Heterocycle-Induced Phase Separation in Conjugated Polymers

Apr 18, 2012 - Grignard metathesis polymerization was used to synthesize a series of poly(3-hexylselenophene)-block-poly(3-hexylthiophene) copolymers ...
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Heterocycle-Induced Phase Separation in Conjugated Polymers Jon Hollinger, Paul M. DiCarmine, Dominik Karl, and Dwight S. Seferos* Lash Miller Chemical Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada S Supporting Information *

ABSTRACT: Grignard metathesis polymerization was used to synthesize a series of poly(3-hexylselenophene)-blockpoly(3-hexylthiophene) copolymers with two different molecular weights and varying selenophene content. These polymers were characterized by optical absorption spectroscopy (film and solution), differential scanning calorimetry, powder X-ray diffraction, variable temperature absorption spectroscopy, and atomic force microscopy (on self-assembled polymer nanofibers). The selenophene to thiophene ratio has a large influence on optical properties, and absorption is tunable across the range of both homochromophores. We observe phase separation in the solid state in both pristine and annealed samples. When allowed to slowly assemble in solution, high molecular weight copolymers have a very sharp transition from the molecularly dissolved to the aggregated state. Most interestingly, increasing polyselenophene content induces the polymer to assemble more readily (at a higher temperature) but also appears to hinder the degree of ordered assembly when the thiophene block is not sufficiently long. This study furthers the understanding of the differences between these structurally similar conjugated polymer building blocks and provides insight into the factors that control heterocycle-induced phase separation.

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INTRODUCTION Conjugated polymers have received increasing attention over the years due to interest in low-cost electronic devices. Within these devices the solid-state morphology of conjugated polymers is one of the most critical properties that controls function. Improvements in device performance that result from factors such as changes in molecular weight,1 regioregularity,2 annealing,3,4 or the addition of additives5 can all be at least partially explained by changes in the morphology of the conjugated polymer within the device’s active layer. McGehee, Fréchet, and co-workers demonstrated that increasing the molecular weight of poly(3-hexylthiophene) (P3HT) increases field-effect mobility. This is because films of high molecular weight P3HT are composed of long nanofibril structures, which allow for more efficient charge transport.1 In another example, Heeger and co-workers demonstrated that annealing a blend of P3HT and a fullerene acceptor significantly increases photovoltaic (PV) performance by altering the morphology of the active layer to better allow charge separation and transport.4 Another important method of controlling morphology in conjugated polymers is by the synthesis of block copolymers. The synthesis of block copolymers that incorporate polythiophene motifs has been explored by several studies.6−14 These studies take advantage of the quasi-living Grignard metathesis15−18 polymerization method to prepare “allconjugated” block copolymers, which are polymers that possess a fully π-delocalized backbone and contain blocks of distinct repeat units. All-conjugated block copolymers phase-separate © 2012 American Chemical Society

into domains that are the approximate length of the distinct blocks, typically on the order of 10−20 nm,7,11 which is roughly the exciton diffusion length for conjugated organic materials.19 Promisingly, in two separate studies, both Jenekhe and coworkers20 and Lin and co-workers21 demonstrated that allconjugated block copolymers have improved power conversion efficiency in PV devices over analogous homopolymers, which is likely the result of their distinct self-assembly properties at the exciton diffusion length scale. Our group has recently reported a new type of block copolymer that consists of blocks of distinct polyheterocycles.22 In our first report we learned that poly(3-hexylselenophene)block-poly(3-hexylthiophene) (P3HS-b-P3HT) exhibits a narrowed optical band gap and a unique type of phase separation that is controlled by the heteroatom of the heterocycle. We have also learned that this type of phase separation can be used to align nanoparticles along phase boundaries providing a convenient method for the preparation of self-assembled semiconductor composites.23 Herein, we report the effect of both block composition and molecular weight on the selfassembly properties of P3HS-b-P3HT. Using a series of six carefully designed P3HS-b-P3HT copolymers, we are able to better understand why such polymers phase separate and learn Received: February 24, 2012 Revised: March 31, 2012 Published: April 18, 2012 3772

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how to more finely control this phase separation, which is induced by the identity of the heterocycle.

