Influence of Spacers in Tetherable Initiators on Surface-Initiated Atom

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Influence of Spacers in Tetherable Initiators on Surface-Initiated Atom Transfer Radical Polymerization (SI-ATRP) Jiajun Yan,† Xiangcheng Pan,† Zongyu Wang,† Jianan Zhang,†,‡ and Krzysztof Matyjaszewski*,† †

Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States School of Chemistry and Chemical Engineering, Anhui University, Hefei 230601, China

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S Supporting Information *

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INTRODUCTION

RESULTS AND DISCUSSION Synthesis of Tetherable Initiators and Particle Modification. Tetherable initiators bearing a chlorodimethylsilyl anchoring group and a 2-bromoisobutyrate initiating group with three different spacers, C3, C6, and C11, were synthesized in two steps (Scheme 1).12,17

Surface-initiated atom transfer radical polymerization (SIATRP) is one of the most versatile and robust methods for creation of a well-defined and stable interface between the inorganic surface and polymer brushes.1−3 A wide variety of organic−inorganic hybrid materials were successfully fabricated via SI-ATRP during the past decade.4−8 Polymers formed by SI-ATRP, as those prepared by homogeneous ATRP,9,10 have precisely controlled architecture and molecular weight (MW) as well as narrow molecular weight distribution (MWD). In addition, SI-ATRP can generate polymer brushes with very high graft densities.1−4,11,12 Various aspects of SI-ATRP that influence the kinetics and quality of products were reported.13−16 In our previous publication, the influence of tetherable initiator structure and particle size on metal-free SI-ATRP was reported.17 Earlier reports showed that the spacer length within the tetherable initiator influenced the MW, graft density, and stability of grafted polymer brushes.18−20 However, detailed analysis of the polymerization kinetics was lacking. Moreover, new ATRP techniques, including activator (re)generated by electron transfer (A[R]GET) ATRP,21 use of supplementary activator and reducing agent (SARA) ATRP,22 initiator for continuous activator regeneration (ICAR) ATRP,23 photoinduced ATRP (photoATRP),24−29 electrochemically mediated ATRP (eATRP),30 and recently developed metal-free ATRP,31−34 have emerged in the past decade, expanding traditional ATRP methods. Many of these ATRP techniques were successfully applied to grafting polymer brushes from surfaces.8,17,33,35,36 Previously, the effect of initiator structure, scattering (particle size), and catalyst concentration in metal-free SI-ATRP was investigated.17 However, the influence of the spacer length in the tethered initiators was not studied. In order to further understand the effect of the spacers, we synthesized three tetherable initiators with 3, 6, and 11 −CH2− units as spacers (abbreviated as C3, C6, and C11 initiators) between the tethering group and the 2-bromoisobutyrate group. The kinetics and products of normal, photoinduced, and metalfree SI-ATRP of methyl methacrylate (MMA) from silica nanoparticles (SiO2 NPs) modified with these three initiators were studied. PhotoATRP was selected as one of the low ppm of Cu techniques due to its similarity to metal-free ATRP. Normal ATRP, the conventional catalytic system, was chosen as the reference procedure. © XXXX American Chemical Society

Scheme 1. Synthesis of Tetherable Initiators with Various Spacer Lengths

All three ω-vinyl alcohols are commercially available. The esterification followed by Pt(0)-catalyzed hydrosilylation provided three ω-(chlorodimethylsilyl)alkyl 2-bromoisobutyrate tetherable initiators. The chlorodimethylsilyl anchoring group was used to effectively avoid the formation of a multilayer on the surface.37 SiO2 NPs (ca. 16 nm diameter) were surface-modified with the three tetherable initiators, resulting in three batches of functionalized SiO2 NPs with apparent bromine densities of ∼1.5 Br/nm2 (Table S1). The slightly larger C11 initiator resulted in the lowest initiator density. However, since achievable graft densities in SIATRP from SiO2 NPs are usually ca. 1 nm−2,1,6,8,12,38 the accessible initiator densities of all three particles were assumed to be similar, ∼1 nm−2, to simplify the calculations. A relatively high targeted degree of polymerization of 500 was set to provide a sufficiently dilute reaction medium and avoid interparticle cross-linking.2 In both photoATRP and metal-free SI-ATRP, activator (re)generation depends on the photoirradiation. As discussed in our previous report,17 the efficiency of UV irradiation was affected by the scattering from the NPs. The scattering cross section is proportional to a sixth power of the diameter.7 In order to obtain a quantitative estimation of the contribution of spacer lengths to the size, and hence the scattering cross section of the NPs, transmission electron microscope (TEM) images of monolayers of the initiator-modified SiO2 NPs were analyzed (Figure S1). The mean interparticle distances of closely packed SiO2 NPs were measured and counted as effective diameters of the modified SiO2 NPs. The relative scattering cross section, Received: October 19, 2016 Revised: November 12, 2016

