Letter pubs.acs.org/macroletters
Enhancing Initiation Efficiency in Metal-Free Surface-Initiated Atom Transfer Radical Polymerization (SI-ATRP) Jiajun Yan,† Xiangcheng Pan,† Michael Schmitt,‡ Zongyu Wang,† Michael R. Bockstaller,‡ and Krzysztof Matyjaszewski*,† †
Department of Chemistry and ‡Department of Material Science and Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States S Supporting Information *
ABSTRACT: Well-defined polymer-inorganic hybrid materials were prepared via metal-free surface-initiated atom transfer radical polymerization (SI-ATRP) with 10-phenylphenothiazine (PhPTZ) as the photocatalyst and 2-bromo-2-phenylacetate initiator tethered to silica surfaces. Initiation efficiency and, hence, graft density were significantly enhanced by this very reactive initiator. The polymerization kinetics, effect of initiator structures, particle sizes, and catalyst concentrations were investigated. Well-defined hybrid particles were prepared at a low catalyst concentration (0.02 mol % or 0.1 mol % to monomer). Poly(methyl methacrylate) (PMMA) with number-average molecular weight of 3.65 × 104, dispersity of 1.43, and graft density of 0.60 chain/ nm2 was grafted from the surface of silica nanoparticles. The hybrid materials were characterized with size exclusion chromatography (SEC), thermogravimetric analysis (TGA), dynamic light scattering (DLS), and transmission electron microscopy (TEM).
I
acted as the activator and the resulting radical cation served as the deactivator, resembling Cu(I) and Cu(II) species in conventional ATRP (Scheme 1).
n the past two decades, atom transfer radical polymerization (ATRP) has emerged as one of most versatile and robust methods for preparation of polymers with controlled molecular weight (MW), molecular weight distribution (MWD), architectures, and functionality.1 However, the use of ATRP metal catalysts, such as copper complexes, results in the presence of an inevitable metal residue in final product. The presence of metal impurities in the products may limit the use of these polymers in electronic and biomedical applications. Extensive research has been carried out on catalyst removal from ATRP products.2 Indeed, various low-ppm catalyst ATRP methods were developed to avoid time-consuming and costly catalyst removal, including activator regenerated by electron transfer (ARGET) ATRP,3 use of Cu(0) as supplementary activator and reducing agent (SARA) ATRP,4 initiators for continuous activator regeneration (ICAR) ATRP,5 electrochemical ATRP (eATRP),6 or photoinduced ATRP (photoATRP).7 In addition to copper catalysts, other metal complexes, such as ruthenium,8 iridium,9 and iron10 catalysts were used for ATRP. Although some recent developments utilizing less toxic iron-based catalyst in ATRP improved the biocompatibility of the products, they still have some limitations.11 Recently reported, a photoinduced metal-free ATRP using an organic catalyst eliminated residual metal in the product.12 A wide range of monomers, including methacrylates and acrylonitrile were successfully polymerized via metal-free ATRP. A photoexcited 10-phenylphenothiazine (PhPTZ) © XXXX American Chemical Society
Scheme 1. Simplified Activation/Deactivation Mechanism for Metal-Free ATRP Catalyzed by PhPTZ
Surface-initiated ATRP (SI-ATRP) is an important synthetic technique for preparation of polymer−inorganic hybrid materials. 13 The development of metal-free techniques eliminates metal impurities from various functional materials prepared via SI-ATRP, including porous carbon for catalysis14 and porous titania for dye sensitized solar cells.15 Interestingly, photoinduced metal-free ATRP could provide precise temporal Received: April 16, 2016 Accepted: May 12, 2016
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DOI: 10.1021/acsmacrolett.6b00295 ACS Macro Lett. 2016, 5, 661−665
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ACS Macro Letters Scheme 3. Synthesis of BPASiCla
and spatial controls,12a−d which are crucial for surface-initiated polymerizations to control the progress and place of polymerization.13 Recently, metal-free SI-ATRP of methacrylates from flat surfaces and particles was reported.16 However, a tethered initiator based on 2-bromoisobutyrate (BiB) was used, resulting in limited initiation efficiency and, hence, low graft density. Due to its lower reactivity, conventional BiB-based initiators may result in slower initiation and lower initiation efficiency in polymerization of methacrylates. Unreacted initiators, and even oligomers, can be removed during a postpolymerization purification after homogeneous solution polymerization. However, when the initiators are covalently anchored onto a surface in SI polymerization, lower initiation efficiency results in lower graft density, and consequently, aggregation, and formation of ill-defined hybrid materials.13b,c,17 Activation rate in ATRP depends on initiator structure. Thus, ethyl 2-bromo-2-phenylacetate (EBPA) is a much more reactive initiator than ethyl 2-bromoisobutyrate (EBiB).18 Indeed, highly reactive initiators, such as EBPA and 2-bromopropionitrile (BPN) were successfully used for polymerization of methacrylates and acrylonitrile via metal-free ATRP.12a−d EBiB has limited initiation efficiency for MMA due to penultimate effect, whereas EBPA has an activation rate coefficient >1000× larger.18,19 Therefore, to enhance the initiation efficiency and, thus, graft density, we combined the structure of EBPA with a silane anchoring group to create novel BPA-based tetherable initiators and compared them to conventional BiB-based tetherable initiators (Scheme 2). The catalyst concentrations could be significantly reduced to balance the cost and yield.
