Letter pubs.acs.org/macroletters
Tetrakis(dialkylamino)phosphonium Polyelectrolytes Prepared by Reversible Addition−Fragmentation Chain Transfer Polymerization C. Tyler Womble,† Geoffrey W. Coates,‡ Krzysztof Matyjaszewski,† and Kevin J. T. Noonan*,† †
Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213-2617, United States Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States
‡
S Supporting Information *
ABSTRACT: A tetrakis(dialkylamino)phosphonium cation ([P(NR2)4]+) was appended to a styrenic monomer and explored in reversible addition−fragmentation chain transfer polymerization (RAFT) to conduct random copolymerizations of the cationic monomer with styrene. Well-defined polyelectrolytes with molecular weights up to ∼30 100 and dispersities between ∼1.2 and 1.4 were obtained. Up to 18.9 mol % of the ionic monomer could be incorporated into the polymer with hexafluorophosphate or bis(trifluoromethane)sulfonimide acting as the counterion during polymerization. Differential scanning calorimetry of the hexafluorophosphate polymers revealed glass transition temperatures higher than polystyrene likely due to interactions between the anion and the polymer. Thermogravimetric analysis indicated these materials have high thermal stability with decomposition temperatures approaching 400 °C.
P
olyelectrolytes are an important class of materials that have unique chemical and physical properties derived from the electrostatic interactions of ions along a polymer chain.1 They have a wide range of potential uses in water purification, catalysis, antimicrobial coatings, gas separation, and electrochemical cells.2 Cationic polyelectrolytes, in particular, have attracted attention as anion-exchange membranes (AEMs) for alkaline fuel cells (AFCs).3 AFCs are promising alternatives to proton exchange membrane fuel cells (PEMFCs) since nonnoble metal electrocatalysts can be used in alkaline media.3a−c However, the long-term stability of AEMs under fuel cell operating conditions remains challenging.3a−c,4 Tetraalkylammoniums are the most common cations employed in polyelectrolytes for hydroxide conduction, but [NR4]OH lifetime is an issue.3b,5 Resonance-stabilized organic cations such as the guanidinium6 and imidazolium7 are also being explored in AEMs with concurrent investigations of their durability under alkaline conditions.8 Beyond the second row elements, transition metals9 and phosphorus-based cations10 are also attracting attention. Tetraalkylphosphonium polymers and ionic liquids exhibit lower viscosities, higher thermal stabilities, and greater ionic conductivities as compared to their nitrogen counterparts (Figure 1).11 However, since [PR4]+ cations are susceptible to deprotonation under basic conditions, these ions may not be ideal candidates for AEMs. Alternatively, employing dialkylamino substituents ([P(NR2)4]+) rather than alkyl groups can provide resonance-stabilized phosphorus-based cations (Figure 1). Tetrakis(dialkylamino)phosphoniums are derived from a well-known class of phosphazene superbases.12 These robust cations have already been evaluated as phase transfer catalysts under alkaline conditions,13 making them particularly attractive as pendant ions for polymer materials in harsh chemical © XXXX American Chemical Society
Figure 1. Styrene-based phosphonium monomers for reversible deactivation radical polymerization (RDRP).
environments. Incorporation of these cations into polymers has been explored in two reports by manipulation of polyphosphazenes.14 One of these systems was investigated in the presence of hydroxide,14a but the polymer had stability issues due to the cationic moiety directly in the polymer main chain. Surprisingly few reports have described polymers with tethered [P(NR2)4]+ ions. We recently reported a cyclooctene-based monomer with an appended [P(NR 2 ) 4 ] + substituent that can be copolymerized with cyclooctene using ring-opening metathesis polymerization (ROMP).15 Upon hydrogenation, the resulting polyethylene materials exhibited good conductivity and exceptional alkaline stability. However, the challenging monomer synthesis and limited control over molecular weight prompted us to develop alternative strategies to these phosphonium materials. Received: December 12, 2015 Accepted: January 19, 2016
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ACS Macro Letters Herein, we present a simplified synthetic approach to a styrenic tetrakis(dialkylamino)phosphonium monomer and the preparation of well-defined polymers with tunable ionic content by reversible addition−fragmentation chain transfer (RAFT) polymerization. RAFT is a reversible deactivation radical polymerization (RDRP) protocol that produces polymers with predictable molecular weights and narrow dispersities and offers access to more complex architectures.16 RDRP tools are quite versatile for synthesizing polyelectrolyte materials, as they have wide monomer scope and exceptional functional group tolerance. Monomers with tethered [PR4]+ cations have been explored previously using free radical,17 RAFT,18 nitroxide-mediated,19 and atom-transfer radical polymerization (ATRP).20 Consequently, RAFT was explored for the polymerization of the tetrakis(dialkylamino)phosphonium monomer. Cyclohexylazide was prepared according to a published procedure.21 The azide was combined with PCl3 in a Staudinger reaction to produce the desired trichlorophosphazene in moderate yield (51%). Subsequently, treatment with Nmethylcyclohexylamine afforded a phosphonium with one pendant N−H group. This precursor can be transformed into a polymerizable group by deprotonation and simple substitution. Styrene was chosen since it is suitable for RDRP, and polymerization affords a relatively inert hydrocarbon backbone. Monomer 1 was synthesized by reaction of the phosphonium precursor with 4-(iodomethyl)styrene in a biphasic reaction at room temperature (Scheme 1). 4-(Iodomethyl)styrene was
