What Limits the Molecular Weight and Controlled Synthesis of Poly(3

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What Limits the Molecular Weight and Controlled Synthesis of Poly(3-alkyltellurophene)s? Shuyang Ye,† Marvin Steube,†,‡ Elisa I. Carrera,† and Dwight S. Seferos*,† †

Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada Institute of Organic Chemistry, Johannes Gutenberg-University of Mainz, 10-14 Duesbergweg, 55128 Mainz, Germany

‡

S Supporting Information *

ABSTRACT: Polytellurophenes are an emerging class of conjugated polymers; however, their controlled polymerization leading to high molecular weight materials has been a major challenge. Here we present a systematic investigation of the synthesis of poly(3alkyltellurophene)s using the catalyst transfer polycondensation methodology. Learning that previous syntheses were limited by both polymerization reaction kinetics and polymer solubility, we design new tellurophene monomers to overcome these limitations. Controlled polymerization behavior up to Mn = 25 kDa, chain extension, block copolymerization, external initiation, and well-defined end groups are demonstrated for poly(3alkyltellurophene)s with appropriately designed side chains. We clarify the role that side-chain branching point plays on polymerization kinetics and optical properties for these prototypical regioregular polymers. In addition, the effect that monomer addition sequence has on well-defined tellurophene−thiophene block copolymers was studied. The controlled polymerization of tellurophene should provide access to more complex polymeric architectures involving these and other conjugated monomers. The methods used to optimize the polymerization of alkyltellurophenes should be applicable to other monomers that have been challenging to synthesize in a controlled manner.

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INTRODUCTION The controlled synthesis of polymers with defined and reproducible molecular weight and narrow dispersity (Đ) is a fundamental challenge in chemistry. Catalyst transfer polycondensation (CTP) is arguably the only route to well-defined, high-molecular-weight conjugated polymers (CPs).1−4 CTP is a chain-growth polymerization that leads to polymers with controlled molecular weight and narrow molecular weight distribution and defined chain-end functionality. Although this method was initially demonstrated for the synthesis of poly(3alkylthiophene)s,5−9 CTP has now been extended to a number of monomers with a variety of properties10−20 through extensive mechanistic investigations,21−24 the development of new catalysts,25−33 and the optimization of polymerization conditions.34,35 Because of the quasi-living nature of CTP and selective functionalization at the chain ends, complex polymer architectures are now accessible including block,27,36−43 gradient,44,45 and statistical copolymers46−49 as well as rod− coil50,51 and bottlebrush polymers52 that contain both conjugated and nonconjugated units. The incorporation of tellurophene derivatives into CPs has become increasingly popular in recent years.53−63 Heavier chalcogens (Se and Te) introduce advantageous properties such as red-shifted optical absorption, high polarizability, strong chalcogen−chalcogen interactions, and the ability to form hypervalent complexes.64−68 Polytellurophenes are less explored due to their significantly more challenging monomer synthesis and difficult polymerization. It was not until 2013 that © XXXX American Chemical Society

our group developed a gram-scale route to 3-alkyltellurophene monomers and synthesized a series of poly(3-alkyltellurophene)s (P3ATe).69 In this case, however, CTP-type conditions did not lead to well-defined polymers. For example, the number-average molecular weights (Mn) of the polymers were relatively low and did not match those expected from the monomer:catalyst ratios. The polymers also had broad dispersities indicative of uncontrolled polymerization. Thus, while progress has been made by our group and others in expanding the scope of CTP, including electron-deficient monomers,19,26,27 the polymerization of 3-alkyltellurophenes has lagged behind. To fully evaluate polytellurophenes as materials, the synthesis of well-defined, high-Mn polymers is required. Herein, we report the preparation of well-defined, highmolecular-weight P3ATe for the first time. The origins of previous polymerization limitations were systematically determined through a series of density functional theory (DFT) calculations and experiments. We find that lower temperature is required to ensure narrow dispersity and a branched side chain is required to maintain solubility. The commonly used 2ethylhexyl side chain, however, greatly slows the polymerization kinetics and prevents complete control over the polymerization. To address this, several monomers with new side chains were Received: December 24, 2015 Revised: January 24, 2016

