All-Conjugated ABC-block-copolymer Formation with a Varying

Jul 1, 2014 - Next, all-conjugated thiophene–fluorene–selenophene triblock-copolymers are synthesized in all possible orders using this catalyst. ...
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All-Conjugated ABC-block-copolymer Formation with a Varying Sequence via an Unassociated Catalyst Michiel Verswyvel,† Joost Steverlynck,† Slim Hadj Mohamed,‡,§ Mahmoud Trabelsi,§ Benoît Champagne,‡ and Guy Koeckelberghs*,† †

Laboratory of Polymer Synthesis, University of Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium Laboratory of Theoretical Chemistry, Unit of Theoretical and Structural Physico-Chemistry, University of Namur, Rue de Bruxelles 61, 5000 Namur, Belgium § Laboratory of Applied Chemistry: Heterocyclics, Fats and Polymers, Faculty of Sciences, University of Sfax, 3038 Sfax, Tunisia ‡

S Supporting Information *

ABSTRACT: This manuscript consists of two parts which focus on enhancing control over the polymerization of conjugated polymers. In the first part, the controlled chain-growth character of the polymerization of poly(selenophene) using Pd(Ruphos) as a catalyst system is demonstrated. Next, all-conjugated thiophene−fluorene−selenophene triblock-copolymers are synthesized in all possible orders using this catalyst. Subsequent, the properties of these advanced structures are assessed using GPC chromatography and 1H NMR, UV−vis, and fluorescence measurements. DFT calculations were performed to explain the unusual independence of the monomer sequence during the polymerization, traditionally observed in other chain-growth protocols for conjugated polymers.

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relies on the association of the catalyst with the π-system of the propagating polymer chain after the reductive elimination (catalyst-transfer polycondensation, CTP).35 As a consequence, termination and transfer reactions are avoided and a controlled chain-growth polymerization mechanism is obtained. In principle, if the controlled polymerization of two different monomers is obtained under the same conditions, allconjugated block-copolymers (π-BCPs) are accessible by sequential monomer addition. Nevertheless, since the mechanism relies on the association of the catalyst with the propagating polymer, the synthetic direction is fixed from the monomer with the lowest catalyst affinity to the one with the highest catalyst affinity, as reported by the groups of Yokozawa and Wang.30,32 As a consequence, the number of all-conjugated block-copolymers composed of electronically different blocks prepared by successive monomer addition remains scarce, e.g. fluorene−thiophene, fluorene−phenylene, pyrrole−phenylene,

INTRODUCTION π-Conjugated polymers are well-studied materials during the last decades because of their optoelectronic properties and their potential in low-cost electronics resulting from their conductivity and easy processing.1−10 The quest toward complex and tailored polymeric structures to improve the performance and usability was the topic of research in the recent past and continues nowadays.11 This quest starts with obtaining as much control as possible over the polymerization mechanism, hereby turning it into a controlled chain-growth mechanism with control on end-group functionalization and molar masses.12−15 Moreover, this enables block-copolymerization by successive monomer addition, in which the next monomer is added after the previous monomer is completely consumed. The research groups of McCullough and Yokozawa simultaneously discovered the chain-growth character of poly(3-hexylthiophene) with Ni(dppp)Cl2 (dppp = diphenylphosphinopropane) as a catalyst.16−19 Further research resulted also in control over other conjugated polymers that were formed in a chain-growth fashion using the same or other Ni- and Pd- based catalysts.20−34 The controlled nature of the polymerization © 2014 American Chemical Society

