Facile Preparation of Regioregular Poly(3-hexylthiophene) and Its

Article pubs.acs.org/Macromolecules

Facile Preparation of Regioregular Poly(3-hexylthiophene) and Its Block Copolymers with π‑Allylnickel Complex as External Initiator Long-Mei Gao, Yan-Yu Hu, Zhi-Peng Yu, Na Liu, Jun Yin, Yuan-Yuan Zhu, Yunsheng Ding, and Zong-Quan Wu* Department of Polymer Science and Engineering, School of Chemical Engineering, and Anhui Key Laboratory of Advanced Functional Materials and Devices, Hefei University of Technology, Hefei 230009, China S Supporting Information *

ABSTRACT: Simply prepared π-allylnickel complexes were used as external initiators for promoting the polymerization of 2-bromo-3-hexyl-5-chloromagnesiothiophene in a living/controlled chain growth manner to afford regioregular poly(3hexylthiophene) with an allyl terminus. The nickel species on the other chain end can initiate the block copolymerization of hexadecyloxylallene and 2-bromo-3-hexyl-5-chloromagnesiothiophene to give a well-defined triblock copolymer containing poly(3-hexylthiophene) and poly(hexadecyloxylallene) segments in one pot via mechanically distinct, sequential living polymerization. Furthermore, such π-allylnickel(II) complexes can also catalyze the polymerization of a range of vinyl monomers, including styrene, 1-methoxy-4-vinylbenzene, and 1-chloro-4-vinylbenzene as well as tert-butyl acrylate, in living/controlled fashion. The active nickel unit at the growing chain end of these vinyl polymers can also initiate the block copolymerization of 2bromo-3-hexyl-5-chloromagnesiothiophene to give a series of block copolymers containing vinyl polymer and poly(3hexylthiophene) segments. The new block copolymerizations have been demonstrated to proceed in living/controlled chainextension manner. The well-defined conjugated block copolymers are isolated in high yield with controlled molecular weight and tunable compositions.

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reported by Kiriy,11 Luscombe,12 and Koeckelberghs13 for externally initiated KCTP. In these studies, stable aryl-Ni(II) initiating complexes are used to carry out the polymerizations for well-defined P3HT and other classes of semiconducting polymers.14 In another important development, externally initiated KCTP has been used to prepare surface-attached brushes and thin films of semiconducting polymers.15 Thus, it is obvious that externally initiated KCTP is superior in the preparation of highly regioregular P3HT as well as other classes of conjugated polymers in a controlled manner. However, the reported external initiators for KCTP has been limited to aryl− Ni(II) complexes to date. To the best of our knowledge, other kinds of facile prepared Ni(II) complexes for KCTP have not been reported. One challenge for the design of semiconductor-based optoelectronic devices is the ability for controlling the patterning of active materials on length scales mandated by the charge separation and shuttling processes of the particular application, generally in the 10−100 nm range.16 A promising solution is to make semiconductor block copolymers to control the morphologies through the self-assembly approach.17 In this context, a series of P3HT block copolymers, including P3HT-b-

INTRODUCTION A vast amount of research has been performed on solutionprocessable conjugated polymers due to their applications in optoelectronic and electrochemical devices,1 such as photovoltaic cells,2 light-emitting diodes,3 chemosensors,4 electrochromics, and field-effect transistors.5 Among π-conjugated polymers, poly(3-hexylthiophene) (P3HT) and its derivatives have been widely studied owing to their excellent thermal and environmental stability, high conductivity in doped form, and ability of adjusting their optical and electronic characteristics by structural modifications.6 Well-defined regioregular P3HT is traditionally prepared though a living Kumada catalyst-transfer polymerization (KCTP) based on Ni-catalyzed coupling of 2bromo-3-hexyl-5-chloromagnesiothiophene (also referred to as Grignard metathesis polymerization, GRIM) following the milestone discovery by McCullough7 and Yokozawa.8 The polymerization mechanism has been demonstrated to proceed via the oxidative addition of a Ni(0) catalyst into a Brterminated P3HT, followed by transmetalation with a chloromagnesiothiophene monomer and reduction elimination.9 Participation of an associative Ni(0)−arene π-complex as a key intermediate has been proposed to explain the “quasiliving” character of the polymerization and was confirmed by McNeil through elegant experiments.10 Exploring truly living polymerization thus depends upon the availability of highly reactive catalytic systems. Important milestone works have been © 2014 American Chemical Society

