Synthesis and study of poly(3-hexylthiophenes) and poly(3-dodecylthiophenes) containing halogen and sulfur substituents in the ω -position of the side chain

Poly[3-(6-bromohexyl)thiophene] ( 1 ) and poly[3-(12-bromododecyl)thiophene] ( 2 ) were synthesized by chemical polymerization of their respective monomers, 3 and 4 , using FeCl 3 . NMR spectroscopy showed 81% of head-to-tail couplings and GPC showed M n = 51,500 and 91,100 with PDIs of 5.2 and 1.7 for 1 and 2 , respectively. Conductivities of the I 2-and FeCl 3-doped 1 were 18 and 7.5 S cm -1 and for 2 were 32 and 46 S cm -1 , respectively. Poly[3-(6-iodohexyl)thiophene] ( 8 ) was prepared by S N2 displacement (>97%), using NaI, on 1 . Poly[6-(3-thienyl)-1-hexyl S -thioacetate] ( 10a ) and poly[12-(3-thienyl)-1-dodecyl S -thioacetate] ( 10b ) were prepared from 1 by post-polymerization S N2 reactions. They were insoluble in organic solvents and had 13 C CP-MAS NMR spectra consistent with the structures. Reaction of 1 with thiourea followed by basic hydrolysis gave poly[3-(6-mercaptohexyl)thiophene] ( 11 ). This thiol functionalized polymer was also insoluble in common organic solvents and it, too, had a 13 C CP-MAS NMR spectrum consistent with the structure. Polymers 10a and 11 showed pressed pellet conductivities of 4 x 10 -5 and 2.3 x 10 -2 S cm -1 when doped with FeCl 3 , respectively and 8 x 10 -5 and 0.44 S cm -1 when doped with I 2 , respectively. TGA of polymers 1 , 8 and 11 showed thermal decomposition in two stages.


Introduction
Because of their electrical conductivity and electroluminescent properties, polythiophenes have been under intense investigation for many years.2][3] In this paper we report on the preparation of poly [3-(6bromohexyl)thiophene] and poly [3-(12-bromododecyl)thiophene] (2) by direct chemical polymerization of the monomers, 3-(6-bromohexyl)thiophene (3) and 3-(12bromododecyl)thiophene (4) and on their conversion to other polythiophene derivatives by displacement of the bromine leaving group.][6] Very recently a paper appeared reporting on head-to-tail regioregular polymer 1 and on several other polymers derived from 1 by post-polymerization reactions where the bromine acts as a leaving group. 8