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Figure 1. Monomer (thiophene and tellurophene) and catalyst [Ni(dppp)] structures, calculated reaction coordinates of catalyst association and oxidative addition to tellurophene (purple) and thiophene (blue), and optimized geometries of the coordination complexes.

the oxidative addition stabilization energy is comparable for both monomers, which suggests that propagation should be energetically favorable in both cases. Considering that 3alkylthiophenes are exceptional monomers for CTP, the calculations imply that comparable control, if not more, should be possible for the polymerization of 3-alkyltellurophenes. The regioselectivity of the Grignard metathesis activation step is important for initiation and propagation as well as for molecular weight and regioregularity of the resulting polymers. Therefore, we performed experiments to determine if this step was limiting polymerization control. When testing several monomers with differing side chains (hexyl, dodecyl, or 2ethylhexyl), we found that Grignard metathesis always leads to a 20:80 ratio of isomers 3-alkyl-2-chloromagnesio-5-iodotellurophene and 3-alkyl-5-chloromagnesio-2-iodotellurophene (see Table S1 and Figure S1, Supporting Information). These results are analogous with thiophene activation. The successfully activated monomers were transferred to flasks containing 1 mol % of Ni(dppp)Cl2 catalyst in methyl-THF and heated to 80 °C overnight to induce polymerization. In all cases, the Mn of the polymers obtained was roughly half the expected value, and the dispersities were high (Mn ∼ 10 kDa, Đ = 1.5−2.2), confirming a lack of control under these polymerization conditions (see Table S2). High temperature is usually adopted in the synthesis of challenging polymers due to more rapid polymerization kinetics and increased solubility, which should lead to higher Mn of the resulting polymers. On the other hand, high temperature may be detrimental to the polymerization as catalyst dissociation becomes more favorable, causing chain termination, chain transfer, and the initiation of new polymer chains to occur. Thus, we next tested the synthesis of poly(3-hexyltellurophene) (P3HTe) at room temperature. A polymer with a Mn = 4.7 kDa was obtained using a 2% catalyst loading. While this Mn is much lower than that predicated from the expected degree of polymerization (50), as well as those obtained at high temperature, the dispersity of this polymer was significantly lower (Đ = 1.3), which is a promising result. We next wondered if poor solubility limits the polymerization of tellurophenes. If this is true, Mn should increase and dispersity should remain unchanged (if not decrease) when side chains impart better solubility to the growing chain. Indeed, the Mn of the polymers increases from 4.7 to 5.4 kDa when hexyl side chains are replaced with longer dodecyl side chains (see Table S3). However, the polymerization results in only

synthesized, and the effect on solubility, polymerization kinetics, and resultant polymer properties for each side-chain was evaluated. Specifically, we find that moving the side-chain branching point further away from the heterocycle leads to dramatic improvement in polymerization rate and introduces control over the polymerization. Polymers with narrow dispersities and Mn up to 25 kDa were synthesized. Controlled polymerization of P3ATe is further supported by chainextension experiments, the synthesis of block copolymers, and the introduction of well-defined end groups through external initiation.