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DOI: 10.1021/acs.macromol.6b02273 Macromolecules XXXX, XXX, XXX−XXX

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Table 1. Summary of SI-ATRP Reactions from Tetherable Initiators with Various Spacer Lengths and via Different Methods entry 1 2 3 4 5 6 7 8 9

spacer C3 C6 C11 C3 C6 C11 C3 C6 C11

reaction methoda normal normal normal photo photo photo metal-free metal-free metal-free

reaction timeb 8.5 h 6h 4h 2.7 h 2.5 h 2.75 h 45 min 45 min 25 min

final convc (%)

Mnd

32 28 30 22 26 26 33 31 17

× × × × × × × × ×

2.32 2.58 2.26 2.15 2.39 2.30 3.42 3.14 3.16

4

10 104 104 104 104 104 104 104 104

Mw/Mnd

σe (nm−2)

1.18 1.17 1.17 1.19 1.19 1.16 1.51 1.49 1.38

0.46 0.48 0.59 0.53 0.54 0.52 0.36 0.39 0.31

a Reaction conditions: [SiO2−Br, ∼1 Br/nm2]0/[MMA]0/[CuCl]0/[CuCl2]0/[dNbpy]0 = 1/500/0.475/0.025/1; 50 vol % anisole; 60 °C (normal); [SiO2−Br]0/[MMA]0/[CuBr2]0/[Me6TREN]0 = 1/500/0.05/0.3; 50 vol % anisole; room temperature, UV irradiation at 365 nm (photo); [SiO2− Br]0/[MMA]0/[PhPTZ]0 = 1/500/0.5; 50 vol % DMA; room temperature, UV irradiation at 365 nm (metal-free). bReactions were quenched when a 25 mm × 12 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.

Figure 1. Kinetic plots (a−c) and MW evolutions (d−e) of SI-ATRP from tetherable initiators with C3 (black square), C6 (red circle), and C11 (blue triangle) spacers via normal (a, d), photoinduced (b, e), and metal-free (c, f) ATRP. (a, c) Semilogarithmic linear fitting of the polymerization kinetics was shown in black solid lines (C3), red dashed lines (C6), and blue dotted lines (C11). (b) Data plots were linked with basis spline to guide vision with no mathematical implication. Theoretical MW was plotted as black solid lines in MW evolutions (d−e). Reaction conditions: [SiO2−Br, ∼1 Br/nm2]0/[MMA]0/[CuCl]0/[CuCl2]0/[dNbpy]0 = 1/500/0.475/0.025/1; 50 vol % anisole; 60 °C (normal); [SiO2−Br]0/[MMA]0/ [CuBr2]0/[Me6TREN]0 = 1/500/0.05/0.3; 50 vol % anisole; room temperature, UV irradiation at 365 nm (photo); [SiO2−Br]0/[MMA]0/ [PhPTZ]0 = 1/500/0.5; 50 vol % DMA; room temperature, UV irradiation at 365 nm (metal-free).

phenylphenothiazine (PhPTZ).34 The monomer conversions and MWs of the polymer brushes prepared in all reactions were followed by 1H nuclear magnetic resonance (NMR) and size exclusion chromatography (SEC). The reactions were stopped when a 25 mm × 12 mm oval rare earth extra power stir bar stopped moving. Beyond this point, homogeneity of the reactions could not be ensured, and irreversible interparticle cross-linking might start. The attainment of this point in each reaction relied on multiple factors, such as initial concentration, particle size, MW and MWD of the polymer brushes, temperature, and stirring power. Samples for 1H NMR were taken immediately when the stir bar stopped moving. The final conversion was thus recorded at this point. The purified final products, obtained by precipitation in methanol, were studied with SEC and thermogravimetric