the selenophene and thiophene rings (at 7.12 and 6.98 ppm, respectively; see Supporting Information). In all cases, the composition agreed well with the monomer feed ratios (Table 1), and the polymers were 85−88% regioregular. Polystyrene equivalent molecular weight was determined by gel permeation chromatography (GPC) at 140 °C in 1,2,4trichlorobenzene that was stabilized with butylated hydroxytoluene. Elution times were compared with narrow molecular weight polystyrene standards. All of the polymers had unimodal peaks, confirming that acid quenching is the predominant termination mechanism as opposed to chain coupling reported in some studies (Figure 1).24 Similar retention times were

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RESULTS AND DISCUSSION Synthesis and Monomer Incorporation Determination. Polymer synthesis was carried out by a modified version of the method previously reported (Scheme 1).22 Briefly, 2,5Scheme 1. Synthesis of Poly(3-hexylselenophene)-b-poly(3hexylthiophene)

Figure 1. GPC profile of a representative HWM and LMW block copolymer with equivalent selenophene content (left). Absorption spectra of HMW polymers with varying selenophene content (right).

observed for polymers that were synthesized with a similar monomer:catalyst ratio, and molecular weight is found to scale proportionally with catalyst ratio. The LMW series has Mn values that range from 9.2 to 12.9 kg/mol, approximately half that of the HMW series, which has Mn values that range from 17.8 to 19.1 kg/mol. All polymers have a polydispersity index of 1.5 or less. Solution-Phase Characterization. Absorption Spectroscopy. To record the absorption spectra in solution, all polymers were dissolved in chlorobenzene at ∼0.02 mg/mL (Table 2). The spectrum of each polymer depends only on the

dibromo-3-hexylselenophene monomer was activated by treatment with isopropylmagnesium chloride (i-PrMgCl) in dry tetrahydrofuran (THF) for 1 h at room temperature and then transferred to a separate flask containing [1,3-bis(diphenylphosphino)propane]dichloronickel (Ni(dppp)Cl2) for polymerization at 40 °C for 20 min. In a separate flask, 2,5-dibromo-3-hexylthiophene was activated in an analogous manner for 60 min and then transferred dropwise to the polymerization mixture after the consumption of selenophene (20 min). The polymerization was allowed to continue for an additional 20 min and then quenched with a 5% HCl solution. The mixture was precipitated in methanol, filtered through a Soxhlet thimble, and extracted with methanol, hexanes, and chloroform. The chloroform fraction was concentrated to afford the purified polymers in 54−76% yields (see the Experimental Section). For the purpose of this study, the monomer:catalyst ratio is varied to control molecular weight and the monomer feed ratio is used to control the relative incorporation of selenophene and thiophene in the final polymer products. A series of high and low molecular weight polymers were synthesized. A 100:1 and 100:2 monomer:catalyst ratio is used for high (HMW) and low molecular weight (LMW) polymers, respectively. Both series contain three selenophene:thiophene ratios, approximately 25:75, 50:50, and 75:25. Monomer incorporation ratios were determined by integrating the characteristic aromatic peaks of

Table 2. Summary of Solution Absorption Characteristics polymer

abs onset (nm)

abs max (nm)

fwhm (cm−1)

HMWSe26 HMWSe53 HMWSe81

590 610 620

465 475 491

6325 6482 6221

relative monomer content, meaning that the trends observed in the LMW samples are identical to those in the HMW samples and that the solution optical properties lie in the molecular weight independent regime. Both the absorption onset and maximum absorption shift to lower energy as the selenophene content increases. The onset of absorption for the HMW sample with 26 mol % selenophene (HMWSe26) is ∼590 nm, which lies between the reported onsets for P3HT and P3HS

Table 1. Summary of Polymer Synthesis Conditions and Characterization Results polymer

monomer:catalyst (mol:mol)

yield (%)

incorporation ratio (Se:S mol:mol)

Mn (kg/mol)

PDI

regioregularity (%)

