Article pubs.acs.org/Macromolecules
Enhanced Spin Capturing Polymerization of Ethylene Cedric Dommanget,† Christophe Boisson,† Bernadette Charleux,† Franck D’Agosto,† Vincent Monteil,*,† Fernande Boisson,‡ Thomas Junkers,*,§ Christopher Barner-Kowollik,*,∥ Yohann Guillaneuf,⊥ and Didier Gigmes*,⊥ †
Laboratoire de Chimie Catalyse Polymères et Procédés (C2P2), LCPP Team, Université de Lyon 1, CPE Lyon, CNRS, UMR 5265, Bat 308F, 43 Bd du 11 novembre 1918, F-69616 Villeurbanne, France ‡ Service de RMN Polymères de l’ICL, Université de Lyon, CNRS, UMR 5223, INSA-Lyon, F-69621, Villeurbanne, France § Institute for Materials Research, Polymer Reaction Design Group, Universiteit Hasselt, Agoralaan, Gebouw D, B-3590 Diepenbeek, Belgium ∥ Preparative Macromolecular Chemistry, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstr. 18, 76128 Karlsruhe, Germany ⊥ Aix-Marseille Université, CNRS, Institut de Chimie Radicalaire, UMR 7273, Av. Esc. Normandie Niemen, 13397 Marseille Cedex 20, France S Supporting Information *
ABSTRACT: Enhanced spin capturing polymerization (ESCP)a recent and versatile technique in the field of controlled radical polymerizationachieves control over molecular weights and the synthesis of complex copolymer structures for a wide range of monomers. In the present work, the use of ESCP was extended to the radical polymerization of ethylene under mild conditions (low temperature and medium ethylene pressure) using a nitrone as spin trapping agent. It was demonstrated that the evolution of polyethylene (PE) molecular weight can be accurately described by classical ESCP kinetic equations. A PE bearing a midchain alkoxyamine function was thus obtained with high selectivity (90%). A more complex structure was produced from the radical polymerization of ethylene in the presence of a midchain alkoxyamine-functionalized polystyrene (PS) synthesized by ESCP in the form of ABA triblock copolymer (where A is polystyrene and B polyethylene).
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INTRODUCTION One of the recent developments in the field of controlled polymerization mechanisms is the enhanced spin capturing polymerization (ESCP) method, which achieves control over molecular weight and polymer functionality via spin trapping agents, usually nitrones.1,2 In the ESCP process, growing macroradical chains are captured by the spin trap and transformed into stable macronitroxide species. Since these radicals are unable to propagate, effective chain length control is obtained, and the average molecular weight of material can be tuned via the spin trap concentration.3 At the same time, however, the macroradical spin trap adducts can themselves trap other transient (macro)radicals. As a consequence, midchain-functionalized polymers are obtained that carry initiator-derived fragments at the chain ends and an alkoxyamine functionality (derived from the reaction nitrone− nitroxide−alkoxyamine) in the middle of the chain.4 Various poly(meth)acrylates, polystyrene, polyacrylamides, poly(vinyl acetate), and polyacrylonitrile5,6 have so far been synthesized via the ESCP process and the related nitrone-mediated radical coupling technique,7 also with the aim of constructing complex star-shaped materials, which become available via functional nitrones that allow for the generation of a midchain-anchor point.8 © XXXX American Chemical Society
In the case of ESCPas for other controlled radical polymerization mechanismsinvestigations have focused essentially on liquid monomers such as acrylic or styrenic monomers. However, the most frequently used monomer for the large scale production of polymer via a free radical mechanism is ethylene. About 16 Mt of low-density polyethylene (LDPE; ∼ 20% of polyethylene production) are produced every year. LDPE is a branched homopolymer of ethylene prepared since 1920 at high temperatures (250 °C) and high pressures (2000 bar) via a free radical polymerization (FRP) process.9−14 In the field of polyolefins synthesis, the development of Ziegler−Natta (or Phillips) catalysis15,16 to produce under mild experimental conditions (T < 100 °C and P < 100 bar) higher density polyethylenes (HDPE or LLDPE) has nevertheless greatly eclipsed the classical radical process, and research devoted to free radical polymerization of ethylene became very scarce after the 1950s. The construction of complex polymer structures, such as block copolymers, based on ethylene segments and other more Received: July 24, 2012 Revised: September 18, 2012
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Table 1. Polymerizations of Ethylene via FRP or ESCP Using Various Quantities of Nitronea entry
solvent
AIBN (mmol L−1)
PBN (mmol L−1)
yield (g)
Mn (g mol−1)b
Đb,c
Tm (°C)d
cryst (%)d
1 2 3 4 5 6 7
THF DMC DMC DMC DMC DMC DMC
6.1 6.1 24.4 24.4 24.4 24.4 24.4
0 0 0 2.4 12.2 18.3 24.4
4.0 2.1 5.7 5.3 0.4 0.1 0.04
1000 20000 12500 11500 2200 1440 1190
2.0 3.3 7.2 4.3 1.5 1.4 1.4
116 118 117 119 113 108 105
70 52 46 49 41 35 30
no. branches/1000 Ce % long branchese 5.0
51
5.2 3.1 3.1
52 33 26
t = 4 h, T = 70 °C, 200 bar ethylene pressure, 50 mL of solvent. bMeasured by high-temperature SEC. cĐ = Mw/Mn. dMeasured by DSC. Measured by 13C NMR (see calculation given in the Supporting Information).