Results and Discussion
Bromine and iodine containing polymers The monomers, (6-bromohexyl)thiophene (3) and 3-(12-bromododecyl)thiophene (4), were prepared as shown in Scheme 1 using a modification of the synthesis reported by Bäuerle. 9The synthetic steps we employed were similar to those of Bäuerle except for the alcohol protecting group.We used the tetrahydropyranyl (THP) protecting group whereas Bäuerle used the pmethoxyphenyl group.1-Bromo-6-(2-tetrahydropyranyloxy)hexane and 1-bromo-12-(2tetrahydropyranyloxy)dodecane were prepared by a modification of the literature procedure from the corresponding ω-bromo-1-alkanol. 10,11Amberlyst H-15, 12 an ion exchange resin containing strongly acidic -SO 3 H groups, was used and yields of 90% and 97% of 5a and 5b, respectively, were obtained.Coupling of the Grignard reagents obtained from 5a and 5b with 3bromothiophene using Ni(dppp)Cl 2 [dppp=1,2-bis(diphenylphosphino)propane] as catalyst gave compounds 6a and 6b.Methanolysis, using Amberlyst H-15, produced 6-(3-thienyl)-1-hexanol (7a) and 12-(3-thienyl)-1-dodecanol (7b) and conversion of -OH to -Br to give 3 and 4 was carried out using PBr 3 following the procedure of Smith, 13 and also with HBr in the case of 7b.For the oxidative polymerization, 4 equivalents of anhydrous FeCl 3 were used to give polymers 1 and 2 (Scheme 1) after dedoping with NH 4 OH. 1 and 2 were soluble in common organic solvents such as chloroform, toluene, THF, DMF and DMSO.The chemical structures and degrees of regioregularity of 1 and 2 were studied by 1 H and 13 C NMR spectroscopy.The 1 H NMR spectra show two resonances at δ 2.83 and 2.60 for 1 and δ 2.81 and 2.60 for 2. On the basis of the shielding effect of the thiophene ring and comparison with the spectrum of poly(3-hexylthiophene) 14 and poly(3-dodecylthiophene) 15 the peaks Scheme 1 at δ 2.60 and δ 2.83/2.81were assigned to the CH 2 groups adjacent to the thiophene ring for the head-to-head (HH) and head-to-tail (HT) configurations, respectively.Furthermore, the intensity ratio of these two peaks suggests that both 1 and 2 consist of 81% HT dyads and 19% HH dyads.Analysis of the spectral region at δ 6.98-7.06provides additional configurational and regiochemical information on the polymers.The four resonances for 1/2 at δ 6.98, 7.01, 7.03 and 7.06/7.05are assigned to HT-HT, HT-TT, HT-HH, TT-HH triad configurations, respectively. 14,15t should be noted that the HT-HT peak at δ 6.98 compares quite favorably with the value of δ 6.98 reported by Iraqi, et al. 6 and δ 6.95 reported recently by Zhai, et al. for the HT-regioregular version of 1. 8 In addition, the peaks reported by Ng, et al. agree with those we observed and, while in our hands the HT dyad is 81% of the polymer, they report that their polymer 1 has only 68-73% HT dyads. 7The reasons for their lower regioregularity is not clear.The relative percentage of triad configurations and 1H NMR data for polymers 1 and 2 are shown in Table 1.The signal for the methylene protons linked to the -Br group are at δ 3.43 for 1 and 3.39 for 2.There is, in addition, a fairly small (<5%) broad peak centered at δ 0.86 ppm for both 1 and 2 which corresponds to a terminal methyl group which must have been produced by reduction of the ω-CH 2 Br in one of the synthetic steps.The structure of the polymers was also revealed in part by the 13   M , obtained by gel permeation chromatography (GPC) using polystyrene standards, for 1 and 2 were 51,500 and 91,100 with polydispersities of 5.2 and 1.7, respectively.This is to be compared with the significantly lower number average molecular weights of 12,300-23,400 obtained by Ng, et al. for 1 also using FeCl 3 polymerization. 7Films of 1 and 2 were deep red/burgundy in color with very similar UV-vis absorption maxima at 492 nm and 505 nm, respectively, and THF solutions of these two polymers also showed similar absorption spectra with λ max of 436 and 438 nm, respectively, all close to the values reported for poly(3-hexylthiophene) [17][18][19] and poly(3-dodecylthiophene) 19 prepared using FeCl 3 .The fluorescence properties of 1 and 2 were also examined.THF solutions of 1 and 2 showed emission maxima at 573 and 576 nm while films cast from THF showed maxima at 670 and 680 nm, respectively.The UV-vis maxima reported by Ng, et al. was 434 nm in CHCl 3 solution and 488 nm for the film while the fluorescence maximum reported was 558 nm for a film of 1. 7 These values show that the polymers prepared here have significantly greater conjugation lengths than those prepared by Ng.The deep red/burgundy polymer films of 1 and 2 turned dark blue upon doping with iodine and 0.05 M FeCl 3 in dry nitromethane.Doping of 1 with I 2 gave a 4-point probe electrical conductivity of σ = 18 Scm -1 while the FeCl 3 -doped polymer showed σ = 7.5 Scm -1 .Films of 2 gave somewhat higher conductivities.With I 2 doping σ = 32 Scm -1 and with FeCl 3 doping σ = 46 Scm -1 .Ng, et al. reported iodine doped conductivities for 1 of 7.9-8.2Scm -1 . 7n order to investigate the effectiveness of S N2 type displacements on these bromo polymers we examined the displacement of bromide by iodide using polymer 1.Since the polymer was soluble in CHCl 3 but insoluble in acetone and the displacement reaction (Finkelstein reaction) 20 is generally run in acetone where NaI is soluble, but it is insoluble CHCl 3 , it was determined that a 3.8:1 (v/v) mixture of CHCl 3 and acetone would keep the reactants in solution.In order to see if the displacement reaction went to completion and produced poly [3-(6-iodohexyl)thiophene] (8) 1 H NMR spectra were examined.The peak in 1 at δ 3.43 was replaced by a peak at δ 3.21 due to the hydrogen atoms on the carbon α-to the halogen.Integration of the peak at δ 3.21 vs the area around δ 3.43 showed that the displacement S (CH 2 ) 6 I n 8 had occurred to at least 97% completion.Figure 1 shows the 1 H NMR spectra of 1 and 8.This 97% displacement is larger than the 87% reported by Iraqi et al. 6 in displacement on the HTregioregular version of 1 with a carboxylate nucleophile and is similar to the report of Zhai, et al. 8

