TEMPO-controlled free radical suspension polymerization

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1 62 Die Angewane Makromolekulare Chemie 265 (1999) (Nr. 4629) TEMPO-controlled free radical suspension polymerization Gudrun Schmi-Naake*, Marco Drache, Carsten Taube Institut für Technische Chemie, TU Clausthal, Erzstr. 18, Clausthal-Zellerfeld, Germany (Received 15 October 1998) SUMMARY: The free radical polymerization controlled by 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) was carried out in a water/oil suspension at C. Suspension polymerization is a polymerization reaction with high conversion and allows high-molecular-weight polymers to be synthesized. The polymerization was started by a styrene macroinitiator. The block length is determined by the level of conversion and the initial macroinitiator concentration. A kinetic model is introduced which predicts a drastic reduction of polymerization time (at ca. 90% conversion a reduction from ca. 20 h to 5 3 h) and improved polydispersity through selective addition of initiator. The agreements between the simulated results and the experimental findings are surprisingly good. From model calculation, the percentage and molar mass distributions of the dead polymers can also be derived. ZUSAMMENFASSUNG: Die von 2,2,6,6-Tetramethylpiperidin-N-oxyl (TEMPO) kontrollierte radikalische Polymerisation von Styrol wurde in einer Wasser/Öl-Suspension bei C durchgeführt. Die Suspensionspolymerisation ist eine Polymerisationstechnik für hohe Umsätze und gestattet, Polymere mit hohen Molmassen zu synthetisieren. Die Polymerisation wurde mit einem Styrol-Makroinitiator gestartet. Die Blocklänge wurde durch den Umsatz und die eingesetzte Makroinitiatorkonzentration bestimmt. Mit Modellberechnungen können Rezepturen postuliert werden, die bei gezielter Initiatorzugabe zu einer drastischen Reduzierung der Polymerisationszeit (bei ca. 90% Umsatz von ca. 20 h auf 5 bis 3 h) bei Verringerung der Polydispersität führen. Die Übereinstimmung der simulierten Resultate mit den Experimenten ist überraschend gut. Aus den Modellrechnungen lassen sich auch Anteil und Molmassenverteilungen der toten Polymeren ableiten. Introduction The controlled radical polymerization is at present of exceptional interest 1 4). The advantage of the radically initiated polymerization lies in its easy and economical handling, i. e. the demands on the purity of substances and the sensitivity to water and oxygen are considerably lower as by an anionic polymerization. For the polymerization, an initiator (e.g. dibenzoyl peroxide (BPO)) and a terminator (e.g. N-oxyls, N*, in this publication 2,2,6,6-tetramethyl-piperidine-N-oxyl (TEM- PO)) are employed, hence two different radicals are formed that differ strongly in their reactivities. The more reactive radical starts the polymerization with monomers (e.g. styrene) while the less reactive one (N*) can only react by recombination with a reactive radical (the growing polymer chain). This reaction is reversible at sufficiently high temperatures and the free radicals reform, i. e. the polymer chain P* is then able to grow, thereby consuming the available monomers. * Correspondence author. k c P* + N* agggg ggggs k d PN (1) K=k d /k c (2) The adduct is a dormant species, k d and k c being the rate constants of dissociation and combination, respectively. The controlled radical polymerization is characterized by a linear increase of molar mass with conversion and a low polydispersity, Pd = M w /M n, where M n and M w are number and weight-average molar masses, respectively. The reaction product is a macroinitiator (P n N), which, by conversion with another monomer without any further initiator, forms a block copolymer. The polymerization of the adduct PN is started by thermal polymerization according to Fukuda 5) and chain propagation is controlled by the equilibrium reaction shown in Eq. (1). Consequently, the precondition for a successful, controlled radical polymerization is the thermal sensitivity to initiation of the used monomers and the dissociation of the PN adducts, i. e. low bond enthalpy of the N-oxyl/polymer chain bond. In this way, it is possible to synthesize a new type of block copolymers, for example by connecting homopolymer with copolymer blocks 6 8). Every thought on a technical realization of this type of reaction requires knowledge about the polymerization up to high conversions and a control on the block lengths. Until now, most of the reported N-oxyl-controlled radical polymerizations were carried out as bulk polymerization, Die Angewane Makromolekulare Chemie 265 i WILEY-VCH Verlag GmbH, D Weinheim /99/ $ /0