a e
valves and a stirrer.24 Ethylene was introduced until the desired pressure (200 bar) and at the same time the mixture was heated at 70 °C under stirring (300 rpm). An intermediate 1.5 L tank filled with ethylene (300 bar of ethylene) was used to charge the reactor and to maintain a constant pressure of ethylene in the reactor by successive manual ethylene additions. After 4 h of polymerization the reactor was slowly cooled down and degassed. For the kinetic investigations, several polymerizations were carried out under identical conditions and to different reaction times, since it is impossible to withdraw samples from the reaction medium under 200 bar pressure. Block Copolymers Formation Based on PE Using ESCP Synthesized Polystyrene. Macroinitiators of polystyrene containing midchain alkoxyamine functionality were obtained by ESCP of styrene initiated by AIBN (40.0 × 10−3 mol L−1) in the presence of PBN (80.0 × 10−3 mol L−1) as reported in the literature.1 Initiator and nitrone were freed from oxygen by vacuum−argon cycles and subsequently dissolved in styrene (40 mL) under an argon atmosphere. The polymerizations were performed at 60 °C for 5 h. The reactions were stopped by cooling the reaction medium in an ice bath, and residual monomers were removed by evaporation. Any remaining initiator and nitrone were eliminated by precipitation of the polystyrene (PS) in methanol followed by filtration. Solutions of these PS (0.75 g) in DMC (50 mL) were used for NMP chain extensions. The reactions were carried out in the same reactor used for the ESCP of ethylene under 200 bar at different temperatures (80, 100, and 120 °C) within 4 h. Size Exclusion Chromatography (SEC). High temperature size exclusion chromatography analyses of polyethylene were performed using a Waters Alliance GPCV 2000 instrument that incorporates two detectors: a differential refractive index (RI) and a viscosimeter. 1,2,4Trichlorobenzene (TCB) was used as the mobile phase at a flow rate of 1 mL min−1 at 150 °C. It was stabilized with 2,6-di(tert-butyl)-4methylphenol. The separation was carried out on three Agilent columns (PL gel Olexis 7 × 300 mm) and a guard column (PL gel 5 μm). Columns and detectors were maintained at 150 °C. The Empower software was used for data acquisition and data analysis. The molecular weight distributions were calculated with a calibration curve based on narrow polyethylene standards from Polymer Standard Service (Mainz). Size exclusion chromatography measurements of polystyrene were performed in THF at 40 °C, at a flow rate of 1 mL min−1, using toluene as a flow rate marker. They were analyzed at a concentration of 3 mg mL−1 after filtration through a 0.45 μm pore size membrane. The separation was carried out on three Polymer Laboratories columns [3 × PLgel 5 μm Mixed C (300 × 7.5 mm) and a guard column (PL gel 5 μm)]. The setup (Viscotek TDA 305) was equipped with a refractive index (RI) detector (λ = 930 nm). The average molecular weights (number-average molecular weight Mn and weightaverage molecular weight Mw) and the dispersity value (Đ = Mw/Mn) were derived from the RI signal by a calibration curve based on polystyrene standards (PS from Polymer Laboratories). Nuclear Magnetic Resonance (NMR). All NMR experiments were performed at 90 °C in 2:1 tetrachloroethylene/deuterated benzene solutions using a Bruker Avance 400 spectrometer (1H: 400.13 MHz; 13C: 100.61 MHz). Temperature was calibrated with a
polar comonomers is, however, an attractive aim as the soobtained materials combine the intrinsic properties of polyethylene originating from its semicrystalline character and properties given by the second monomer. Diblock copolymers were obtained by successive catalytic olefin polymerization and living/controlled polymerization of a polar vinyl monomer.17,18 Strategies based on the use of well-defined semicrystalline PE building blocks19 in macromolecular design can also be employed.20−22 To the best of our knowledge, no well-controlled architectures based on ethylene segments were reported to date using only conventional free radical polymerization mechanisms. The polymerization conditions of the free radical polymerization of ethylene are indeed too severe to allow for fine control over the macromolecular architecture. This was confirmed by investigations using RAFT technique