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Fine polymer chemicals with conductive properties I. The development of conductive polymers Since the Dutch physicist H. Konnes discovered the phenomenon of superconductivity, materials have been classified into four main categories based on their conductivity: insulators, semiconductors, conductors, and superconductors. The electrical conductivity of insulators is roughly between that of semiconductors and that of conductors ； A superconductor is a conductor with resistance close to 0. In 1977, American scientists A.F. Heeger and Macdiarmid discovered that crystalline polyacetylene exhibited significant electrical conductivity. Subsequently, Macdiarmid carried out electron-acceptor doping of polyacetylene films, increasing its conductivity by 12 orders of magnitude; its electrical properties thus reached those of metallic conductors. Prior to this, the preparation of conductive polymer materials was mainly achieved through the use of doped metal powders, metal wires, carbon fibers, and conductive carbon black, and such materials were widely used in antistatic and electric shielding applications. The discovery of the electrical conductivity of polyacetylene opened up entirely new manufacturing methods and broader application areas for conductive polymer materials. Today, the applications of these materials have expanded from antistatic and electric shielding uses to full-solid-state batteries, polymer display materials, polymer information conversion materials, and organic semiconductor materials. II. Definition and classification of conductive polymer materials. Also known as conductive polymers. Therefore, materials that possess the structural characteristics of polymer repeat units and can exhibit current conduction under an electric field are all referred to as conductive polymer materials. Conductive polymer materials differ from metallic conductors; metallic conductors are conductive substances in the form of metal crystals, whereas polymer conductors are conductive substances at the molecular level, and their mechanisms of conductivity are different. Conductive polymer materials can be classified into the following 3 categories based on their structural characteristics and conductivity mechanisms: electron-conductive polymer materials whose charge carriers are free electrons ； Conductive polymer materials in which the charge carriers are positive and negative ions that can migrate between polymer molecules ； Redox-type conductive polymer materials whose electron transfer mechanism is based on redox reactions. III. Preparation of electron-conducting polymers. The mechanisms of conductivity and doping in electron-conducting polymers have been well studied; among the three types of polymers that belong to this category, they are the most numerous and have been the subject of research earliest among conductive materials. It holds prospects for application, which has motivated many researchers to focus on the preparation and application of conductive polymers. Due to the low electron mobility of structural conductive polymers themselves, doping is often required to obtain electronically conductive polymers that are useful in practice. Therefore, the preparation of electronically conductive polymers involves two steps: first, the synthesis of structural conductive polymers; and second, the further functionalization of these polymers through doping. The methods and processes used for preparing the conductive polymer matrix determine its cost, as well as its practical value and future prospects. The characteristic of the matrix macromolecules in electronically conductive polymers is their highly conjugated structure; therefore, the preparation of such matrix macromolecules involves designing and using appropriate starting materials to form macromolecular compounds with conjugated structures. The choice of raw materials generally determines the synthesis route and process, thereby determining the procedures and equipment as well. The preparation of conjugated macromolecules can be achieved through direct polymerization of monomers, or via addition and elimination reactions on existing macromolecules. In recent years, electrochemical polymerization methods have also been developed. The former belongs to the category of chemical polymerization methods; the resulting conjugated macromolecules require doping in order to obtain conductive polymer materials with conductivity similar to that of metal conductors. The latter relies on electrochemical mechanisms, allowing for direct doping without the need for external dopants. Moreover, the resulting conductive polymers precipitate as conductive polymer films on the electrodes. When used to modify electrodes, such modified electrodes not only possess respiratory catalytic functions but can also serve as sensors for electronic devices. 1. Preparation of structural polyacetylene conjugated polymers. Polyacetylene is the earliest-developed and most widely studied and applied structural conductive polymer. In 1971, Shirakawa in Japan successfully produced conductive polyacetylene films through gas-phase bulk polymerization using a Ziegler-Natta catalyst. The conductivity of these films, obtained through directed polymerization and subsequently doped with iodine, fell within the range between that of semiconductors and metal conductors, thus paving the way for research on the synthesis of structural conductive polymers. (1) Gas-phase bulk polymerization of acetylene using Z-N catalysts; (2) Elimination of HCl from polyvinyl chloride. This method involves the use of conjugated polymers generated through elimination reactions of saturated polymers, but cross-linking structures tend to form during heating, resulting in defects in the conjugated structure. Polyacetylene can also be synthesized by adding and then eliminating units from polybutadiene. (3) Using polybutadiene as a raw material. 