2 TEMPO-controlled free radical suspension polymerization 63 which is getting problematic with increasing viscosity, especially if the first block has a high molar mass. Suspension polymerization is a polymerization technique for high conversion that allows polymers with high molar masses to be synthesized. Works on the synthesis of polystyrene latex via controlled radical polymerization have recently been reported, for example, the TEMPO-controlled radical polymerization in non-aqueous (n-decane) dispersions at 1358C 9), as well as that in alcoholic and aqueous alcoholic media using poly(n-vinylpyrrolidone) as a steric stabilizer at C 10). In the present work, we report on the controlled radical polymerization in a water/oil suspension initiated with a styrene macroinitiator (this styrene macroinitiator was previously terminated by TEMPO). The bead polymerization, which corresponds to a water-cooled bulk polymerization, takes care of the viscosity problem at high conversions and high molar masses. The molar masses of the employed terminated macroinitiators can be chosen freely. This experimental study was accompanied by model calculations, which also determined the choice of experiments to be carried out. Experimental Polystyrene macroinitiator (PN) synthesis For the synthesis of the lower-molecular-weight PN sample, the reactor was charged with styrene and TEMPO (15 mmol N L 1 ). After 30 min under gentle nitrogen purge, 10 mmol N L 1 of BPO was added and the reaction mixture was adjusted at 958C for 1 h to ensure a complete BPO decomposition. Subsequently the temperature was raised to 125 8C, thus starting the polymerization (time zero). After 390 min (43.5% conversion) the product was removed. Size exclusion chromatography (SEC) (see below) detected a M n value of g N mol 1 and a polydispersity (M w /M n ) of Suspension polymerization The suspension, which consists of 700 ml of boiling deionized water and 900 mg (12.5 mmmol) of poly(vinylalcohol), 60 mg (0.208 mmol) of sodium dodecylsulfate, 3 g (21 mmol) of sodium sulfate and 750 mg (2.636 mmol) of stearic acid, was boiled for 5 min. The styrene macroinitiator and, if necessary, the initiator (dicumyl peroxide, DCP) were dissolved in styrene and poured into the suspension solution which was tempered at 508C (30 ml styrene solution for 100 ml suspension solution). The reaction proceeded discontinuously in a 200 ml glass autoclave equipped with a six-bladed turbine stirrer. After stirring at 350 rpm for 30 min under nitrogen flush, the stirring speed was accelerated to 500 and 1000 rpm for experiments without initiator and in the presence of DCP, respectively, and the solution was then brought to the polymerization temperature with maximum heating rate. After the polymerization, the product was poured into a 5 10 wt.-% poly(vinylalcohol) aqueous solution to avoid agglomeration of the beads, and finally washed with warm methanol solution and dried in a vacuum oven. For the determination of molar mass distribution, the polymerisate dissolved in toluene and precipitated in methanol. Product characterization Molar masses and molar mass distributions were determined by SEC using a Waters 510 gel chromatograph equipped with a differential refractometer and Waters/Styragel HR3 and HR4 columns in series. Measurements were carried out with tetrahydrofuran as eluent; polystyrene standards were used for calibration. All scanning electron microscopic experiments were performed using a Zeiss Gemini DSM 982 operated at 10 kv. The sputter instrument was a Balzer MED 020. The polymer balls were stuck to the conduct patch and framed by a conduct carbon paste. Chromium was used as the target. All samples were covered with a 30 nm coat of Cr. Reaction model In the model presented in this work the following reactions are considered: thermal start, initiator decomposition, chain start (initiation), reversible termination of growing chains, propagation of living chains, transfer to monomers and the irreversible decomposition of the adduct PN. Autopolymerization of styrene: 3M ggggs 2 P 1 k th M 3 (3) Initiator decomposition and initiation of chains: I ggggs 2 I k 1 N I (4) I M ggggs P 1 2k 1 N f N I (5) P 1 N Propagation: agggg ggggs P N k 1 c N P N 1 N k d P 1 N (6) M ggggs 1 k p N N M (7) N agggg ggggs P j N k c N N N k d P j N (8) Transfer to monomers: M ggggs D P 1 k m N N M (9) Termination by combination: P k ggggs D k tc N N P k (10) Irreversible decomposition of PN 11, 12) : k decomp N PN (11)