at high temperature and high pressure, where only limited control over molecular weights was achieved.23 Under more appropriate milder conditions (150 °C). The alkoxyamine bond stability was proven by NMR: After being heated to 150 °C for 3 h, there was no noticeable change in the 13C spectrum. Additionally, the cleavage will occur preferentially for the N−O bond,30,31 dissociation of O−C bond is however the prerequisite for the synthesis of block copolymers (e.g., PE− NO−PS−PE) in an NMP process and thus difficult to implement. In the following, an alternative pathway was explored to obtain block copolymers containing polyethylene segments andto avoid problems with the alkoxyamine stabilitythe inverse block structure PS−NO−PE−PS was targeted. Incorporating PE segments by the use of PS synthesized from ESCP is an appealing method. Radicals are readily formed via thermolysis of the O−C bond, which can be followed by rapid ethylene monomer addition. After the addition of ethylene units, the recombination of the growing macroradical with a free nitroxide will stop its growth; thus, the establishment of an NMP equilibrium is not strictly required, and the temperature of the block extension reaction is indeed too low to cleave the NO−C bond formed after the incorporation of ethylene units. Ethylene polymerization without any conventional initiator and in the presence of purified ESCP-derived PS was conducted at 80, 100, and 120 °C. Because of differences in the propagation rate, polymers with varying block molecular weights might be produced as sketched in Scheme 3. Again, as
To establish if indeed block copolymers are obtained from the ethylene polymerizations, DSC thermograms were recorded, as depicted in Figure 6. The characteristic melting
Figure 6. DSC analyses of copolymers prepared by ethylene polymerization performed at 80, 100, and 120 °C in the presence of PS synthesized via ESCP of styrene.
temperature of 112 °C attests to the presence of polyethylene when the polymerization is performed at 120 °C. In this case, the PE segments had a molecular weight which is sufficiently high to let the polymer crystallize. No clear transition is, however, observed for the polymer prepared at 80 or 100 °C, suggesting that either no PE is formed or the molecular weight of the PE segment is too low. Likewise, 13C NMR analysis confirms the formation of PE in the polymer prepared at 100 and 120 °C. An intense signal at 30 ppm which is characteristic of polyethylene and weaker signals corresponding to the branches (between 14 and 40 ppm) (see Figures 7 and 4) are readily noticeable, F
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ization of the PE segments is 24 at 100 °C and 140 at 120 °C (Figure 9).
unambiguously proving the presence of ethylene sequences obtained by a free radical polymerization mechanism.
Figure 9. Expected degrees of polymerization (DPn) of PS and PE blocks in copolymer prepared at 100 and 120 °C.
Figure 7. C NMR spectra in TCE/C6D6 at 90 °C of PS synthesized by ESCP and copolymers prepared by ethylene polymerization performed at 100 and 120 °C in the presence of PS synthesized via ESCP of styrene. 13
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CONCLUSIONS The current study has demonstrated the efficiency of the ESCP technique in ethylene polymerization under mild conditions (low temperature and medium ethylene pressure) using a nitrone as spin trap agent. PE molecular weights have been successfully controlled and can be predicted by classical ESCP kinetic equations. In addition, the ESCP technique allows for the synthesis of very attractive structures: midchain alkoxyamine-functionalized polyethylenes (90% functionality). Block copolymers based on polyethylene segments can be also produced from via the ESCP technique. As a proof of concept, ABA triblock copolymers of styrene (A) and ethylene (B) were successfully prepared by radical polymerization of ethylene from midchain alkoxamine-functionalized polystyrene. As ESCP can be applied to the polymerization of a wide range of monomers, such as (meth)acrylate or vinyl acetate, these promising results pave the way to the production of new polyethylene-based architectures. The successful preparation of midchain-functionalized polyethylenes is thus an attractive method for e.g. PE-based star copolymers syntheses.