2. Preparation of structurally defined conductive polymers based on substituted polyacetylene. The synthesis of such structurally defined conductive polymers typically involves ring-forming reactions of diacetylenes; the rings in these polymers with a polyacetylene structure act as substituents, which results in relatively lower electrical conductivity. The polyacetylene-based conductive polymers obtained using the above methods suffer from issues such as poor solubility and difficulty in molding and processing. Therefore, there is a need for a polymer that lacks a conjugated structure before processing, yet possesses solubility and processability, so that it can form macromolecules with a conjugated structure after being processed and molded. The most promising synthetic method at present is translocation polymerization. Translocation polymerization can be employed using conventional Ziegler-Natta catalysts to effect the stereoregular polymerization of norbornene and its derivatives; the emergence of active translocation polymerization has opened up promising applications for such conductive polymer materials in electronic components. Although the polymers produced by these two reactions involve ordinary translocation polymerization, both types of polymers are easy to process; upon molding and heating, conductive polyacetylene is obtained, and after doping, transistors and capacitors can be fabricated. Through active translocation polymerization, soluble polymers can be obtained; by formulating these polymers into solutions, conductive films or fibers that can be stretched can be produced. 3. Preparation of polyaromatic conductive polymers. These conductive polymers are mainly prepared using oxidative coupling polymerization; their polymerization mechanism belongs to condensation polymerization, and the reactions employed are traditional organic chemical reactions such as Friedel-Crafts alkylation and Ullmann coupling reactions. 4. Preparation of conductive carbon fibers Conductive carbon fibers are the most widely used conductive polymers, with well-developed industrial production methods. They are employed in various applications such as electrical distributor components in aerospace and automotive engines, and they can also serve as reinforcing agents for polymer materials. Its production is achieved through processes such as polymerization of acrylonitrile, high-temperature cracking, and cyclization. 5. Electrochemical method for preparing conductive polymers: The electrochemical polymerization method is another approach for producing electron-conducting polymers that has been developed in recent years. This method uses electrode potential as the trigger and driving force for the polymerization reaction; the polymerization takes place on the electrode surface, resulting in the direct formation of a conductive polymer film. After the reaction is complete, the resulting conductive polymer film has been oxidized (or reduced) by the electrode potential used during the reaction, thereby simultaneously completing the so-called “doping” process without the need for any second substance. Compared with the previous preparation methods, it features high electrical conductivity, and the product can be directly used to fabricate conductive films, which can be applied not only to the preparation of polymer-modified electrodes. Moreover, it can be used to create bonding conductive films of larger area. Its polymerization mechanism involves the polymerization of monomers triggered by electron transfer during electrochemical reactions. 6. Doping of electron-conducting polymers and factors affecting their electrical conductivity – Currently, electrochemical methods have become one of the main approaches for preparing various conductive polymers. The monomers are mainly alkylpyrroles. Polymerization mainly proceeds via a free radical coupling mechanism. Except for the electrochemical method used in the preparation of the above five electron-conductive polymers, their electrical conductivity does not meet practical requirements in all cases, necessitating doping. “The term “doping” originates from crystal semiconductor chemistry and refers to a process in which the lattice is disrupted to create vacancies. In the case of conductive polymers, doping involves a redox reaction. P-type doping is carried out using oxidizing agents, with iodine, bromine, ferric trichloride, and arsenic pentafluoride being the most common ones; these substances act as typical electron acceptors. N-type doping, on the other hand, uses reducing agents, with alkali metals typically serving as electron donors. After doping, the delocalization of the π-electrons in the structured conductive polymers increases significantly, thereby effectively enhancing the conductivity of the polymers. As a result of doping, the conductivity of conjugated polymers increases by several orders of magnitude, reaching levels comparable to those of