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RESULTS AND DISCUSSION Previous syntheses of P3ATe under CTP-type conditions had significantly uncontrolled polymerization behavior.69 In order to improve this, it is crucial to understand the limitations of the current methodology. These limitations could be inherent to the structure of the tellurophene monomer itself or may be due to the polymerization mechanism. Possible limitations are (1) poor association between the polymer and catalyst that leads to the dissociation and diffusion of the catalyst, (2) strong association between the polymer and catalyst may slow down the polymerization and encourage chain termination, (3) poor selectivity of the Grignard metathesis reaction for the active monomer species, (4) high polymerization temperature that disturbs polymer−catalyst association, and (5) poor solubility that causes aggregation and prevents propagation. Each of these was investigated in turn. The association complex between the catalyst and π-system of the growing polymer chain is critical for controlled CTP.26,70,71 Stronger association prevents the dissociation and diffusion of the catalyst from the backbone, which leads to termination, chain transfer, and initiation of new polymer chains. To predict the stabilization energy between the catalyst and the tellurophene heterocycle, we used gas-phase, singlepoint energy (SPE) DFT calculations (Figure 1).27 The analogous thiophene monomer was used as a reference. For both monomers the side chains were replaced with methyl groups to reduce computation time. Interestingly, these calculations predict that the tellurophene−catalyst complex is 23.3 kJ/mol more stable upon complexation than the thiophene−catalyst complex. It is important to note that the catalyst association strength has an upper limit at which oxidative addition becomes energetically unfavorable, preventing chain propagation.72 However, the calculations predict that B

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Macromolecules oligomeric materials when the most soluble branched 2ethylhexyl side chain is installed. It should be noted that bulky side chains significantly decrease the rate of Ni(dppp)Cl2catalyzed polymerizations of thiophenes.37,73 In order to tackle this problem, the more reactive Ni(dppe)Cl2 was used as catalyst. We find that the polymerization proceeds smoothly, and after 12 h, the Mn reaches 10 kDa (Đ = 1.3). We next monitored the monomer conversion of 5-chloromagnesio-3-(2-ethylhexyl)-2-iodotellurophene as a function of polymerization time. We find that monomer conversion is linear up to 75% conversion as a function of polymerization time (Figure 2a). The nonlinearity at the later stage of the

Scheme 1. Synthesis of 3-(3-Ethylhepyl)-2,5diiodotellurophene

available. The synthesis begins with treating 2-ethylhexylmagnesium bromide with carbon dioxide to install one additional carbon and afford 3-ethylheptanoic acid (1a). The carboxylic acid was reduced followed by bromination to give 1bromo-3-ethylhelptane (3a). The bromide was treated with magnesium turnings to generate the desired Grignard reagent in situ. From here the monomer was prepared in the same manner as before.69 After optimization, the overall yield of the eight-step synthesis was 31%. With 3-(3-ethylheptyl)-2,5-diiodotellurophene in hand, detailed studies on its polymerization were carried out. Here, monomer conversion as a function of polymerization time is linear up to 85% (Figure 3a). The first-order rate constant from the linear regime is 9.8 × 10−3 min−1, which is a 3.4-fold increase compared to the 2-ethylhexyl monomer. The SEC elution profiles have a unimodal distribution, and the curves shift entirely to the higher molecular weight regions without broadening or the appearance of low-mass shoulders (Figure 3b). The Mn increases linearly as a function of monomer conversion without deviation (Figure 3c). Dispersity remains low and under 1.2 throughout the polymerization. At 2% catalyst loading the final polymer had a 11.5 kDa Mn, which is very close to the expected value. All evidence strongly supports that controlled chain growth is achieved. In addition, the concentration of reverse activated (unreactive) monomer remains constant throughout the polymerization and is not incorporated into the chain. This results in a high regioregularity of 93% for the final polymer. Further evidence of controlled chain growth comes from polymerizations using various monomer:catalyst ratios. One of the defining hallmarks of chain growth polymerization is that one catalyst initiates one chain; therefore, the Mn of resulting polymers is inversely proportional to catalyst:monomer ratio. When we carried out a series of polymerizations with different catalyst loadings, the molecular weights increase linearly as a function of [M]0/[Ni] and dispersities remain under 1.2 (Figure 4). Surprisingly, we find that polymers with a molecular weight of 24.9 kDa can be obtained with this low dispersity. This high-molecular-weight polytellurophene dissolves in THF