3 64 G. Schmi-Naake, M. Drache, C. Taube The thermal initiation R th of styrene and the gel effect were described by Hamielec 13). I is the initiator, I* are initiator radicals, N* is the terminator used (N-oxyl), PN is the growing chain previously terminated by N-oxyl ( dormant chain ), P j * and P k * denote polymer radicals of chain lengths j and k, respectively. M is the monomer (styrene), D represents the dead macromolecules. For styrene polymerization started by the adduct PN, the balance equation system can be formulated as follows (brackets [] indicating concentrations have been omitted): di ˆ k 1 N I 12 dm ˆ 2k 1 N f N I k p k m M N P 6 N k th N M 3 with : P ˆ Xv jˆ1 13 Fig. 1. Conversion-time dependence of suspension polymerization of styrene with 4 mmol N L 1 styrene macroinitiator, c = 1228C, [styrene] = 8.73 mol N L 1. Tab. 1. Model constants of styrene. dp 1 d dp ˆ 2k 1 N f N I R th k p N N M k d k c P 1 N N k tc P N 1 P 2 N k th N M 3 14 ˆ k p k m N M k p N P N M k j 1 tc N N P k d k c P N 1 N 15 ˆ 2k 1 N f N I R th k tc N P 2 k d N PN k c N P N N k I (s 1 ) N exp ( 19030/T) 15) k p (L mol 1 s 1 ) N 10 7 exp ( 4100/T) 14) k trm0 (L mol 1 s 1 ) 2.31 N 10 6 exp( 6377/T) 13) k tc0 (L mol 1 s 1 ) N 10 9 exp ( 844/T) 13) k d (s 1 ) 2.0 N exp ( 14940/T) 12) k decomp (L mol 1 s 1 ) 5.8 N exp ( 18430/T) 12) k c (L mol 1 s 1 ) 7.11 N 10 6 exp ( 824/T) k th (L mol 1 s 1 ) 4.3 N 10 5 exp ( 14240/T) 13) Model constants by Hamielec 13) : k tc = k tc0 exp[ 2(A 1 X + A 2 X 2 + A 3 X 3 )] mit A 1 = N 10 3 T; A 2 = N 10 2 T; A 3 = N 10 3 T. k trm = k trm0 bx mit b = N 10 3 exp[( T)/202.5] dd dn 2 N k th N M 3 16 ˆ k m N P N M 0; 5k tc P 2 k decomp N PN ˆ k d N PN k c N P N N dpn ˆ k c N P N N k d N PN k decomp N PN 19 To solve these model equations, the program PREDICI was used. Results and discussion TEMPO-terminated polystyrene ([PN] = 4 mmol N L 1 with M n = g N mol 1, Pd = 1.16) was dissolved in styrene and polymerized in a water/oil suspension. Since the dissociation of the terminated styrene macroinitiator (PN e P* + N*) proceeds at temperatures higher than 110 8C, the polymerization temperature is always above the glass transition temperature of polystyrene (T g = C). Fig. 1 shows the conversion(x)-time(t) behavior of the suspension polymerization of styrene at 122 8C. Every single experimental point corresponds to an autoclave experiment carried out up to the specified period of time. The solid line indicates the X-t-dependence calculated with the proposed model. The parameters required for the model calculation are listed in Tab. 1. Number and weight-average molar masses of polymerization products taken at different levels of conversion show a linear increase with increasing level of conversion, indicating that the course of the polymerization in suspension was controlled. The solid lines correspond to the simulated results. Polymerization products with average molar masses higher than g N mol 1 and polydispersities lower than 1.5 were obtained. Fig. 3 shows the experimentally

