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

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1 Die Angewandte Makromolekulare Chemie 265 (1999) (Nr. 4630) 69 Controlled free radical polymerization of styrene initiated by a [BPO-polystyrene-(4-acetamido-TEMPO)] macroinitiator Chang Hun Han, Sören Butz, Gudrun Schmidt-Naake* Institut für Technische Chemie, TU Clausthal, Erzstr. 18, Clausthal-Zellerfeld, Germany (Received 15 October 1998) SUMMARY: The bulk polymerization of styrene at 125 8C was studied using a [BPO-polystyrene-(4-acetamido-TEMPO)] macroinitiator synthesized by a styrene polymerization in the presence of 4-acetamido- 2,2,6,6-tetramethylpiperidine-N-oxyl (4-acetamido-TEMPO) and benzoyl peroxide (BPO). The rates of polymerization were independent of the initial macroinitiator concentration and they were very similar to that for the thermal autopolymerization of styrene. Additionally, different types of N-oxyls did not have any effect on the polymerization rate. The number-average molecular weights (M n ) of the obtained polymers agreed very well with theoretical predictions, deviations were observed only at low macroinitiator concentrations. Increasing macroinitiator concentrations resulted in lower magnitudes of the growing molecular weights and reduced polydispersities (M w /M n ) at the initial stage of the polymerization. The concentration of the polymer chains was calculated, and it was recognized that the concentration of polymer chains increased during the polymerization as a result of an additional radical formation due to the thermal self-initiation of styrene. This thermal self-initiation could be proved qualitatively by the addition of N-oxyl to a macroinitiator polymerization system. ZUSAMMENFASSUNG: Ausgehend von [BPO-Polystyrol-(4-Acetamido-TEMPO)]-Makroinitiatoren wurde die Substanzpolymerisation von Styrol bei 125 8C untersucht. Die Polymerisationsgeschwindigkeit war unabhängig von der eingesetzten Makroinitiator-Konzentration und stimmte im Rahmen der Meßgenauigkeit mit der der thermisch initiierten Autopolymerisation von Styrol überein. Bezüglich der Art des verwendeten N-Oxyls wurde kein Einfluß auf die Polymerisationsgeschwindigkeit festgestellt. Die mit Gelpermeationschromatographie (GPC) ermittelten Molmassen stimmten gut mit den theoretisch geschätzten Molmassen überein, Abweichungen ließen sich nur bei geringen Makroinitiator-Konzentrationen beobachten. Zunehmende Makroinitiator-Konzentrationen resultierten in einer Reduktion der Molmassenzunahme sowie der Polydispersität, wobei eine Beeinflussung der Polydispersität insbesondere in der Anfangsphase der Polymerisation beobachtet wurde. Durch Berechnung von Kettenkonzentrationen ließen sich eindeutig Radikal- Neubildungen durch die thermische Styrol-Selbstinitiierung feststellen. Zusätzlich wurde diese Selbstinitiierung durch spezielle Reaktionsführung mit N-Oxyl-Zudosierung qualitativ nachgewiesen. Introduction The controlled free radical polymerization is of particular academic and industrial interest. This type of polymerization does not require a high purity of the monomers and does not need special precautions to ensure anhydrous conditions. Moreover, it allows one to synthesize polymers with a narrow molecular weight distribution and block copolymers. The N-oxyl-controlled free radical polymerization is one of the most extensively studied methods to control a radical polymerization. The key to success of this polymerization is the reversible deactivation of the growing radicals (P*) by N-oxyls (N*) (Eq. (1)): P9 +N9 k c agggg ggggs k d P1N K=k d /k c (1) where k c and k d are the rate constants of combination and dissociation, respectively. The essentially simultaneous initiation and the small contribution of irreversible termination result in controlled molecular weights and narrow molecular weight distributions. N-oxyls can be employed in three different ways to control the radical polymerization. The first method is the combination of an N-oxyl with a traditional initiator such as benzoyl peroxide (BPO) 1 9). Radicals formed by the decomposition of the initiator initiate the polymerization and the N-oxyls terminate this growing radicals reversibly. The second method, alkoxyamines and polyalkoxyamines such as [polymer-(n-oxyl)] adducts which are synthesized by N-oxyl-controlled radical polymerization can be used as unimolecular initiators 10 19, 24 29). In this case, the initia- * Correspondence author. Die Angewandte Makromolekulare Chemie 265 i WILEY-VCH Verlag GmbH, D Weinheim /99/ $ /0