Sulfur containing polymers
Approaches to the synthesis of sulfur containing polymers included attempted polymerization of sulfur containing monomers and displacement reactions on the bromoalkyl substituted polythiophenes.Initially we prepared 6-(3-thienyl)-1-hexyl S-thioacetate (9a) and 12-(3-thienyl)-1-dodecyl S-thioacetate (9b) by displacement on bromoalkyl monomers 3 and 4 using potassium thioacetate in ethanol.Polymerization was carried out using 4 equivalents of anhydrous FeCl 3 in CHCl 3 (Scheme 2).The polymers precipitated and were washed with MeOH and dedoped with NH 4 OH or hydrazine to give red-orange solids of 10a and 10b.Surprisingly, it was found that both ω-SCOCH 3 functionalized polyalkylthiophenes 10a and 10b were insoluble in CHCl 3 , THF, DMSO, CH 3 OH, toluene, odichlorobenzene and tetramethylene sulfone, and only very slightly Scheme 2 soluble in boiling thiophene (84 °C).The reasons for the insolubility of these polymers is unknown.However, since Zhai, et al. report that HT-regioregular 10a is soluble in organic solvents, the insolubility of the samples we prepared by FeCl 3 oxidation may be due to a higher molecular weight.As discussed above, the number average molecular weights of 1 and 2 were 51,500 and 91,100 respectively whereas that reported for the HT-regioregular 1 was 15,000-18,000 using a lower monomer concentration or 25,000-30,000 using a higher monomer concentration. 8Thus, our molecular weight was at least double those reported by Zhai, et al. 8 for 1 and are 3-4 times that for polymer 2. Polymers 10a and 10b were characterized by IR and 13 C CP-MAS NMR spectroscopy along with elemental analysis.Thus, for example, 10a showed the ring C-H stretch at 3058 cm -1 , the aliphatic C-H stretching at 2852 and 2926 cm -1 and the carbonyl stretch at 1688 cm -1 .The 13 C CP-MAS spectra of 10a and 10b are shown in Figure 2. The spectra are consistent with the structures in that the integrated ratios of the aromatic peaks (at δ ≈ 120-145) to aliphatic peaks (at δ ≈ 20-45) are 0.54:1.00for 10a and 0.26:1.00(including spinning side-bands) for 10b.The theoretical integrated ratios should be 0.57:1.00and 0.31:1.00for 10a and 10b respectively.The pressed pellet conductivities for polymer 10a were σ = 4 x 10 -5 Scm -1 doped with FeCl 3 and σ = 8 x 10 -5 Scm -1 doped with I 2 .The next approach to poly[3-(6-mercaptohexyl)thiophene] (11) involved post-polymerization substitution of bromo-polymer 1.A standard method for preparing thiols from a halide is by S N2 reaction with thiourea to produce an isothiouronium salt, which, in turn, can be hydrolyzed with aqueous Na 2 CO 3 21,22 to produce a thiol. 23Scheme 3 shows this sequence of reactions as applied to polymer 1. Polymer 11 was produced as a red powder which was insoluble in standard organic solvents Our structure proof rests on the IR and 13C CP-MAS spectra as well as elemental analysis.The thiophene and aliphatic C-H stretching frequencies were at 3059, 2853 and 2927 cm -1 .Unfortunately there was not an obvious S-H stretching band in the range 2550-2600 cm -1 but because it is characteristically rather weak it may go undetected in thin films or in dilute solutions. 24The 13 C CP-MAS spectrum of 11 is shown in Figure 3.The integrated ratio of the aromatic peaks (at δ ≈ 120-145) to aliphatic peaks (at δ ≈ 20-45) is 0.58 (including spinning sidebands):1.00whereas the required ratio for 11 is 0.67:1.00.The pressed pellet conductivities for polymer 11 were σ = 2.3 x 10 -2 Scm -1 doped with FeCl 3 and σ = 0.44 Scm -1 doped with I 2 .

