52.7 55.0
a
Amount of stabilizer relative to monomer. Intensity weighted diameter from DLS. c Number average diameter from TEM. b
the case of the SDS latexes the diameters determined with DLS and TEM agree within the experimental errors. 3.4. Influence of chain transfer agents The control of the molecular weight distribution of polymer dispersions is crucial for many technical applications. A CTA influences all three intervals of an emulsion polymerization as chain transfer events lead: Ž1. to more hydrophobic end groups thus influencing particle nucleation; Ž2. to a higher concentration of small radicals in the particles, thus leading to a lower n due to en-
hanced exit; and Ž3. to a lower viscosity inside the particles, thus influencing the gel effect. Figs. 6 and 7 summarize the results for dodecyl mercaptan and for carbon tetrabromide as CTA, respectively. Especially the heat flow]time curves Žcf. Fig. 6a and Fig. 7a. show that in the presence of a CTA the reaction starts earlier. In the case of dodecyl mercaptan, a dependence on the amount of CTA is still visible, whereas in the case of the more hydrophobic carbon tetrabromide, the calorimeter is unable to distinguish between 1 and 2 mol.% CTA. A possible explanation is an earlier nucleation due to an enhanced hydrophobicity of the oligomers born after a change transfer event. Another common feature of both CTAs is that the RRPs show a broader region of a high polymerization rate with increasing CTA concentration. This is a consequence of the lower molecular weights and hence a lower viscosity inside the particles with increasing CTA concentration, rather than of larger particle sizes as the average particle diameters show only for carbon tetrabromide a weak dependence on the CTA concentration Žcf. Table 3.. 3.5. Polymeric radicals with stabilizing properties The use of water-soluble polymeric radicals to start heterophase polymerizations of hydrophobic monomers leads to the formation of particles
Fig. 5. Influence of the stabilizer properties on the RRP of an STY emulsion polymerization recipe: 80 g of water, 20 g of STY, 400 mg PEGA200, 828C; varying amount of SDS; Žb. varying amount of 1-H52.
K. Tauer et al. r Colloids Surfaces A: Physiochem. Eng. Aspects 153 (1999) 143]151 Table 3 Influence of the kind and the concentration of CTA on polymer molecular weights and particle sizes of polyŽ2EHMA. dispersions CTA
MN Žg moly1 .a
Di Žnm.b
Without CTA 1 mol.% dodecyl mercaptan 2 mol.% dodecyl mercaptan 1 mol.% carbon tetrabromide 2 mol.% carbon tetrabromide
69.000 25.000 25.000 13.000 8.600
99.1 104.6 99.8 103.6 105.2
a b
MN , number average molecular weight. Di , intensity weighted diameter ŽDLS..
consisting of amphiphilic block copolymers. These particles possess some interesting properties with respect to particle morphology and redispersibility depending on the particular chemical structure
149
of the block copolymers w21,22x. Furthermore, the application of polymeric radicals leads to RRPs with a very pronounced gel effect as a typical pattern. The results collected in Fig. 8 show this for an MMA polymerization started with the redox system PEG10000rceric ions Žcurve 1. and for two STY polymerizations started with PEGA2000 and PEGA200 Žcurve 2 and 3., respectively. The heat flow]time plots Žcf. Fig. 8a. reveal for both STY polymerizations the influence of the primary radical chain length on the duration of the polymerization. The longer the primary radical chain length, the slower the overall polymerization kinetics. In contrast, the heat flow]conversion plots Žcf. Fig. 8b. emphasize the common features and both curves look very similar, indicating the same mechanism.
Fig. 6. Influence of dodecyl mercaptan on the RRP of a 2EHMA emulsion polymerization recipe: 80 g of water, 20 g of 2EHMA, 200 mg of SDS, 100 mg of KPS, 708C; concentration of CTA related to monomer; Ža. heat flow]time plots; Žb. heat flow]conversion plots.
Fig. 7. Influence of carbon tetrabromide on the RRP of a 2EHMA emulsion polymerization recipe: 80 g of water, 20 g of 2EHMA, 200 mg of SDS, 100 mg of KPS, 708C; concentration of CTA related to monomer; Ža. heat flow]time plots; Žb. heat flow]conversion plots.
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Fig. 8. Influence of different kinds of polymeric radicals on RRPs of heterophase polymerizations recipes: Ž1. 100 g of water, 5 g of MMA, 3.5 g of PEG10000, 0.33 g of HNO3 , 3.278 g of CAN, 558C; Ž2. 60 g of water, 7.8 g of STY, 9.14 g of PEGA2000, 828C; Ž3. 60 g of water, 7.8 g of STY, 0.471 g of PEGA200, 828C, 2.24 g of SE1030; Ža. heat flow]time plots; Žb. heat flow]conversion plots.