2 70 C. H. Han, S. Butz, G. Schmidt-Naake tors do not only act as an initiator but also as a terminator. The third method, when the monomers undergo a thermal autopolymerization, a controlled free radical polymerization is practicable in the absence of additionally added initiating systems, only the N-oxyl is needed for the control of the polymerization process 20 23, 30). The use of [polymer-(n-oxyl)] adducts in the N-oxylcontrolled radical polymerization is of special interest 16, 18, 26, 27) due to their ability to undergo a chain extension and to synthesize block copolymers 17, 19, 24, 25). Moreover, this system is advantageous for the investigation of kinetics since the [polymer-(n-oxyl)] adducts do not lead to any significant side reactions, for instance, for the BPO/TEMPO system 2) at the initiation stage. The [polymer-(n-oxyl)] adducts also provide a perfect balance between the scavenger (N-oxyl) and the polymer chains and eliminate any induction period. Fukuda et al. 16), for example, used [PS-(TEMPO)] adducts as a macroinitiator in the polymerization of styrene and reported their results in several kinetics works 16, 18, 26, 27). Besides TEMPO, other N-oxyls can be used in the controlled radical polymerization 3 6, 12, 13, 20, 25, 28 30), and the synthesized polymers can also be used as macroinitiators for a chain extension polymerization and for the synthesis of block copolymers. In our present work, 4-acetamido-2,2,6,6-tetramethylpiperidine-N-oxyl (4-acetamido-TEMPO) was used as a scavenger. The [BPO-PS-(4-acetamido-TEMPO)] adduct, synthesized through the polymerization of styrene initiated by BPO in the presence of 4-acetamido-TEMPO, was used as a macroinitiator in the polymerization of styrene. In addition, the results are compared with those obtained for the TEMPO system and are discussed in this paper. (TEMPO, Fluka) were used as received. Benzoyl peroxide (BPO, Merck) was purified by recrystallization from trichloromethane/methanol. Polymerization All polymerizations were performed in bulk and carried out in batch reactors under gentle nitrogen purge and with stirring throughout the reaction. The polymers were recovered as precipitants from an excess of methanol, purified, and dried up to weight constancy. Synthesis of the macroinitiators The reactor was charged with styrene ( g) and 4-acetamido-TEMPO (1.015 g). After 30 min under gentle nitrogen purge, BPO (0.678 g) was added and the reaction mixture was preheated for 1 h at 958C to ensure complete BPO decomposition 8). Subsequently, the temperature was quickly increased to 135 8C. The polymer was recovered from the reactor after 2 h. The synthesized polymer was found to have M n of 4900 (GPC) and polydispersity (M w /M n ) of Two [BPO-PS-TEMPO] macroinitiators were synthesized in analogy to the [BPO-PS-(4-acetamido-TEMPO)] macroinitiator. These polymers have M n 7000 (M w /M n = 1.16) and (M w /M n = 1.24), respectively. Polymerization of styrene with macroinitiator The reactor was charged with styrene (45.45 g) and various amounts of macroinitiator. After 30 min under gentle nitrogen purge, the reaction was started with a rapid temperature increase up to 1258C. Determination of molecular weight and molecular weight distribution Molecular weights and molecular weight distributions were estimated by gel permeation chromatography (GPC) equipped with two Nucleogel columns (GPC and 104 5, Macherey-Nagel) in series and with a RI-detector (Knauer). [BPO-PS-(4-acetamido-TEMPO)] macroinitiator Experimental part Materials Styrene (BASF) was distilled under reduced pressure. 