a
Reagents and conditions: (a) 2-bromo-2-phenylacetic acid, N,N′dicyclohexylcarbodiimide (DCC), 4-dimethylaminopyridine (DMAP), dry methylene chloride (DCM), room temperature; (b) chlorodimethylsilane, Karstedt’s catalyst.
Six metal-free SI-ATRP reactions were performed to compare the influence of different parameters, such as initiator type, particle size, and catalyst concentration on the “graftingfrom” polymerizations. Two of the reactions were conducted from BiB-based tetherable initiators on either 16 or 120 nm silica nanoparticles. The reactions were compared with other two reactions utilizing BPA-based tetherable initiators under the same conditions. The last two reactions were designed to examine the influence of catalyst concentration. The results are summarized in Table 1. The accessible initiation sites for surface-initiated polymerization depend upon various reaction parameters, including bulkiness of monomers and initiation efficiency.17 Typically, the grafting density lies between 0.1−0.8 nm−2, although initiator density can exceed 1 nm−2.13b,22,24 Elemental analysis showed ∼1 Br/nm2 for modified 16 nm silica nanoparticle. Since it is difficult to accurately measure the density of accessible initiation sites on large nanoparticles, the initiator density was assumed as the same value as 16 nm particle to ensure a fair comparison between different samples. A high target degree of polymerization (DP) of 500 was chosen in order to provide a dilute reaction medium and avoid interparticle cross-linking.13b Four parameters: final conversion, number-average molecular weight (Mn), dispersity (Mw/Mn), and graft density (σ) were measured or calculated, which are listed in Table 1. The final monomer conversion was determined when the 10 mm × 5 mm oval rare earth extra power stir bar could no longer stir the polymerization mixture. Beyond this point, the reaction could start uncontrolled cross-linking. Multiple factors influence the attainment of this point including concentration, molecular weight, and dispersity. Graft densities were calculated as a measure of initiation efficiency. These four parameters reflected overall control over the polymerization (vide infra). Polymerization of MMA from BiB-based initiator reached 6.4% conversion at 1 h, providing only 0.03 nm−2 graft density (entry 1). Switching from BiB-based initiator to BPA-based initiators significantly improved graft densities to 0.15 nm−2 and resulted in higher final conversion (18%; entry 1 vs 2). The kinetic data are presented in Supporting Information. Despite significant improvement in graft density after switching to BPAbased initiators, the graft density was lower than from Cumediated systems, plausibly due to scattering from large 120 nm silica nanoparticles (Table 1). Thus, metal-free SI-ATRP from 16 nm silica nanoparticle modified with BiBSiCl and BPASiCl was investigated and the results are summarized in entries 3 and 4 in Table 1 and Figure 1. The overall rate of metal-free SI-ATRP from silica nanoparticles modified with BiBSiCl was slower than the same reaction from silica nanoparticles modified with BPASiCl (Figure 1a). In addition, the MW of grafted polymer from BiBSiCl deviated significantly from the theoretical prediction and did not increase linearly with conversion (Figure 1b). In contrast, the polymerization from BPASiCl displayed a progressive increase in MW and approached the theoretical
Scheme 2. Structures of BiB-Based and BPA-Based Tetherable Initiators
Silica nanoparticles with diameters of 16 and 120 nm were used as the inorganic core for the SI-ATRP reactions.20 BiBbased tetherable initiators (BiBSiCl and BiBSiOEt) and BPAbased tetherable initiators (BPASiCl and BPASiOEt) were selected for each size of nanoparticles. Chlorodimethylsilane was used as anchoring group for the 16 nm silica nanoparticles to avoid formation of multiple layers.21 Triethoxysilane moiety was used for the 120 nm silica nanoparticles to ensure more stable bonding.22 A BPA-based tetherable initiator was previously reported as a precursor for a tetherable chain transfer agent for surfaceinitiated reversible addition−fragmentation chain transfer (SIRAFT) polymerization.23 However, BPA moiety was never used for SI-ATRP. An example of synthesis of BPA-based tetherable initiator is given in Scheme 3. 662