Received: March 25, 2014 Revised: June 6, 2014 Published: July 1, 2014 4668

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Scheme 1. Structure of the Six Synthesized π-BCPs

dithienosilole−thiophene, and selenophene−thiophene blockcopolymers.22,28,32,34,36−48 Furthermore, ABC π-BCPs should also be achievable by sequential monomer addition using the CTP method as long as the next monomer added has a stronger association with the catalyst then the previous one, however, in practice this has never been challenged. In this manuscript, we investigate a synthetic way to compose π-BCPs, consistent out of three different monomers (ABC πBCP), in a chain-growth way and in any order. Even though this seems irrelevant for di-π-BCP since AB π-BCP are identical to BA π-BCP, this can be of critical importance as ABC π-BCP could behave completely different then e.g. ACB π-BCP. To meet this requirement, a Negishi-coupling based polymerization protocol with Pd(Ruphos) as a catalyst, that was previously reported by our research group, is used.20 The ability of the catalyst to dissociate from the propagating polymer chain as a stable moiety and reinsert at the end of the same or another chain is the key aspect in this mechanism. The intramolecular deactivation concept in AB-functionalized monomers wherein an A-function deactivates the B-function toward reaction in the polymerization, as introduced by Yokozawa for a polyamide polymerization in 2007, is of major importance in the Pd(Ruphos)-mechanism.49 The deactivation by the carbon−metal (C−Zn) bond (A-function) for the oxidative addition of the Pd(Ruphos) moiety in the carbon−bromine (B-function) bond prevents transfer of the catalyst to the unreacted monomer and allows dissociation from the π-system during polymerization. This renders the mechanism independent of the limitations associated with catalyst complexation and, consequently, the sequential order seen for CTP. We reported previously the successful synthesis of poly(3-hexylthiopene), poly(9,9-dioctylfluorene) and their block-copolymers in both directions with the Pd(Ruphos) mechanism. To extend the protocol, this report first investigates the chain-growth behavior of poly(3-octylselenophene) with a Pd(Ruphos) catalyst. The red-shift of the absorption band of this polymer in comparison with poly(thiophene) and poly(fluorene) makes this polymer attractive to combine it in triblock-copolymers to expand the absorption spectrum and consequently enhance the performance in applications, e.g. in photovoltaic cells. In a next step, the synthesis of poly(3-hexylthiopene)-block-poly(9,9-dioctylfluorene)-block-poly(3-octylselenophene) in all six possible combinations is presented. The resulting polymers (see Scheme 1) are analyzed with GPC, 1H NMR, UV−vis absorption, and emission spectroscopy.

Chain-Growth Character Poly(3-octylselenophene). The 2-bromo-5-iodo-3-octylselenophene 4 was prepared from 3-iodoselenophene 1 (see Scheme 2). Treating 3-iodoselenophene 1 with n-octylmagnesium bromine under Kumada-type coupling conditions afforded 2 in 62% yield. The longer n-octyl side chain was selected to ensure the solubility of the poly(3alkylselenophene). Subsequent bromination on the 2-position Scheme 2. Synthesis of the Selenophene Precursor Monomer 4 and Initiator 5

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RESULTS AND DISCUSSION The chain-growth polymerization mechanism based on Pd(Ruphos) uses monobromo-monobromozincio functionalized monomers prepared via a Grignard metathesis on the monobromo−monoiodo precursor, followed by a transmetalation with dry ZnBr2. The Negishi-coupling polymerization is initiated by addition of an aryl−Pd(Ruphos)−Br compound. Complete conversion of the precursor monomer to the actual monomer is essential to prevent any transfer reactions to the monomer if the catalyst is dissociated from the growing polymer chain. The bulky and electron-rich Ruphos ligand promotes the reductive elimination and the oxidative addition, respectively. The Pd(0)−Ruphos catalyst moiety is known to be solution stable and therefore any termination by catalyst degradation is prevented.50 4669