4-acetamido-2,2,6,6-tetramethylpiperidine-N-oxyl (4-acetamido- TEMPO, Hüls) and 2,2,6,6-tetramethylpiperidine-N-oxyl Results and discussion To study the influence of the macroinitiator concentration on the polymerization process, five different macroinitiator concentrations were chosen, ranging from to mmol L 1. The time-conversion plots for the various macroinitiator concentrations are shown in Fig. 1. The rate of polymerization is independent of the [BPO- PS-(4-acetamido-TEMPO)] macroinitiator concentration and, therefore, at the same polymerization time, all conversions are nearly the same. Fig. 2 shows a comparison of time-conversion plots for the polymerizations initiated by [BPO-PS-(4-acetamido-

3 Controlled free radical polymerization of styrene 71 Fig. 1. Percent conversion as a function of time for the polymerization of styrene initiated by [BPO-PS-(4-acetamido- TEMPO)] macroinitiator (M n = 4900, M w /M n = 1.21) at 1258C; [styrene] 0 = 8,73 mol L 1, [macroinitiator] 0 : (g) 3 mmol L 1, (h) 5 mmol L 1, (f) 8 mmol L 1, (0) 10 mmol L 1. The dotted line shows the data obtained for the thermal autopolymerization of styrene at 1258C. Fig. 2. Comparison of time-conversion plots for the polymerization of styrene initiated by (h) [BPO-PS-(4-acetamido- TEMPO)] macroinitiator (M n = 4900, M w /M n = 1.21) and (0) [BPO-PS-TEMPO] macroinitiator (M n = 7000, M w /M n = 1.16), and the thermal autopolymerization of styrene at 125 8C; [styrene] 0 = 8,73 mol L 1, [macroinitiator] 0 = 8 mmol L 1 (dotted line). TEMPO)] and [BPO-PS-TEMPO] macroinitiators, and the thermal autopolymerization of styrene. All three rates of the polymerizations (R p ) are remarkably similar and they are independent of the type of N-oxyls. The important role of the thermal autopolymerization of styrene in the N-oxyl-controlled radical polymerization of styrene is indicated and this result agrees well with the hypothesis of Fukuda et al. 16, 19) described in Eq. (2): R p ˆ k p P9Š MŠ ˆ k2r 1=2 p i MŠ 2 k t where k p is the rate constant of propagation, [P9] is the concentration of the growing radicals, [M] is the concentration of the monomer, R i is the rate of initiation due to the initiator and/or the thermal autopolymerization of styrene and k t is the rate constant of termination. The GPC-determined number-average molecular weights (M n ) of the obtained polymers are plotted in Fig. 3 as a function of conversion. The molecular weights increase linearly with conversion and they are very similar to the theoretical predictions (M n, th ) defined by Eq. (3), within an experimental error (l10%). M n,th = (M n ) MI + ([St] 0 /[MI] 0 )6(MW) St 6Conversion (3) where (M n ) MI is the number-average molecular weight of the macroinitiator, [St] 0 is the initial concentration of styrene, [MI] 0 is the initial concentration of the macroinitiator and (MW) St is the molecular weight of styrene. At low macroinitiator concentrations, deviations of M n from M n, th are observed. This can be partly explained by Fig. 3. Dependence of the molecular weight (M n ) on the conversion for the polymerization of styrene initiated by [BPO-PS- (4-acetamido-TEMPO)] macroinitiator (M n = 4900, M w /M n = 1.21) at 1258C; [styrene] 0 = 8,73 mol L 1, [macroinitiator] 0 : (g) 3 mmol L 1, (h) 5 mmol L 1, (f) 8 mmol L 1, (0) 10 mmol L 1. The dotted lines present the theoretical predictions. the uncontrolled polymerization process due to the low N-oxyl concentration. In this case, the concentration of the N-oxyl is too low for an ideal, controlled free radical polymerization. At higher macroinitiator concentrations (A5 mmol L 1 ), the effects of the thermal self-initiation, the irreversible termination by combination, and the chain transfer to the monomers on the control of the molecular weights seem to be negligible up to 30% conversion. Increasing macroinitiator concentration results in the reduction of the magnitude of the growing molecular weights at an equivalent conversion due to the increasing