3.6. Polymerization temperature
4. Conclusions
The influence of the polymerization temperature on the RRPs should be very complex, as the polymerization mechanism comprises a lot of partial reactions and each partial reaction has a unique temperature dependence. The results in Figs. 4 and 9 show how strongly a temperature difference of approx. 108 can alter the RRPs. The changes are especially visible in the heat flow]time plots whereas the heat flow]conversion plots have almost a similar shape. The reasons for these drastic changes are not clear. Differences in the particle size are obviously not responsible, as in both cases almost the same particle diameters of approx. 100 nm were estimated with DLS for all temperatures.
The evaluation of heterophase polymerizations is either possible by means of heat flow]time or heat flow]conversion plots. The heat flow]time plots are suited for a direct comparison of the kinetics, whereas the heat flow]conversion plots are more suited for comparison of general features of heterophase polymerizations under different conditions. Eq. Ž2. describes a relation between the heat flow]time curve and the amount of monomer used Ž m M . where MM is the molecular weight of the monomer and D HR is the reaction enthalpy. ts`
Hts0
HFdt s D HR ? Ž m M rMM .
Ž2.
Fig. 9. Influence of the reaction temperature on RRPs of STY emulsion polymerization recipe: 60 g of water, 7.8 g of STY, 0.465 g of PEGA200, 2.24 g of SE1030, 0.180 g of DIPB, varying temperature; Ža. heat flow]time plots; Žb. heat flow]conversion plots.
K. Tauer et al. r Colloids Surfaces A: Physiochem. Eng. Aspects 153 (1999) 143]151
Furthermore, Eq. Ž2. is a link between reaction calorimetry and other methods to follow the polymerization conversion. The validity of Eq. Ž2. is a prerequisite for the calculation of conversionbased RRPs and, hence, is very important for the comparison and systematic investigations of heterophase polymerizations. Eq. Ž2. has been checked for all the monomers used in this contribution with D HR values published in Moritz w7x. It was found that the accuracy of the calculated conversion data with the given equipment and experimental procedure is approx. 5%. The RRP of an emulsion polymerization contains, at least theoretically, all information about the course of the reaction, provided that the reaction calorimeter is sensitive enough. With the RM-2S reaction calorimeter, a heat flow of around 0.5 W is still reproducibly detectable that corresponds to an instantaneous monomer conversion rate of approx. 7 = 10y6 mol sy1 and 9 = 10y6 mol sy1 for STY and MMA, respectively w7x. The evaluation of RRPs is complicated as: Ž1. most of the information is hidden and not so clearly visible as, for instance, gel peaks; and Ž2. today there is still a lack of reaction rate profiles of heterophase polymerizations, with respect to a theoretical prediction or interpretation. It is of note that a detailed recalculation of RRPs is only possible with sophisticated models considering the colloid chemical as well as the kinetic features of heterophase polymerizations. More systematic investigations with reaction calorimetry alone or in combination with on-line turbidity or FT-IR measurements promise for the future a better understanding of heterophase polymerizations. Acknowledgements The authors thank the Federal Ministry of Education and Research of Germany ŽBMBF. for providing scholarships to H.M. and L.R. through grant 03 D 0039 E 0 as well as to C.S. through
