An efficient method to prepare a new class of regioregular graft copolymer via a click chemistry approach

Abstract

Herein, we report the synthesis of a novel regioregular poly-norbornene anhydride-g-(3-hexyl thiophene) (PNBA-g-PHT) graft copolymer. Grignard metathesis polymerization was used to obtain a PHT homopolymer. Then, PHT-N₃ was prepared via a chemical modification reaction of the hydroxyl terminated PHT. A click chemistry approach was then used to synthesis MM 1. Finally, ring-opening metathesis polymerization (ROMP) of MM 1 produced the final copolymer. NMR and COSY studies clearly showed that the regioregularity was retained in the newly synthesized copolymer.

---

In recent trends in the literature, poly(3-hexyl thiophene) (PHT) has attracted considerable interest of researchers due to its high optoelectronic properties with applications in photovoltaics, light emitting diodes, and solar cells.^1,2 The synthesis of head to tail coupled regioregular poly(3-hexyl thiophene) polymer has been well established in the literature by McCullough and co-workers using Grignard metathesis.^3–5 However, regioregular PHTs still have issues, such as poor mechanical and processing properties relative to typical flexible polymers.⁶ Recently, block copolymers have emerged as interesting materials due to their self-assembly properties; they can form conventional superstructures as nanostructures.^7,8 This problem can be overcome by making a copolymer of PHT with a conducting polymer or oligomer units.⁹ Such a block copolymer can be self-assembled into interesting morphologies such as lamellar, spherical, cylindrical and vesicular structures.^10,11 Most importantly, in the block copolymer, the π-conjugated segment helps to assemble novel morphologies, which enhances the mechanical as well as electronic properties of the polymeric materials.⁷ There are studies reporting varieties of poly(3-hexyl thiophene)-based block copolymers that have been synthesized via ATRP and RAFT polymerization techniques.^1,3,8,11,12 However, there are some limitations such as the difficulties found in their synthesis and purification, along with limitations associated with the existing procedures including controlling the molecular weight of the polymers formed and the removal of the catalysts used from the final products. Therefore, ring-opening metathesis polymerization (ROMP) was utilized in this study, because the Grubbs' catalyst displays a wide range of functional group tolerance.^13–20

To overcome these problems, herein, we envisioned a simple synthetic route to prepare regioregular poly-norbornene anhydride-g-(3-hexyl thiophene) (PNBA-g-PHT) graft copolymer using ring-opening metathesis polymerization¹⁶ and Grignard metathesis polymerization.^21–23 A click chemistry approach was used to graft PHT onto norbornene backbone (Scheme 1).

Scheme 1 Synthesis scheme for PNBA-g-PHT copolymer.

First, we synthesized regioregular vinyl-terminated poly(3-hexyl thiophene) (PHT) 1 by the Grignard metathesis method, using 1,3-bis(diphenyl phosphino)propane nickel(II) chloride (Ni(dppp)Cl₂) as the catalyst.³ The formation of this product was confirmed by NMR spectroscopy. In ¹H NMR spectrum, the signal at δ = 6.8 ppm indicated the presence of aromatic protons (Fig. 1a). In addition, in solid state ¹³C CP-MAS NMR spectrum, the signals at δ = 10–40 ppm were ascribed to the carbons of hexyl group and the signals at δ = 130–145 ppm were assigned to the aromatic thiophene ring (Fig. 1b). A COSY spectrum showed a strong correlation between the HT α-CH₂ protons and the HT β-CH₂, which were located as a broad peak at δ = 1.69 ppm. The HT σ-CH₂ protons were also correlated with the HT ξ-CH₂ resonance at δ = 1.20 (Fig. S2 & S3†). Moreover, in the HMQC NMR spectrum, a broad signal at δ = 2.81 ppm overlapped with a broad peak having lower intensity at δ = 2.6 ppm (Fig. S5†). These results confirmed the regioregularity of poly(3-hexyl thiophene) prepared (Fig. S1–S8†). Then, PHT 1 was reduced using 9-BBN, followed by a reaction with 2-bromopropionyl bromide to obtain molecule 3 (Fig. S10†), which was further functionalized to obtain PHT-azide (PHT-N₃) (Fig. S12†).