Figure 2. (a) Semilogarithmic kinetic plots of 2-ethylhexyltellurophene monomer consumption as a function of polymerization time. (b) Number-average molecular weight (black) and dispersity (red) as a function of monomer conversion for 2-ethylhexyltellurophene carried out with a 1:50 catalyst:monomer ratio at 0.1 M monomer concentration.

polymerization indicates the occurrence of termination. The first-order rate constant from the linear regime is k = 2.9 × 10−3 min−1. Deviation from linearity is concurrent with the increase in the dispersity (Figure 2b). At the same time, low-mass shoulders appear in the size exclusion chromatography (SEC) elution profiles (see Figure S2). These combined results suggest that most of the polymer chains initially propagate in a chain-growth manner; however, at longer times, catalysts dissociate and initiate new polymer chains. Since both short and long linear side chains are not able to afford sufficient solubility, a branched side chain is imperative to polytellurophene synthesis. However, the increased steric bulk imparted by the branched chain may limit polymerization rate due to steric hindrance. To probe this, we designed and synthesized 3-(3-ethylheptyl)-2,5-diiodotellurophene, for which the branching point is one carbon further away from the tellurophene, thus decreasing steric crowding while maintaining improved solubility (Scheme 1). This side chain has been seldom used,74 presumably because it is not commercially C

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polythiophene synthesis. When the same polymerization reaction was quenched with ethynylmagnesium bromide,75 selective functionalization with an ethynyl end group was achieved (Figure 5b). If the significant improvement in polymerization is due to reduced steric bulkiness, the polymerization rate should continue to increase and maximize when the steric effects are excluded. With this in mind we designed another new monomer with a 4-ethyloctyl side chain. The synthesis begins with commercially available 4-ethyloctanoic acid and from that point is analogous to the 3-ethylheptyl side chain synthesis with comparable yields (see Scheme S1). Polymerizations were carried out, and we again observe a linear correlation between monomer conversion and polymerization time, which indicates the absence of termination events (see Figure S3). The firstorder rate constant is 9.5 × 10−3 min−1, which is very close to the 3-ethylheptyl monomer. The comparison shows that moving the ethyl branch further away does not result in increased polymerization rate, suggesting that steric effects are completely overcome with a one-carbon extension of the branching position. In living (or quasi-living) polymerizations, the ends of the propagating polymer chain remain active after the monomer has been consumed, and chain-extended polymers can be obtained by adding more activated monomers. As such, a selfextension experiment was carried out to confirm quasi-living character. When the polymerization of 3-(3-ethylheptyl)-2,5diiodotellurophene was initiated at a 20:1 monomer:catalyst ratio and allowed to proceed to completion (3 h), we observe a Mn = 5.2 kDa (Đ = 1.1). When more activated monomer was added to this polymerization to achieve a final 50:1 monomer:catalyst ratio and allowed to proceed for an additional 3 h, we observe a Mn = 11.7 kDa (Đ = 1.2) (Figure 6a). The results were analogous to single-step control polymerizations with the same monomer:catalyst ratio and confirm the propagating polymer chains remain active and that living (or quasi-living) behavior is achieved. Similarly, block copolymers can be obtained in living polymerizations by the chain extension of a macroinitiator with a different activated monomer. To test this, a P3HT macroinitiator was synthesized from an o-tolyl-functionalized Ni catalyst with an expected degree of polymerization of 50. After complete consumption (30 min), 50 mol equiv of activated tellurophene monomer was added to the reaction mixture. The SEC elution curve shifts entirely to the expected higher molecular weight region (Mn = 20 kDa; Đ = 1.2) (Figure 6b). The progressive incorporation of two repeat units was examined by 1H NMR. The thiophene and tellurophene repeat units have distinct 1H NMR resonances in the aromatic region (6.98 and 7.40 ppm, respectively). The block copolymer contained resonances consistent with these repeat units without evidence of statistical copolymers, showing that a block copolymer has formed (see Figure S4). Interestingly, block copolymerization failed when the polymerization was carried out using the reverse monomer addition sequence (see Figure S5). Based on DFT predictions, catalyst association should be stronger with the tellurophene block, and this may create a resting state that prevents catalyst transfer to thiophene, which explains these observations.76,77 Finally, the absorption properties of all P3ATe with comparable molecular weights were studied by solution optical absorption spectroscopy. Poly(3-(2-ethylhexyl)tellurophene), poly(3-(3-ethylheptyl)tellurophene), and poly(3-(4-ethyl-