4 TEMPO-controlled free radical suspension polymerization 65 Fig. 2. Dependence of M n (f) and M w (g) on conversion in the suspension polymerization of styrene with 4 mmol N L 1 styrene macroinitiator, c = 1228C, [styrene] = 8.73 mol N L 1, in comparison with calculated data (11). Fig. 4. Comparison of (a) experimental and (b) calculated molar mass distribution after 6.5 h at 41% conversion in suspension polymerization of styrene with 4 mmol N L 1 styrene macroinitiator (c), c = 1228C, [styrene] = 8.73 mol N L 1, part of dead polymer (black): 27%. Fig. 3. Experimentally determined molar mass distribution with respect to conversion (16%, 30%, 41%, 77% (11) with increasing line wih, peaks from left to right) in the suspension polymerization of styrene with 4 mmol N L 1 styrene macroinitiator (- - -), c = 1228C, [styrene] = 8.73 mol N L 1. determined molar mass distributions of the products with increasing conversion, starting from the macroinitiator. With increasing reaction time (up to 20 h) the polydispersity (Pd) of the distribution is also increased, which is caused by the increasing extent of occurring side reactions, such as irreversible decomposition of the terminated chains (see Eq. (11)). In Fig. 4 and Fig. 5 the molar mass distributions at 41% and 72% conversion, respectively, obtained from model calculations are compared with those obtained experimentally. The calculated molar mass distributions (Fig. 4, Fig. 5), the X-t behavior (Fig. 1), as well as the dependence of M w and M n on conversion (Fig. 2) show that the simulation of the controlled polymerization of the macroinitiators can well describe the experimental data. The percentage of dead polymers is calculated to be 27% Fig. 5. Comparison of experimental (a) and (b) calculated molar mass distribution after 20 h at 72% conversion in suspension polymerization of styrene with 4 mmol N L 1 styrene macroinitiator (c), c = 1228C, [styrene] = 8.73 mol N L 1, part of dead polymer (black): 36%. at 41% conversion after 6.5 h, and 36% at 72% conversion after ca. 20 h. The termination of the living chains or the adduct PN can proceed through combination reaction (Eq. (10)), transfer to monomers (Eq. (9)) and irreversible thermal decomposition of PN (Eq. (11)). The part of dead polymers increases with increasing reaction time. Fig. 6 shows the particle size distribution and Fig. 7 shows the SEM image of the suspension polymerization product (X = 72%). Fig. 8 shows beads with crater-like contact areas and those sticking together resulting from an unfavorable stirring rate, which can occur especially in the polymerization above the glass transition temperature. As a further proof of the occurrence of controlled radical polymerization under the condition of the suspension

66 G. Schmi-Naake, M. Drache, C. Taube Fig. 6. Particle size distribution of suspension polymerization of styrene with 0.004 mol N L 1 styrene macroinitiator, c = 1228C, [styrene] = 8.

73 mol N L 1. Fig. 7. Scanning electron micrograph of suspension polymerization beads of styrene with 4 mmol N L 1 styrene macroinitiator, c = 1228C, [styrene] = 8.73 mol N L 1, X = 72%.

25) was isolated and, as a macroinitiator, used to initiate the bulk polymerization of styrene at 1208C. The low conversion was necessary to regulate the viscosity during the bulk polymerization.