Scheme 2. Copolymerization of Phosphonium Monomer with Styrene using RAFT
Molecular weights up to 30 100 were observed for the random copolymers along with low dispersities (Đ = 1.16− 1.37) (Table 1). 1H NMR spectroscopy was used to determine monomer conversion and compare differences between the initial phosphonium monomer feed and the final phosphonium composition in the resultant polymer (Table 1 and Supporting Information). Successful incorporation up to 20 mol % of 1[NTf2] was achieved with final incorporation of the cation up to 18.9 mol %; however, an induction period was observed at this higher concentration of 1. Attempts to increase the ionic content further by conducting a homopolymerization of 1[NTf2] and a 1:1 random copolymerization of styrene:1[NTf2] both failed to produce high-molecular weight polymer. The large steric bulk of the phosphonium core likely contributes to the difficulty of these reactions. Considering some bulky ethyl ferrocene phosphonium salts have been polymerized free radically,23 we also attempted a free radical polymerization of 1[NTf2] using AIBN at 75 °C in anisole (5 M). Unfortunately, the polymerization of these tetrakis(dialkylamino)phosphoniums still did not proceed and provided further evidence that steric congestion is an issue. More studies are necessary to probe this behavior in detail. A slight increase in the apparent rate constant, kapp, was observed with increasing ionic content (from 5 to 15%, Supporting Information). The observed rate increases are likely due to a change in dielectric constant of the reaction medium. The rate did not increase further from 15 to 20 mol % which could be due to the increase in viscosity with more cationic monomer in the solution. Changes in the apparent rate constant have been observed previously in free radical and RAFT polymerization in the presence of ionic liquids, while added salts have been shown to dramatically affect ATRP with styrene.24 All polymerization reactions were terminated at moderate conversions (32.4−58.4%, Table 1), and the RAFT process was well controlled as indicated by the first-order kinetic behavior and linear Mn versus conversion plot (Figure 2). The degree of polymerization in a RAFT process can be estimated from the monomer consumed and RAFT agent (Mn, 16 theo, Table 1). While two of the Mn, theo values were quite close to the experimentally determined Mn (entries 1 and 8, Table 1), significant deviation from the expected value was observed. It could be partially attributed to differences in cationic monomer incorporation along the chain as well as inaccurate estimation of the Mn using neutral polystyrene standards. The thermal stability of the synthesized polyelectrolytes was investigated using TGA. The polymers demonstrated high thermal stabilities (between 350 and 400 °C) with slightly higher decomposition temperatures for the −[NTf2] polyelec-
Scheme 1. Phosphonium Monomer Synthesis
employed to ensure alkylation could be conducted at room temperature, preventing any autoinitiation of the styrene moiety. A final workup using lithium bis(trifluoromethane)sulfonimide (Li[NTf2]) or potassium hexafluorophosphate (K[PF6]) produced the desired 1[PF6] or 1[NTf2]. 19F NMR and 31P NMR spectroscopy were used to confirm anion metathesis from −[PF6] to the −[NTf2] salt. In comparison to the previous monomer synthesis, this route requires only four synthetic steps and does not necessitate toxic alkylating agents such as dimethylsulfate.15 A random copolymerization of styrene with monomer 1 was conducted with 2-cyano-2-propyl dodecyl trithiocarbonate as the chain transfer agent or CTA (Scheme 2). Preparation of the polyelectrolyte materials could be conducted in a controlled manner using anisole as the solvent. GPC traces of the synthesized materials were obtained using THF doped with 10 mM Li[NTf2], and molecular weights were determined relative to polystyrene standards.22 These standards may not be ideal candidates for direct comparison to ionic polymers, but they provide a good basis for comparison of all synthesized materials. 254
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ACS Macro Letters Table 1. Random Copolymerization of Styrene and Phosphonium Monomer 1 Using RAFT entry
[styrene]/[1] feed ratioa
1 2 3 4 5 6 7 8
95/5 90/10 85/15 80/20 90/10 90/10 90/10 85/15
X
mol % CTA/mol % AIBN
time (h)
total monomer conv. (%)
% 1 in polymerb
[NTf2] [NTf2] − [NTf2] − [NTf2] − [NTf2] − [NTf2] − [PF6] − [PF6]
0.5/0.25 0.5/0.25 0.5/0.25 0.5/0.25 0.2/0.1 0.1/0.05 0.5/0.25 0.5/0.25
10 10 10 10 10 10 8 8
46.9 57.0 57.5 58.4 32.4 32.6 48.9 56.4
