Citation: Kholkhoev, B.C.; Matveev, Z.A.; Bardakova, K.N.; Timashev, P.S.; Burdukovskii, V.F. Aliphatic Polybenzimidazoles: Synthesis, Characterization and High- Temperature Shape-Memory Performance. Polymers 2023, 15, 1399. https://doi.org/10.3390/ polym15061399 Academic Editor: Tao Chen Received: 21 January 2023 Revised: 23 February 2023 Accepted: 9 March 2023 Published: 11 March 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 Aliphatic Polybenzimidazoles: Synthesis, Characterization and High-Temperature Shape-Memory Performance Bato Ch. Kholkhoev 1, *, Zakhar A. Matveev 1 , Kseniia N. Bardakova 2,3 , Peter S. Timashev 2,3,4,5, * and Vitaliy F. Burdukovskii 1 1 Baikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences, 670047 Ulan-Ude, Russia 2 Research Center “Crystallography and Photonics”, Russian Academy of Sciences, 108840 Moscow, Russia 3 Institute for Regenerative Medicine, Sechenov University, 119991 Moscow, Russia 4 Semenov Institute of Chemical Physics, Russian Academy of Sciences, 119991 Moscow, Russia 5 Chemical Department, Lomonosov Moscow State University, 119991 Moscow, Russia * Correspondence: holh_bat@mail.ru (B.C.K.); timashev.peter@gmail.com (P.S.T.) Abstract: A series of aliphatic polybenzimidazoles (PBIs) with methylene groups of varying length were synthesized by the high-temperature polycondensation of 3,3 ′ -diaminobenzidine (DAB) and the corresponding aliphatic dicarboxylic acid in Eaton’s reagent. The influence of the length of the methylene chain on PBIs’ properties was investigated by solution viscometry, thermogravimetric analysis, mechanical testing and dynamic mechanical analysis. All PBIs exhibited high mechanical strength (up to 129.3 ± 7.1 MPa), glass transition temperature (≥200 ◦ C) and thermal decomposition temperature (≥460 ◦ C). Moreover, all of the synthesized aliphatic PBIs possess a shape-memory effect, which is a result of the presence of soft aliphatic segments and rigid bis-benzimidazole groups in the macromolecules, as well as strong intermolecular hydrogen bonds that serve as non-covalent crosslinks. Among the studied polymers, the PBI based on DAB and dodecanedioic acid has high adequate mechanical and thermal properties and demonstrates the highest shape-fixity ratio and shape-recovery ratio of 99.6% and 95.6%, respectively. Because of these properties, aliphatic PBIs have great potential to be used as high-temperature materials for application in different high-tech fields, including the aerospace industry and structural component industries. Keywords: polybenzimidazoles; high-temperature shape-memory polymers; mechanical properties; thermally stable polymers; physical crosslinking; hydrogen bonding 1. Introduction Polybenzimidazoles (PBIs) are a class of high-performance polymers that, due to their chain stiffness and strong intermolecular hydrogen bonds, are characterized by excellent heat resistance, wear resistance, mechanical strength, chemical stability, radiation resistance and dielectric properties [1–6]. Due to these properties, PBIs appear to be attractive materials for use in various fields, such as aerospace, the petrochemical industry, electronic devices, gas separation, fuel cells, etc. [1–3,7–18]. In order to expand the possibilities of the practical application of PBIs, it is necessary to impart additional functional properties to materials based on them, such as the high- temperature shape-memory effect. High-temperature shape-memory polymers (SMPs) have great potential for practical applications in the development of deployable space structures, smart jet propulsion systems, engine controls and high-temperature actua- tors [19–24]. SMPs contain rigid segments that can be formed by covalent crosslinking or hydrogen bonding, as well as crystalline, liquid-crystalline or amorphous soft block domains. However, traditional PBIs such as poly-2,2 ′ -m-phenylene-5,5 ′ -dibenzimidazole have highly rigid backbones and do not contain any flexible groups, so they do not pos- sess a shape-memory effect. Previously, we showed that high-temperature SMPs could Polymers 2023, 15, 1399. https://doi.org/10.3390/polym15061399 https://www.mdpi.com/journal/polymers