Received: July 1, 2014 Revised: July 21, 2014 Published: July 31, 2014 5010

dx.doi.org/10.1021/ma5013539 | Macromolecules 2014, 47, 5010−5018

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Scheme 1. Synthesis of P3HT, P3HT-b-PHA, and P3HT-b-PHA-b-P3HT Block Copolymers with π-Allylnickel(II) Complexes as External Initiators

poly(dimethylsiloxane),18 P3HT-b-polylactide,19 P3HT-b-poly(pyridinium phenylene),20 P3HT-b-poly(alkyl acrylate),21 P3HT-b-polystyrene,22 P3HT-b-poly(perylene bisimide acrylate),23 P3HT-b-poly(γ-benzyl-L-glutamate),24 and others,25 have been synthesized. In addition, P3HT triblock block copolymers, such as poly(methyl methacrylate)-b-P3HT-bpoly(methyl methacrylate),21d polystyrene-b-P3HT-b-polystyrene,22b poly(4-vinyltriphenylamine)-b-P3HT-b-poly(4-vinyltriphenylamine),26 and P3HT-b-poly(pyridinium phenylene)-bP3HT,27 have also been developed. The synthesis of these P3HT di- and triblock copolymers can be generally classified into either graf ting-to or graf ting-f rom strategies. Such methodologies, however, can be complex and inefficient, frequently resulting in materials that contain inseparable homopolymer impurities. We recently developed a facile synthetic strategy for P3HT-b-poly(phenyl isocyanide) block copolymers in one pot using Ni complex as a single catalyst.28 By using this method, P3HT-b-poly(alkyl isocyanide),29 P3HT-b-poly(hexadecyloxyallene),30 and P3HT-b-poly(6,7-dimethyl-quinoxaline-2,3-diyl)31 block copolymers can be readily prepared in one pot with controlled molecular weights and tunable compositions. However, this polymerization method is limited to the preparation of diblock copolymers and only applicable to some specific monomers. Therefore, the development of a versatile synthetic method for P3HT di- and triblock copolymers with structurally diverse monomers, such as vinyl derivatives, is of highly desirable. Vinyl monomers are usually polymerized though living radical polymerization, while the research on catalytic vinyl oligomerization and polymerization mediated by Ni(II) complexes has rarely been reported. It is well-known that π-allylmetals such as π-allylpalladium and πallylnickel complexes are important intermediates in organic synthesis for providing selective reaction systems. π-Allylnickel(II) complexes have been documented to initiate living polymerization of butadiene,32 allene,33 and isocyanide monomers34 to yield the respective high polymers. Although Brookhart and co-workers reported the living polymerization of some olefins with Ni(II) catalysts containing α-diimine ligand with methylaluminoxane (MAO) as cocatalyst,35 examples of πallylnickel complex as catalyst/initiator for the preparation of conjugated polymers are very rare.36 Herein, we report a novel synthetic procedure for the preparation of regioregular P3HT by using π-allylnickel complex as an external initiator in living/controlled chain growth manner. The allyl group originating from the initiator is simultaneously installed on the P3HT chain end which may be further modified.37 The nickel species resides at the other

P3HT terminus can initiate living block copolymerization of the hexadecyloxylallene (HA) and 2-bromo-3-hexyl-5-chloromagnesiothiophene to give a series of well-defined di- and triblock copolymers containing P3HT and poly(hexadecyloxylallene) (PHA) segments in a single pot via sequential monomer additions. Moreover, such π-allylnickel complex can also promote the polymerization of some vinyl monomers in living/controlled fashion. The Ni(II) species residing at the chain ends of these vinyl polymers are active enough to further initiate the living block copolymerization of 2-bromo-3-hexyl-5-chloromagnesiothiophene, affording welldefined P3HT block copolymers in high yields and with controlled molecular weights and tunable compositions.

