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, So¨ren Butz, Gudrun Schmidt-Naake* Institut fu¨r Technische Chemie, TU Clausthal, Erzstr. 18, 38678 Clausthal-Zellerfeld, Germany (Received 15 October 1998) SUMMARY: The bulk polymerization of styrene at 1258C was studied using a [BPO-polystyrene-(4-acet- amido-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 poly- merization 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 (Mn) 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 (Mw/Mn) 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 polymeriza- tion system. ZUSAMMENFASSUNG: Ausgehend von [BPO-Polystyrol-(4-Acetamido-TEMPO)]-Makroinitiatoren wurde die Substanzpolymerisation von Styrol bei 1258C untersucht. Die Polymerisationsgeschwindigkeit war unabha¨ngig von der eingesetzten Makroinitiator-Konzentration und stimmte im Rahmen der Meß- genauigkeit mit der der thermisch initiierten Autopolymerisation von Styrol u¨berein. Bezu¨glich der Art des verwendeten N-Oxyls wurde kein Einfluß auf die Polymerisationsgeschwindigkeit festgestellt. Die mit Gel- permeationschromatographie (GPC) ermittelten Molmassen stimmten gut mit den theoretisch gescha¨tzten Molmassen u¨berein, Abweichungen ließen sich nur bei geringen Makroinitiator-Konzentrationen beobachten. Zunehmende Makroinitiator-Konzentrationen resultierten in einer Reduktion der Molmassenzunahme sowie der Polydispersita¨t, wobei eine Beeinflussung der Polydispersita¨t insbesondere in der Anfangsphase der Poly- merisation beobachtet wurde. Durch Berechnung von Kettenkonzentrationen ließen sich eindeutig Radikal- Neubildungen durch die thermische Styrol-Selbstinitiierung feststellen. Zusa¨tzlich wurde diese Selbstinitiie- rung durch spezielle Reaktionsfu¨hrung mit N-Oxyl-Zudosierung qualitativ nachgewiesen. Introduction where k and k are the rate constants of combination and The controlled free radical polymerization is of particular c d dissociation, respectively. The essentially simultaneous academic and industrial interest. This type of polymeriza- initiation and the small contribution of irreversible termi- tion does not require a high purity of the monomers and nation result in controlled molecular weights and narrow does not need special precautions to ensure anhydrous molecular weight distributions. conditions. Moreover, it allows one to synthesize poly- N-oxyls can be employed in three different ways to con- mers with a narrow molecular weight distribution and trol the radical polymerization. The first method is the block copolymers. combination of an N-oxyl with a traditional initiator such The N-oxyl-controlled free radical polymerization is as benzoyl peroxide (BPO)1–9). Radicals formed by the one of the most extensively studied methods to control a decomposition of the initiator initiate the polymerization radical polymerization. The key to success of this poly- and the N-oxyls terminate this growing radicals reversibly. merization is the reversible deactivation of the growing The second method, alkoxyamines and polyalkoxyamines radicals (P*) by N-oxyls (N*) (Eq. (1)): such as [polymer-(N-oxyl)] adducts which are synthesized kc by N-oxyl-controlled radical polymerization can be used 9 9 aggggggggs 1 P +N P NK=kd/kc (1) 10–19, 24–29) kd as unimolecular initiators . In this case, the initia- * Correspondence author. Die Angewandte Makromolekulare Chemie 265 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 1999 0003-3146/99/0203–0069$17.50+.50/0 70 C. H. Han, S. Butz, G. Schmidt-Naake tors do not only act as an initiator but also as a terminator. (TEMPO, Fluka) were used as received. Benzoyl peroxide The third method, when the monomers undergo a thermal (BPO, Merck) was purified by recrystallization from tri- autopolymerization, a controlled free radical polymeriza- chloromethane/methanol. tion is practicable in the absence of additionally added initiating systems, only the N-oxyl is needed for the con- trol of the polymerization process20–23, 30). Polymerization The use of [polymer-(N-oxyl)] adducts in the N-oxyl- All polymerizations were performed in bulk and carried out controlled radical polymerization is of special interest in batch