Figure 8. Molecular weight distributions of copolymers measured by high temperature size exclusion chromatography: ĐPS: 1.9; Đ80 °C: 2.2; Đ100 °C: 1.8; Đ120 °C: 1.6.
Figure 8 illustrates the evolution of molecular weight distributions (MWD) of the formed polymers obtained from high temperature SEC. Ambient temperature SEC was not used because the copolymers prepared at 100 and 120 °C are not soluble in THF. When the polymerization is performed at 80 °C, the MWD of the starting PS and of the product isolated after polymerization are superimposable, indicating that no reaction has taken place. At 80 °C, the midchain alkoxyamine of PS cannot efficiently dissociate and polymerization of ethylene is not occurring.32 When polymerizations are carried out at higher temperatures (100 and 120 °C) the MWDs are successfully shifted toward higher MW, leaving no initial PS material behind as can be seen from the monomodality of the product distribution. Formation of homopolymer (also due to the lack of addition of any conventional initiator) can thus be excluded, and ABA triblock copolymers are exclusively obtained from the chain extension. The degree of polymerization of the PE block was determined by using 1H NMR spectroscopy. The polystyrene prepared by ESCP and used as an initiator in the copolymerization has a Mn of 12 000 g mol−1 (measured in SEC THF at 30 °C), which approximately corresponds to a degree of polymerization of 120. According to 1H and 13C NMR, the ratios of ethylene/styrene in the copolymer prepared at 100 and 120 °C are respectively close to 0.2 and 1.2. Thus, if no side reaction occurs (Scheme 3), the degree of polymer-
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ASSOCIATED CONTENT
S Supporting Information *
Calculations for branching determination and for the midchain functionality from NMR analyses as well as COSY and HSQC NMR contour maps. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (V.M.); thomas.junkers@ uhasselt.be (T.J.);
[email protected] (C.B.K.);
[email protected] (D.G.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS T.J. is grateful for funding from the Fonds Wetenschappelijk Onderzoek (FWO) via project G.0491.11N and the Odysseusscheme. C.B.-K. acknowledges continued support from the Karlsruhe Institute of Technology (KIT) in the context of the Excellence Initiative for leading German universities as well as the German Research Council (DFG). C.D. thanks the G
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“Ministère de la Recherche et de l’Enseignement Supérieur” for fellowship. V.M. acknowledges ANR and competitiveness clusters AXELERA and PLASTIPOLIS for financial support (project FRaPE; ANR 2011 JS08 008 01).
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(31) Godoy Lopez, R.; Boisson, C.; D’Agosto, F.; Spitz, R.; Boisson, F.; Gigmes, D.; Bertin, D. J. Polym. Sci., Polym. Chem. 2007, 45, 2705− 2718. (32) Junkers, T.; Zang, L.; Wong, E. H. H.; Dingenouts, N.; BarnerKowollik, C. J. Polym. Sci., Polym. Chem. 2011, 49, 4841−4850.