Fig. 1 (a) ¹H NMR spectrum of poly(3-hexyl thiophene). (b) Solid state ¹³C CP-MAS spectrum of poly(3-hexyl thiophene).

Subsequently, the macromonomer (MM 1) was synthesized by a click reaction between PHT-azide and norbornene–alkyne. The completion of the reaction was confirmed by FT-IR spectroscopy. In the FT-IR spectrum, the stretching frequency at 1633 cm⁻¹ was due to aromatic C=C stretching, and the stretching frequency at 2926 cm⁻¹ and 3329 cm⁻¹ were due to aliphatic and aromatic C–H stretching, respectively. The stretching frequency for the azide functional group at 2110 cm⁻¹ disappeared completely due to the formation of MM 1 (Fig. 2 and S14†). Finally, the macromonomer (MM 1) was polymerized by a ROMP technique using the second-generation Grubbs' catalyst to yield poly-norbornene anhydride-g-(3-hexyl thiophene) (PNBA-g-PHT) copolymer that was soluble in THF as well as chloroform. In the NMR spectrum, the appearance of a new signal at δ = 5.34 ppm confirmed the polymer formation. The molecular weights of the macromonomer and polymers formed were obtained using GPC techniques with polystyrene as a standard and THF as the solvent. The observed molecular weight (M_n) was 20 300 Da with a polydispersity index of 1.14 (Fig. 3, S15† and Table 1).

Fig. 2 FT-IR spectra of PHT-N₃ (black) and the macromonomer MM 1 (red).

Fig. 3 GPC traces for molecule 1 (black), PHT-N₃ (red), macromonomer MM 1 (blue) and PNBA-g-PHT copolymer (pink).

Table 1 Molecular weights of the polymers (GPC using THF as the solvent and calibration using a polystyrene standard)

Sr. no.	Polymer	Molecular weight (Mn) by GPC (Da)	PDI
1	Molecule 1	5200	1.03
2	PHT-N3	5500	1.09
3	MM 1	5700	1.12
4	PNBA-g-PHT	20 300	1.14

---

The electronic properties of the polymeric materials were studied using UV-vis spectroscopy, where the band at 449 nm was attributed to the thiophene segment; a similar band was observed in the copolymer as well (Fig. S16 and S17†). Moreover, the mechanical properties, for example, the thermal stability as well as the rigidity of the polymer, were investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (Fig. S18 and S19†), respectively.

Finally, to study the morphology of the PNBA-g-PHT copolymer, atomic force microscopy (AFM) and scanning electron microscopy (SEM) were utilized. A PNBA-g-PHT copolymer solution in chloroform was prepared using a spin coating method. A rod-like nanostructured morphology was observed using AFM (Fig. 4a), which was further supported by SEM (Fig. 4b).

Fig. 4 Morphology study of PNBA-g-PHT copolymer: (a) AFM and (b) SEM images.

In summary, we have successfully demonstrated the synthesis of a novel regioregular poly-norbornene anhydride-g-(3-hexyl thiophene) (PNBA-g-PHT) graft copolymer via Grignard metathesis polymerization and a click chemistry approach, followed by ring-opening metathesis polymerization. The molecular weight of PNBA-g-PHT copolymer was obtained using GPC with a narrow polydispersity index. AFM as well as SEM confirmed the rod-like morphology. To the best our knowledge, this is the first report on the efficient preparation of regioregular PNBA-g-PHT copolymer using a metathesis polymerization in combination with click chemistry.

Acknowledgements

SRM, SS and RNV thank CSIR New Delhi, for research fellowship and RS thanks DST, New Delhi, for Ramanujan Fellowship, CARS/NPOL project for funding and IISER-Kolkata for the infrastructure and start-up funding. We thank Dr T. Mukundan and Ms Shiny Nair (NPOL, Kochi) for valuable discussion.