reactors under gentle nitrogen purge and with stir- due to their ability to undergo a chain extension16, 18, 26, 27) ring throughout the reaction. The polymers were recovered and to synthesize block copolymers17, 19, 24, 25). Moreover, as precipitants from an excess of methanol, purified, and this system is advantageous for the investigation of dried up to weight constancy. kinetics since the [polymer-(N-oxyl)] adducts do not lead to any significant side reactions, for instance, for the Synthesis of the macroinitiators BPO/TEMPO system2) at the initiation stage. The [poly- mer-(N-oxyl)] adducts also provide a perfect balance The reactor was charged with styrene (127.26 g) and 4-aceta- between the scavenger (N-oxyl) and the polymer chains mido-TEMPO (1.015 g). After 30 min under gentle nitrogen purge, BPO (0.678 g) was added and the reaction mixture and eliminate any induction period. Fukuda et al.16), for was preheated for 1 h at 958C to ensure complete BPO example, used [PS-(TEMPO)] adducts as a macroinitiator decomposition8). Subsequently, the temperature was quickly in the polymerization of styrene and reported their results increased to 1358C. The polymer was recovered from the 16, 18, 26, 27) in several kinetics works . reactor after 2 h. The synthesized polymer was found to have Besides TEMPO, other N-oxyls can be used in the con- Mn of 4900 (GPC) and polydispersity (Mw/Mn) of 1.21. trolled radical polymerization3–6, 12, 13, 20, 25, 28–30), and the Two [BPO-PS-TEMPO] macroinitiators were synthesized synthesized polymers can also be used as macroinitiators in analogy to the [BPO-PS-(4-acetamido-TEMPO)] macro- for a chain extension polymerization and for the synthesis initiator. These polymers have Mn 7000 (Mw/Mn = 1.16) and of block copolymers. 37800 (Mw/Mn = 1.24), respectively. In our present work, 4-acetamido-2,2,6,6-tetramethylpi- peridine-N-oxyl (4-acetamido-TEMPO) was used as a sca- venger. The [BPO-PS-(4-acetamido-TEMPO)] adduct, Polymerization of styrene with macroinitiator synthesized through the polymerization of styrene initiated The reactor was charged with styrene (45.45 g) and various by BPO in the presence of 4-acetamido-TEMPO, was used amounts of macroinitiator. After 30 min under gentle nitro- as a macroinitiator in the polymerization of styrene. In addi- gen purge, the reaction was started with a rapid temperature 8 tion, the results are compared with those obtained for the increase up to 125 C. TEMPO system and are discussed in this paper. 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 103–5 and 104–5, Macherey-Nagel) in series and with a RI-detector (Knauer). Results and discussion To study the influence of the macroinitiator concentration on the polymerization process, five different macroinitia- tor concentrations were chosen, ranging from 0.003 to [BPO-PS-(4-acetamido-TEMPO)] macroinitiator 0.010 mmol L–1. The time-conversion plots for the var- ious macroinitiator concentrations are shown in Fig. 1. Experimental part The rate of polymerization is independent of the [BPO- PS-(4-acetamido-TEMPO)] macroinitiator concentration Materials and, therefore, at the same polymerization time, all con- Styrene (BASF) was distilled under reduced pressure. 4-acet- versions are nearly the same. amido-2,2,6,6-tetramethylpiperidine-N-oxyl (4-acetamido- Fig. 2 shows a comparison of time-conversion plots for TEMPO, Hu¨ls) and 2,2,6,6-tetramethylpiperidine-N-oxyl the polymerizations initiated by [BPO-PS-(4-acetamido- Controlled free radical polymerization of styrene 71 Fig. 1. Percent conversion as a function of time for the poly- Fig. 2. Comparison of time-conversion plots for the polymeri- merization of styrene initiated by [BPO-PS-(4-acetamido- zation of styrene initiated by (h) [BPO-PS-(4-acetamido- TEMPO)] macroinitiator (Mn = 4900, Mw/Mn = 1.21) at 1258C; TEMPO)] macroinitiator (Mn = 4900, Mw/Mn = 1.21) and (0) –1 –1 [styrene]0 = 8,73 mol L , [macroinitiator]0: (g) 3 mmol L , (h) [BPO-PS-TEMPO] macroinitiator (Mn = 7000, Mw/Mn = 1.16), 5 mmol L–1, (f) 8 mmol L–1, (0) 10 mmol L–1. The dotted line and the thermal autopolymerization of styrene at 1258C; [styr- –1 –1 shows the data obtained for the