Thermogravimetric analysis
Thermogravimetric analyses (TGA) were run on polymers 1, 8, and 11 to examine their thermal stabilities.All three showed two stage weight losses (Figure 4).For 1, decomposition began at about 330 °C, and the second step began at about 470 °C and ended at about 580 °C.For 8, which was less stable than 1, decomposition began slowly at about 150 °C and the second step began at about 370 °C and ended at about 510 °C.For 11, decomposition began at about 210 °C and the second step began at about 360 °C and ended at about 520 °C which makes this polymer of intermediate stability.From the weight loss it appears that the first step in each case is loss of HBr, HI and H 2 S respectively.For polymer 1 the first weight loss is about 36% and loss of HBr requires 33%, for polymer 8 the first weight loss is about 41-45% and loss of HI requires 44% while for polymer 11 the first weight loss is about 26% and loss of H 2 S requires 17%.The second weight loss is most likely due to side chain loss.

Experimental Section
General Procedures. 1 H and 13 C NMR spectra were obtained on either a Bruker MSL 300 spectrometer operating at 300.13 MHz for 1 H and 75.97 MHz for 13 C or on a JEOL Eclipse 500 spectrometer operating at 500.16 MHz for 1 H and 125.78 MHz for 13 C. CDCl 3 was used as the solvent with TMS (δ = 0.00 ppm) and CDCl 3 (δ = 77.0ppm) used as internal reference for 1 H and 13 C spectra, respectively.CP-MAS 13 C NMR spectra were taken on a Bruker Avance 400 instrument operating at 100.63 MHz by Bruker Instruments, Billerica, MA.FT-IR spectra were obtained on a Biorad-Digilab FTS-40 or a Bruker VECTOR22 FT-IR instrument using powdered samples (approximately 1-2 weight %) with KBr in a diffuse reflectance unit or liquid samples between NaCl plates.Gel permeation chromatography (GPC) was carried out on a Waters GPC system, using a Waters Model 510 HPLC pump, a Model 490 multiwavelength detector λ = 254 nm), Millennium 2010 Software, a serial combination of 103, 104, and 105 Å Ultrastyragel columns and THF with a flow rate of 1.0 mL/min.The calibration curve was established by use of polystyrene standards with a molecular weight range of 800 to 9x105 g/mol.HPLC was performed on a Waters HPLC system, using a Waters Model 501 HPLC pump, a Lambda-Max Model 481 LC UV-vis detector (254 nm), Maxima 820 Chromatography Software, and an Econosil C18 10U (10 µm) reverse phase column (250 mm x 4.6 mm).Methanol was used as the eluent with a flow rate of 1.0 mL/min.UV-vis-NIR spectra were recorded on a Cary 5E UV-vis-NIR spectrophotometer using tetrahydrofuran solutions and polymer thin films cast onto quartz cuvettes from polymer-THF solutions.Fluorescence spectra were measured on a Perkin-Elmer Model 204 fluorescence spectrophotometer using a Perkin-Elmer 150 Xenon power supply.Samples were either polymer-THF solutions or polymer thin films on glass substrates.TGA was carried out on a DuPont model 9900 Thermal Analysis system fitted with a Model 951 Thermogravimetric Analyzer, under nitrogen with a heating rate of 10 °C/min.Elemental analyses were obtained either on a Perkin-Elmer 2400 CHN analyzer or determined by Texas Analytical Laboratories, Stafford, Texas.Melting points were determined using a Thomas-Hoover capillary melting point apparatus and are uncorrected.EI mass spectra were obtained at 70 eV on a Finnigan MAT TSQ-70 instrument.The electrical conductivity of doped polymer films was measured using the standard four-in-line probe method. 25,26The thickness of polymer films was determined with an Alpha-Step 200 profilometer or Mitutoyo Digimatic Digital Micrometer.

