A formal preparation of regioregular and alternating thiophene−thiophene copolymers bearing different substituents

Differently substituted thiophene−thiophene alternating copolymer was formally synthesized employing a halobithiophene as a monomer. Nickel-catalyzed polymerization of bithiophene, which substituent at the 3-position involves alkyl, fluoroalkyl, or oligosiloxane containing alkyl group, afforded the corresponding copolymer in good to excellent yield. The solubility test to organic solvents was performed to reveal several copolymers show a superior solubility. X-ray diffraction analysis of the thin film of the alternating copolymer composed of methyl and


Introduction
Polythiophenes attract much attention in materials science because of their extended π-conjugation, which is applied for a wide range of electronic materials. In particular, the regioregular polymers with head-to-tail (HT) orientation by the substituent at the 3-position are extensively studied to date since those generally show superior performances as material. [1][2][3][4][5][6] Cross-coupling polymerization catalyzed by a transition metal complex has been recognized as an effective tool to afford the regioregular polythiophene, in which 2,5-dihalo-3-substituted thiophene 1 is employed as a monomer precursor converting to the corresponding organometallic monomer by the halogen−magnesium exchange reaction with a Grignard reagent. Employment of 1 leading to polythiophene is shown to proceed in a dehalogenative manner. 3 We have recently shown that generation of the organometallic monomer species can also be achieved alternatively by a deprotonative method with 2-halo-3-substituted thiophene 2 and 3 with a bulky magnesium amide Knochel-Hauser base (TMPMgCl·LiCl) 7 and following polymerization catalyzed by a nickel complex leads to the regioregular HTpolythiophene. 8,9 An additional remark of the deprotonative protocol for polythiophene is the use of chlorothiophene 3, in which the use of a nickel NHC (N-heterocyclic carbene) complex has found effective. 10,11 We have also engaged in the design of the side chain of polythiophenes and several functionalities have been successfuly introduced. 12-14 Our further concern is turned to the copolymerization of thiophene employing differently substituted thiophene monomers, with which several copolymerizations are plausible to give thiophene/thiophene copolymers of random 15 (statistical), gradient, 16,17 block, 18,19 alternating, 20-23 etc. 24,25 We are thus interested in the preparation of alternating polythiophene bearing two kinds of different substituent.
We envisaged that such an alternating copolymer in the perfect regularity is achieved by deprotonative polymerization employing a bithiophene with different substituents at the 3,3'-positions.

Scheme 1: Cross-coupling polymerization of thiophene
We have recently shown that the coupling of 2-chloro-3-substituted thiophene 2 with 2-bromo-3-substituted thiophene 3. 25 The use of a palladium catalyst 26 efficiently surpressed the undesired polymerization to afford the HT halobithiophene with different substituents. Polymerization of 4 (R 1 = n hexyl; R 2 = (CH2)4Si(Me2)OSiMe3) was also examined preliminary and it was confirmed that the formal alternating copolymer was obtained with extremely high regularity. We herein wish to study 4 polymerization of bithiophene 4, which possesses several kinds of substituents with a variety of functionalities. Since a part of homopolymer is well-recognized as rather insoluble in most of organic solvents, the improved solubility in the related alternating copolymer is discussed.

Results and Discussion
Synthesis of chlorobithiophenes with different substitents at the 3,3'-positions were carried out in a manner as we described previously. 25 We chose five kinds of chlorobithiophenes as a monomer precursor for the alternating copolymer as summarized in Scheme 3. Cross coupling proceeded smoothly as shown in Scheme 2 to afford bithiophene 4 in 48-92% yield.