HMWSe26 HMWSe53 HMWSe81 LMWSe27 LMWSe53 LMWSe83

100:1

71 72 54 67 57 70

26:74 53:47 81:19 27:73 53:47 83:17

17.8 18.2 19.1 9.2 8.4 12.9

1.52 1.46 1.47 1.42 1.28 1.49

87 88 86 88 85 88

100:2

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homopolymers (560 and 620 nm, respectively).22 The HMW samples with 53 and 81% P3HS content (HMWSe53 and HMWSe81) both have absorption onsets (610 and 620 nm, respectively) that are closer to that of P3HS. We also note that the full width at half-maximum (fwhm), when measured in wavenumbers (cm−1), reaches a maximum in the 50:50 copolymers. This is expected if both polythiophene and polyselenophene were contributing to the chromophore equally. Indeed, we find that the two different blocks have comparable extinction coefficients (7.3 × 103−8.3 × 103 and 7.5 × 103−8.5 × 103 L molrepeat unit−1 cm−1 for P3HT and P3HS, respectively) when measured as separately prepared homopolymers (see Supporting Information). Further, we note that P3HT has a broader fwhm than P3HS, which is consistent with what is observed in the block copolymers. Solid-State Characterization. Absorption Spectroscopy. We next examined the self-assembly of block copolymers by measuring the optical absorption of thin films. For reference, thin films of P3HT have a strong peak at 550 nm and characteristic vibronic structure, most recognizable as a shoulder at ∼610 nm.22 This structure becomes more pronounced with annealing and is attributed to interchain π−π interactions that are enhanced by the annealing process. Absorption profiles of P3HS behave in a similar manner, with a peak at 600 nm and a shoulder appearing at 700 nm. Films of block copolymers were spin-cast onto glass substrates, and absorbance was measured before and after annealing for 30 min at 200 °C. No further changes in absorption properties were observed when the samples were annealed beyond this temperature or time. In all block copolymers we observe the characteristic vibronic shoulder of P3HS, which is consistent with our previous report and indicates selenophene π-stacking in the films (Figure 2). The

are better ordered than LMW before annealing. After annealing all samples have more structure in their absorption profiles. As is the case in the pristine samples, more structure is present in the high molecular weight polymers; the total absorption in the red region is increased and contains a higher intensity shoulder, which is likely due to overall increased selenophene content as well as increased selenophene−selenophene π-interactions, respectively. Taken together, the annealed and pristine absorption spectra suggest that there is a higher degree of organization in the higher molecular weight block copolymers than the lower molecular weight polymers; however, the optical properties are governed mainly by the selenophene content in the material. Cyclic Voltammetry. In order to better understand the effect of copolymer composition on electronic properties, we carried out cyclic voltammetry experiments on thin films. Broad reversible oxidation peaks were observed for all polymers (see Supporting Information); however, reduction peaks were unobtainable, likely due to polymer instability. For reference, P3HT and P3HS were examined first and found to have HOMO levels of 5.19 and 5.43 eV, respectively. P3HT is in the accepted range of reported values; however, P3HS is less studied, reportedly having a HOMO similar to that of P3HT.25 Block copolymers with high and moderate P3HT content (HMWSe26 and HMWSe53) show a similar onset of oxidation as P3HT, while the block copolymer with low P3HT content (HMWSe81) has a HOMO between that of P3HT and P3HS. The lowest unoccupied molecular orbital (LUMO) energy is calculated by the estimation of the optical band gap by the onset of absorption in thin films (Table 3). Table 3. Electrochemical Characterization of Block Copolymers polymer

HOMO (eV)a

optical band gap (eV)b

LUMO (eV)c

HMWSe26 HMWSe53 HMWSe81

−5.15 −5.16 −5.25

1.66 1.63 1.63

−3.49 −3.53 −3.62

a

Determined by the onset of oxidation vs ferrocene. bDetermined by the onset of absorption of thin films. cCalculated by the HOMO and optical band gap.

X-ray Diffraction. In order to further investigate the thinfilm organization of the block copolymers, we carried out wideangle X-ray diffraction (WAXD) measurements on the annealed thin films. The crystallinity of both P3HT and P3HS homopolymers has been characterized previously.25,26 Despite the equivalent side chain lengths, several studies report that the alkyl side chain interdigitation length (the so-called dspacing) is slightly different for P3HT and P3HS homopolymers (∼16.0−16.7 Å for P3HT26,27 and 15.2−15.5 Å for P3HS25,28). This difference is quite small, however, and given the line width of X-ray scattering (∼1 Å), we would expect a single diffraction peak from a crystalline P3HS-b-P3HT block copolymer thin film. All of the block copolymer samples show a single sharp diffraction (100) as well as higher order diffractions (200) and (300), indicating a high degree of crystallinity for all of the block copolymer lengths and compositions (Figure 3). In the LMW series we observe a single spacing that decreases with increasing selenophene content and is consistent with a slightly smaller spacing for P3HS relative to P3HT. This trend, however, is not observed in the HMW series, which suggests that the diffraction of P3HS-b-P3HT is influenced by other