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RESULTS AND DISCUSSION

First, π-allylnickel complex 1a with triphenylphosphine (PPh3) as ligand was prepared in THF following the reported procedure by simply mixing Ni(COD)2 (COD = 1,5-cyclooctadiene) with allyltrifluoroacetate in dry THF, followed by addition of PPh3 at 25 °C.33 The resulting solution of 1a was used directly for the polymerization reaction without further isolation. The catalytic activity of 1a for KCTP was then investigated. A solution of 2-bromo-3-hexyl-5-chloromagnesiothiophene (2), which was generated in situ from 2,5-dibromo3-hexylthiophene and iPrMgCl, was treated with 1a ([2]0 = 0.10 M, [2]0/[1a]0 = 20) in THF at 25 °C (Scheme 1). The colorless solution of 2 immediately turned to orange upon the addition of 1a. The polymerization was followed by size exclusion chromatography (SEC) measurements of small aliquots withdrawn from the reaction mixture at appropriate time intervals. When no further molecular weight increase was observed, the reaction mixture was poured into excess amount of methanol, and the precipitated solids were isolated by filtration in 81% yield. As shown in Figure 1a, SEC analysis of the isolated polymer indicated polymerization occurred. The number-average molecular weight (Mn) and its distribution (Mw/Mn) were estimated to be 3.29 kDa and 1.04, respectively (run 1 in Table 1). 1H NMR spectrum of the obtained P3HT was displayed in Figure 2a, which confirmed the structure. The regioregularity of the isolated P3HT was then evaluated by integrating the two signals belonging to the α-CH2 protons of the 3-alkyl chains in the 1H NMR spectrum.7−10 It was found that the regioregularity of the isolated P3HT is not very high as comparing with the typical regioregular P3HT (poly-240) prepared from 2 with Ni(dppp)Cl 2 (dppp = 1,3-bis(diphenylphosphino)propane) as catalyst (Figure 2b). In 5011


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Figure 1. Size exclusion chromatograms of poly-a220 (a), poly-b240 (b), poly(b240-b-340) (c), and poly(b240-b-340-b-240) (d). SEC conditions: eluent = THF, temperature = 40 °C.

Table 1. Selected Results for the Polymerization of 2 with 1a or 1b as External Initiator in THF at 25 °Ca run

initiatorb

[2]0/[1]0c

polymer

Mnd (kDa)

Mw/Mnd

yielde (%)

1 2 3 4 5 6 7

1a 1a 1b 1b 1b 1b 1b

20 45 35 50 60 70 85

poly-a220 poly-a245 poly-b235 poly-b250 poly-b260 poly-b270 poly-b285

3.29 7.44 5.61 8.20 9.21 11.2 13.4

1.04 1.16 1.16 1.14 1.15 1.18 1.17

81 83 80 82 84 86 85

Figure 2. 1H NMR spectra of poly-a220 (a), poly-240 (b), poly-b240 (c), poly(b240-b-340) (d), and poly(b240-b-340-b-240) (e) in CDCl3 at 25 °C.

a

The polymers were synthesized according to Scheme 1. bThe solution of the complexes of 1a and 1b were used directly without further isolation. cThe initial feed ratio. dThe Mn and Mw/Mn were determined by SEC and reported as equivalent to standard polystyrene. eIsolated yield.

allyl group can be observed on an enlarged 1H NMR spectrum of a shorter P3HT prepared by using this method (Figure S3, Supporting Information). The allyl resonances appeared at 6.00, 5.18, and 3.51 ppm with 1:2:2 integral area ratios. These results indicated the polymerization of 2 with 1b as initiator may proceed in a living/controlled chain-growth manner, giving regioregular P3HT with expected structures. To explore if the polymerization of 2 with 1b as external initiator proceeded in a living/controlled chain growth manner, a series polymerizations were carried out with different initial feed ratios of monomer 2 to catalyst 1b. SEC chromatograms of the generated P3HT are shown in Figure 3a. All the isolated polymers exhibited single model elution peaks. The Mn was found to increase linearly and in proportion to the initial feed ratio of 2 to 1b (Figure 3b). Moreover, all samples showed narrow molecular weight distributions with Mw/Mn < 1.20. These results supported the polymerization of 2 with allylnickel complex 1b as external initiator proceeded in a living/ controlled chain growth manner. Because of the living nature of the polymerization, a broad range of regioregular P3HT (poly-b2m’s) with different Mns and narrow Mw/Mns were prepared by simply varying the initial feed ratio of monomer to initiator. The results are summarized in Table 1. To get deep insight into this new polymerization system, the relationship between the conversion of monomer 2 with Mn and Mw/Mn of the isolated P3HT was also examined. The polymerization was performed in THF at 25 °C in the presence of naphthalene as internal standard to facilitate the calculation of the monomer conversion. The polymerization was followed