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grant 03 D 0044 A 2 and the Max-Planck-Society for partial support of the project. The authors thank Dr J. Hartmann and Mrs R. Pitschke for preparing the TEM micrographs. References w1x G. Markert, Angew. Makromol. Chem. 123r124 Ž1984. 285. w2x R.G.W. Norrish, R.R. Smith, Nature 150 Ž1942. 336. w3x E. Trommsdorff, H. Kohle, P. Lagally, Makromol. Chem. ¨ 1 Ž1948. 169. w4x W.D. Harkins, J. Am. Chem. Soc. 69 Ž1947. 1428. w5x W.V. Smith, R.H. Ewart, J. Chem. Phys. 16 Ž1948. 592. w6x H. Gerrens, DECHEMA Monogr. 49 Ž1964. 53. w7x H.U. Moritz, in: K.H. Reichert, W. Geiseler ŽEds.., Polymer Reaction Engineering, VCH Verlag, Weinheim, 1989, p. 248. w8x C.M. Miller, P.J. Blythe, E.D. Sudal, C.A. Silebi, M.S. El-Aasser, J. Polym. Sci. Part A } Polym. Chem. 32 Ž1994. 2365. w9x C.M. Miller, E.D. Sudol, C.A. Silebi, M.S. El-Aasser, Macromolecules 28 Ž1995. 2754. w10x S. Fengler, K.-H. Reichert, Angew. Makromol. Chem. 225 Ž1995. 139. w11x L.V. Delarosa, E.D. Sudol, M.S. El-Aasser, A. Klein, J. Polym. Sci. Part A } Polym. Chem. 34 Ž1996. 461. w12x I.S. Deburuaga, M. Arotcarena, P.D. Armitage, L.M. Gugliotta, J.R. Leiza, J.M. Asua, Chem. Eng. Sci. 51 Ž1996. 2781. w13x C.M. Miller, P.A. Clay, R.G. Gilbert, M.S. El-Aasser, J. Polym. Sci. Part A } Polym. Chem. 36 Ž1997. 989]1006. w14x W.D. Hergeth, in: J.M. Asua, Polymeric Dispersion: Principles and Applications, Kluwer, Dodrecht, 1997, p. 267. w15x H. Muller, W. Leube, K. Tauer, S. Forster, M. Antoni¨ ¨ etti, Macromolecules 30 Ž1997. 2288. w16x R. Walz, B. Bohmer, W. Heitz, Makromol. Chem. 178 ¨ Ž1977. 2527. w17x R.S. Corley, in: E.R. Blout, H. Mark ŽEds.., Monomers, Interscience, New York, 1951. w18x A.L. Ward, W.J. Roberts, in: E.R. Blout, H. Mark ŽEds.., Monomers, Interscience, New York, 1951. w19x K. Tauer, B.-R. Paulke, I. Muller, W. Jaeger, G. ¨ Reinisch, Acta Polym. 33 Ž1982. 287. w20x K. Tauer, S. Kosmella, Polym. Int. 30 Ž1993. 253. w21x K. Tauer, Polym. Adv. Technol. 6 Ž1995. 435. w22x K. Tauer, M. Antonietti, L. Rosengarten, H. Muller, ¨ submitted to Macromol. Chem. Phys., 1997.
Evaluation of heterophase polymerizations by means of reaction calorimetry Klaus Tauer a,U , Harmin Muller ¨ a , Carsten Schellenberg a , Lutz Rosengartenb a
Max-Planck-Institut fur Kantstraße 55, D-14513 Teltow-Seehof, Germany ¨ Kolloid und Grenzflachenforschung, ¨ b PolymerLatex GmbH& Co. KG, D-41538 Dormagen, Germany
Abstract Reaction calorimetry is a powerful tool for systematic investigations of heterophase polymerizations. The heat flow]time or heat flow]conversion profiles clearly reflect any changes of the recipe components. Results of batch heterophase polymerizations are presented proving the dependence of the reaction rate profiles on the water solubility of the monomers, on the presence of a chain transfer agent, on the type and concentration of the stabilizer and the initiator, respectively, and on the polymerization temperature. A complete mechanistic interpretation of this data collection is nowadays still impossible. Q 1999 Elsevier Science B.V. All rights reserved. Keywords: Reaction calorimetry; Emulsion polymerization; Stabilizer; Initiator; Chain transfer agent
1. Introduction Two main advantages of heterophase polymerizations are, firstly, the ease of removing the polymerization heat through the continuous phase of low viscosity and, secondly, the possibility, in the case of feeding procedures, to polymerize most of the time at a maximum polymerization rate w1x. The complete exploitation of these advantages requires knowledge of the polymeriza-
U
Corresponding author. Tel.: q49 3328 46258; fax: q49 3328 46255. E-mail address: [email protected] ŽK. Tauer.
tion kinetics up to very high conversions, i.e. the change of the rate of polymerization during the whole polymerization period, the so called reaction rate profile ŽRRP.. The overall rate of polymerization of a heterophase polymerization is given by Eq. Ž1.. r p s k p ? N ? CM ? n
Ž1.