Figure 3. (a) Semilogarithmic kinetic plots of 3-ethylheptyltellurophene monomer consumption as a function of polymerization time. (b) SEC elution profiles for each aliquot. (c) Number-average molecular weight (black) and dispersity (red) as a function of monomer conversion for 3-ethylheptyltellurophene carried out with a 1:50 catalyst:monomer ratio at 0.1 M monomer concentration.

or chloroform at room temperature, which shows that improved solubility and polymerization control are achieved concurrently by moving the side-chain branching point one carbon further from the tellurophene heterocycle. To gain insight into the end groups of the polymer, a polymerization was carried out using an o-tolyl-functionalized Ni complex as the external initiator. The end groups of polymer were analyzed by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry. The mass spectrum shows that the predominant distribution corresponds to the o-tolyl/H-terminated chains (Figure 5a), which are expected for this type of externally initiated polymer.29 A small fraction of H/H terminated chains are observed that we assume results from polymerization catalyzed by a small number of Ni complexes that did not react with the o-tolyl Grignard reagent, and this is consistent with other externally initiated D

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Figure 4. (a) SEC elution profiles of poly(3-ethylheptyl)tellurophene prepared with different monomer:catalyst ratios. (b) Number-average molecular weight (black) and dispersity (red) as a function of 3-ethylheptyltellurophene monomer:catalyst ratio.

side chains undergo polymerization with significant living character. The controlled polymerization of tellurophene will provide access to more complex polymeric architectures involving these monomers. The effects of the side-chain branching point were systematically investigated. Moving the branch point to the 3-position is best to eliminate the influence of steric hindrance on polymerization kinetics. Further extension did not improve polymerization kinetics but lead to changes in optical properties. This establishes that CTP rate can be manipulated by subtle modification of the side chains. 2Ethylhexyl groups are widely used in the synthesis of conjugated polymers, which may have detrimental effects on both the rate of polymerization and quality of the polymers. This effect of the side-chain branch point likely extends to other types of monomers and polymerization mechanisms and should be taken into consideration when designing conjugated polymers. Finally, we emphasize that the order of successive monomer addition is important for successful synthesis of welldefined block copolymers.

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EXPERIMENTAL SECTION

General Considerations. [Caution: the MSDS should be consulted before handling organotellurium compounds. Although the toxicity of these compounds is low, they should be handled with proper personal protective equipment.] Tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) were purchased from Fisher Scientific, degassed, stored under argon, and dried over molecular sieves prior to use. Hexanes, 1,2,4-trichlorobenzene (spectrophotometric grade), ethyl acetate, methanol, 2-propanol, potassium carbonate, and magnesium turnings were purchased from Fisher Scientific. Sodium thiosulfate, potassium carbonate, sulfuric acid, diethyl ether, and dichloromethane (DCM) were purchased from Caledon Laboratories. N-Iodosuccinimide (NIS) and lithium aluminum hydride were purchased from Alfa Aesar. Anhydrous ethanol was purchased from Commercial Alcohols. trans-2-[3-(4-tert-Butylphenyl)2-methyl-2-propenylidene]malononitrile (DCTB) was purchased from Fluka Analytical. All other solvents and reagents were purchased from Sigma-Aldrich and used as received. Schlenk techniques were employed for experiments conducted under argon. 2-Chloro-Nmethoxy-N-methylacetamide78 and o-tolyl external initiator79 were prepared according to literature procedures. Instrumentation. Nuclear magnetic resonance (NMR) spectra were recorded on a Varian Mercury 400 spectrometer (400 MHz). Masses were determined on a Waters GCT Premier TOF mass spectrometer (EI). Gas chromatography was conducted using a PerkinElmer Clarus gas chromatograph. Polymer molecular weights were determined using a Viscotek HT-SEC module 350A (1,2,4trichlorobenzene stabilized with butylated hydroxytoluene, 140 °C)