3.6 7.5 11.6 18.9 8.2 7.8 8.8 12.8
− − −
Mn, theoc
Mn, GPCd
13 20 25 29 29 58 16 22
14 16 19 15 18 30 15 18
300 500 100 900 100 700 300 300
800 700 000 600 300 100 500 400
Đ
Tg (°C)
1.18 1.22 1.22 1.16 1.20 1.37 1.20 1.23
100 104 105 102 102 107 126 137
All polymerization reactions were conducted at 5 M in anisole at 75 °C. bPolymer composition was determined using 1H NMR spectroscopy. Theoretical molecular weights were estimated using a weighted average of the initial monomer feeds according to the equation: p × [( fstyrene· MWstyrene + fcation·MWcation) ÷ % CTA], where p is the percent monomer conversion determined from 1H NMR spectroscopy; f is the initial monomer feed; and MW is the monomer molecular weight. dGPC profiles were recorded at 40 °C versus polystyrene standards using THF with 10 mM Li[NTf2] as the eluent. a c
Figure 2. Left: plot of ln([M]o/[M]) vs time. Middle: GPC traces illustrating molecular weight evolution with time. Right: plot of Mn and dispersity vs conversion. Polymerization conditions: 5 M in anisole at 75 °C with 15% 1[NTf2] (Entry 3, Table 1).
free-standing membranes did not have sufficient integrity for ion-exchange measurements. We suspect that chain-entanglement molecular weights have not yet been reached, and further efforts will focus on higher molecular weights with these systems. Several recent reports have illustrated that block copolymers can be used as AEM materials to enhance performance.28 To further highlight the degree of control over phosphonium polymerization, we synthesized a polyisoprene macro-RAFT agent (Mn = 10 100, Đ = 1.23)29 and used this macroinitiator to copolymerize 1[PF6] with styrene (Mn = 26 000, Đ = 1.27). GPC data confirmed formation of the desired copolymer (Supporting Information). Block copolymers of this type will be explored as a method to improve the mechanical integrity of these phosphonium materials. In conclusion, we have developed a new synthetic route to a styrene-based tetrakis(dialkylamino)phosphonium monomer. Well-defined random copolymers were prepared with styrene using 2-cyano-2-propyl dodecyl trithiocarbonate as the RAFT agent. Kinetic data and GPC traces indicate controlled behavior of the polymerization reaction. Ion content in the polymers could be tuned by adjusting the monomer feed prior to polymerization, and up to 18.9 mol % phosphonium incorporation was achieved. TGA curves demonstrate the high thermal stability of these materials, and DSC analysis illustrates the glass transition temperature can be tuned by varying the counterion. The reported synthesis expands the scope of polymerizable cations by RDRP methods and has potential for making durable and stable anion exchange membranes. Further efforts will focus on higher molecular weights, block copolymers, and investigation of the stability of this class of materials. The benzylic position will be explored in
trolytes (Figure 3). The primary decomposition pathway is likely polymer backbone degradation.25 An appropriate glass transition temperature (Tg) is necessary for optimizing ion transport in polyelectrolyte membranes. Operating at or above the Tg the material has been shown to increase ion conductivity by orders of magnitude.26 DSC curves show one broadened glass transition, which is indicative of a random copolymer (Figure 3). A second transition was not observed, which would have suggested partial formation of an ion-rich microphase. The polymer samples with −[NTf2] as the counterion exhibit Tg values similar to polystyrene (104 °C), and these values do not change significantly with increasing ion content. Interestingly, the Tg increased to 126 °C with 10% 1[PF6] (Figure 3) and up to 137 °C with 15% 1[PF6]. This behavior could be due to a stronger interaction between the cation−anion pair for a −[PF6] counterion and is consistent with previous reports on cationic polyelectrolytes.27 We attempted to cast thin-film membranes of these materials by slow evaporation from acetone and chloroform. The resulting films were optically transparent; however, they were brittle, and
Figure 3. TGA (left) and DSC (right) curves of phosphonium polyelectrolytes. 255
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ACS Macro Letters
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depth to investigate how this tether influences stability of the tetrakis(dialkylamino)phosphonium cation.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.5b00910. Complete descriptions of experimental procedures including monomer and polymer synthesis, NMR spectra, GPC traces, and TGA and DSC curves (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS KJTN is grateful to the NSF for support through a Career Award (CHE-1455136). This technical effort was also performed in support of the U.S. Department of Energy’s National Energy Technology Laboratory’s ongoing research on CO2 capture under the contract DE-FE0004000. NMR Instrumentation at Carnegie Mellon was partially supported by the NSF (CHE-0130903 and CHE-1039870).
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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on February 1, 2016, with text errors in the abstract and references. The corrected version was republished on February 2, 2016.
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