Eq. Ž1. describes with the assumptions that after the nucleation period Žinterval I. the particle concentration, N Žmol cmy3 ., as well as the mean number of radical per particle, n, and the monomer concentration in the particle, CM Žmol cmy3 . are almost constant a constant polymeriza-
0927-7757r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 Ž 9 8 . 0 0 4 3 6 - 1
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K. Tauer et al. r Colloids Surfaces A: Physiochem. Eng. Aspects 153 (1999) 143]151
tion rate during interval II as long as a free monomer phase is present Žcf. Fig. 1.. During interval I the rate of polymerization increases due to increasing values of N, CM and n. In contrast, during interval III Žmonomer starvation. the rate of polymerization decreases due to a decreasing value of CM . As the decrease in CM leads to an increasing viscosity inside the polymer particles a Norrish w2x or Trommsdorff w3x or gel effect frequently occurs leading to a temporary increase in r p during interval III. Fig. 1 sketches the time development of the rate of polymerization, the conversion, and the latex surface tension, respectively, for a batch styrene emulsion polymerization with a weak gel effect at an emulsifier concentration above the critical micelle concentration ŽCMC. according to the classical assumptions of Harkins w4x and SmithrEwart w5x. Such a behaviour is indeed sometimes observed as it was shown by Gerrens w6x. But it is also known that for more water soluble monomers the observed RRPs strongly deviate from that classical picture w6x. For example, the RRP of a methyl methacrylate ŽMMA. emulsion polymerization is characterized by the absence of a period with a constant r p and by a very strong gel effect w6x. The reaction calorimetry offers nowadays the unique possibility of an on-line monitoring of RRPs even in combination with an on-line infrared or turbidimetric analysis with a high accuracy and reaction volumes of up to several litres. It is a powerful tool for basic experiments on a laboratory scale, as well as experiments on a
Fig. 1. Schematic drawing of generalised features of an emulsion polymerization Ž r p , rate of polymerization; X, monomer conversion; s, latex surface tension..
semi-technical scale for product optimizations. However, it is astonishing that with respect to the economic importance of heterophase polymerizations only a relatively small number of papers have been published concerning systematic investigations of heterophase polymerizations with reaction calorimetry w7]14x. A systematic calorimetric investigation of the styrene emulsion polymerization with sodium dodecyl sulfate ŽSDS. as stabilizer and potassium persulfate ŽKPS. as initiator revealed that the classical constant rate period Žinterval II. must not necessarily occur w11x. Instead, the authors concluded that the end of the nucleation period Žinterval I. and the start of the monomer starvation Žinterval III. take place in the same conversion range between 36% and 40%. The aim of this contribution is to report on the beginning of systematic investigations into the dependence of RRPs of heterophase polymerizations on changes of the particular recipe components, i.e. the kind of monomer, kind of initiator, kind of stabilizer, kind of chain transfer agent ŽCTA. and the temperature, respectively. It is mainly still a report on experimental data, as a detailed mechanistic discussion is, alas, not possible at the moment. However, the acquisition of this kind of data will be useful for a better understanding of heterophase polymerizations. 2. Experimental section 2.1. Materials Styrene Ž STY . , M M A, 2,29-azobisiso butyronitrile ŽAIBN., SDS, dodecyl mercaptan, carbon tetrabromide, 1,3-diisopropenylbenzene ŽDIPB. all from Sigma-Aldrich, 2-ethyl hexyl methacrylate Ž2EHMA., butyl methacrylate ŽBMA. both from Huls ¨ AG, polyŽethylene glycol. ŽPEG. of different molecular weight, cerium ammonium nitrate ŽCAN., sodium hydrogen carbonate, and nitric acid Ž70%., all from FLUKA were used all except the monomers as received. The polystyrene-b-polyŽethylene glycol. stabilizer with a molecular weight of the STY block and the PEG block of 1000 g moly1 and 3000 g moly1 , respectively, SE1030, was a gift of the Th. Gold-
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schmidt AG, Essen, Germany. The polyŽethylethylene.-b-polyŽstyrenesulfonate . ionic block copolymer stabilizer with a degree of sulfonation of the polystyrene block of 52% Ž1-H52. was synthesized as described elsewhere w15x. The synthesis of symmetrical polyŽethylene glycol.-azo-initiators ŽPEGA where the number indicates the molecular weight of the PEG chains. was carried out according to w16x. De-ionized water was taken from a REWA HQ 5 system Ž18 MV cmy1 . and degassed prior to use. STY, MMA and BMA were distilled under reduced pressure to remove inhibitors and 2EHMA was purified by passing it through a basic activated Al 2 O 3 column. The monomers were stored in a refrigerator. Prior to use, the monomers were checked for oligomer formation by instilling a drop into an excess of methanol. Only oligomer-free monomers were used. 2.2. Emulsion polymerization Emulsion polymerizations were carried out batchwise in a reaction calorimeter RM2-S from ChemiSens ŽLund, Sweden. with a volume of 100 ml equipped with a stainless steel stirrer and a heating facility through the reactor bottom. The standard procedure was as follows: the water, the monomer, the stabilizer, in the case of the cerium ionrPEG redox system also 0.33 g of HNO3 and 3.5 g of PEG with a molecular weight of 10 000 g moly1 , and if necessary the chain transfer agent or the cross-linker were added to the reactor and heated up to reaction temperature. After the thermal equilibrium was reached the reactor was calibrated with an input of 3 W of electric power, and after re-equilibration the reaction was started by injecting the initiator dissolved in water. In several experiments we observed an induction period, which is reproducible with a standard deviation of 5%. The reaction was finished until no further heat development was detected by the calorimeter. 2.3. Latex and polymer characterization The particle size of the final latexes was analysed either by dynamic light scattering ŽDLS. with
