Controlled Free Radical Polymerization of Styrene Initiated by a [BPO-Polystyrene-(4-Acetamido-TEMPO)] Macroinitiator

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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-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 (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 polymerization 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 control 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 [polymer-(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-tetramethylpiperidine-N-oxyl (4-acetamido-TEMPO) was used as a scavenger. 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 macroinitiator 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 various 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 thermal autopolymerization of ene]0 = 8,73 mol L , [macroinitiator]0 = 8 mmol L (dotted styrene at 1258C. line).

TEMPO)] and [BPO-PS-TEMPO] macroinitiators, and the thermal autopolymerization of styrene. All three rates of the polymerizations (Rp) are remarkably similar and they are independent of the type of N-oxyls. The impor- tant 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):

2 1=2 kpRi Rp kp P9 M M 2 kt where kp is the rate constant of propagation, [P9] is the concentration of the growing radicals, [M] is the concentration of the monomer, Ri is the rate of initiation due to Fig. 3. Dependence of the molecular weight (Mn) on the con- the initiator and/or the thermal autopolymerization of version for the polymerization of styrene initiated by [BPO-PS- (4-acetamido-TEMPO)] macroinitiator (Mn = 4900, Mw/Mn = styrene and kt is the rate constant of termination. –1 1.21) at 1258C; [styrene]0 = 8,73 mol L , [macroinitiator]0: (g) The GPC-determined number-average molecular 3 mmol L–1, (h) 5 mmol L–1, (f) 8 mmol L–1, (0) 10 mmol L–1. weights (Mn) of the obtained polymers are plotted in The dotted lines present the theoretical predictions. Fig. 3 as a function of conversion. The molecular weights increase linearly with conversion and they are very similar to the theoretical predictions (Mn, th) defined by Eq. the uncontrolled polymerization process due to the low (3), within an experimental error (l10%). N-oxyl concentration. In this case, the concentration of the N-oxyl is too low for an ideal, controlled free radical

Mn,th = (Mn)MI + ([St]0/[MI]0)6(MW)St6Conversion (3) polymerization. At higher macroinitiator concentrations (A5 mmol L–1), the effects of the thermal self-initiation, where (Mn)MI is the number-average molecular weight of the irreversible termination by combination, and the chain the macroinitiator, [St]0 is the initial concentration of sty- transfer to the monomers on the control of the molecular rene, [MI]0 is the initial concentration of the macroinitia- weights seem to be negligible up to 30% conversion. tor and (MW)St is the molecular weight of styrene. Increasing macroinitiator concentration results in the

At low macroinitiator concentrations, deviations of Mn reduction of the magnitude of the growing molecular from Mn, th are observed. This can be partly explained by weights at an equivalent conversion due to the increasing 72 C. H. Han, S. Butz, G. Schmidt-Naake