Scheme 3: Preparation of chlorobithiophenes
The formal synthesis of alternating copolymer was carried out with monomer precursor 4 by deprotonation with Knochel−Hauser base followed by addition of nickel catalyst NiCl2(PPh3)IPr to initiate the polymerization of bithiophene.
We first carried out the polymerization of chlorobithiophene 4a bearing hexyl and methyl substituents at the 3-and 3'-positions, respectively. Although the polymerization took place in a slightly low yield (34%) indeed, formation of hardly soluble precipitates was observed during the reaction and the thus obtaind solid was found to fail to dissolve in any of organic solvents. As studied in references on the regioregular polythiophene synthesis, P3HT (poly-3-hexylthiophene) can be smoothly dissolved in several organic solvents. In contrast, there has been few reports on the preparation of regioregular polythiophene bearing a methyl group at the 3-position. 27 Incorpration of the alternating methyl substituent would resulted in much inferior solubility of the alternating copolymer, accordingly. Several kinds of chlorobithiophene 4 was then subjected to the polymerization in a similar manner.

6
The result concerning the formal alternating copolymerization is summarized in Table   1. The deprotonation by the Knochel−Hauser base was carried out at room temperature for 3 h. Addition of the nickel catalyst 5 and further stirring at room temperature followed. The reaction proceeded smoothly to afford the corresponding formal alternating copolymers in 48-84% yields. 28 The molecular weight was found controllable based on the ratio of monomer/catalyst feed ratio and the molecular weight distributions were relatively narrow. Solubility tests of the obtained polymer was studied as summarized in Figure 1.
Although the alternating copolymer composed of methyl and C4 alkyl with terminal pentamethyldisiloxane group 6b was obtained with slightly low melecular weight suggesting improved solubility compared with 6a (R 2 = Me; R 1 = n hexyl), attempted dissolution of the obtained polythiophene 6b to chloroform was found unsuccessful.
Switching the oligosiloxane moiety to the branched derivative (R 2 = (CH2)4Si(Me)(OSiMe3)2) remarkably improved the solubility and the copolymer 6c was soluble in chloroform whereas dissolution to hexane was unsuccessful.
Copolymers bearing a fluoroalkyl substituent ((CH2)3 n C4F9), whose solubility of the corresponding homopolymer was relatively worse than that of the long-chained alkyl derivative, was then examined. The alternating copolymer 6d bearing fluoroalkyl and non-branched disiloxane, respectively, was nicely dissolved in chloroform, while attempted dissolution of 6e in hexanes was shown to be unsuccessful. Remarkable solubility in hexanes concerning copolymer bearing a partial substituent of the fluoroalkyl group was achieved when copolymer composed of branched oligosiloxane 6e was employed. XRD analysis of the copolymer 6c bearing a branched oligosiloxane and methyl groups was carried out. Two remarkable peaks were observed in 2θ = 3.94° and 12.18°, respectively, as shown in Fig 2 (a). The result suggests that the thin film of the alternating copolymer 6c shows bilayer lamellar structure involving 7.3 Å and 22.4 Å distances, respectively, 13,29,30 (Fig 2(b)) The molecular modeling of the alternating copolymer 6c suggests the chain lengths of 11.6 Å and 2.  (4-(1,1,3,3,3-pentamethyldisiloxy)butan-

3-yl)thiophen-2,5-diyl)-alt-poly(3-methylthiophen-2,5-diyl) (6b): To 20 mL
Schlenk tube equipped with a magnetic stirring bar were added 4b (104 mg, 0.25 mmol) and 1 M THF solution of TMPMgCl·LiCl (0.3 mL, 0.3 mmol) was added at room temperature. After stirring at room temperature for 3 h, THF (2.5 mL) and NiCl2(PPh3)IPr (5, 3.9 mg, 6.0 µmol) was then added to initiate polymerization. The color of the solution was turned to light orange. After stirring at room temperature for 24 h, the reaction mixture was poured into a mixture of hydrochloric acid (1.0 M, 2 mL) and methanol (10 mL) to form a precipitate, which was filtered off to leave a dark purple solid. After washing with methanol and hexanes repeatedly, the solid was dried under reduced pressure to afford 79.6 mg of 6b (84% isolated yield). The headto-tail (HT) regioregularity was confirmed by 1  Other polymers 6c-6e were synthesized in a similar manner. Properties and spectroscopic data were summarized below.

Supporting Information
Synthesis of chlorobithiophene 4 was carried out in a manner as shown our previous report. Spectroscopic properties and analytical data for 4 were summarized below. Hz, 1H), 7.14 (d, J = 5.0 Hz, 1H). 13