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a Nicomp particle sizer ŽModel 370, PSS Santa Barbara, USA. at a fixed scattering angle of 908 or by enumeration of transmission electron microscopy ŽTEM. pictures. The polymer molecular weights were calculated from gel permeation chromatography ŽGPC. data. The GPC analysis was performed on a P1000 pump with a UV1000 detector Ž l s 260 nm. both from Thermo Separation Products with 5-mm 8 = ˚ from 300 mm SDV columns with 10 6 , 10 5, 10 3 A Polymer Standard Service in THF with a flow rate of 1 ml miny1 at 308C. The molecular weights were calculated with a calibration relative to PS standards. 3. Results 3.1. Kind of monomer As it has been known for a long time that the water solubility of the monomers has a strong influence on emulsion polymerization kinetics w4,6x, one might also expect differences in the corresponding RRPs. The results depicted in Fig. 2 show that this is really the case. BMA and STY differ slightly in water solubility, whereas STY has a slightly higher water solubility than BMA w17,18x. Curve 1 of Fig. 2a and curve 2 of Fig. 2b represent RRPs for almost the same polymerization conditions. The different water solubility is very likely the reason for the faster rate during interval I in the case of BMA compared to STY, as a lower water solubility lead to an earlier nucleation and subsequently to a higher polymerization rate compared to a not yet nucleated system. Furthermore, the results in Fig. 2 prove both the influence of the polymerization temperature and of the surfactant concentration on the resulting rate profiles. Curve 1 of Fig. 2b corresponds to a SDS concentration well above the CMC, whereas curve 4 corresponds to an SDS concentration well below the CMC. A direct comparison with the data published in Delarosa et al. w11x is not possible as the initiators are different. Fig. 2 describes the case of a non-ionic watersoluble initiator leading to oligomeric radicals whereas the data in Delarosa et al. w11x were obtained with KPS leading to low molecular
146
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Fig. 2. RRPs of BMA Ža. and STY Žb. emulsion polymerization recipes: 80 g of water, 20 g of monomers, 400 mg PEGA200; Ža. 200 mg of SDS, varying temperature; Žb. 828C, varying amount of SDS.
weight sulfate ion radicals. However, the results of both investigations agree in the point that the SDS concentration has a strong influence on the rate profiles. 3.2. Initiator influence The importance of the kind of initiator for emulsion polymerization, as compared to that of the emulsifier, is still underestimated today. To prove the initiator influence, emulsion polymerizations of BMA were carried out also with KPS instead of PEGA200. The use of an ionic and low molecular weight initiator changes the RRP significantly Žcf. Figs. 3 and 4.. The most ostentatious and most interesting change occurs at low conversions during interval I. If KPS is used, a distinct rate maximum at conversions lower than 10% is observed Žcf. Fig. 3.. It is straightforward to conclude that this maximum is linked with the particle nucleation. Such a distinct maximum in the polymerization rate in connection with a maximum in the particle concentration was also observed during the emulsion polymerization of vinyl chloride with SDS and KPS w19x. In contrast, the non-ionic PEGA200 initiator does not lead to a distinct rate maximum during interval I. The results depicted in Figs. 3 and 4 reveal an initiator influence on the particle nucleation. It is not possible to decide whether the chemical structure
of the nucleating species ŽPEG vs. sulfate end group. or the increase in the ionic strength is responsible for the measured differences of the RRPs. Figs. 3 and 4 represent two sets of polymerizations with varying rates of initiation adjusted either by changing the initiator concentration or by changing the polymerization temperature. It is interesting to note that in the case of KPS as initiator the final particle size clearly increases with decreasing rate of initiation, whereas in the case of PEGA200 as initiator at temperatures above 708C the final particle size remains almost unchanged Žcf. Table 1.. These differences are in agreement with earlier published results on the temperature dependence of the particle concentration for different initiator]emulsifier systems in STY emulsion polymerization w20x. Table 1 Dependence of the final particle size on the rate of initiation for a BMA emulsion polymerization with different initiators Polymerization condition
Di Žnm.a
KPS s 50 mg; T s 708C KPS s 100 mg; T s 708C KPS s 200 mg; T s 708C
109.2 91.9 78.9
PEGA200 s 400 mg; T s 608C PEGA200 s 400 mg; T s 708C PEGA200 s 400 mg; T s 808C
108.1 91.2 91.4
a
Di intensity weighted diameter ŽDLS..