Figure 5. MALDI-TOF mass spectrum of externally initiated poly(3ethylheptyl)tellurophene (top) and externally initiated and end-capped poly(3-ethylheptyl)tellurophene.

octyl)tellurophene) have maximum absorption peaks at 514, 527, and 538 nm, respectively (Figure 6c). A notable red shift is observed in the three absorption profiles when the ethyl branch point is moved further from the heterocycle likely due to decreased degree of twisting and thus increased effective conjugation length. This is further supported by the predicted dihedral angle between central bitellurophene in the optimized geometry of corresponding hexamers. These results demonstrate that the nature of the side chain not only affects solubility and polymerization kinetics but also can affect optoelectronic properties as well.

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CONCLUSION We have demonstrated the first controlled synthesis of welldefined, high-molecular-weight P3ATe using catalyst transfer polycondensation through a systematic investigation of polymerization conditions, computation, and molecular design. Results from kinetics studies, molecular weight control experiments, self-extension, and block copolymerizations all indicate that 3-alkyltellurophenes with appropriately designed E

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MS (DART + ) C13H22Te [M + H]+ Expected: 309.08620. Found 309.08620. Δ = −4.06 ppm. 3-(4-Ethyloctyl)tellurophene. 1H NMR (CDCl3, 400 MHz): δ 8.78 (dd, J = 6.6, 1.9 Hz, 1H), 8.34 (s, 1H), 7.76 (dd, J = 6.6, 1.4 Hz, 1H), 2.60 (t, 7.6 Hz, 2H), 1.64−1.54 (m, 2H), 1.33−1.17 (m, 11H), 0.94−0.78 (m, 6H). 13C NMR (CDCl3, 400 MHz): δ 153.2, 140.2, 124.2, 118.0, 38.9, 35.4, 33.0, 33.0, 29.1, 27.6, 26.0, 23.3, 14.3, 11.0. TOF MS (DART + ) C14H24Te [M + H]+ Expected: 323.10185. Found 323.10306. Δ = 3.74 ppm. Monomer Synthesis. Preparation of the diiodinated monomers was carried out under optimized reaction conditions according to the following procedure. 3-Alkyltellurophene (1.0 g, 1 equiv) was added to a Schlenk flask charged with N-iodosuccinimide (NIS) (2 equiv) and dimethylformamide (10 mL) under argon. The reaction was protected from light and stirred at 40 °C overnight, then cooled to room temperature (rt), and quenched with water. The aqueous layer was diluted with a 10% sodium thiosulfate solution (100 mL) and extracted with diethyl ether (2 × 120 mL). The combined organic layer was washed with water (2 × 150 mL) and brine (150 mL), dried over MgSO4, and concentrated under reduced pressure to give a brown oil. Purification by column chromatography (hexanes) afforded a bright yellow liquid. 3-(3-Ethylheptyl)-2,5-diiodotellurophene. Yield 1.37 g, 75%. 1 H NMR (CDCl3, 400 MHz): δ 7.72 (s, 1H), 2.54−2.46 (m, 2H), 1.49−1.40 (m, 2H), 1.39−1.20 (m, 9H), 0.94−0.83 (m, 6H). 