using C3-initiator-modified SiO2 NPs as reference, is listed in Table S1. At the same concentration, SiO2 NPs with C6 and C11 initiators scattered 13% and 47% more incident light at the initial stage. SI-ATRP. Three representative ATRP techniques, normal ATRP, photoATRP, and metal-free ATRP, were selected to investigate the contribution of spacer length on kinetics and outcomes of SI-ATRP (Table 1). To improve the control of polymerization, halogen exchange was used in normal ATRP to mitigate the influence of penultimate unit effect;39,40 i.e., a CuCl2/CuCl pair was used to modulate the polymerization. In both photoATRP and metal-free ATRP, a UV irradiation at 365 nm was used. The excess of ligand, Me6TREN added at the beginning, served as the sacrificial activator regenerator in photoATRP for the 100 ppm of Cu(II),29 whereas the metalfree ATRP was controlled by a photoredox reaction of 10B

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the future selection of tetherable initiators for applications of SI-ATRP under various conditions.

analysis (TGA) to determine the number-average MW (Mn), dispersity (Mw/Mn), and graft density (σ). Kinetic Studies. As shown in Table 1, all three normal ATRP reactions displayed almost identical final conversion, Mn, and Mw/Mn, indicating similar good control over the polymerization. However, the times required for the three reactions to reach their final conversions were different. The rate of polymerization (Rp) was significantly higher for the initiator with a longer spacer (Figure 1a), indicating the presence of a higher equilibrium radical concentration in the reaction. As there should not be much difference in Cu(I)/ Cu(II) ratio for the three reactions, a higher effective alkyl halide concentration, especially at the initial stage of the reaction, could be responsible for the higher equilibrium radical concentration. Therefore, the longer and more accessible tetherable initiator might have contributed to the higher Rp. This hypothesis was further supported by the higher graft density achieved from the C11 initiator. In photoATRP, an increase of the Rp was observed in all three cases (Figure 1b). This could be due to a gradual photogeneration of Cu(I) and hence slow increase of Cu(I)/ Cu(II) ratio.29,41 The graft densities of all three products were similar and comparable to the highest graft density obtained via normal ATRP (Table 1). The Cu/Me6TREN complex is much less bulky than Cu/dNbpy2, and it could reach and activate initiating sites even for a short spacer C3. In contrast, the much bulkier Cu/dNbpy2 complex used for normal ATRP plausibly could not reach initiators with C3 and C6 shorter spacers after brushes started to grow. In metal-free ATRP the lowest graft densities, highest MWs, and highest dispersities were observed. Limited concentrations of activators and deactivators in the reaction rendered the initiation and deactivation less efficient. Consequently, all three reactions had the lowest levels of control over the polymerization, in comparison to the previous two methods (Figure 1f), especially in the reaction using the C3 initiator.17 The dependence of the Rp on the length of the spacer was smaller than that in the other two methods (Figure 1c). The scattering from the C11 initiator-modified silica (Table S1) could plausibly neutralize the contribution of higher accessibility of C11 alkyl halide to the overall Rp. A slightly lower conversion and graft density with C11 initiator could be due to a collapsed nonpolar C11 spacer in polar DMA.42 In metal-free ATRP the lowest graft densities were observed since longer brushes may hinder original initiating sites in a surface initiated polymerization with a large dispersity43 (Table 1).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02273. Experimental procedures; measurement of interparticle distances; TGA traces (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.M.). ORCID

Krzysztof Matyjaszewski: 0000-0003-1960-3402 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Nissan Chemical for their generous donation of SiO2 NPs. We thank the NSF (DMR 1501324), the DoE (DE-EE0006702), and the CRP Consortium of CMU for the financial support. J.Y. acknowledges the support from the Richard King Mellon Foundation Presidential Fellowship and J.Z. the support from the CSC Scholarship.

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CONCLUSION Tetherable ATRP initiators with three different lengths of alkyl spacers, C3, C6, and C11, were synthesized and utilized to functionalize SiO2 NPs. Three representative SI-ATRP conditions were used to graft PMMA brushes from the SiO2 NPs. Polymerization kinetics and graft densities were measured. The influence of the spacer lengths on kinetics and outcomes of SI-ATRP were demonstrated. In both normal ATRP and metalfree ATRP, longer spacer resulted in more accessible alkyl halide and hence higher graft density. In addition, the Rp in normal ATRP was faster than in photoATRP. In contrast, the contribution of spacer was smaller in photoATRP due to Cu/ Me6TREN catalyst smaller than Cu/dNbpy2 catalysts used for normal ATRP. The detailed understanding of how the polymerization was affected by the spacers may contribute to C

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