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Fig. 3. Influence of the KPS concentration on the RRPs of a BMA emulsion polymerization recipe: 80 g of water, 20 g of BMA, 200 mg of SDS, 708C; Ža. heat flow]time plot; Žb. heat flow]conversion plot.
3.3. Stabilizer influence The kind and concentration of the stabilizer used in a heterophase polymerization determines the particle size and hence the particle concentration. According to Eq. Ž1., one might expect a strong influence of the chemical structure of the stabilizer on the RRPs. It is to note that n strongly depends on both the particle size and the particle concentration. Fig. 5 compares the influence of SDS with that of a polymeric stabilizer Ž1-H52.. The polymeric stabilizer 1-H52 has, up to concentrations of approx. 5 wt.% relative to monomer, almost no influence on the RRP Žcf. Fig. 5b., whereas the SDS concentration has a strong influence on the resulting rate profiles Žcf. Fig. 5a..
This behaviour can be attributed to differences in the particle size stabilizer concentration dependence for both systems Žcf. Table 2.. In the case of SDS the particle size decreases with increasing stabilizer concentration, whereas for 1-H52 the DLS data suggest an unusual stabilizer concentration dependence w15x. However, for 1-H52 latexes the TEM data reveal that at stabilizer concentrations between 1 and relative to 10 wt.% relative to monomer the hard core particle diameters are almost constant. The difference between TEM and DLS data Žcf. Table 2. is attributed to the chemical structure of the polymeric stabilizer that allows an interparticular adsorption at higher concentrations, leading to a limited aggregation and consequently to higher DLS values w15x. In
Fig. 4. Influence of the polymerization temperature on the RRPs of a BMA emulsion polymerization recipe: 80 g of water, 20 g of BMA, 200 mg of SDS, 400 mg of PEGA200; Ža. heat flow]time plot; Žb. heat flow]conversion plot.
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Table 2 Dependence of average particle diameters on stabilizer concentration Stabilizer Ž%. a
Di Žnm. b
DN Žnm. c
SDS 0.2 SDS 0.7 SDS 1.0 SDS 1.7
227.0 99.7 71.7 62.6
] ] 68.2 ]
1-H52 0.4 1-H52 1.0 1-H52 5.1
94.2 78.5 129.3
] 52.7 55.0
a
Amount of stabilizer relative to monomer. Intensity weighted diameter from DLS. c Number average diameter from TEM. b
the case of the SDS latexes the diameters determined with DLS and TEM agree within the experimental errors. 3.4. Influence of chain transfer agents The control of the molecular weight distribution of polymer dispersions is crucial for many technical applications. A CTA influences all three intervals of an emulsion polymerization as chain transfer events lead: Ž1. to more hydrophobic end groups thus influencing particle nucleation; Ž2. to a higher concentration of small radicals in the particles, thus leading to a lower n due to en-
hanced exit; and Ž3. to a lower viscosity inside the particles, thus influencing the gel effect. Figs. 6 and 7 summarize the results for dodecyl mercaptan and for carbon tetrabromide as CTA, respectively. Especially the heat flow]time curves Žcf. Fig. 6a and Fig. 7a. show that in the presence of a CTA the reaction starts earlier. In the case of dodecyl mercaptan, a dependence on the amount of CTA is still visible, whereas in the case of the more hydrophobic carbon tetrabromide, the calorimeter is unable to distinguish between 1 and 2 mol.% CTA. A possible explanation is an earlier nucleation due to an enhanced hydrophobicity of the oligomers born after a change transfer event. Another common feature of both CTAs is that the RRPs show a broader region of a high polymerization rate with increasing CTA concentration. This is a consequence of the lower molecular weights and hence a lower viscosity inside the particles with increasing CTA concentration, rather than of larger particle sizes as the average particle diameters show only for carbon tetrabromide a weak dependence on the CTA concentration Žcf. Table 3.. 3.5. Polymeric radicals with stabilizing properties The use of water-soluble polymeric radicals to start heterophase polymerizations of hydrophobic monomers leads to the formation of particles
Fig. 5. Influence of the stabilizer properties on the RRP of an STY emulsion polymerization recipe: 80 g of water, 20 g of STY, 400 mg PEGA200, 828C; varying amount of SDS; Žb. varying amount of 1-H52.