addition, chain termination and/or transfer may take place during the polymerization of 2 with 1a as initiator. However, these results undoubtedly demonstrated the catalyst-transfer polymerization of 2 with π-allylnickel complex as external initiator was feasible. It has been reported that catalytic initiators of square-planar Ni(II) complexes stabilized by bidentate phosphine ligands exhibited better performance in the KCTP than those with monodentate ligands.11−13 Therefore, new π-allylnickel complex 1b was prepared under a procedure similar to that of 1a by using dppp as ligand instead of PPh3. The coordination structure of 1b was further examined by 31P NMR. The appearance of two signals around 26.5 and 15.4 ppm indicated 1b possessed a cis-geometry (Figure S2, Supporting Information). The polymerization behavior of 1b was then investigated by adding a solution of 1b to monomer 2 in THF at 25 °C ([2]0/[1b]0 = 40, [2]0 = 0.10 M), following the procedure described above for 1a. The resulting poly-b240 exhibited a monomodal elution peak on SEC chromatogram (Figure 1b). The Mn and Mw/Mn were estimated to be 6.31 kDa and 1.15, respectively, by SEC analysis versus standard polystyrene (PS). These results suggest the polymerization of 2 with 1b as external initiator also proceeded well. The 1H NMR spectrum of the isolated poly-b240 is shown in Figure 2c, which further confirmed the structure of the generated P3HT. In addition, the regioregularity of the polymer was estimated to be ≥96%, considering the 1H NMR resolution. Moreover, resonances of 5012


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Figure 3. (a) SEC chromatograms of poly-b2ms prepared from 2 with 1b as initiator in THF at 25 °C with different initial feed ratios of 2 to 1b. (b) Plot of Mn and Mw/Mn values of the generated poly-b2ms as a function of the initial feed ratios of 2 to 1b. Mn and Mw/Mn were determined by SEC with polystyrene standard (SEC conditions: eluent = THF, temperature = 40 °C).

Figure 4. (a) Plot of Mn and Mw/Mn values of the isolated P3HT as a function of 1b-initiated conversion of 2. (b) Plot of the conversion of 2 with the polymerization time initiated by 1b in THF at 25 °C ([2]0 = 0.1 mM, [2]0/[1b]0 = 60).

by SEC and 1H NMR analyses of small aliquots taken from the reaction solution at appropriate time intervals to estimate Mn and Mw/Mn of the generated P3HT and monomer conversion. Both conversion−Mn and conversion−Mw/Mn relationships are plotted in Figure 4a. It can be found that the Mn values of the isolated P3HT are linearly correlated with the conversion of 2 and kept narrow distributions, further confirming the living nature of the polymerization. Furthermore, the polymerization was complete within 60 min after the addition of 1b to the THF solution of 2 (Figure 4b). Recently, we have demonstrated that the nickel species residing at P3HT terminus can initiate the living block copolymerization of hexadecyloxylallene 3 to afford a welldefined P3HT-b-poly(hexadecyloxylallene) (P3HT-b-PHA) block copolymer in one-pot via sequential monomer additions.30 The nickel complex 1b can also promote the tandem polymerization of monomer 2 and 3 to give a Ni(II)terminated P3HT-b-PHA block copolymer poly(b240-b-340) in living/controlled manner ([3]0/[2]0 /[Ni]0 = 40/40/1) (Scheme 1). As shown in Figure 1c, the resulting poly(b240b-340) block copolymer showed a single modal SEC chromatogram at higher-molecular-weight region as compared with macroinitiator precursor poly-b240 (Figure 1b). The Mn and Mw/Mn of poly(b240-b-340) were estimated to be 11.3 kDa and 1.19 by SEC analysis. The nickel complex at the chain end of PHA segment of poly(b240-b-340) possessing a similar structure to that of 1b, which may promote the further polymerization of chloromagnesiothiophene 2. Therefore, the one-pot synthesis