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with N-bromosuccinimide and iodation on the 5-position with I2 and PhI(OAc)2 afforded the precursor monomer 4 in 70% yield. Initiator 5 was synthesized by combining Pd2(dba)3, Ruphos and 2-bromo-3-octyl-selenophene in THF for 2 h at 70 °C. Consecutive addition of pentane to precipitate the initiator complex, followed by Soxhlet extraction with pentane and drying under vacuum, yielded the initiator 5 in 37% yield. To verify whether the polymerization of monomer 4 with Pd(Ruphos) proceeds via a controlled chain-growth mechanism, the monomer consumption as a function of time and the molar masses and polydispersities as a function of conversion were investigated. Therefore, the precursor monomer 4 was treated with t-BuMgCl (1 equiv) in THF (0.1 M) at 0 °C and under argon atmosphere. After 45 min, an aliquot of the reaction mixture was quenched in D2O and analyzed with 1H NMR to verify the complete conversion of 4 to the organomagnesium derivative. Next, the reaction mixture was cannulated to dried ZnBr2 and 1,3,5-trimethoxybenzene (internal reference) and allowed to transmetalate at room temperature for 10 min to obtain the organozinc monomer 10 (see also Scheme 3). Initiator 5 (2.5

relationship between ln([M]0/[M]) and time (until 50 s, at which time the polymerization is finished) is in line with the absence of termination if the transmetalation is ratedetermining. Further proof for the absence of termination is provided by the successful block-copolymerization later in this manuscript. Furthermore, polydispersity values are steady during the polymerization. The rather high (>1.1) polydispersities are explained by the small number of propagation events. Indeed, the polydispersity index for a chain-growth polymerization process equals 1 + “propagation events”−1. During each propagation event, several monomers are added while the catalyst remains complexed with the polymer chain for a very short time where after it decomplexes and moves on to another chain. This behavior was also observed for the synthesis of poly(3-hexylthiophene) and poly(9,9-dioctylthiophene) in previous studies with Pd(Ruphos), notwithstanding the living character.20 Synthesis of the Triblock-Copolymers. Since a controlled chain-growth mechanism is now demonstrated for poly(3-hexylthiophene) (PT), poly(9,9-dioctylfluorene) (PF) and poly(3-octylselenophene) (PS) using the Pd(Ruphos) protocol under the same conditions, ABC π-BCP have come within reach.20 Therefore, the precursor monomers 4, 6 and 8 are converted in a two-step reaction to the monomers 7, 9, and 10 using t-BuMgCl (for 4 and 6) or (t-Bu)3MgLi (for 8) as shown in Scheme 3. The conversion is monitored using 1H NMR spectroscopy as complete conversion is essential for the controlled character of the Pd(Ruphos) mechanism. The 6 possible ABC π-BCP are initiated by addition of the monomer to the appropriate initiator (5 mol %) (see Scheme 4). After 50 min, an aliquot is withdrawn from the reaction mixture and quenched in acidified THF (HCl, 2 M) and an equimolar amount of monomer of the second block is added. Again, after 50 min, an aliquot is withdrawn from the reaction mixture and quenched and the monomer of the third block is added. Finally, the polymerization is quenched 90 min after the last monomer addition using acidified THF (HCl, 2 M). A prolonged reaction time is chosen to account for the larger dilution. The resulting polymers are precipitated in methanol and washed with methanol, acetone and hexane using a Soxhlet extraction to quantitatively remove the leftover monomers, Ruphos ligands, Pd and contamination of homopolymers and/or diblockcopolymers. Indeed, traces of leftover precursor monomer or protonated monomer (formed by reaction of the monomer with traces of moisture) act as a transfer reagent and can initiate new homopolymers, which can in their turn also result on diblock-copolymers. For the homopolymerization of PT, PF and PS, this no real issue−these molecules act then as initiator, resulting in lower molar masses, but it becomes important in the block-copolymerization. Finally, after the extraction with chloroform, the polymer solution is again precipitated in methanol, filtered and dried in vacuum. GPC Analysis. The final ABC π-BCPs poly(3-hexylthiophene)-block-poly(9,9-dioctylfluorene)-block-poly(3-octylselenophene) (PT-b-PF-b-PS), poly(3-hexylthiophene)-blockpoly(3-octylselenophene)-block-poly(9,9-dioctylfluorene) (PTb-PS-b-PF), poly(9,9-dioctylfluorene)-block-poly(3-hexylthiophene)-block- poly(3-octylselenophene) (PF-b-PT-b-PS), poly(9,9-dioctylfluorene)-block-poly(3-octylselenophene)-blockpoly(3-hexylthiophene) (PF-b-PS-b-PT), poly(3-octylselenophene)-block-poly(3-hexylthiophene)-block-poly(9,9-dioctylfluorene) (PS-b-PT-b-PF) and poly(3-octylselenophene)block-poly(9,9-dioctylfluorene)-block-poly(3-hexylthiophene)