13C NMR (CDCl3, 400 MHz): δ 157.7, 148.8, 70.2, 69.6, 38.9, 33.5, 33.4, 32.8, 29.1, 26.0, 23.3, 14.4, 11.1. TOF MS (DART + ) C13H20I2Te [M + H]+ Expected: 560.87948. Found 560.88038. Δ = 1.61 ppm. 3-(4-ethyloctyl)-2,5-diiodo-tellurophene. Yield 1.38 g, 77%. Synthesized as described above. 1H NMR (CDCl3, 400 MHz): δ 7.73 (s, 1H), 2.53−2.47 (m, 2H), 1.53−1.42 (m, 2H), 1.35−1.19 (m, 11H), 0.95−0.80 (m, 6H). 13C NMR (CDCl3, 400 MHz): δ 157.5, 148.9, 70.6, 69.5, 38.7, 36.4, 32.9, 32.8, 29.1, 27.2, 26.0, 23.3, 14.4, 11.0. TOF MS (DART + ) C14H22I2Te [M + H]+ Expected: 574.89513. Found 574.89406. Δ = −1.87 ppm. General Procedure for Grignard Quenching Experiments. A Schlenk flask was charged with a 0.1 M solution of 3-alkyl-2,5diiodotellurophene in THF. Isopropylmagnesium chloride (1.00 equiv) was added, and the mixture was stirred at rt. Aliquots were removed after 15 and 30 min and quenched with dilute HCl. The aqueous layer was extracted with diethyl ether. The organic phase was washed with a saturated solution of NaHCO3 and brine, dried over MgSO4, and concentrated under reduced pressure to afford a brown liquid that was examined by 1H NMR spectroscopy (see Supporting Information). General Procedure for Polymerizations. Isopropylmagnesium chloride (0.98 equiv) was added to a solution of 3-(3-ethylheptyl)-2,5diiodotellurophene (120.6 mg, 0.22 mmol) in THF (1.7 mL). The mixture was stirred at rt for 15 min, then transferred to a Schlenk flask containing Ni(dppe)Cl2 catalyst (2.28 mg, 0.0043 mmol, 2 mol %), and stirred at rt for 3 h before quenching with dilute HCl. The polymer was precipitated into methanol and purified by sequential Soxhlet extraction (methanol, hexanes, and chloroform). The chloroform fraction was concentrated under reduced pressure to give a dark purple solid (43.2 mg, 65%). SEC profiles (Figure S7) and 1 H NMR spectra (Figure S8) are provided in the Supporting Information. 3-(3-Ethylheptyl)tellurophene polymers synthesized for the molecular weight control experiments were also prepared according to this procedure using various catalyst loadings. The molecular weight and dispersity were determined without purification. General Procedure for Kinetic Studies. A solution of activated monomer (1.06 g, 1.90 mmol) was prepared as described above and transferred to a Schlenk flask containing Ni(dppe)Cl2 catalyst (20.03 mg, 0.04 mmol, 2 mol %). Aliquots were removed during the polymerization (t = 1 min to t = 180 min), and all samples were quenched with dilute HCl and extracted with chloroform immediately after removal. Each aliquot was analyzed using gas chromatography, and the monomer consumption was determined relative to a docosane internal standard. The molecular weight and dispersity were determined by SEC without purification.