of P3HT-b-PHA-b-P3HT triblock copolymer was performed by using 1b as a single catalyst via sequential monomer additions. Freshly generated 2 was added to the reaction mixture of Ni(II)-terminated poly(b240-b-340) after monomer 3 was completely consumed ([2]0/[Ni]0 = 40). The resulting mixture was stirred at 25 °C for 2 h, affording P3HT-b-PHA-b-P3HT triblock copolymer poly(b240-b-340-b-240). The SEC chromatogram of the isolated triblock copolymer is shown in Figure 1d. It can be seen that the elution peak of the isolated polymer shifted to higher-molecular-weight region compared with diblock copolymer precursor poly(b240-b-340), indicating the block copolymerization of 2 with Ni(II)-terminated P3HT-bPHA diblock copolymer as macroinitiator did take place. The Mn and Mw/Mn of poly(b240-b-340-b-240) were estimated to be 17.6 kDa and 1.25 by SEC analysis. 1 H NMR analyses of the isolated polymers further demonstrated the efficiency of the one-pot syntheses of P3HT-b-PHA and P3HT-b-PHA-b-P3HT block copolymers with 1b as a single catalyst. The 1H NMR spectrum of the block copolymer poly(b240-b-340) is provided in Figure 2d, from which signals attributable to both P3HT and PHA segments can be clearly observed. Characteristic signals of double bond protons of PHA appeared at 5.8−6.0 ppm, and the presence of two peaks in this region indicates the existence of both cis and trans geometry for the double bond. No signals appeared around 5.1 ppm assignable to the exo-methylene moiety in the 1,2-polymerized unit, which suggests the block copolymerization region-selectively took place on the 2,3-position of the 5013


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Table 2. Polymerization Results of P3HT, P3HT-b-PHA, and P3HT-b-PHA-b-P3HT through Sequential Living Block Copolymerization Using 1b as Catalysta P3HTb (poly-b2m)

P3HT-b-PHAb (poly(b2m-b-3n))

P3HT-b-PHA-b-P3HT (poly(b2m-b-3n-b-2o))

run

Mnc (kDa)

Mw/Mnc

Mnc (kDa)

Mw/Mnc

Mnc (kDa)

Mw/Mnc

yieldd (%)

block ratio (m:n:o)e

1 2 3 4

6.31 7.18 9.74 7.69

1.15 1.16 1.19 1.17

11.3 19.6 12.2 10.7

1.19 1.25 1.27 1.19

17.6 31.2 18.6 12.5

1.25 1.27 1.30 1.27

78 80 77 76

40:40:40 45:100:70 60:20:40 50:30:10

a The polymers were synthesized according to Scheme 1. bThe Mn and Mn/Mn was determined by analysis via SEC of aliquots removed from the respective reaction mixtures prior to the addition of a new monomer. cMn and Mw/Mn were determined by SEC and reported as their polystyrene equivalents. dIsolated yields over the three steps. eEstimated by integral analysis on 1H NMR spectroscopies.

Scheme 2. Polymerizations of 4a−c and 5 with 1b as Initiator and Their Block Copolymerization with 2

Figure 5. (a) Size exclusion chromatograms of poly-b4ams prepared form 4a with 1b as initiator in THF at 55 °C with different initial feed ratios. (b) Plots of Mn and Mw/Mn values of poly-b4ams as a function of the initial feed ratio of 4a to 1b. Mn and Mw/Mn were determined by SEC with polystyrene standard (SEC conditions: eluent = THF, temperature = 40 °C).