metallic conductors. Doping has brought electron-conducting polymers into the practical stage. When carrying out doping, the amount of dopant must be considered. This is because the dopants are closely related to the conductivity of polymers; the results of iodine doping experiments using polyacetylene serve as an example. Additionally, it should be noted that when using conductive polymer materials, the effect of temperature on conductivity must be taken into account. Unlike metal conductors, the conductivity of polymers increases as the temperature rises, and this is a factor that users must consider carefully. IV. Preparation of ionically conductive polymer materials 1. Briefly, a conductive polymer framework that uses positive and negative ions as charge carriers is known as an ionically conductive polymer, and it represents an important class of conductive polymer materials. Ion-conducting polymers are generally solid ion conductors composed of polymer electrolytes. Since polymer electrolytes are easy to process, they can be utilized more readily than electron-conducting polymers. They are currently widely used in sensor elements, as well as in lightweight and high-energy secondary batteries. Since ionic conduction is an electrochemical process, ionic conductors play the role of electrolytes in this process. Therefore, the development of solid-state ionic conductors can address issues such as short service life and corrosion of other components caused by leakage and evaporation of traditional liquid electrolytes used in batteries. It also solves the problem that liquid electrolytes cannot be shaped or turned into films. Moreover, solid-state electrolytes enable a reduction in the weight and volume of batteries while maintaining high energy density, making them suitable for use in aerospace, defense, automotive, and other fields. From the perspective of solid-state ionic conduction mechanisms, polymer electrolytes belong to the category of ion-conducting materials that rely on diffusion in amorphous regions. The following are the types of ion-conducting polymers that can be used: (1) Sol-gel type polyelectrolytes, also known as ionopolymers, which are substances in which ionic groups are connected through covalent bonds within their polymer molecular structure; (2) Polymeric salts; (3) Solvated polymers; (4) Solvated ionopolymers; (5) Glass transition temperature; (6) Solvation capacity; (7) Other influencing factors. 2. Preparation of ion-conducting polymers: Currently, ion-conducting polymers mainly fall into three categories: polyethers, polyesters, and polyimides. Polyepoxide polymers are the most commonly used polyether-type ion-conducting polymers, primarily made from ethylene oxide and propylene oxide. The preparation process of such polymers involves the ring-opening polymerization or copolymerization of epoxides and/or propylene oxides, with cations, anions, or coordination complexes all capable of initiating such reactions. For the preparation of ion-conducting polymers, it is required that the resulting polymers have a high molecular weight; however, side reactions such as chain transfer occur easily during cationic polymerization, which reduces the molecular weight of the polymers, leading to their limited use in the production of conductive polymers. In the anionic polymerization of ethylene oxide, hydroxides, alkoxy compounds, and others can all be used as initiators for anionic ring-opening polymerization. The anionic polymerization of epoxides is a step-growth reaction, and the molecular weight of the resulting polymer increases as the conversion rate rises. Polyesters and polyamides are another common class of ion-conducting polymers, among which polyesters of ethylene glycol are generally prepared through polycondensation reactions. Polymerization using dibasic acids and diols yields linear polymers; the resulting polymers are highly flexible and have a low glass transition temperature, making them suitable for use as polyelectrolytes. Polyamides obtained by reacting dibasic acid derivatives with diamines also possess similar properties. Their production process is the same as that used for polyamide fibers, except that careful consideration must be given to controlling the softening point and the regularity of the polymer structure. Ion-conducting polymers should also meet the following requirements. 1. It should possess good mechanical strength at the operating temperature; generally, the mechanical strength of a polymer is proportional to its molecular weight ； 2. It should have good chemical stability and should not react with lithium and oxidizing anodes in solid-state batteries ； 3. It has good processability, especially being easy to be processed into films for use. 5. Applications of conductive polymers: 1. Applications of electron-conductive polymers. The development of applications for electron-conductive polymers is based primarily on their following physicochemical properties: high electrical conductivity, reversible redox behavior, light absorption characteristics at different oxidation states, charge storage capacity, and the ability to switch between conductive and non-conductive states. They can be used to prepare materials for organic rechargeable batteries, optical display materials, information storage materials, shielding and antistatic materials, as well as molecular electronic devices. Among them, its application in the production of