Scheme 3. Conversion of the Precursor Monomers 6, 8, and 4 to monomers 7, 9, and 10

mol %) in THF was added to the reaction mixture and aliquots were taken at the appropriate time and quenched in acidified THF (HCl, 2 M) during the polymerization and analyzed with GPC and GC-MS. The extremely fast polymerization rate explains why only 4 withdrawals could be achieved. After less than 1 min, complete conversion of the monomer was achieved and polymers with M n ∼ 8.0 kg/mol and Đ = 1.6 were obtained. The spectra are shown in Figure 1. The linear relationship between the molar mass and conversion points at a chain-growth polymerization without transfer. The linear

Figure 1. Monomer consumption as a function of time and the molar masses and polydispersities as a function of conversion for the polymerization of 10 in THF with Pd(Ruphos) (c = 0.10 M in THF). 4670

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Scheme 4. Synthesis of the Triblock-Copolymers in a One-Pot Synthesis by Sequential Monomer Addition

(PS-b-PF-b-PT) were subjected to GPC with an UV−vis detector set around λmax of the different blocks, i.e. 370 nm for poly(9,9-dioctylfluorene), 440 nm for poly(3-hexylthiophene) and 490 nm for poly(3-octylselenophene) and at 254 nm, a wavelength at which all polymers absorb. The M n values at the different wavelengths are all observed in the same range and Đ values around 1.5, a typical value if Pd(RuPhos) is used,20 are obtained. The unimodal chromatograms of the six ABC πBCPs detected at different wavelengths, displayed in Figure 2,

confirm the successful block-copolymer formation. This is further supported by the fact that the GPC traces recorded at different wavelengths nicely coincide (see also Figures S16− S21, Supporting Information), except for PF-b-PT-b-PS and to a smaller extent PT-b-PF-b-PS. This can be ascribed to the fact that the removal of homopolymers and/or diblock-copolymers by extraction with hot hexane was less effective in these samples. Therefore, the small fluctuations observed for the M n values at the different wavelengths (see Table 1) might indicate the presence of small amounts of homopolymers and diblockcopolymers, caused by incorrect initiation, besides the triblockcopolymers. Already a small presence of this compounds cause different absorbance values in the tails of the spectra of the triblock-copolymers at the different wavelengths and affect the M n values dramatically. Despite the more or less similar content of the different ABC π-BCPs, slight variation among the M n values are noticed and can be ascribed to either real small variations in molar mass or to the sequential variations of the three blocks which result in different hydrodynamic volumes. Indeed, when blocks A and B exhibit a coil-like structure and C a rod-like structure, the hydrodynamic volume of an ABC block-copolymer will be different than in the case of an ACB block-copolymers. The GPC chromatograms of the homopolymer and diblock-copolymer taken during the polymerizations (detected at 254 nm) are available in the Supporting Information (see Figure S16). These spectra need to be considered with care. Again, the different polymer blocks all have their particular hydrodynamic volume and varying the sequence or the content during the polymerization can have significant effects on the M n values. Moreover, they can be contaminated with homopolymers and/or diblock-copolymers. 1 H NMR Analysis. To further determine the structure, the block-copolymers were analyzed with 1H NMR spectroscopy (see Figures S9−S14). A representative spectrum of PT-b-PFb-PS in CDCl3 is shown in Figure 3. Since the repeating units of the three different blocks have clearly distinct 1H NMR aromatic resonances (poly(fluorene): δ = 7.90−7.35 ppm; poly(thiophene): δ = 6.98 ppm; poly(selenophene): δ = 7.12 ppm), this technique can be used to evaluate the content of the different blocks in the synthesized block-copolymers (see Table 2). The theoretical fractions of the different blocks, controlled