K. Tauer et al. r Colloids Surfaces A: Physiochem. Eng. Aspects 153 (1999) 143]151 Table 3 Influence of the kind and the concentration of CTA on polymer molecular weights and particle sizes of polyŽ2EHMA. dispersions CTA
MN Žg moly1 .a
Di Žnm.b
Without CTA 1 mol.% dodecyl mercaptan 2 mol.% dodecyl mercaptan 1 mol.% carbon tetrabromide 2 mol.% carbon tetrabromide
69.000 25.000 25.000 13.000 8.600
99.1 104.6 99.8 103.6 105.2
a b
MN , number average molecular weight. Di , intensity weighted diameter ŽDLS..
consisting of amphiphilic block copolymers. These particles possess some interesting properties with respect to particle morphology and redispersibility depending on the particular chemical structure
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of the block copolymers w21,22x. Furthermore, the application of polymeric radicals leads to RRPs with a very pronounced gel effect as a typical pattern. The results collected in Fig. 8 show this for an MMA polymerization started with the redox system PEG10000rceric ions Žcurve 1. and for two STY polymerizations started with PEGA2000 and PEGA200 Žcurve 2 and 3., respectively. The heat flow]time plots Žcf. Fig. 8a. reveal for both STY polymerizations the influence of the primary radical chain length on the duration of the polymerization. The longer the primary radical chain length, the slower the overall polymerization kinetics. In contrast, the heat flow]conversion plots Žcf. Fig. 8b. emphasize the common features and both curves look very similar, indicating the same mechanism.
Fig. 6. Influence of dodecyl mercaptan on the RRP of a 2EHMA emulsion polymerization recipe: 80 g of water, 20 g of 2EHMA, 200 mg of SDS, 100 mg of KPS, 708C; concentration of CTA related to monomer; Ža. heat flow]time plots; Žb. heat flow]conversion plots.
Fig. 7. Influence of carbon tetrabromide on the RRP of a 2EHMA emulsion polymerization recipe: 80 g of water, 20 g of 2EHMA, 200 mg of SDS, 100 mg of KPS, 708C; concentration of CTA related to monomer; Ža. heat flow]time plots; Žb. heat flow]conversion plots.
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Fig. 8. Influence of different kinds of polymeric radicals on RRPs of heterophase polymerizations recipes: Ž1. 100 g of water, 5 g of MMA, 3.5 g of PEG10000, 0.33 g of HNO3 , 3.278 g of CAN, 558C; Ž2. 60 g of water, 7.8 g of STY, 9.14 g of PEGA2000, 828C; Ž3. 60 g of water, 7.8 g of STY, 0.471 g of PEGA200, 828C, 2.24 g of SE1030; Ža. heat flow]time plots; Žb. heat flow]conversion plots.
3.6. Polymerization temperature
4. Conclusions
The influence of the polymerization temperature on the RRPs should be very complex, as the polymerization mechanism comprises a lot of partial reactions and each partial reaction has a unique temperature dependence. The results in Figs. 4 and 9 show how strongly a temperature difference of approx. 108 can alter the RRPs. The changes are especially visible in the heat flow]time plots whereas the heat flow]conversion plots have almost a similar shape. The reasons for these drastic changes are not clear. Differences in the particle size are obviously not responsible, as in both cases almost the same particle diameters of approx. 100 nm were estimated with DLS for all temperatures.
The evaluation of heterophase polymerizations is either possible by means of heat flow]time or heat flow]conversion plots. The heat flow]time plots are suited for a direct comparison of the kinetics, whereas the heat flow]conversion plots are more suited for comparison of general features of heterophase polymerizations under different conditions. Eq. Ž2. describes a relation between the heat flow]time curve and the amount of monomer used Ž m M . where MM is the molecular weight of the monomer and D HR is the reaction enthalpy. ts`
Hts0
HFdt s D HR ? Ž m M rMM .
Ž2.
Fig. 9. Influence of the reaction temperature on RRPs of STY emulsion polymerization recipe: 60 g of water, 7.8 g of STY, 0.465 g of PEGA200, 2.24 g of SE1030, 0.180 g of DIPB, varying temperature; Ža. heat flow]time plots; Žb. heat flow]conversion plots.