DOI: 10.1021/acsmacrolett.6b00295 ACS Macro Lett. 2016, 5, 661−665
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ACS Macro Letters Table 1. Summary of Metal-Free SI-ATRPa entry 1 2 3 4 5 6
initiator BiBSiOEt BPASiOEt BiBSiCl BPASiCl BiBSiCl BPASiCl
particle size (nm) 120 120 16 16 16 16
[PhPTZ]0/[MMA]0 (ppm)
reaction timeb (h)
200 200 200 200 1000 1000
final conversionc (%)
Mnd
6.4 18 16 36 31 57
× × × × × ×
1.0 3.0 2.0 3.0 3.0 1.4
1.12 9.20 8.27 4.70 3.60 3.65
5
10 104 104 104 104 104
Mw/Mnd
σe (nm−2)
1.91 2.06 1.93 1.62 1.60 1.43
0.03 0.15 0.09 0.39 0.38 0.60
a Reaction conditions: [SiO2−Br, ∼1 Br/nm2]0/[MMA]0/[PhPTZ]0 = 1:500:0.1/0.5; DMA = 50 vol %; UV irradiation at 365 nm, room temperature. bReactions were quenched when a 10 mm × 5 mm oval rare earth extra power stir bar stopped moving in the reaction. cCalculated from 1H NMR integral of protons on MMA double bond. dMeasured with SEC using THF as eluent and toluene as internal standard and calibrated to linear PMMA standards. eCalculated from TGA inorganic fraction and eq S1.
dielectric index contrast between particles and the reaction medium.25 To assess the role of scattering and absorption on the reaction process, extinction spectra of both 120 and 16 nm silica nanoparticles were measured in N,N-dimethylacetamide (DMA) at corresponding reagents concentrations. Since the scattering cross section of spherical particles scales with the sixth power of particle size we expect the scattering cross section of 120 nm particles to be about a factor 7.56 ∼ 1.5 × 105 greater than of 16 nm particle analogs.26 This is approximately consistent with the increase of the extinction coefficient that is measured for solutions of both particle systems (Table 2 and Figure S4). Table 2. Extinction and Extinction Coefficients of Silica Nanoparticle Dispersions at 365 nma particle size (nm) 16 120
extinction 0.030 0.67c
b
extinction coefficient (mL g−1 cm−1) 0.94 4.3
a
Measured in DMA dispersion, 1.0 cm path. b0.032 g/mL. c0.155 g/ mL.
The absolute extinction and extinction coefficient at irradiation wavelength (365 nm) for 16 and 120 nm silica nanoparticle dispersion are listed in Table 2 and reveal that 120 nm silica nanoparticle dispersion attenuated light at 365 nm significantly stronger than its 16 nm counterpart. Thus, for larger particles, both initiation and deactivation should be less efficient. This prediction from UV−vis spectra agreed with observations from polymerization (Table 1). Under the same reaction conditions, large silica nanoparticles resulted in slower polymerization and hybrid particles with lower graft densities. Although surface curvature may affect graft density in an SIATRP reaction,27 it had small contribution to the final graft density here. In entries 1 and 2, each polymer chain occupied an average area of 33 and 7 nm2, respectively, far larger than the cross-sectional area of a PMMA brush.28 Therefore, the graft densities were predominantly determined by low initiation efficiency, affected by the external quantum yield of the photocatalyst, rather than surface curvature. Previous report on metal-free SI-ATRP used a significantly higher catalyst concentration (10 mg/mL) and achieved a moderate MWD of the polymers obtained with BiB-based tetherable initiator.16 This indicated that a higher catalyst concentration might mitigate the slow initiation and improve graft density. Therefore, two experiments, with higher concentrations of catalyst (1.3 mg/mL), were performed to evaluate the influence of catalyst concentration.