Figure 2. GPC spectra of the synthesized triblock-copolymers detected at 254, 370, 440, and 490 nm. The peaks at 570 and 650 s originate from Ruphos and butylhydroxytoluene (stabilizer THF), respectively. 4671

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Table 1. Molar Masses ( M n ) and Molar Mass Distributions (Đ) for the Synthesized Triblock-Copolymers at 254, 370, 440, and 490 nm 254 nm

a

polymer

M n (kg/mol)

PT-b-PF-b-PS PT-b-PS-b-PF PF-b-PT-b-PS PF-b-PS-b-PT PS-b-PT-b-PF PS-b-PF-b-PT

9.1 9.6 12.1 9.4 7.9 9.7

a

370 nm Đ

a

M n (kg/mol)

1.5 1.5 1.6 1.5 1.5 1.4

440 nm Đ

a

11.4 10.2 12.3 10.5 8.6 11.7

a

1.5 1.5 1.6 1.5 1.5 1.4

M n (kg/mol)

a

8.3 9.8 13.9 8.7 7.6 9.5

490 nm Đ

a

M n (kg/mol)a

Đa

9.4 11.5 17.1 10.7 9.0 11.1

1.5 1.5 1.7 1.5 1.5 1.4

1.5 1.5 1.7 1.5 1.5 1.4

Determined by GPC in THF against poly(styrene) standards.

the 1H NMR spectra of the block-copolymers (see Figures S9− S14) support the fact that pure blocks were built in during the synthesis and the thiophene or selenophene monomer was completely consumed before the next monomer was added. DFT Calculations. DFT calculations were exploited to support our hypotheses on the differences in association behavior between the Ni or Pd catalyst and the conjugated system to a further extent. The stability of the various π- and σcomplexes between Ni(dppp) or Pd(Ruphos) and the end of the polymer chains described above were theoretically determined by using the M05 DFT exchange-correlation functional (see Supporting Information for more details). The corresponding brominated monomers of the respective polymers, as a model for the chain ends, were used to simplify and speed up the calculations. In addition, the length of the side-chain was reduced to only one carbon since we are only interested in the interaction between the catalyst and the πconjugated system. Yoshikai et al. showed that the coordination position of a Ni catalyst on an asymmetric aromatic ring of bromotoluene and the activation barrier to move to a new position on the ring are minimal.51 Therefore, the Gibbs free energies for the π-complexes were determined for the association on the C−C bond next to the C−Br bond. The dissociated monomer−catalyst pair is set to an energy of 0 kJ/ mol, and therefore, the energy change indicates the strength of the π- and σ-complex. The results are given in Scheme 5 and clearly illustrate our assumptions. All the Ni-complexes experience solid energy stabilization upon π-complexation prior to σ-complexation. The subsequent oxidative addition stabilizes the complex to a further degree. This preliminary πstabilization is clearly significantly less in all the Pd-complexes or even slightly disfavored. This is perfectly in line with the fact that the Ni-catalyst remains associated with the growing polymer chain through the strong interaction, while this tendency is far less pronounced with the Pd-catalyst and that dissociation from the polymer chain is possible. Moreover, if the different monomers are compared, it is clear that the

Figure 3. 1H NMR spectrum of PT-b-PF-b-PS recorded in CDCl3.

