Citation: Bao, F.; Liu, Y.; Shi, L.; Cui, J.; Ji, M.; Liu, H.; Yu, J.; Zhu, C.; Xu, J. Non-Coplanar Diphenyl Fluorene and Weakly Polarized Cyclohexyl Can Effectively Improve the Solubility and Reduce the Dielectric Constant of Poly (Aryl Ether Ketone) Resin. Polymers 2023, 15, 962. https://doi.org/10.3390/ polym15040962 Academic Editors: Zhikang Yuan, Sichen Qin and Geng Chen Received: 29 December 2022 Revised: 7 February 2023 Accepted: 9 February 2023 Published: 15 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Non-Coplanar Diphenyl Fluorene and Weakly Polarized Cyclohexyl Can Effectively Improve the Solubility and Reduce the Dielectric Constant of Poly (Aryl Ether Ketone) Resin Feng Bao 1, * ,† , Yanxing Liu 1,† , Ludi Shi 2 , Jinze Cui 1 , Muwei Ji 3 , Huichao Liu 1 , Jiali Yu 1 , Caizhen Zhu 1, * and Jian Xu 1 1 Institute of Low-Dimensional Materials Genome Initiative, College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518060, China 2 Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, China 3 Department of Chemistry, College of Science, Shantou University, Shantou 515063, China * Correspondence: bfisvip@163.com (F.B.); czzhu@szu.edu.cn (C.Z.) † These authors contributed equally to this work. Abstract: With the rapid development of high-frequency communication and large-scale integrated circuits, insulating dielectric materials require a low dielectric constant and dielectric loss. Poly (aryl ether ketone) resins (PAEK) have garnered considerable attention as an intriguing class of engineering thermoplastics possessing excellent chemical and thermal properties. However, the high permittivity of PAEK becomes an obstacle to its application in the field of high-frequency communication and large-scale integrated circuits. Therefore, reducing the dielectric constant and dielectric loss of PAEK while maintaining its excellent performance is critical to expanding the PAEK applications mentioned above. This study synthesized a series of poly (aryl ether ketone) resins that are low dielectric, highly thermally resistant, and soluble, containing cyclohexyl and diphenyl fluorene. The effects of cyclohexyl contents on the properties of a PAEK resin were studied systematically. The results showed that weakly-polarized cyclohexyl could reduce the molecular polarization of PAEK, resulting in low permittivity and high transmittance. The permittivity of PAEK is 2.95–3.06@10GHz, and the transmittance is 65–85%. In addition, the resin has excellent solubility and can be dissolved in NMP, DMF, DMAc, and other solvents at room temperature. Furthermore, cyclohexyl provided PAEK with excellent thermal properties, including a glass transition temperature of 239–245 ◦ C and a 5% thermogravimetric temperature, under a nitrogen atmosphere of 469–534 ◦ C. This makes it a promising candidate for use in high-frequency communications and large-scale integrated circuits. Keywords: poly (aryl ether ketone); permittivity; solubility; cyclohexyl; diphenyl fluorene 1. Introduction The rapid development of high-frequency communication and large-scale integrated circuits has raised new requirements for interlayer insulating dielectric materials [1–3]. Insulating dielectric materials are required to have a low dielectric constant and low dielectric loss under high frequency, as well as excellent thermal and mechanical properties, processability, dimensional stability, and low water absorption [4–6]. Traditional general polymeric materials, such as polyethylene (PE), polypropylene (PP), etc., have low permittivity and dielectric loss but fail to meet requirements due to application–temperature mismatch. Numerous special engineering plastics, such as traditional polyimide (PI) and liquid crystal polymer (LCP), were expected to meet re- quirements due to their superior thermal and mechanical performance and dielectric properties [7,8]. However, the dielectric constant (D k ) of 3.0–3.5 is insufficient for appli- cation in high-frequency communication and large-scale integrated circuits. Based on the Clausius–Mossotti equation, low-dielectric materials can be prepared by reducing Polymers 2023, 15, 962. https://doi.org/10.3390/polym15040962 https://www.mdpi.com/journal/polymers