Notes and references

1. A. de Cuendias, M. le Hellaye, S. Lecommandoux, E. Cloutet and H. Cramail, J. Mater. Chem., 2005, 15, 3264–3267.
2. S. Kang, R. J. Ono and C. W. Bielawski, J. Polym. Sci., Part A: Polym. Chem., 2013, 51, 3810–3817.
3. R. S. Loewe, S. M. Khersonsky and R. D. McCullough, Adv. Mater., 1999, 11, 250–253.
4. R. D. McCullough, S. Tristram-Nagle, S. P. Williams, R. D. Lowe and M. Jayaraman, J. Am. Chem. Soc., 1993, 115, 4910–4911.
5. R. D. McCullough and S. P. Williams, J. Am. Chem. Soc., 1993, 115, 11608–11609.
6. R. H. Lohwasser and M. Thelakkat, Macromolecules, 2012, 45, 3070–3077.
7. H. O. Rashid, M. Seo, S. Y. Kim, Y. S. Gal, J. M. Park, E. Y. Kim, W. K. Lee and K. T. Lim, J. Polym. Sci., Part A Polym. Chem., 2011, 49, 4680–4686.
8. M. C. Iovu, M. Jeffries-El, E. E. Sheina, J. R. Cooper and R. D. McCullough, Polymer, 2005, 46, 8582–8586.
9. Z. Wu, A. Petzold, T. Henze, T. Thurn-Albrecht, R. H. Lohwasser, M. Sommer and M. Thelakkat, Macromolecules, 2010, 43, 4646–4653.
10. A. C. Kamps, M. H. M. Cativo, X. J. Chen and S. J. Park, Macromolecules, 2014, 47, 3720–3726.
11. K. Yuan, F. Li, Y. Chen, X. Wang and L. Chen, J. Mater. Chem., 2012, 21, 11886–11894.
12. K. Palaniappan, N. Hundt, P. Sista, H. Nguyen, J. Hao, M. P. Bhatt, Y. Y. Han, E. A. Schmiedel, E. E. Sheina, M. C. Biewer and M. C. Stefan, J. Polym. Sci., Part A: Polym. Chem., 2011, 49, 1802–1808.
13. D. M. Lynn, B. Mohr and R. H. Grubbs, J. Am. Chem. Soc., 1998, 120, 1627–1628.
14. A. E. Madkour, J. M. Grolman and G. N. Tew, Polym. Chem., 2011, 2, 114–119.
15. Z. M. Al-Badri and G. N. Tew, Macromolecules, 2008, 41, 4173–4179.
16. S. R. Mane, V. N. Rao and R. Shunmugam, ACS Macro Lett., 2012, 1, 482–488.
17. S. R. Mane, K. Chatterjee, H. Dinda, J. Das Sarma and R. Shunmugam, Polym. Chem., 2014, 5, 2725–2735.
18. S. R. Mane and R. Shunmugam, ACS Macro Lett., 2014, 3, 44–50.
19. S. R. Mane, H. Dinda, A. Sathyan, J. Das Sarma and R. Shunmugam, ACS Appl. Mater. Interfaces, 2014, 6, 16895–16902.
20. S. R. Mane, N. V. Rao, K. Chatterjee, H. Dinda, S. Nag, A. Kishore, J. Das Sarma and R. Shunmugam, J. Mater. Chem., 2012, 22, 19639–19642.
21. R. S. Loewe, P. C. Ewbank, J. Liu, L. Zhai and R. D. McCullough, Macromolecules, 2001, 34, 4324–4333.
22. Y. Mao, Y. Wang and B. L. Lucht, J. Polym. Sci., Part A: Polym. Chem., 2004, 42, 5538–5547.
23. O. F. Pascui, R. Lohwasser, M. Sommer, M. Thelakkat, T. Thurn-Albrecht and K. Saalwächter, Macromolecules, 2010, 43, 9401–9410.

---

Footnote

† Electronic supplementary information (ESI) available: Details of the synthetic procedures and other analytical data. See DOI: 10.1039/c5ra12510c

---