4 72 C. H. Han, S. Butz, G. Schmidt-Naake Fig. 4. Molecular weight distributions of the obtained polymers from the polymerization of styrene initiated by [BPO-PS- (4-acetamido-TEMPO)] macroinitiator ((a) M n = 4900) at 1258C in dependence on reaction time: (b) 20 min (4.1%, M n = 8600), (c) 40 min (8.3%, M n = 12900), (d) 60 min (12.6%, M n = 17300), (e) 80 min (16.7%, M n = 19900), (f) 100 min (20.8%, M n = 23300), (g) 120 min (24.4%, M n = 26300), (h) 150 min (30.3%, M n = 29900); [styrene] 0 = 8,73 mol L 1, [macroinitiator] 0 = 10 mmol L 1. number of polymer chains. This dependence of the growth of the molecular weights on the macroinitiator concentration can also be observed in the polymerization of styrene initiated by a [BPO-PS-TEMPO] macroinitiator. The molecular weight distributions as a function of the polymerization time in the polymerization of styrene with 10 mmol L 1 [BPO-PS-(4-acetamido-TEMPO)] macroinitiator are presented in Fig. 4. It can be seen that the molecular weight distributions are shifted to higher molecular weights and a high efficiency of the [BPO-PS-(4-acetamido-TEMPO)] macroinitiator is indicated. The polydispersities (M w /M n ) of the obtained polymers are plotted in Fig. 5. In all cases, the polydispersities at the initial stage of the polymerization (at low conversions) are higher than those at high conversions. This general tendency is legitimate for the Poisson distribution in the living polymerization. However, the significantly high polydispersities at low macroinitiator concentrations are related to a relatively low concentration of the N- oxyl. The polydispersities at low conversions decrease dramatically with increasing macroinitiator concentration. But at high conversions, no significant influence of the macroinitiator concentration on the polydispersity was observed. Knowing both the conversion and M n obtained from the GPC curve, one can estimate the concentration of polymer chains ([P n ]) by means of Eq. (4) 30). Eq. (4) consists of the sum of the N-oxyl terminated ([PN]), dead ([D]), and stationary active radical ([P9] stat ) chains. Fig. 5. Dependence of polydispersity (M w /M n ) on the conversion for the polymerization of styrene initiated by [BPO-PS-(4- acetamido-tempo)] macroinitiator (M n = 4900, M w /M n = 1.21) at 1258C; [styrene] 0 = 8,73 mol L 1, [macroinitiator] 0 : (g) 3 mmol L 1, (h) 5 mmol L 1, (f) 8 mmol L 1, (0) 10 mmol L 1. [P n ] = [PN] + [D] + [P9] stat = {(M n ) MI 6[MI] 0 /(M n ) Polym } + {[St] 0 6(MW) St 6conversion/(M n ) Polym } (4) where (M n ) MI is the number-average molecular weight of the macroinitiator, [MI] 0 is the initial concentration of the macroinitiator, (M n ) polym is the number-average molecular weight of the obtained polymer, [St] 0 is the initial concentration of styrene and (MW) St is the molecular weight of styrene. The concentration of stationary active radical chains is very small in comparison with [PN] and [D]. The calculated concentrations of the polymer chains in the polymerization initiated by the [BPO-PS-(4-acetamido- TEMPO)] macroinitiator are plotted in Fig. 6. [P n ] increases slightly with increasing conversion. This can clearly be observed at low macroinitiator concentrations (3 and 5 mmol L 1 ). The increase of [P n ] with increasing conversion can be explained by the additional formation of radicals due to the thermal self-initiation of styrene. In order to prove the thermal self-initiation of styrene during the polymerization and to quantify its portion in the obtained polymers, additional amounts of N-oxyl were given to the polymerization system initiated by a macroinitiator. This reaction was carried out at 125 8C and with quite low concentration of the [BPO-PS- TEMPO] macroinitiator (M n = 37800, M w /M n = 1.24; 2 mmol L 1 ). In addition, 8 mmol L 1 TEMPO was added to the reaction mixture (styrene + macroinitiator). After 30 min under gentle nitrogen purge the reaction was started by a rapid temperature increase to 125 8C. As expected, the addition of TEMPO resulted in a long induction period (ca. 120 min), which was very similar to that of the thermal autopolymerization of styrene in the