32 show that the macroinitiator produced through suspension polymerization is an adduct (PN) reversibly terminated by TEMPO, which above the dissociation temperature can be further chain-extended by

5 66 G. Schmi-Naake, M. Drache, C. Taube Fig. 6. Particle size distribution of suspension polymerization of styrene with mol N L 1 styrene macroinitiator, c = 1228C, [styrene] = 8.73 mol N L 1, X = 72%. Fig. 8. Scanning electron micrograph of suspension polymerization product with unfavorable stirring rate, 4 mmol N L 1 styrene macroinitiator, c = 1228C, [styrene] = 8.73 mol N L 1. Fig. 7. Scanning electron micrograph of suspension polymerization beads of styrene with 4 mmol N L 1 styrene macroinitiator, c = 1228C, [styrene] = 8.73 mol N L 1, X = 72%. polymerization, a polystyrene sample obtained at low conversion (X = 15%, M n = g N mol 1, Pd = 1.25) was isolated and, as a macroinitiator, used to initiate the bulk polymerization of styrene at 1208C. The low conversion was necessary to regulate the viscosity during the bulk polymerization. The linear dependences of X-t (Fig. 9) and M n X (Fig. 10), as well as the low polydispersity of show that the macroinitiator produced through suspension polymerization is an adduct (PN) reversibly terminated by TEMPO, which above the dissociation temperature can be further chain-extended by controlled polymerization. The polymerization rates in Fig. 1 (6.5% N h 1 ) and in Fig. 8 (6.6% N h 1 ) correspond to the rate of thermal polymerization of styrene. In a controlled polymerization with a macroinitiator, the stationary radical concentration is determined by the thermal autopolymerization of the monomers used 12). An increase in the rate of polymerization is necessary because such long polymerization times Fig. 9. Conversion-time behavior of bulk polymerization of a terminated suspension polymerization product (5 mmol N L 1 ), M n = g N mol 1, c = 1208C, [styrene] = 8.73 mol N L 1. are impractical and increase the part of dead polymers (see Eq. (11)). With the help of model calculations it can be postulated that in a controlled radical polymerization it is possible with the use of 10 mmol N L 1 styrene macroinitiator and addition of 5 mmol N L 1 dicumyl peroxide (DCP) (k I = exp( 19030/T) s 1 ) 15) at 1258C to reach 90% conversion in ca. 5.5 h, and with an addition of 3 mmol N L 1 DCP at 1358C to reach 80% conversion in ca. 3 h. The experimental results confirm the prediction made using model calculations. Through addition of an initiator which decomposes at the polymerization temperature, the stationary radical concentration responsible for the rate of polymerization is increased. Fig. 11 shows the molar mass distributions of the initial polystyrene macroinitiator ([PN] = 10 mmol N L 1 with

6 TEMPO-controlled free radical suspension polymerization 67 Fig. 10. Dependence of M n (f) and M w (g) on conversion in the bulk polymerization of a terminated suspension polymerization product (5 mmol N L 1 ), M n = g N mol 1, c = 1208C, [styrene] = 8.73 mol N L 1. Fig. 12. Comparison of (a) experimental and (b) calculated molar mass distribution at 75% conversion in suspension polymerization of styrene with 10 mmol N L 1 styrene macroinitiator (c) and initiator DCP; [DCP] = 3 mmol N L 1, c = 1358C, [styrene] = 8.73 mol N L 1, part of dead polymer (black) 34%, Pd = r R P th 2fk 1 I Š stat ˆ k tc 20 Fig. 11. Comparison of (a) experimental and (b) calculated molar mass distribution at 90% conversion in suspension polymerization of styrene with 10 mmol N L 1 styrene macroinitiator (c) and initiator DCP; [DCP] = 5 mmol N L 1, c = 1258C, [styrene] = 8.73 mol N L 1, part of dead polymer (black) 32%, Pd = M n = g N mol 1, Pd = 1.16), of the polymerization product after 350 min of reaction (X = 90%) at 1258C with added initiator DCP ([DCP] = 5 mmol N L 1 ), and of the polymerization product after 150 min of reaction (X = 75%) at 1358C with added initiator DCP ([DCP] = 3 mmol N L 1 ) (Fig. 12). The calculated molar mass distributions are in surprisingly good agreement with those obtained experimentally. The percentages of dead polymers determined through model calculations are 32% at c = 1258C and 90% conversion, and 34% at c = 1358C and 75% conversion. The initiator increases the stationary radical concentration [P*] stat (see Eq. (20)) and increases, therefore, the rate of polymerization drastically. with R th ˆ 2 k th M 3 The polymerization proceeds further controlled with Pd = 1.35 at conversions of 75 and 90%, respectively. Through selective and controlled addition of initiators, planned using the model calculations, the polymerization time required for high conversion can be strongly shortened. The part of dead polymers can be lowered through a shortening of the reaction time as well as through an increase in the macroinitiator concentration [PN]. With increasing [PN] the stationary N-oxyl concentration [N*] stat also increases, the latter is responsible for the controlled radical polymerization. N Š stat ˆ k d PNŠ=k c P Š Stat 21 The macroinitiator concentration determines also the possible block length: M nexp ˆ M n0 MŠ 0 N X N MM= PNŠ DŠ P Š 22 with M n0 ˆ M n of macroinitiator, [M] 0 ˆ 8.73 mol N L 1 and molar mass of styrene MM styrene ˆ 104 g mol 1. With increasing macroinitiator concentration, the possible block length, i. e. the achievable growth of molar mass, decreases. At the same time, the part of dead polymers is reduced through the increase in stationary N-oxyl concentration (reduction of termination and transfer reactions), e. g. at a macroinitiator concentration of 20 mol N L 1 and an addition of 3 mmol N L 1 DCP at 1358C the part of dead polymers is decreased (ca. 20%).

