## 4 Conclusions  
To summarise, Stille polymerization was used to synthesise two new low band-gap conjugated copolymers, such as dithienosilole (DTS), flanked by thienyl moieties as donor units and by benzothiadiazole dicarboxylic imide (BTDI) as acceptor units. To prepare PDTSDTBTDI-DMO and PDTSDTBTDI-8 (yielding 92% and 57%, respectively) distannylated DTS monomer (M3) was copolymerised with dibrominated BTDI monomers (M1 and M2). The BTDI units were attached to two different side chains, _n_-octyl and 3,7-dimethyloctyl, to study the impact of these substituents on the resulting polymers' optical, electrochemical, thermal, and structural properties, as well as their solubility and molecular weight. Anchoring 3,7-dimethyloctyl chains in PDTSDTBTDI-DMO and _n_-octyl chains in PDTSDTBTDI-8 to the BTDI units negatively affected solubility and molecular weight. The sokhlet extraction process was conducted in the separation of PDTSDTBTDI-8, and its toluene fraction showed a low \(M_{\mathrm{n}}\) value (5700 g mol\({}^{-1}\)) because of its limited solubility. Soxhlet extraction of PDTSDTBTDI-DMO gave two fractions; a toluene fraction with a slightly higher \(M_{\mathrm{n}}\) value than for PDTSDTBTDI-8 and a chloroform fraction with a \(M_{\mathrm{n}}\) value more than twice as high. The absorption maxima of PDTSDTBTDI-8 in the solution and the thin-film were blue-shifted compared to PDTSDTBTDI-DMO absorption. For both polymers, the absorption spectra showed bathochromic shift absorption maxima in the thin-films in contrast to their absorption in the solution. It was observed that both polymers had similar optical band gaps at approximately 1.4 eV. The HOMO levels of the polymers were found to be similar, and the LUMO levels of the PDTSDTBTDI-DMO and PDTSDTBTDI-8 were found to be \(-3.56\) and \(-3.45\) eV, respectively. The attaching of different alkyl chains to the BTDI units slightly affected the band gaps and HOMO levels, whereas LUMO energy levels were influenced by approximately 0.1 eV. PDTSDTBTDI-DMO and PDTSDTBTDI-8 showed good thermal stability with decomposition temperatures at 357 and 394 \({}^{\circ}\)C, respectively. By changing substituents on the imide functionality of BTDI units, the polymers' thermal properties were significantly affected. The powder XRD studies of the polymers exhibited diffraction peaks at around 24\({}^{\circ}\) in proportion to the \(\pi\)-\(\pi\) stacking distance of about 3.6 A. The two polymers were found to be amorphous in nature.  
Supplementary Materials: Supplementary Materials are available online at [[https://www.mdpi.com/article/10.3390/app1114866/s1](https://www.mdpi.com/article/10.3390/app1114866/s1)]([https://www.mdpi.com/article/10.3390/app1114866/s1](https://www.mdpi.com/article/10.3390/app1114866/s1)).  
Figure 4: Powder XRD of PDTSDTBTDI-DMO and PDTSDTBTDI-8.  
**Author Contributions:** Conceptualization, A.R.M., S.B.A. and A.I.; Formal analysis, A.R.M.; Funding acquisition, E.M.A.D.; Methodology, A.R.M. and S.N.A.; Project administration, S.B.A. and A.I.; Supervision, A.I.; Validation, E.M.A.D., S.B.A., A.I., S.N.A., M.M.N.; R.M.A.; Visualization, M.M.N.; Writing--original draft, A.R.M.; Writing--review & editing, E.M.A.D., S.B.A., A.I, S.N.A., M.M.N., R.M.A. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Exclude this statement because the study did not report any data.  
We would like to acknowledge all support for this work by the University of Sulaimani, Prince Sultan University, Charmo University and Komar University of Science and Technology. The authors would like to acknowledge the support of Prince Sultan University for paying the Article Processing Charges (APC) of this publication and for their financial support.  
The authors declare no conflict of interest.