Figure 2. Thin film absorption spectra of pristine (top row) and annealed (bottom row) HMW (left) and LMW (right) polymers.

structure of the absorption profile depends on the selenophene content, molecular weight, and history of the sample. Pristine films of both low and high molecular weight P3HS-b-P3HT with comparable selenophene content have similar maxima; however, the intensity of the selenophene π-stacking shoulder increases with molecular weight, suggesting that HMW films 3774

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Figure 3. Representative WAXD spectra of an annealed polymer film (HMWSe53). Inset: summary of interlayer spacing for all polymer samples.

factors than simply the incorporation ratio of monomer units. Generally, the diffraction of all of the copolymers falls within 1 Å of each other (15.3−16.3 Å) and within the accepted range of P3HT and P3HS homopolymers. Another peak is often reported for poly(3-alkylthiophene)s at 2θ ∼ 23° and is associated with π-stacking distances between chains. This peak is a relatively weak diffraction and not always visible in an XRD profile.29 With the exception of LMWSe53, this diffraction is weakly visible or not present, likely due to variations in film thicknesses (see Supporting Information). The presence of πstacking is evident, however, from the vibronic structure in the thin-film absorption spectra. Differential Scanning Calorimetry. Differential scanning calorimetry was carried out to determine the thermal properties of the polymers. Additionally, to further probe the effects of annealing, we carried out a heat/cool/heat cycle, heating to 300 °C at 10 °C/min. We first established the melting transitions of the two separate homopolymers. P3HT shows a melting transition at 240 °C, in agreement with other reported values. The behavior of P3HS has been less thoroughly studied and only one melting transition has been reported at 255 °C.25 We found two melting transitions for P3HS in the first heating cycle, at 125 and 229 °C; however, only the 229 °C transition occurs in the second heating cycle (Figure 4a). In the first heating cycle, the high molecular weight copolymers exhibit melting features that are similar to both of the homopolymers. HWMSe53, for example, has two major melting transitions at 125 and 229 °C and a minor transition at 239 °C (Figure 4b). We suspect that the low-temperature transition in both P3HS and the block copolymers is due to a kinetically stable type-2 polymorph which is converted to thermodynamically stable type-1 polymorph upon annealing. The two polymorphs can be distinguished by their different interlayer stacking distances; the alkyl side chains are more closely packed in the type-2 polymorph (12.1 Å d-spacing, Figure 4d).28 When a pristine film of HWMSe53 was examined by WAXD, the presence of the type-2 polymorph is observed as a small peak at 2θ = 7.5° (see Supporting Information). The observation of this selenophene−selenophene interaction provides evidence that phase separation occurs prior to annealing. The two blocks are phase separated after annealing as well (which is apparent from optical studies noted above); however, because of the close melting transitions of the homopolymers (229 and 239 °C for P3HS and P3HT, respectively), these two peaks are difficult to resolve in the second heating cycle for the

Figure 4. (a) DSC profile of P3HS, (b) HMWSe53 block copolymer, and (c) HMWSe26 block copolymer (two heating cycles are offset for clarity). (d) Packing schemes for type-1 and type-2 polymorphs.

block copolymers. The enthalpy of melting also appears to be higher for polyselenophene, which further obscures the 239 °C transition of the pure polythiophene phase. Two distinct melting transitions are best observed, then, in the low selenophene content HMWSe26 (Figure 4c), which has two distinct transitions at 224 and 239 °C. These transitions can be assigned to the two separated phases of polyselenophene- and polythiophene-rich domains, respectively. The behavior of the low molecular weight samples is dependent on polymer composition. Selenophene-rich or thiophene-rich copolymers have profiles that are very similar to the corresponding homopolymers (see Supporting Information). LMWSe53 has a single transition in the second heating cycle (189 °C) that does not correspond to any of the high molecular weight transitions. Low molecular weight P3HT is known to have a reduced melting transition,30 and it is likely that the low molecular weights of both of the blocks in this sample cause this transition to occur at this lower temperature. Self-Assembly. Variable Temperature Absorption Measurements. The self-assembly of poly(3-alkylthiophene) (P3AT) homopolymers and block copolymers can be visualized by cooling the polymers in a moderate solvent and monitoring the change in optical properties by absorption spectroscopy (the so-called whisker method developed by Samitsu, Ito, and others 31,32 ). Self-assembled or self-aggregated poly(3alkylthiophene)s have a distinct absorption profile. This characteristic absorption profile should also be visualized in selenophenes and should be distinct from thiophenes.25,28 This motivated us to conduct a series of variable temperature experiments on the series of block copolymers in this study in order to better understand their self-assembly properties. Self-assembly experiments were carried out in anisole (0.02 mg polymer/mL solvent) while controlling the temperature and monitoring the absorption profile. Unimodal peaks are observed for all samples at 80 °C (Figure 5), indicating that the polymer chains are molecularly dissolved and that little aggregation is occurring. The solutions were then cooled at 4 °C/min, and the spectra were recorded in 5 °C intervals with 20 min equilibration time. In this experiment, self-assembly is visualized by a sharp change in absorption from a Gaussian 3775

