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Functionalization of Single-Walled Carbon Nanotubes via the Piers− Rubinsztajn Reaction Ryan C. Chadwick, John B. Grande, Michael A. Brook, and Alex Adronov* Department of Chemistry, McMaster University, Hamilton, ON L8S 4M1, Canada S Supporting Information *
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silicone−nanotube composite that can participate in hydrosilylative cross-linking of bulk PDMS. In order to test the efficiency of the Piers−Rubinsztajn reaction on the surface of nanotubes, anisole-grafted SWNTs (aSWNTs) were synthesized via the Tour reaction, using panisidine and isoamyl nitrite (Scheme 1).43,44 After removal of
he silane and siloxane functionalization of carbon nanotubes has been widely investigated, and a number of silylation strategies have been employed.1 These include the extensive use of halo- or alkoxysilanes to react with surface hydroxyls,2−12 the use of polysilazanes,13 direct hydrosilylation of the carbon nanotube surface,14 silylesterification,15 photochemical silylation,16 and the formation of supramolecular complexes.17 The interest in these conjugate materials stems from several potential applications including heat or charge dissipative elastomers,2,18 biocompatible and flexible electrodes or sensors,19,20 Li-ion batteries,21 robust elastomers,22 capacitive energy harvesting,23 artificial muscles,24 skin-like stretch sensors,25−27 and other electronics.28 However, despite this extensive research into the use of smallmolecule silanes to decorate carbon nanotube side-walls, the functionalization of carbon nanotubes or graphene with polymeric silicones, such as PDMS, is still remarkably rare.5,29 Likewise, there is little precedent for covalently anchoring singlewalled carbon nanotube (SWNT) composites in a PDMS host material, despite having been shown to be an effective technique in this and similar elastomers.30,31 The high solubility of silicone chains within a wide variety of common organic solvents makes them ideal solubilizing agents for insoluble materials, such as SWNTs, where they can operate as steric stabilizers. However, virtually all reports of PDMS−SWNT composites use slow-tocure commercial silicone kits, leading to a high degree of nanotube flocculation or phase separation, processes that occur faster than the silicone chemistry.2 This lack of activity in synthesizing PDMS−SWNT complexes is particularly surprising considering the interest in silicone composites for flexible electronics, such as artificial skin and other touch-based sensors,32,33 where elastomer robustness is a key factor, but may have to do with the absence of facile processes for silicone− SWNT grafting. The recently developed Piers−Rubinsztajn (PR) reaction34−36 is an efficient (C6F5)3B-catalyzed hydrosilylative coupling between an siloxy, silanol, alcohol, or alkoxy group (generally methoxy, ethoxy, and propoxy) and a hydrosilane (Si−H). This reaction has been used for the production of well-defined macromolecules, silicone elastomers, and silicone foams.37−41 It is compatible with alkenes and alkyl halides, potentially enabling the sequential use of other common silicone chemistry.42 However, to our knowledge, the Piers−Rubinsztajn reaction has not yet been explored for the modification of surfaces or graphitic materials. Here, we demonstrate the viability of the Piers−Rubinsztajn reaction for the functionalization of singlewalled carbon nanotubes (SWNTs) by producing a series of PDMS−SWNT composites and develop an easily dispersible © XXXX American Chemical Society
Scheme 1. Functionalization of Single-Walled Carbon Nanotubes via Tour Reaction, Followed by Piers− Rubinsztajn Reaction
excess p-anisidine through extensive rinsing with DMF and toluene, the resulting a-SWNTs were redispersed in toluene and then treated with an excess of α,ω-dihydride-terminated PDMS (PDMS-H2) in the presence of (C6F5)3B as catalyst (Scheme 1). In each case, the addition of catalyst was followed by a short (ca. 15 s) induction period and then by rapid gas evolution. In all, a series of five molecular weights of PDMS-H2, ranging from 134 g/mol (tetramethyldisiloxane, TMDS) to 28 kDa were employed (Table 1). In general, we found the Piers−Rubinsztajn reaction was very effective for the functionalization of carbon nanotubes. If sufficiently thorough mixing was achieved, such as via light “milling” with glass beads, 2−12 h reaction times for each step of the reaction were sufficient to yield highly soluble materials (Table 1). Dispersibility in THF and toluene was measured for all samples using a modified literature method (details in Supporting Information).45 The materials functionalized using the Piers−Rubinsztajn reaction showed very high solubilities in THF (typically >350 mg L−1). Further characterization of the functionalized SWNTs was carried out using Raman spectroscopy (Figure 1). The introduction of anisole functionalities on Received: June 23, 2014 Revised: August 27, 2014
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dx.doi.org/10.1021/ma501297v | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Note
Table 1. Solubility and Graft Weights for Silicone−Carbon Nanotube Composites graft anisole TDMS PDMS-1 PDMS-6 PDMS-17 PDMS-28 control a
PDMS Mn (g mol−1) a
108 134 1.1K 6.0K 17K 28K 1.1K
PDMS av DP
solubility in THF (mg L−1)
solubility in toluene (mg L−1)
total mass loss (%, TGA)
mass PDMS (%)
N/A 2 14 80 230 380 14
0 358 ± 20 335 ± 20 588 ± 30 730 ± 40 540 ± 40