allene unit of monomer 3.33 Based on the integral analysis of the double bond signal of PHA and the signal of aromatic protons of P3HT, the block ratio of P3HT to PHA was estimated to be 1:1, which was in good agreement with the initial feed ratio of the two monomers and the results of SEC analysis. As shown in Figure 2e, the 1H NMR profile of the triblock copolymer poly(b240-b-340-240) is similar to that of the P3HT-b-PHA diblock copolymer. The number of the double bonds of PHA segment was determined quantitatively on the basis of the integral ratio of the peaks of double bonds to OCH2. Thus, the double bonds in the P3HT-b-PHA block

copolymer did not undergo any side reactions during the triblock copolymerization of 2 with Ni(II)-terminated P3HT-bPHA as macroinitiator. However, the block ratio of P3HT to PHA of the triblock copolymer increased to 2:1, larger than that (1:1) of the diblock copolymer precursor. These results again supported that the polymerization of 3 with Ni(II)terminated P3HT as macroinitiator proceeded in a living/ controlled chain-growth manner, and the afforded P3HT-bPHA with active living chain end could initiate further living block copolymerization of 2 even through distinct mechanism. Thus, a series of P3HT-b-PHA-b-P3HT triblock copolymers 5014


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Table 3. Polymerization Results of 4a−c and 5 with πAllylnickel Complex 1b as Initiator in THF at 55 °Ca

with different Mns and narrow Mw/Mns could be prepared in one pot by just changing the initial feed ratios of monomers and catalyst through the sequential monomer additions using 1b as catalyst (Table 2). We envisioned that this new polymerization system with πallylnickel complex as initiator may be extended to the preparation of vinyl polymers. The polymerization of styrene (4a) with 1b as initiator/catalyst was then investigated (Scheme 2). The polymerization was first carried out in THF solution at 25 °C ([4a] = 0.05 M, [4a]0/[1b]0 = 80), and the process was followed by SEC. However, no polymer product was isolated after even 20 h. At 55 °C, the polymerization took place smoothly as the viscosity of the solution increase considerably. After 20 h, a large amount of methanol was added to quench the polymerization. The formed precipitate was isolated by centrifugation to afford polystyrene (PS) polyb4a80 in 77% yield. As shown in Figure 5a, SEC analysis revealed the isolated material was of high Mn with narrow Mw/ Mn (Mn = 9.21 kDa, Mw/Mn = 1.14). The 1H NMR spectrum (Figure 6) also supports the formation of the desired materials.

run

monomer

feed ratiob

polymer

Mnc (kDa)

Mw/Mnc

yieldd (%)

1 2 3 4 5 6 7 8 9 10

4a 4a 4a 4a 4b 4b 4c 4c 5 5

50 70 80 120 45 65 30 50 75 90

poly-b4a50 poly-b4a70 poly-b4a80 poly-b4a120 poly-b4b45 poly-b4b65 poly-b4c30 poly-b4c50 poly-b575 poly-b590

6.01 7.35 9.21 14.2 7.64 10.4 5.44 8.41 10.6 13.0

1.14 1.21 1.14 1.16 1.23 1.25 1.19 1.25 1.26 1.29

73 75 77 78 77 80 75 78 78 82

a The polymers were synthesized according to Scheme 2. bThe initial feed ratio of monomer to initiator. cThe Mn and Mw/Mn were determined by SEC and reported as equivalent to standard polystyrene. dIsolated yield.