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Figure 5. Variable-temperature absorbance spectra for HMW and LMW copolymers.

Figure 6. Tapping-mode AFM height images of solution-assembled fibers of high molecular weight (top row) and low molecular weight (bottom row) block copolymers.

profile to one that contains many new well-defined peaks. This distinct profile is attributed to a transition from a disordered molecularly dissolved state to an ordered, aggregated state. In the HMW series, a clean transition with a single isosbestic point is observed in all samples. The low temperature spectra are nearly identical to the thin films, suggesting the aggregates and thin films are similar in structure. The temperature at which the sample transitions between these two states depends on the selenophene content in the polymer. Polymers with increasing selenophene content transition to the aggregated state at higher temperatures. This can be explained by the greater tendency of selenophene to crystallize, relative to thiophene, as we and others have reported.22,33 During the cooling process fibril structures form that are similar to those reported for solution-assembled P3HT.31

Fibrils are several micrometers long and 15−20 nm wide (Figure 6) and can be visualized with tapping-mode atomic force microscopy (AFM). Many of these structures are large aggregates, which is likely due to drying effects. Interestingly, the phase images of the HMW fibrils show some bright and dark areas, suggestive of different hardnesses within these regions (see Supporting Information). When conducting variable temperature absorption experiments on the LMW series, we observe that they behave similarly, although assembly does not occur as readily, and a Gaussian absorption peak remains even at the lowest temperatures. The fibers are also qualitatively shorter, although approximately the same width. Previous work indicates that self-assembly depends on molecular weight;31 however, this does not appear to be the only factor in this instance. For 3776

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for 20 min. In a separate flask, 2,5-dibromo-3-hexylthiophene (136 mg, 0.42 mmol) was treated with THF (2.9 mL) and i-PrMgCl (0.22 mL) and stirred for 1 h at room temperature. The thiophene monomer solution was transferred to the polymerization flask after the selenophene monomer had polymerized for 20 min. The reaction was continued for another 20 min and then quenched with 5% HCl and precipitated in methanol to afford a dark purple solid. This was filtered through a Soxhlet thimble and extracted with methanol, hexanes, and chloroform. The chloroform fraction was collected, and the chloroform was removed under reduced pressure to afford the polymer as a dark purple solid (114 mg, 72%). The other polymers were synthesized using this method, changing the monomer feed ratio and catalyst loading as appropriate. Cyclic Voltammetry. CV experiments were carried out in anhydrous acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate. The acetonitrile was degassed with nitrogen prior to use. Polymer samples were drop-cast onto a 2 mm diameter Pt-disk working electrode. A Pt-wire counter electrode was used with a Ag pseudoreference electrode. After an experiment was performed, ferrocene was added, and the reported potential is referenced to the ferrocene oxidation peak. X-ray Diffraction Analysis. Polymer samples were drop-cast from hot filtered chlorobenzene solutions onto silicon substrates. These samples were then annealed at 200 °C for 30 min. Solution-Assembly of Fibrils. Polymer samples were dissolved in anisole at a concentration of 0.02 mg/mL, heated to 80 °C, and filtered through a 0.45 μm Teflon syringe filter into a quartz cuvette. The solution was heated to 80 °C, and spectra were recorded in 5 °C increments, allowing 20 min between temperature changes for equilibration. For imaging purposes, solutions were spin-coated onto glass substrates once no further changes were observed in the absorption spectra upon a decrease in temperature.

example, P3HT with a comparable molecular weight assembles readily in solution without any evidence of molecularly solvated chains. A similar molecular weight P3HS homopolymer, however, does not assembly readily, suggesting that the presence of selenophene, in the absence of P3HT, hinders assembly (see Supporting Information). This shows that the aggregation of P3HS-b-P3HT is governed by both molecular weight and monomer proportion and suggests a method of independently controlling both optical properties (by changing the selenophene content) and morphological properties (by changing molecular weight).