by the feed during the synthesis, were compared with the experimentally obtained values and some deviations from the intended values were found. This is especially the case if the fluorene blocks are polymerized as the last block, because of their slower polymerization with the used catalyst system compared to the polymerization of thiophene and selenophene. Furthermore, the high dilution of the monomer concentration after the last monomer addition reinforce this effect. Indeed, for comparison, all the polymerizations were quenched after the same polymerization time and resultantly no full conversion of the fluorene monomers was obtained and consequently, shorter fluorene blocks are achieved. Hollinger et al. showed that the aromatic resonances of thiophene and selenophene appear at different chemical shifts when a different heterocycle is present at the 5-position, as is the case in alternating or random copolymers but also at the junctions of different blocks in our block-copolymers.47 In the latter case, only very small, or not well-resolved signals, should be present when compared to the internal aromatic resonances. The absence of resolved signals at this particular resonances in

Table 2. Theoretical and Experimental Fractions after Fractionation of the Poly(thiophene) (PT), Poly(fluorene) (PF), and Poly(selenophene) (PS) Blocks in the Synthesized Block-Copolymers

a

polymer

PT (mol %)

PF (mol %)

PS (mol %)

PTa (mol %)

PFa (mol %)

PSa (mol %)

PT-b-PF-b-PS PT-b-PS-b-PF PF-b-PT-b-PS PF-b-PS-b-PT PS-b-PT-b-PF PS-b-PF-b-PT

35 35 35 30 35 30

35 30 35 35 30 35

30 35 30 35 35 35

33 46 37 33 36 35

16 6 18 25 9 7

51 48 45 42 56 58

Determined by 1H NMR spectroscopy in CDCl3. 4672

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Scheme 5. Energy Profiles of the Ni- and Pd-Catalyst πAssociation and the Subsequent Oxidative Addition for Thiophene (Red), Selenophene (Blue), and Fluorene (Green)

Figure 4. UV−vis spectra of the homopolymers PT, PF and PS and the synthesized block-copolymers.

measurements. Since it is know from literature that the fluorescence intensity of PS is only very weak in comparison to that of PT and that the interaction of PS with PT quenches the fluorescence of the latter,47 we use the poor emission of PS to demonstrate that the emission of the block-copolymer depends on the order of the blocks. If the π-BCP is excited at a wavelength at which PF is not excited but PT (and PS) is, the emission will largely result from the PT block, as PS hardly emits.47 However, if PS is situated next to PT, efficient energy transfer can occur, which result in a strong decrease of the PT emission. If, however, the PF block is between the PT and the PS block, energy transfer is complicated, resulting in a much smaller decrease of the emission. In order to exclude the excitation of the PF block, and consequently, the possible energy transfer from the PF block to the PT block or the PS block, an excitation spectrum of poly(fluorene) was recorded at 415 nm (415 nm was determined to be the λmax,em of poly(fluorene) with excitation at 370 nmsee Figure S17). It was found that the PF block does not experience any excitation at 425 nm and further. This wavelength was used to excite the block-copolymers to ensure the major part of the absorbed light was used to excite the PT block and not the PS block. Indeed, both blocks absorb around the same region but the PS absorption is red-shifted (see Figure 5). Figure 5 shows the emission spectra of the PT homopolymer and the synthesized block-copolymers excited at 425 nm, normalized for the absorption of the PT block and corrected for absorption of the