electrodes for secondary batteries (rechargeable batteries) has reached a practical stage. Since the conductive polymers that have been developed to date are difficult to process, they are hard to use as conductive materials for power transmission. However, as antistatic and shielding materials, conductive polymers have shown great competitiveness. In addition, conductive carbon fibers can be used as components in electrical distributors, as well as in household heaters with decorative effects similar to wallpaper. (1) Electrode materials and batteries: Conductive polymers also show good prospects as electrode materials. Compared to inorganic electrode materials, batteries made with electron-conducting polymers as electrode materials are not only much lighter when the capacitance remains constant, but also exhibit better voltage characteristics. The development of special rechargeable batteries for aerospace and electric vehicles is of great significance. At the same time, conductive polymer electrodes serve both as a conductor and as a reactant in electrochemical processes. (2) As electric display materials, electron-conducting polymers have also achieved significant results in this field. This display function arises from the electrochemical reaction that occurs in the conductive polymer under the influence of electrode voltage, causing a change in its oxidation state and thereby leading to a change in color. This color change is correlated with voltage. Compared to liquid crystal displays, the advantage of this display device is that it has no viewing angle limitations. Polypyrrole, polythiophene, and polyaniline are electro-display materials with good colorability and stability. At present, such conductive polymer displays are difficult to prepare using conventional film-forming methods, and therefore are all produced by electrochemical polymerization. (3) Chemical reaction catalysts: Conducting polymers are used in analytical chemistry, as catalysts in organic synthesis, and in the fabrication of chemical sensors. Since p-type doped polymers have the function of electron acceptors, and n-type doped polymers have the function of electron donors, the doped conductive polymers possess redox catalytic properties. Curing conductive polymers on the electrode surface can produce polymer-modified electrodes, which can be used as electrocatalytic materials in electrochemical reactions. (4) In terms of electronic devices, conductive polymers prepared by electrochemical polymerization are doped through the gain or transfer of electrons at the electrodes. The conductivity of these conductive polymers in their doped state differs greatly from that in their undoped state, with a difference of seven orders of magnitude. Therefore, this property of conductive polymers can be utilized to create organic molecular switch devices. Multi-layer compounding of conductive polymers with different electrical conductivity properties on the surface of microelectrodes is another approach for fabricating organic molecular diodes, transistors, and simple logic circuits; it will become an important direction in the research of molecular electronic materials. To overcome the limitations of conductive polymers in terms of their electrochemical properties, it is often necessary to attach specific redox groups to their conductive frameworks when manufacturing organic molecular microelectronic devices, in order to improve their physicochemical properties. At this point, the redox charge transfer mechanism acts as the main electronic switch, while the electronic-conducting polymer backbone only serves to facilitate electron transport during the preparation stage. 2. Applications of ion-conducting polymers: The main application areas of ion-conducting polymers are their use as substitutes for liquid electrolytes in various electrochemical devices, as well as in the fabrication of sensor elements. As a substitute for liquid electrolytes, although the conductivity of most of the polymer electrolytes currently available is not yet on par with that of liquid electrolytes, their superior mechanical strength allows them to be made into thin films with large areas. As a result, the structural constants of electrochemical devices made from these materials can attain high values, enabling the absolute conductivity between the electrodes to be comparable to that of liquid electrolytes and thus meeting practical requirements. Currently, the thickness of polyelectrolyte films is generally 10–100 micrometers, with a conductivity that can reach 100 S/m2. All-solid-state batteries, composed of solid polymer electrolytes and polymer electrodes, have entered the practical application stage. Its characteristics are summarized as follows. 1 It is easy to process into shape, and it has good mechanical properties, being strong and durable ； Leak-proof, splash-proof, and causes no corrosion to other components ； 2 The electrolytes are non-volatile, resulting in a long service life for the devices ； 3 It is easy to fabricate electrochemical devices with large structural constants, and thus high energy density. Batteries made from solid-state electrolytes are particularly suitable for applications such as implantable pacemakers, power supplies for computer memory, and self-powered large-scale integrated circuits. In the manufacturing of trace gas sensors, ion-conducting polymers have begun to be used both domestically and internationally.