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CONCLUSION The properties of P3HS-b-P3HT block copolymers were examined as a function of both molecular weight and relative monomer incorporation. We found that some properties scale with relative selenophene content, such as absorption onset. Phase separation in the bulk state was observed even in pristine samples, evidenced by a characteristic type-2 melting transition in P3HS. Solid-state organization was studied by solution phase self-assembly. When allowed to slowly assemble in solution, high molecular weight copolymers have a very sharp transition from the molecularly dissolved to the aggregated state. Most interestingly, increasing polyselenophene content induces the polymer to assemble more readily (at a higher temperature) but also appears to hinder the degree of ordered assembly when the thiophene block is not sufficiently long (i.e., in the case of low molecular weight copolymers). Overall, this set of experiments furthers our understanding and appreciation of the fundamental differences in the physical and morphological properties of P3HS versus P3HT and offers a promising route to allow for independent control of optical and morphological properties in conjugated polymers that contain distinct polymer heterocycles.

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ASSOCIATED CONTENT

S Supporting Information *

Polymer NMR spectra, GPC profiles, DSC curves, WAXD plots, and additional AFM imaging. This material is available free of charge via the Internet at http://pubs.acs.org.

EXPERIMENTAL SECTION

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General Considerations. All reagents were used as received unless otherwise specified. Selenophene, 2,5-dibromo-3-hexylthiophene, i-PrMgCl (2.0 M in THF), and Ni(dppp)Cl2 were purchased from Sigma-Aldrich. Solvents were purchased from Caledon Laboratory Chemicals and dried using an Innovative Technology solvent purification system. 2,5-Dibromo-3-hexylselenophene was synthesized from selenophene as previously reported.22 The concentration of Grignard reagents was determined by titration of diphenylacetic acid in diethyl ether. Instrumentation. 1H NMR was performed using either a Varian Mercury 300 (300 MHz) or Varian Mercury 400 (400 MHz) spectrometer. GPC measurements were carried out using a Malvern 350 HT-GPC system at 140 °C using 1,2,4-trichlorobenzene stabilized with butylated hydroxytoluene as the mobile phase. Molecular weights were determined using a set of narrow molecular weight polystyrene standards. Absorption spectra were recorded using a Varian Cary 5000 spectrometer fitted with a Peltier controlled autosampler. Wide-angle X-ray scattering spectra were recorded on a Bruker AXS D8 Discovery with GADDS area detector. AFM imaging was carried out using a Veeco Dimension 3000 microscope. DSC measurements were taken using a TA Instruments Q2000 differential scanning calorimeter. Synthesis. A representative synthesis of a block copolymer (HMWSe53) is as follows: In a nitrogen-filled glovebox, a solution of Ni(dppp)Cl2 (83 mg, 0.15 mmol) in dichloromethane (6 mL) was prepared. 0.32 mL of this solution (4.4 mg, 0.0081 mmol of Ni(dppp)Cl2) was then transferred to a separate flask, and the solvent was removed under reduced pressure. A Schlenk flask was loaded with 2,5-dibromo-3-hexylselenophene (153 mg, 0.41 mmol) and evacuated for 20 min. THF (2.9 mL) was then added followed by i-PrMgCl (0.22 mL, 1.8 M in THF). After 1 h the mixture was transferred to the flask containing the Ni(dppp)Cl2 for polymerization and heated to 40 °C

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada, the Canadian Foundation for Innovation, the Ontario Research Fund, an Ontario Early Researcher Award, and a DuPont Young Professor Grant for D.S.S. The authors thank Hani Naguib and Reza Rizvi for their assistance with DSC measurements and Gilbert Walker and Claudia Grozea for their assistance in AFM imaging. J.H. is grateful for an NSERC PGS-D scholarship.

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REFERENCES

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dx.doi.org/10.1021/ma300394u | Macromolecules 2012, 45, 3772−3778