Fig. 4. Molecular weight distributions of the obtained poly- Fig. 5. Dependence of polydispersity (Mw/Mn) on the conver- mers from the polymerization of styrene initiated by [BPO-PS- sion for the polymerization of styrene initiated by [BPO-PS-(4- (4-acetamido-TEMPO)] macroinitiator ((a) Mn = 4900) at acetamido-TEMPO)] macroinitiator (Mn = 4900, Mw/Mn = 1.21) –1 1258C in dependence on reaction time: (b) 20 min (4.1%, Mn = at 1258C; [styrene]0 = 8,73 mol L , [macroinitiator]0: (g) 3 –1 –1 –1 –1 8600), (c) 40 min (8.3%, Mn = 12900), (d) 60 min (12.6%, Mn = mmol L , (h) 5 mmol L , (f) 8 mmol L , (0) 10 mmol L . 17300), (e) 80 min (16.7%, Mn = 19900), (f) 100 min (20.8%, Mn = 23300), (g) 120 min (24.4%, Mn = 26300), (h) 150 min –1 9 6 (30.3%, Mn = 29900); [styrene]0 = 8,73 mol L , [macroinitia- [Pn] = [PN] + [D] + [P ]stat = {(Mn)MI [MI]0/(Mn)Polym} –1 tor]0 = 10 mmol L . + {[St]06(MW)St6conversion/(Mn)Polym} (4) number of polymer chains. This dependence of the where (Mn)MI is the number-average molecular weight of growth of the molecular weights on the macroinitiator the macroinitiator, [MI]0 is the initial concentration of the concentration can also be observed in the polymerization macroinitiator, (Mn)polym is the number-average molecular of styrene initiated by a [BPO-PS-TEMPO] macroinitia- weight of the obtained polymer, [St]0 is the initial concen- tor. tration of styrene and (MW)St is the molecular weight of The molecular weight distributions as a function of the styrene. polymerization time in the polymerization of styrene with The concentration of stationary active radical chains is 10 mmol L–1 [BPO-PS-(4-acetamido-TEMPO)] macroini- very small in comparison with [PN] and [D]. The calcu- tiator are presented in Fig. 4. It can be seen that the mole- lated concentrations of the polymer chains in the poly- cular weight distributions are shifted to higher molecular merization initiated by the [BPO-PS-(4-acetamido- weights and a high efficiency of the [BPO-PS-(4-acet- TEMPO)] macroinitiator are plotted in Fig. 6. [Pn] amido-TEMPO)] macroinitiator is indicated. increases slightly with increasing conversion. This can

The polydispersities (Mw/Mn) of the obtained polymers clearly be observed at low macroinitiator concentrations –1 are plotted in Fig. 5. In all cases, the polydispersities at (3 and 5 mmol L ). The increase of [Pn] with increasing the initial stage of the polymerization (at low conver- conversion can be explained by the additional formation sions) are higher than those at high conversions. This of radicals due to the thermal self-initiation of styrene. general tendency is legitimate for the Poisson distribution In order to prove the thermal self-initiation of styrene in the living polymerization. However, the significantly during the polymerization and to quantify its portion in high polydispersities at low macroinitiator concentrations the obtained polymers, additional amounts of N-oxyl are related to a relatively low concentration of the N- were given to the polymerization system initiated by a oxyl. The polydispersities at low conversions decrease macroinitiator. This reaction was carried out at 1258C dramatically with increasing macroinitiator concentra- and with quite low concentration of the [BPO-PS- tion. But at high conversions, no significant influence of TEMPO] macroinitiator (Mn = 37800, Mw/Mn = 1.24; the macroinitiator concentration on the polydispersity 2 mmol L–1). In addition, 8 mmol L–1 TEMPO was added was observed. to the reaction mixture (styrene + macroinitiator). After

Knowing both the conversion and Mn obtained from 30 min under gentle nitrogen purge the reaction was the GPC curve, one can estimate the concentration of started by a rapid temperature increase to 1258C. As 30) polymer chains ([Pn]) by means of Eq. (4) . Eq. (4) con- expected, the addition of TEMPO resulted in a long sists of the sum of the N-oxyl terminated ([PN]), dead induction period (ca. 120 min), which was very similar to

([D]), and stationary active radical ([P9]stat) chains. that of the thermal autopolymerization of styrene in the Controlled free radical polymerization of styrene 73

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 GPC-

determined Mn 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 Fig. 6. Dependence of the polymer chain concentration ([Pn]) on the conversion for the polymerization of styrene initiated by value of 1.36 with increasing conversion. After 330 min, [BPO-PS-(4-acetamido-TEMPO)] macroinitiator (Mn = 4900, the polydispersity remained constant (Mw/Mn = 8 –1 Mw/Mn = 1.21) at 125 C; [styrene]0 = 8,73 mol L , [macroinitia- 1.33l1.36). Clearly, this experiment indicates the occur- g –1 h –1 f –1 0 tor]0: ( ) 3 mmol L , ( ) 5 mmol L , ( ) 8 mmol L , ( ) 10 rence of the thermal autopolymerization of styrene during mmol L–1, (*) data obtained from the thermal autopolymerization of styrene at 1258C, dotted lines: initial macroinitiator con- the N-oxyl-controlled radical polymerization initiated by centration. a macroinitiator.

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