interaction with thiophene is stronger than with fluorene, which is in line with the previously observed fact that, in case the catalyst remains associated with the growing polymer chain, fluorene must be polymerized first, followed by thiophene. UV−Vis Absorption and Fluorescence Spectroscopy. To further assess the electronic properties the synthesized block-copolymers, optical studies in chloroform were carried out. Also the homopolymers PT, PF, and PS were investigated as a reference. All the block-copolymer spectra bear the peak at 370 nm, reflecting the presence of a poly(fluorene) block, and a broadened absorption band centered at 470 nm (see Figure 4), which originates from the combination of a poly(thiophene) block and a poly(selenophene) block, since a larger full width at half-maximum is obtained as for the two homopolymers PT and PS. This is further supported for the poly(selenophene) block by the appearance of congruent edges of the blockcopolymers with the PS at 620 nm. Furthermore, the simulated absorption spectra, based on the absorption of the homopolymers and the fractional content determined in the 1H NMR analysis (vide supra), closely approximate the experimental spectra (see Figure S16). In order to prove the benefits of the possibility to synthesize block-copolymers in all directions, it is necessary to analyze how the sequence of the monomer types affects the physical behavior. In order to target this, we performed fluorescence

Figure 5. Fluorescence spectra of the homopolymers PT, and the synthesized block-copolymers. 4673

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on the computing facilities of the Consortium des Équipements de Calcul Intensif (CÉCI), in particular those of the Plateforme Technologique de Calcul Intensif (PTCI) installed in the University of Namur, for which we gratefully acknowledge financial support of the FNRS-FRFC (Convention No. 2.4.617.07.F and 2.5020.11) and of the University of Namur. S.H.M. and M.T. thank BELSPO (IUAP No. P7-05 “Functional Supramolecular Systems”) for financially supporting their stays at UNamur.

PS block. The experiment demonstrates in particular that the fluorescence is notably quenched with ∼40% for the blockcopolymers carrying a PT block and a PS block next to each other (PT-b-PS-b-PF, PF-b-PT-b-PS, PF-b-PS-b-PT and PSb-PT-b-PF). In PT-b-PF-b-PS and PS-b-PF-b-PT, the concerning blocks are separated by a PF block, which results in a limited interaction, yielding quenching to a much lesser extent, and the signal approaches the fluorescence of the PT homopolymer. The results of the optical study are in line with the proposed structures of the block-copolymers and strongly supports correct block-copolymer formation during the polymerization.

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CONCLUSION This manuscript demonstrates a successful controlled chaingrowth polymerization mechanism for poly(3-octylselenophene) using Ar−Pd(Ruphos)Br as an initiating moiety. Since polymerization of poly(3-hexylthiophene) and poly(9,9dioctylfluorene) both also exhibit a chain-growth character using the same catalyst, all-conjugated ABC-block-copolymers were synthesized by sequential monomer addition in a one-pot synthesis. This approach has not yet been used before for the synthesis of ABC-block-copolymers. The synthesized blockcopolymers were characterized using GPC chromatography and 1 H NMR, UV−vis, and fluorescence spectroscopy to elucidate the different physical behavior resulting from the different order, albeit the same constituting building blocks. Taking together the results of the used techniques, a successful synthesis of the block-copolymers is demonstrated and is consistent with the proposed structures. Theoretic modeling was used to probe the association energies of the π-complexes and revealed a far stronger association of the Ni catalyst with the polymer chain-end compared to the Pd catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental section, GPC chromatograms detected at 370, 440 and 490 nm of the samples withdrawn during the polymerizations, GPC chromatograms of the final blockcopolymers at 254 nm, UV−vis and 1H NMR spectra of the final block-copolymers, 1H NMR and 13C NMR spectra of 2, 3, 4 and 5, GPC spectra of the block-copolymers recorded at different wavelengths, and all geometries of the optimized structures in the DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(G.K.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.V. thanks the Fund for Scientific Research (FWOVlaanderen) for a doctoral fellowship and J.S. thanks the IWT for a doctoral fellowship. We are also grateful to the Onderzoeksfonds KU Leuven/Research Fund KU Leuven and the Air Force Office of Scientific Research. The authors acknowledge Dr. Pieter Willot for the GC−MS measurements and Prof. Koen Binnemans and Dr. Sophie Carron for the use of the spectrofluorimeter and the help concerning the measurements, respectively. The calculations were performed 4674

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