REFERENCES
(1) Wong, E. H. H.; Junkers, T.; Barner-Kowollik, C. J. Polym. Sci., Polym. Chem. 2008, 46, 7273−7279. (2) Wong, E. H. H.; Barner-Kowollik, C.; Junkers, T. Polym. Chem. 2011, 2, 1008−1017. (3) Wong, E. H. H.; Stenzel, M. H.; Junkers, T.; Barner-Kowollik, C. J. Polym. Sci., Polym. Chem. 2009, 47, 1098−1107. (4) Junkers, T.; Wong, E. H. H.; Stenzel, M. H.; Barner-Kowollik, C. Macromolecules 2009, 42, 5027−5035. (5) Zang, L.; Wong, E. H. H.; Barner-Kowollik, C.; Junkers, T. Polymer 2010, 51, 3821−3825. (6) Detrembleur, C.; Debuigne, A.; Altintas, O.; Conradi, M.; Wong, E. H. H.; Jerôme, C.; Barner-Kowollik, C.; Junkers, T. Polym. Chem. 2012, 3, 135−147. (7) Wong, E. H. H.; Boyer, C.; Stenzel, M. H.; Barner-Kowollik, C.; Junkers, T. Chem. Commun. 2010, 46, 1959−1961. (8) Wong, E. H. H.; Stenzel, M. H.; Junkers, T.; Barner-Kowollik, C. Macromolecules 2010, 43, 3785−3793. (9) Aggarwal, S. L.; Sweeting, O. J. Chem. Rev. 1957, 57, 665−742. (10) Doak, K. W. In Encyclopedia of Polymer Science and Engineering, 2nd ed.; Mark, H. F., Bikales, N. M., Overberger, C. G., Menges, G., Eds.; Wiley-Interscience: New York, 1985; Vol. 6, p 386. (11) Buback, M. Prog. Polym. Sci. 2002, 27, 191−254. (12) Buback, M.; Schweer, J. Z. Phys. Chem. (Muenchen, Ger.) 1989, 161, 153−165. (13) Buback, M. Macromol. Symp. 2009, 275−276, 90−101. (14) Barth, J.; Buback, M. Macromol. React. Eng. 2010, 4, 288−301. (15) Mülhaupt, R. Macromol. Chem. Phys. 2003, 204, 289−327. (16) Beach, D. L.; Kissin, Y. V. In Encyclopedia of Polymer Science and Engineering, 2nd ed.; Mark, H. F., Bikales, N. M., Overberger, C. G., Menges, G., Eds.; Wiley-Interscience: New York, 1985; Vol. 6, p 454. (17) Lopez, R. G.; D’Agosto, F.; Boisson, C. Prog. Polym. Sci. 2007, 32, 419−454. (18) Zhao, Y.; Wang, L.; Xiao, A.; Yu, H. Prog. Polym. Sci. 2010, 35, 1195−1216. (19) Mazzolini, J.; Espinosa, E.; D’Agosto, F.; Boisson, C. Polym. Chem. 2010, 1, 793−800. (20) Li, Q. Z.; Zhang, G. Y.; Chen, J. Z.; Zhao, Q. L.; Lu, H. C.; Huang, J.; Wei, L. H.; D’Agosto, F.; Boisson, C.; Ma, Z. J. Polym. Sci., Polym. Chem. 2011, 49, 511−517. (21) Lefay, C.; Gle, D.; Rollet, M.; Mazzolini, J.; Bertin, D.; Viel, S.; Schmid, C.; Boisson, C.; D’Agosto, F.; Gigmes, D.; Barner-Kowollik, C. J. Polym. Sci., Polym. Chem. 2011, 49, 803−813. (22) Mazzolini, J.; Boyron, O.; Monteil, V.; D’Agosto, F.; Boisson, C.; Sanders, G. C.; Heuts, J. P. A.; Duchateau, R.; Gigmes, D.; Bertin, D. Polym. Chem. 2012, 3, 2383−2392. (23) Bush, M.; Roth, M.; Stenzel, M. H.; Davis, T. P.; BarnerKowollik, C. Aust. J. Chem. 2007, 60, 788−793. (24) Grau, E.; Broyer, J. P.; Boisson, C.; Spitz, R.; Monteil, V. Macromolecules 2009, 42, 7279−7281. (25) Grau, E.; Broyer, J. P.; Boisson, C.; Spitz, R.; Monteil, V. Polym. Chem. 2011, 2, 2328−2333. (26) Grau, E.; Dugas, P. Y.; Broyer, J. P.; Boisson, C.; Spitz, R.; Monteil, V. Angew. Chem. 2010, 49, 6810−6812. (27) Grau, E.; Broyer, J. P.; Boisson, C.; Spitz, R.; Monteil, V. Phys. Chem. Chem. Phys. 2010, 12, 11665−11669. (28) Ranieri, K.; Conradi, M.; Chavant, P. Y.; Blandin, C.; BarnerKowollik, C.; Junkers, T. Aust. J. Chem. 2012, 65, 1110−1116. (29) Galland, G. B.; de Souza, R. F.; Mauler, R. S.; Nunes, F. F. Macromolecules 1999, 32, 1620−1625. (30) Godoy Lopez, R.; Boisson, C.; D’Agosto, F.; Spitz, R.; Boisson, F.; Bertin, D.; Tordo, P. Macromolecules 2004, 37, 3540−3542. H
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