Figure 1. Kinetics (a), molecular weight (black), and dispersity (red) evolutions (b) of metal free SI-ATRP from 16 nm silica nanoparticle modified with BiBSiCl (200 ppm PhPTZ: entry 3, open circle, dashed line; 1000 ppm PhPTZ: entry 5, open triangle, dash-dotted line) or BPASiCl (200 ppm PhPTZ: entry 4, solid square, solid line; 1000 ppm PhPTZ: entry 6, solid diamond, dotted line). The solid line in molecular weight evolution is the theoretical value calculated from target degree of polymerization. Reaction conditions: [SiO2−Br, ∼1 Br/nm2]0/[MMA]0/[PhPTZ]0 = 1:500:0.1/0.5; DMA = 50 vol %; UV irradiation at 365 nm, room temperature.
values as well as a lower dispersity, both indicating a moderate level of control over the reaction. The efficiency of photoinduced polymerization strongly depends on the probability of conversion of incident photons to excite the photocatalyst.12d In a surface-initiated polymerization from nanoparticles, light may be absorbed or scattered. Scattering by nanoparticles is affected by particle size and the 663
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ACS Macro Letters
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Both reactions with higher catalyst concentrations displayed profound improvement in control of polymerization and graft densities. The reaction from BPASiCl (entry 6) progressed for 1.4 h before it turned too viscous to stir at a conversion of 57%. The higher catalyst concentration resulted in a faster polymerization. The significant acceleration can be ascribed to increased amount of excited catalyst at the surface. Consequently, the lower activation rate was compensated by an increased catalyst concentration. A linear semilogarithmic kinetic plot indicated efficient initiation (Figure 1). Both the regular increase of MW and decrease of dispersity with the conversion demonstrated good control over the polymerization. The final product with graft density of 0.60 nm−2 is comparable to values achievable via copper-catalyzed SI-ATRP.13b,24 TEM images confirmed successful grafting of PMMA from silica nanoparticles and show a small fraction of aggregated nanoparticles. The broad size distribution, observed in DLS, originated in broad size distribution of the pristine silica nanoparticles (Figure 2).
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
We acknowledge Nissan Chemical for their generous donation of silica nanoparticles. We acknowledge the financial support from the NSF (DMR 1501324), the Department of Energy (DE-EE0006702), and the CRP Consortium of Carnegie Mellon University. NMR instrumentation at CMU was partially supported by NSF (CHE-0130903).
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Figure 2. (a) Bright field TEM images of 16 nm silica nanoparticles modified with BPASiCl used to form grafted PMMA (entry 4). Inset: magnified TEM image to show more details. Scale bars: 200 nm. (b) DLS hydrodynamic size distribution by intensity of the same nanoparticles in THF.
To summarize, two tetherable initiators based on 2-bromo-2phenylacetate were prepared and utilized for surface modification of silica nanoparticles. Comparison of metal-free SI-ATRP reactions from silica nanoparticles modified with BiBbased tetherable initiators and BPA-based tetherable initiators demonstrated the superior initiation properties of the BPAbased initiators. The hybrid materials prepared via metal-free SI-ATRP at low catalyst concentration (0.02 or 0.1 mol %) were of quality comparable to hybrid particles prepared by Cucatalyzed SI-ATRP. The development of BPA-based tetherable initiator enabled application of metal-free SI-ATRP for the preparation of hybrid materials without transition metal catalyst. The BPA-based tetherable initiators are capable of initiating metal-free SI-ATRP, and should be also applicable to SI-ATRP with Cu-catalyzed systems.
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Letter
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00295. Experimental procedures, TGA plots of the final samples, additional kinetic plots, SEC traces and size evolutions, and UV−vis spectra of silica nanoparticle dispersions (PDF). 664
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