5 Controlled free radical polymerization of styrene 73 Fig. 6. Dependence of the polymer chain concentration ([P n ]) on the conversion for the polymerization of styrene initiated by [BPO-PS-(4-acetamido-TEMPO)] macroinitiator (M n = 4900, M w /M n = 1.21) at 1258C; [styrene] 0 = 8,73 mol L 1, [macroinitiator] 0 : (g) 3 mmol L 1, (h) 5 mmol L 1, (f) 8 mmol L 1, (0) 10 mmol L 1, (*) data obtained from the thermal autopolymerization of styrene at 125 8C, dotted lines: initial macroinitiator concentration. by TEMPO until the point at which the stationary TEMPO concentration is established (the end of the induction period). At the end of the induction period, there are two main types of growing chains in the system, the first type is the growing chains originated from the macroinitiator (L2 mmol L 1 ), and the second type is the growing chains originated from the thermal self-initiation of styrene (L8 mmol L 1 ). These two different types of chains propagate uniformly and result in a bimodal molecular weight distribution. At the initial stage of the polymerization, the GPCdetermined M n decreased due to the portion of polymers obtained from the thermal self-initiation, then, it subsequently increased linearly with increasing conversion. This resulted in an initial increase of the polydispersity up to a value of 2.53, and subsequently decreased to a value of 1.36 with increasing conversion. After 330 min, the polydispersity remained constant (M w /M n = 1.33 l 1.36). Clearly, this experiment indicates the occurrence of the thermal autopolymerization of styrene during the N-oxyl-controlled radical polymerization initiated by a macroinitiator. Fig. 7. Molecular weight distributions of the obtained polymers in the polymerization of styrene initiated by [BPO-PS- TEMPO] macroinitiator ((a) M n = 37800) and with additionally added TEMPO in dependence on reaction time: (b) 120 min (0.1%, M n = 17700), (c) 180 min (13.3%, M n = 23100), (d) 240 min (24.5%, M n = 34300) at 1258C; [styrene] 0 = 8,73 mol L 1, [macroinitiator] 0 = 2 mmol L 1, [TEMPO] = 2 mmol L 1. presence of 8 mmol L 1 TEMPO 30). Moreover, the rate of polymerization was similar to that of the thermal autopolymerization of styrene. The GPC-determined molecular weight distributions are presented in Fig. 7. At the initial stage of the polymerization, bimodal distributions are observed. These bimodal molecular weight distributions in Fig. 7 indicate a thermal self-initiation of styrene during the polymerization. The thermally self-initiated radicals are terminated 1) M. K. Georges, R. P. N. Veregin, P. M. Kazmaier, G. K. Hamer, Macromolecules 26 (1993) ) R. P. N. Veregin, M. K. Georges, P. M. Kazmaier, G. K. Hamer, Macromolecules 26 (1993) ) K. Matyjaszewski, S. Gaynor, D. Greszta, D. Mardare, T. Shigemoto, Macromol. Symp. 98 (1995) 73 4) R. D. Puts, D. Y. Sogah, Macromolecules 29 (1996) ) E. Yoshida, T. Fujii, J. Polym. Sci., Part A: Polym. Chem. 35 (1997) ) E. Yoshida, J. Polym. Sci., Part A: Polym. Chem. 34 (1996) ) M. K. Georges, R. P. N. Veregin, P. M. Kazmaier, G. K. Hamer, Trends Polym. Sci. 2 (1994) 66 8) R. P. N. Veregin, M. K. Georges, G. K. Hamer, M. Kazmaier, Macromolecules 28 (1995) ) H. Baethge, S. Butz, G. Schmidt-Naake, Macromol. Rapid Commun. 18 (1997) ) C. J. Hawker, J. Am. Chem. Soc. 116 (1994) ) C. J. Hawker, J. L. Hedrick, Macromolecules 28 (1995) ) J. M. Catala, F. Bubel, S. O. Hammouch, Macromolecules 28 (1995) ) S. O. Hammouch, J. M. Catala, Macromol. Rapid Commun. 17 (1996) ) D. Greszta, K. Matyjaszewski, Macromolecules 29 (1996) ) C. J. Hawker, G. G. Barclay, A. Orellana, J. Dao, W. Devonport, Macromolecules 29 (1996) ) T. Fukuda, T. Terauchi, A. Goto, K. Ohno, Y. Tsujii, T, Miyamoto, S. Kobatake, B. Yamada, Macromolecules 29 (1996) ) S. Butz, H. Baethge, G. Schmidt-Naake, Macromol. Rapid Commun. 18 (1997) ) A. Goto, T. Terauchi, T. Fukuda, T. Miyamoto, Macromol. Rapid Commun. 18 (1997) ) K. Ohno, M. Ejaz, T. Fukuda, T. Miyamoto, Y. Shimizu, Macromol., Chem. Phys. 199 (1998) 291

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