3-[6-(2-Tetrahydropyranyloxy
)hexyl]thiophene (6a). 28Into a 100 mL, three-necked flask equipped with a magnetic stirrer, a pressure-equalizing dropping funnel, and a reflux condenser attached to an argon gas inlet was put 0.85 g (35 mg-atom) of magnesium turnings.A solution of 1.00 g of 1-bromo-6-(2-tetrahydropyranyloxy)hexane (5a; 3.80 mmol) in 20 mL of dry THF was added rapidly and an exothermic reaction occurred.Then an additional 7.08 g (total = 8.08 g; 30.0 mmol) of 5a in 50 mL of dry THF was added over ca.1.5 h at room temperature.The mixture was stirred at room temperature for 2 h, then heated at about 45 °C with stirring for 1 h.Then, into a 100 mL three-necked flask, equipped in the same manner as above, was put 4.20 g (26.0 mmol) of 3-bromothiophene, 20 mL of dry ether and 20 mg of Ni(dppp)Cl 2 [Ni(Ph 2 P(CH 2 ) 3 PPh 2 Cl 2 ].The Grignard reagent prepared above was transferred to a dropping funnel and added over 2 h with stirring to the mixture cooled in an ice bath.After the addition was complete, stirring was continued for an additional 3 h at the ice bath temperature.A white precipitate formed and the resulting mixture was allowed to warm to room temperature.Stirring was continued for another 18 h and then the mixture was heated with stirring at 50 °C for 1 h.The mixture was cooled to room temperature and poured into 100 mL of 1 M NaOH solution.This was extracted with diethyl ether (2x50 mL) and the combined ether extracts were washed with water (4x60 mL) and dried (MgSO 4 ).After filtration, the ether was removed in vacuum to give a crude brown product which was purified by silica gel flash column chromatography using hexane/ethyl acetate (33:1 v/v) as eluent.4.80 g (70%) of 6a was obtained as a clear, colorless liquid with a purity >98% (HPLC area percent, t R = 4.22 min.). 1

12-(3-Thienyl)-1-bromododecane (4).
In a 100 mL three-necked flask fitted with a pressure equalizing dropping funnel, and a condenser capped with a drying tube was placed 0.490 g (1.80 mmol) of PBr 3 and 20 mL of freshly distilled benzene.A mixture of 0.10 mL of dry pyridine and 2 mL of benzene was added dropwise via syringe with stirring over a period of 10 minutes.The flask was placed in an ice-salt bath, the contents were cooled to -10 °C and a mixture of 1.32 g of 7b (4.92 mmol) and pyridine (0.10 mL) in 5 mL of benzene was added slowly over a period of 1 h keeping the temperature between -10 °C and -5 °C.Stirring was continued for 1 additional h in the cooling bath and then the mixture was allowed to warm to room temperature and stirred for 46 h.The product was purified by silica gel flash column chromatography using hexane/ethyl acetate (100:1 v/v) as eluent to give 0.47 g (29%) of the colorless liquid product 4. 48% HBr was also used in place of PBr 3 with benzene as solvent.The reaction was carried out for 6 h under reflux.1.50 g of 7b, 13 mL of 48% HBr, and 15 mL of benzene were placed in a 100 mL round bottomed flask under argon.The reaction was run for 6 h under reflux and gave 37% (0.66 g) of 4. The chief advantages of this latter method for preparing 4 were that not only did it give improved yields (to 37-46%) but also that 35-47% of unreacted 7b could be completely recovered from the reaction mixture for reuse.UV-vis (hexane): λ max 235 nm (ε = 4.71 x 10 3 ). 1 H NMR δ: 1.25-1.50(m, 16 H), 1.51-1.70(quintet, 2 H), 1.83 (quintet, 2 H, J = 7 Hz), 2.61 (t, 2 H, J = 8 Hz), 3.38 (t, 2 H, J = 7 Hz), 6.86-7.00(m, 2 H), 7.10-7.23(m, 1 H). 13