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Furthermore, Eq. Ž2. is a link between reaction calorimetry and other methods to follow the polymerization conversion. The validity of Eq. Ž2. is a prerequisite for the calculation of conversionbased RRPs and, hence, is very important for the comparison and systematic investigations of heterophase polymerizations. Eq. Ž2. has been checked for all the monomers used in this contribution with D HR values published in Moritz w7x. It was found that the accuracy of the calculated conversion data with the given equipment and experimental procedure is approx. 5%. The RRP of an emulsion polymerization contains, at least theoretically, all information about the course of the reaction, provided that the reaction calorimeter is sensitive enough. With the RM-2S reaction calorimeter, a heat flow of around 0.5 W is still reproducibly detectable that corresponds to an instantaneous monomer conversion rate of approx. 7 = 10y6 mol sy1 and 9 = 10y6 mol sy1 for STY and MMA, respectively w7x. The evaluation of RRPs is complicated as: Ž1. most of the information is hidden and not so clearly visible as, for instance, gel peaks; and Ž2. today there is still a lack of reaction rate profiles of heterophase polymerizations, with respect to a theoretical prediction or interpretation. It is of note that a detailed recalculation of RRPs is only possible with sophisticated models considering the colloid chemical as well as the kinetic features of heterophase polymerizations. More systematic investigations with reaction calorimetry alone or in combination with on-line turbidity or FT-IR measurements promise for the future a better understanding of heterophase polymerizations. Acknowledgements The authors thank the Federal Ministry of Education and Research of Germany ŽBMBF. for providing scholarships to H.M. and L.R. through grant 03 D 0039 E 0 as well as to C.S. through
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grant 03 D 0044 A 2 and the Max-Planck-Society for partial support of the project. The authors thank Dr J. Hartmann and Mrs R. Pitschke for preparing the TEM micrographs. References w1x G. Markert, Angew. Makromol. Chem. 123r124 Ž1984. 285. w2x R.G.W. Norrish, R.R. Smith, Nature 150 Ž1942. 336. w3x E. Trommsdorff, H. Kohle, P. Lagally, Makromol. Chem. ¨ 1 Ž1948. 169. w4x W.D. Harkins, J. Am. Chem. Soc. 69 Ž1947. 1428. w5x W.V. Smith, R.H. Ewart, J. Chem. Phys. 16 Ž1948. 592. w6x H. Gerrens, DECHEMA Monogr. 49 Ž1964. 53. w7x H.U. Moritz, in: K.H. Reichert, W. Geiseler ŽEds.., Polymer Reaction Engineering, VCH Verlag, Weinheim, 1989, p. 248. w8x C.M. Miller, P.J. Blythe, E.D. Sudal, C.A. Silebi, M.S. El-Aasser, J. Polym. Sci. Part A } Polym. Chem. 32 Ž1994. 2365. w9x C.M. Miller, E.D. Sudol, C.A. Silebi, M.S. El-Aasser, Macromolecules 28 Ž1995. 2754. w10x S. Fengler, K.-H. Reichert, Angew. Makromol. Chem. 225 Ž1995. 139. w11x L.V. Delarosa, E.D. Sudol, M.S. El-Aasser, A. Klein, J. Polym. Sci. Part A } Polym. Chem. 34 Ž1996. 461. w12x I.S. Deburuaga, M. Arotcarena, P.D. Armitage, L.M. Gugliotta, J.R. Leiza, J.M. Asua, Chem. Eng. Sci. 51 Ž1996. 2781. w13x C.M. Miller, P.A. Clay, R.G. Gilbert, M.S. El-Aasser, J. Polym. Sci. Part A } Polym. Chem. 36 Ž1997. 989]1006. w14x W.D. Hergeth, in: J.M. Asua, Polymeric Dispersion: Principles and Applications, Kluwer, Dodrecht, 1997, p. 267. w15x H. Muller, W. Leube, K. Tauer, S. Forster, M. Antoni¨ ¨ etti, Macromolecules 30 Ž1997. 2288. w16x R. Walz, B. Bohmer, W. Heitz, Makromol. Chem. 178 ¨ Ž1977. 2527. w17x R.S. Corley, in: E.R. Blout, H. Mark ŽEds.., Monomers, Interscience, New York, 1951. w18x A.L. Ward, W.J. Roberts, in: E.R. Blout, H. Mark ŽEds.., Monomers, Interscience, New York, 1951. w19x K. Tauer, B.-R. Paulke, I. Muller, W. Jaeger, G. ¨ Reinisch, Acta Polym. 33 Ž1982. 287. w20x K. Tauer, S. Kosmella, Polym. Int. 30 Ž1993. 253. w21x K. Tauer, Polym. Adv. Technol. 6 Ž1995. 435. w22x K. Tauer, M. Antonietti, L. Rosengarten, H. Muller, ¨ submitted to Macromol. Chem. Phys., 1997.