7 68 G. Schmi-Naake, M. Drache, C. Taube In our next paper we will report on the synthesis of different styrene block copolymers prepared through suspension polymerization under optimum conditions. 1) G. Moad, E. Rizzardo, D. H. Solomon, Macromolecules 15 (1982) 909 2) M. K. Georges, R. P. N. Veregin, P. M. Kazmaier, G. K. Hamer, Macromolecules 26 (1993) ) E. E. Malmström, C. J. Hawker, Macromol. Chem. Phys. 199 (1998) 923 4) T. Otsu, A. Matsumoto, Adv. Polym. Sci. 136 (1998) 75 5) T. Fukuda, T. Terauchi, A. Goto, Y. Tsujii, T, Miyamoto, Y. Shimizu, Macromolecules 29 (1996) ) S. Butz, H. Baethge, G. Schmi-Naake, Macromol. Rapid Commun. 18 (1997) ) S. Butz, J. Hillermann, G. Schmi-Naake, J. Kressler, R. Thormann, B. Heck, B. Stühn, Acta Polym. 49 (1998) 693 8) H. Baethge, S. Butz, G. Schmi-Naake, Macromol. Rapid Commun. 18 (1997) 911 9) M. Hölderle, M. Baumert, R. Mülhaupt, Macromolecules 30 (1997) ) L. I. Gabaston, R. A. Jackson, S. P. Armes, Macromolecules 31 (1998) ) D. Greszta, K. Matyjaszewski, Macromolecules 29 (1996) ) T. Fukuda, T. Terauchi, A. Goto, Y. Tsujii, T. Miyamoto, Y. Shimizu, Macromolecules 29 (1996) ) W. Hui, A. E. Hamielec, J. Appl. Polym. Sci. 16 (1972) ) M. Buback, R. G. Gilbert, R. A. Hutchinson, B. Klumperman, F.-D. Kuchta, B. G. Manders, K. F. O Driscoll, G. T. Russell, J. Schweer, Macromol. Chem. Phys. 196 (1995) ) J. C. Masson, Polym. Handb., 3. ed. (1989), p. II/25

Controlled free radical polymerization of styrene initiated by a [BPO-polystyrene-(4-acetamido-TEMPO)] macroinitiator

Die Angewandte Makromolekulare Chemie 265 (1999) 69 74 (Nr. 4630) 69 Controlled free radical polymerization of styrene initiated by a [BPO-polystyrene-(4-acetamido-TEMPO)] macroinitiator Chang Hun Han,