Poly[3-(6-iodohexyl)thiophene] (8).
Poly [3-(6-iodohexyl)thiophene] (8) was obtained by a modification of the Finkelstein reaction 9 from 1.The solvent selection was critical for the completion of substitution of bromine by iodide.A solubility study showed that 1 was completely soluble in CHCl 3 and insoluble in acetone, while sodium iodide was soluble in acetone and insoluble in CHCl 3 .However, both 1 and NaI were soluble in a mixture of CHCl 3 and acetone in the ratio of 3.8 to 1. Poly [3-(6-bromohexyl)thiophene] (1; 30.0 mg; 0.120 mmol) and CHCl 3 (25 mL) were placed in a 100 mL three-necked round bottomed flask equipped with magnetic stirrer, and nitrogen inlet.A solution of NaI (0.18 g, 1.22 mmol) in acetone (6.50 mL) was prepared in a small vial.The CHCl 3 and acetone were dried over 4 Å molecular sieves before they were used.To the flask containing 1/CHCl 3 was added the NaI/Me 2 CO solution via syringe.The mixture was heated at 65 °C for 22 h, then 3 mL of acetone was added and the mixture was heated for an additional 24 h at 65 °C.The solvent was removed in vacuum and 50 mL of CHCl 3 was added.The solution was washed four times with water, dried (MgSO 4 ) and the solvent was removed in vacuum to give dark red poly [3-(6-iodohexyl)thiophene] (8).Further purification was done by adding a concentrated CHCl 3 solution of 8 to 40 mL of methanol from which 32.9 mg (92%) of 8 was obtained. 1H NMR δ: 6.98 (m, 1 H), 3.43 (m, about 2-3%, this peak belongs to -CH 2 Br of 1), 3.21 (m, 2 H), 2.51-2.89(br m, 2 H), 1.20-1.92(m, 8 H). Integration of the peaks at δ 3.43 (-CH 2 Br) and δ 3.21 (-CH 2 I) indicated >97% of the bromine had been displaced by iodide. 13

12-(3-Thienyl)-1-dodecyl S-thioacetate (9b).
Potassium thioacetate (0.420 g, 3.66 mmol) and absolute ethanol (20 mL) were put into a 50 mL three-necked round bottomed flask equipped CHCl 3 .40.0 mg (0.520 mmol) of thiourea was dissolved in 10 mL of EtOH and both solutions were combined in a 50 mL three-necked flask equipped with a magnetic stirring bar, condenser capped with a drying tube and nitrogen inlet.The reaction mixture was stirred for 24 h under reflux, cooled to room temperature and the solvent was removed in vacuo.20 mL of CHCl 3 and 6 mL of saturated Na 2 CO 3 were added and the mixture was stirred under nitrogen at room temperature for 24 h.The product 11 was separated and washed with an aqueous HCl solution (pH = 4.00), methanol and dried under vacuum.The red powder of 11 was insoluble in CHCl 3 , THF, DMF, DMSO, toluene, xylene, o-dichlorobenzene and very slightly soluble in refluxing thiophene (84 °C). 13 The pressed pellet conductivities for polymer 11, obtained as described for polymers 1 and 2, but on pellets pressed in a KBr IR die, were σ = 2.3 x 10 -2 Scm -1 doped with FeCl 3 and σ = 0.44 Scm -1 doped with I 2 .Attempted hydrolysis of 10a.0.02 g (0.083 mmol) of poly[6-(3-thienyl)-1-hexyl S-thioacetate] 10a, 5 mL of 2M KOH solution and 10 mL of CH 2 Cl 2 were placed in a 50 mL three-necked round bottomed flask equipped with a magnetic stirrer and nitrogen inlet.The mixture was refluxed for 32 h and the solid material did not go into solution during this time.After hydrolysis, the solid was filtered and its IR spectrum was identical to that of the starting material 10a.Another hydrolysis experiment was performed using 0.5 g NaBH 4 in 5 mL of 1M KOH and 10 mL of C 2 H 5 OH.This reaction was carried out for 48 h under reflux.The solid material 10a did not go into solution during the hydrolysis process and after hydrolysis, the product again was shown to be the starting material 10a by IR spectroscopy.
C NMR spectra.The signals at δ 139.6 and 139.8 are assigned to the carbons in position 3 of the thiophene rings containing the alkyl substituents in 1 and 2, respectively, the signals at δ 133.7 and 133.6 to the carbon in position 5, those at δ 130.6 and 130.4 to the carbons in position 2 and those at δ 128.7 and 128.6 to the unsubstituted carbons at position 4, analogous to the assignments that have been made for poly(alkylthiophenes) by Hotta, et al.16

Figure 2 .
Figure 2. 13 C CP-MAS NMR spectra of 10a and 10b.The peaks marked with * are spinning sidebands.

Scheme 3 such
Scheme 3

Figure 3 .
Figure 3. 13 C CP-MAS NMR spectrum of 11.The peaks marked with * are spinning sidebands.