(4c), and nonaromatic vinyl monomer t-butyl acrylate (5) in THF at 55 °C. All these polymerizations gave respective vinyl polymers in high yield with controlled Mns and narrow Mw/ Mns. These polymerizations all proceeded in living/controlled chain-growth fashion as the Mn of the generated materials could be easily controlled by just varying the initial feed ratios of the monomer to catalyst (Figures S6−S8, Supporting Information). The results are summarized in Table 3. Given the high activity of π-allylnickel complex 1b in initiating the living/controlled polymerization of 2, styrene, and tert-butyl acrylate, we envisioned that 1b might also be used for the preparation of conjugated block copolymers containing P3HT and vinyl polymers in one pot via mechanically distinct, sequential living polymerization. Block copolymerizations of 4a, 4b, 4c, and 5 with Ni(II)-terminated P3HT as macroinitiator were thus examined at 25 °C. SEC analyses showed that no copolymerization took place because no molecular weight increase was observed. Since Ni(II)-terminated P3HT was reported to be unstable at elevated temperature,21c block copolymerization at elevated temperature was not further investigated. Instead, we changed the polymerization sequence of the two monomers by initiating block polymerization of 2 with Ni(II)-terminated poly-b4am as a macroinitiator. The block copolymerization was performed by adding a THF solution of 2 to freshly generated Ni(II)-terminated poly-b430 (Mn = 3.75, Mw/Mn = 1.14) in THF at 25 °C ([2]0/[Ni]0 = 30). Upon the addition of 2, the colorless solution turned to orange immediately, indicating block copolymerization took place. After 2 h, a large amount of methanol was added and the formed precipitate was collected by filtration to afford PS-bP3HT block copolymer poly(b4a30-b-230) in 82% yield. SEC measurements also showed that the chromatogram of the resulting material shifted to the higher-molecular weight region (Figure 7a). The Mn and Mw/Mn were estimated to be 9.16 and 1.32 by SEC analysis. Block copolymerizations of Ni(II)terminated poly-b4a60 (Mn = 6.28 kDa, Mw/Mn = 1.23) with 2 of different amount were also performed in THF at 25 °C. The resulting block copolymers were isolated all in high yield and exhibited monomodal elution peaks on SEC chromatograms (Figure S9, Supporting Information). Moreover, a linear correlation between Mn of the isolated poly(b4am-b-2n) block copolymers and the initial feed ratio of monomer 2 to macroinitiator poly-b4a60 was observed (Figure 7b). These

Figure 6. 1H NMR spectra of poly-b4am and the respective poly(b4amb-2n) block copolymer measured in CDCl3 at 25 °C.

It is worthy to note that the radical inhibitor 4-tertbutylcatechol containing in the monomer has no influence on polymerization, which suggests the polymerization did not take place in radical polymerization mechanism. To study the living nature of the polymerization, polymerization of 4a with 1b as initiator was carried out with different initial feed ratios. SEC chromatograms (Figure 5a) showed that all the obtained polymers exhibited single model SEC traces and narrow molecular weight distributions (Mw/Mn < 1.30). The Mn of the afforded poly-b4am also increased linearly and was in proportion to the initial feed ratio of monomer 4a to initiator 1b (Figure 5b). These results demonstrated that the polymerization of 4a with π-allylnickel complex 1b as initiator did proceed in a living/controlled chain growth manner. As summarized in Table 3, a broad range of poly-b4am with different Mns and narrow Mw/Mns were then prepared by varying the initial feed ratio of monomer to initiator. In addition, the established polymerization system for 4a can be further extended to the polymerization of other vinyl monomers, such as p-methoxylstyrene (4b), p-chlorostyrene 5015


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Figure 7. (a) SEC chromatograms of PS-b-P3HT block copolymer poly(b4a30-b-230) prepared with Ni(II)-terminated poly-b4a30 (Mn = 3.75, Mw/ Mn = 1.14) as macroinitiator in THF at 25 °C. (b) Plots of Mn and Mw/Mn values of poly(b4am-b-2n) block copolymers measured as a function of the initial feed ratio of 2 to a common macroinitiator poly-b4a60 (Mn = 6.28 kDa, Mw/Mn = 1.23). SEC conditions: eluent = THF, temperature = 40 °C.

Table 4. Selected Molecular Weights and Distribution Data of Vinyl Polymers and the Respective P3HT Block Copolymersa macroinitiatorb c

run

polymer

1 2 3 4 5 6 7 8 9 10

poly-b4a30 poly-b4a40 poly-b4a50 poly-b4a60 poly-b4b30 poly-b4b25 poly-b4c20 poly-b4c30 poly-b540 poly-b570

block copolymer

Mnd (kDa)

Mw/Mnd

polymerc

Mnd (kDa)

Mw/Mnd

yielde (%)

P3HTf (%)

3.75 4.35 5.19 6.28 5.25 4.56 3.97 5.38 6.07 9.71

1.14 1.08 1.10 1.23 1.25 1.14 1.14 1.05 1.18 1.20

poly(b4a30-b-230) poly(b4a40-b-215) poly(b4a50-b-250) poly(b4a60-b-255) poly(b4b30-b-220) poly(b4b25-b-240) poly(b4c20-b-230) poly(b4c30-b-260) poly(b540-b-260) poly(b570-b-230)

