Synthesis of Polythiophene Derivatives with Oligoisobutylene Side Chains
Authors: Miyane Satoshi , Higashihara Tomoya 
Published: February 2020

In recent years, organic electronics including organic light emitting diode, organic photovoltaic cell and organic thin film transistor have received much attention due to their severals advantages based on organic materials, such as mechanical flexibility and solution-based low-cost processability. π-Conjugated polymers are known as mechanically soft and flexible material in organic semiconductors. However, their durability for stretched stress is not enough for practical application. Therefore, the molecular design is required for enabling stretchable organic semiconductor. The low strethcability of π-conjugated polymer is due to high crystallinity which induced their stiff main chain and strong intermolecular interactions. Here, we developed novel polythiophene derivatives, P3IBTs, which contain oligoisobutylene units in their side chains. We expect that several branched points on side chains weaken π-π stacking interaction and decrease crystallinity of the polymers. In addition, we tried to develop cross-linked P3IBT to realize the formation of an inherently stretchable π-conjugated polymer.

We prepared the monomer of polythiophene derivatives with diisobutylene and triisobutylene side chains according to Scheme 1. The oligoisobutylene unit was synthesized based on a carbocationic addition reaction. Finally, polythiophene derivatives with diisobutylene sidechain (P3IB2T) (M
n = 12,000, M
w/M
n = 1.17) and triisobutylene side chain (P3IB3T) (M
n = 13,000, M
w/M
n = 1.21) could be synthesized by Kumada catalyst-transfer polycondensation. P3IB2T is dark red solid and P3IB3T is dark red chewing-gum like solid at room temperature. Subsequently, we characterized their thermal, optical properties and crystalline nature compared with poly(3-hexylthiophene)(P3HT) (M
n = 10,000, M
w/M
n = 1.17). The thermal properties of the polymers were investigated by differential scanning calorimetry (DSC). The melting temperature of P3IB2T (T
m = 130 ˚C) is lower than P3HT (T
m = 242 ˚C). On the other hand, there are neigher endothermic nor exothermic peaks in the DSC curve of P3IB3T. Therefore, this polymer would be more amorphous-like than P3IB2T. These result suggests that oligoisobutylene side chains decrease crystallinity of polymers.

The absorption spectrum of P3IB2T film showed lower absorption intensity around 620 nm than the P3HT film, indicating weaker π-π stacking interaction. Moreover, P3IB3T films showed the blue-shifted absorption spectrum compared to P3HT and P3IB2T ones probably due to larger torsion of main chains affected by triisobutylene side chain.

Next, the crystalline nature was evaluated by Grazing incidence wide-angle x-ray scattering (GIWAXS). P3IB2T and P3IB3T films showed edge-on orientation similar to the P3HT ones, judged from the azimuth plots of (200) diffraction patterns. Moreover, (010) diffraction patterns were not observed for P3IB2T and P3IB3T films, probably due to less π-π stacking interaction by oligoisobutylene side chains.

Finally, we cross-linked P3IB3T to realize the formation of an inherently stretchable p-conjugated polymer. 4,4’-Methylenebis(2,6-bis(methoxymethyl)phenol) and isopropyl p-toluenesulfonate were used as cross-linker and thermal acid generator, respectively. The cross-linking reaction was proceeded by heating at 200 ˚C for 3 h. The mechanical properties of the cross-linked P3IB3T was investigated by the tensile test using a TMA instrument. As a result, P3IB3T showed much lower tensile strength (0.68 MPa) and higher elongation at break (141%) than P3HT (984 MPa, 59 %).

In conclusion, we have synthesized polythiophene derivatives with oligoisobutylene side chains. These polymer showed lower crystallinity and weaker π-π stacking interaction than P3HT. These properties might be caused by the effect of several branch points on oligoisobutylene side chains. In addition, the cross-linked P3IB3T film could be formed, possessing good mechanical durability.

Figure 1