Figure 6. (a) SEC elution profiles of the self-extension experiment. (b) SEC elution profiles of block copolymerization. (c) Solution absorption spectra of poly(3-alkyltellurophene)s. with narrow weight distribution polystyrene standards using absorbance at 485 nm. MALDI spectra were obtained using a Bruker Microflex MALDI-TOF mass spectrometer from a matrix of DCTB (2500:1 matrix-to-polymer ratio) cast from chloroform. Optical absorption spectra were recorded using a Varian Cary 5000 spectrometer. 3-Alkyltellurophene Synthesis. 3-Alkyltellurophenes were prepared according to literature procedures69 starting with commercially available Grignard reagents (hexylmagnesium bromide, dodecylmagnesium bromide, and 2-ethylhexylmagnesium bromide) or by preparation of the Grignard bearing the desired side chains (3ethylheptyl and 4-ethyloctyl; see Supplementary Synthetic Methodologies, Supporting Information). 3-(3-Ethylheptyl)tellurophene. 1H NMR (CDCl3, 400 MHz): δ 8.78 (dd, J = 6.6, 1.8 Hz, 1H), 8.34 (s, 1H), 7.76 (dd, J = 6.6, 1.4 Hz, 1H), 2.60 (t, J = 8.0 Hz, 2H), 1.60−1.54 (m, 2H), 1.39−1.19 (m, 9H), 0.93−0.83 (m, 6H). 13C NMR (CDCl3, 400 MHz): δ 153.4, 140.2, 124.2, 117.7, 38.7, 34.0, 32.9, 32.3, 29.0, 25.9, 23.3, 14.3, 11.0. TOF F

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Macromolecules General Procedure for Chain Extension Polymerization. A solution of activated monomer (110.9 mg, 0.20 mmol) was prepared as described above and transferred to a Schlenk flask containing Ni(dppe)Cl2 catalyst (5.25 mg, 0.0099 mmol, 5 mol %) and stirred at rt for 3 h. One aliquot (0.5 mL) was removed and quenched with dilute HCl while additional activated monomer (0.22 mmol) was added to the polymerization mixture. The polymerization mixture was stirred for an additional 3 h before quenching with dilute HCl. Each polymer fraction was extracted with chloroform. The organic layer was dried over MgSO4 and concentrated under reduced pressure to afford a dark purple solid. Molecular weight and dispersity were determined without purification. General Procedure for Block Copolymerization. A 99.3 mg (0.30 mmol) solution of activated monomer 2,5-dibromo-3-hexylthiophene was prepared as described above, and an o-tolyl-functionalized Ni(dppe)Cl2 external initiator solution containing Ni catalyst (0.0061 mmol) was added. The mixture was stirred at rt for 30 min, and an aliquot (1.0 mL) was removed and quenched with dilute HCl followed by extraction with chloroform. The organic layer was dried over MgSO4 and concentrated under reduced pressure. In another flask, 112.1 mg (0.20 mmol) of activated monomer B was prepared in THF (1.0 mL) and then added to the polymerization mixture. The polymerization mixture was stirred at rt for an additional 3 h before quenching with dilute HCl. The polymer was precipitated into methanol and purified by sequential Soxhlet extraction (methanol, hexanes, and chloroform). Density Functional Theory Calculations. All calculations were carried out using the B3LYP hybrid functional in Gaussian 09.80−82 Geometry optimizations were performed on representative polymer structures of six repeat units and on structures associated with the catalytic cycle involving the nickel catalyst and one tellurophene unit. Alkyl chains were replaced with methyl groups to reduce the computation time. The optimizations were carried out using the LANL2DZ83,84 effective core potential (ECP) for tellurium, nickel, and iodine, Ahlrichs SVP85,86 for bromine, and 6-31G(d)82,87 splitvalence basis set for all other atoms. In order to construct the reaction coordinate energy diagram for the catalytic cycle, single-point energy calculations were performed on the optimized geometries using SDD88 for nickel, tellurium, and iodine, TZVP89,90 for bromine, and 6311+G(d,p)91,92 for all other atoms.

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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/acs.macromol.5b02770. Figures S1−S34, Tables S1−S3, and Scheme S1 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (D.S.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSERC, DuPont for a Young Professor Grant, and the A. P. Sloan Foundation for a research fellowship in chemistry. We thank Walid Houry for the use of his MALDI-TOF instrument. The authors thank Huy Huynh and Colin Bridges for helpful discussions and Dan Mathers for assistance with GC measurements.

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DOI: 10.1021/acs.macromol.5b02770 Macromolecules XXXX, XXX, XXX−XXX