9.16 6.90 13.0 14.6 10.6 12.2 10.0 15.1 17.0 14.6

1.32 1.17 1.13 1.23 1.33 1.23 1.32 1.27 1.27 1.22

82 79 77 84 75 76 74 78 81 80

50 30 50 50 40 60 60 70 60 30

Block copolymers were synthesized according to Scheme 2 by first preparing macroinitiator of different Mns, followed by the addition of monomer 2. bThe Mn and Mw/Mn of the macroinitiators were determined by analysis via SEC of aliquots removed from the respective reaction mixtures prior to the addition of 2. cThe footnotes indicate the initial feed ratios of monomer to catalyst ([monomer]0/[Ni]0). dThe Mn and Mw/Mn values were determined by SEC and are reported as their polystyrene equivalents. eIsolated yield over two steps. fDetermined by 1H NMR spectroscopies. a

initiator (Figures S11−S15, Supporting Information). Additionally, all the isolated block copolymers exhibited narrow Mw/ Mns (Table 4).

results support that the block copolymerization of 2 with Ni(II)-terminated PS poly-b4a60 as macroinitiator did proceed in a living/controlled chain growth manner, giving well-defined PS-b-P3HT block copolymers with controlled Mn, narrow Mw/ Mn, and tunable compositions. A variety of PS-b-P3HT block copolymers with different M n s, narrow M w /M n s, and compositions were thus readily prepared in one-pot using this new method (Table 4). The isolated materials were further characterized by 1H NMR and FT-IR. 1H NMR analysis of the PS-b-P3HT block copolymer in CDCl3 undoubtedly confirmed the formation of the expected block copolymer as the signals of PS and P3HT segments could clearly be observed (Figure 6). Block ratios of P3HT to PS segment of the afforded PS-bP3HT block copolymers could be obtained through integral analysis in the 1H NMR spectrum. The result was in good agreement with the initial feed ratio of the two monomers, further supporting the living nature of the block copolymerization. Further study showed that poly-b4bm, poly-b4cm, and polyb5m with nickel species residing at the polymer terminus could also initiate block copolymerization of 2 to give new welldefined P3HT block copolymers. These block copolymerizations all proceeded in living/controlled chain growth manner as the Mn of the resulting block copolymers could be easily controlled by varying the initial feed ratio of monomers to

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CONCLUSION In summary, we have developed a novel synthetic approach for the preparation of regioregular P3HT by using a readily available π-allylnickel complex as external initiator. The polymerization has been revealed to proceed in a living/ controlled chain growth manner. The Ni(II)-terminated P3HT can initiate living block copolymerization of hexadecyloxylallene and chloromagnesiothiophene monomers, affording a series of new well-defined block copolymers containing P3HT and PHA segments in one-pot via mechanically distinct, sequential living polymerization. Moreover, this allylnickel complex can also promote the living/controlled polymerization of discrete vinyl monomers. In particular, nickel species residing at the living chain end of these polymers can also initiate block copolymerization of chloromagnesiothiophene, yielding a variety of P3HT block copolymers in high yields and with controlled molecular weights and tunable compositions. The allyl group originating from the initiator was simultaneously installed at the chain end of the synthetic polymers, which may allow for further modifications. We believe our findings not only provide a facile synthetic approach for the preparation of 5016


Macromolecules

Article

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semiconductor materials and vinyl polymers but also pave a road to the design of novel catalysts for the synthesis of new polymers.

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ASSOCIATED CONTENT

* Supporting Information S

Additional experimental procedures, spectral data, and SEC chromatograms. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (Z.-Q.W.). Author Contributions

L.-M.G. and Y.-Y. H. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21104015, 21172050, 21371043, 51303044, and 21304027), Fundamental Research Fund for the Central Universities of China (2011HGRJ0005, 2012HGZY0012, and 2013HGCH0013), and the Natural Scientific Foundation of Anhui Province (1408085QE80). Z.W. thanks the Thousand Young Talents Program for financial Support. J.Y. expresses his thanks for Specialized Research Fund for the Doctoral Program of Higher Education (20130111120013) and Research Foundation for Returned Overseas Chinese Scholars of the Ministry of Education of China.

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REFERENCES

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Facile Preparation of Regioregular Poly(3-hexylthiophene) and Its

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