Patent Publication Number: US-2007096055-A1

Title: Oxidizing agent useful for oxidative polymerization of high conductive polymers

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an oxidizing agent useful for polymerization of high conductive polymer, more particularly a kind of metal ion complex formed of metal ion salts having oxidizing capability and one or a plurality of nitrogen-containing compounds.  
      2. Description of the Related Art  
      Since it was found that doped polyacetylene can exhibit electrical conductivity, considerable efforts have been invested in the development and application of new types of electrically conductive conjugate polymer. Currently most commonly applied conductive polymers include polypyrrole, polyaniline, polythiophene and the derivatives of those compounds and have been used in anti-static coating, electrolytic capacitor, printed circuit board, solar collector and light-emitting display.  
      Conductive polymers are synthesized mainly by electrochemical polymerization or oxidative polymerization. Electrochemical polymerization is achieved by dissolving monomers in a solution containing electrolytes and passing through current to form conjugate polymer at the anode. The electrolytes in the solution would undergo doping reaction in the process to endow the conjugate polymer with electrical conductivity. Oxidative polymerization entails oxidizing the monomers with oxidizing agent to cause polymerization and subsequently adding in dopant in gaseous or liquid phase to give the resulting polymer electrical conductivity. The electrochemical polymerization process is known to produce polymers with excellent conductivity, and can get free standing films of certain conducting polymers such as polypyrrole. However the application of this method is severely hampered by the requirements of processed object being electrically conductive and having high oxidation potential. Oxidative polymerization does not have such requirements. In this process, monomers, oxidant, dopant and solvent are mixed, and coating or impregnation can be carried out before monomers are massively polymerized into conductive polymer.  
      The practicality of oxidative polymerization and the difficulty of its processing technology are governed by the stability of monomer-oxidant mixture. The mixing of monomers and oxidant will induce the polymerization of monomers which causes the viscosity of the mixture to rise, making it difficult to be coated or impregnated onto the processed object. Thus how to obtain stable monomer-oxidant mixture and high conductive polymer are the focus in the research of oxidative polymerization.  
      In the example of making solid-state electrolytic capacitor, to reduce the reaction rate of oxidant-monomer mixture and prolong the processing time of the mixture, Friedrich Jonas et al. in their U.S. Pat. No.4,910,645 discloses the use of large amount of solvent to dilute the concentration of monomers to below 10 wt % so as to slow down the reaction rate of monomer-oxidant solution at room temperature. But by impregnating a capacitor element with monomer-oxidant mixture, only small amount of conductive polymer is obtained in one impregnation, while the remaining part consists of residual reactants and large amount of solvent. This proposed process needs to go through 16 cycles of impregnation and polymerization to produce sufficient conductive polymers to fill the gap between the positive and negative foils of capacitor element (U.S. Pat. No. 6,136,176). Thus using conductive polymer as an electrode of solid state electrolytic capacitor has the drawbacks of complex process and high manufacturing cost.  
      Friedrich Jonas et al. also disclose in U.S. Pat. No. 4,959,430 a thiophene derivative -3,4-ethylenedioxythiophene which has lower polymerization rate after mixing with oxidant under room temperature and the resulting (poly(3,4-ethylenedioxythiophene) has excellent conductivity and thermal stability. But the polymerization rate of this monomer at room temperature when mixing with oxidant is still controlled by the concentration of oxidant. High oxidant concentration will affect significantly the room-temperature stability of mixture, hence restricting the processing conditions of oxidative polymerization.  
      Philip M. Lessner et al. in U.S. Pat. No. 6,056,899 discloses a complex formed by the mixture of oxygen-containing organic compound with specific low boiling point, e.g. tetrahydrofuran, and Fe (III) oxidizing agent, which reduces the oxidizing power of oxidizing agent so the mixture of monomer and oxidizing agent can be stably preserved for a longer period of time. After the capacitor element is impregnated with this mixture, the low-boiling solvent can be evaporated under high temperature, and monomers and oxidizing agent can react to form conductive polymer. The oxygen-containing compound disclosed in the aforesaid patent, e.g. tetrahydrofuran, acts as a polymerization retardant. But the complex formed of such compound and Fe (III) oxidizing agent does not have much effect on the oxidative capability of ferric ion, hence having limited effect on stabilizing the mixture of monomer and oxidizing agent at room temperature. This process fails to effectively simplify the process for preparing conductive polymer solid state electrolytic capacitor and reduce its cost.  
      Thus the focus of efforts on oxidative polymerization is to develop a process that effectively inhibits the room-temperature polymerization rate of monomer-oxidant mixture without at the same time affecting the conductivity of resulting polymer or adding to process difficulty.  
     SUMMARY OF THE INVENTION  
      To address the aforesaid problems, the present invention provides an oxidizing agent in the form of metal complex, which has weak oxidizing strength at room temperature and proper polymerization capability at high temperature. As such, the mixture of monomers and the oxidizing agent in the form of a complex does not undergo oxidative polymerization apparently under room temperature. Instead, the mixture possesses long-term stability at room temperature, and undergo polymerization reaction at higher temperature to obtain polymer with excellent conductivity.  
      To achieve the aforesaid object, the new oxidizing agent provided by the present invention is an organic metal complex formed of metal ion salts having oxidizing capability and nitrogen-containing compounds having lone pair electrons with partial π-electron character. The organic metal complex reduces the room-temperature oxidizing capability of metal ion without affecting its oxidizing capability at high temperature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows the composition (wt) of different reactants, oxidizing agents and solvents described in Example 1.  
       FIG. 2  shows the room temperature viscosity of mixtures of monomers and different oxidizing agent described in Example 1 versus time.  
       FIG. 3  shows the characteristics of conductive polymers synthesized according to Example 2 and comparative examples.  
       FIG. 4  shows the composition (wt) of different reactants, oxidizing agents and solvents described in Example 3.  
       FIG. 5  shows the room temperature viscosity of mixtures of monomers and different oxidizing agent described in Example 3 versus time.  
       FIG. 6  shows the characteristics of conductive polymers synthesized according to Example 4 and comparative example.  
       FIG. 7  shows the composition (wt) of different reactants, oxidizing agents and solvents described in Example 5.  
       FIG. 8  shows the room temperature viscosity of mixtures of monomers and different oxidizing agent described in Example 5 versus time.  
       FIG. 9  shows the characteristics of conductive polymers synthesized according to Example 6 and comparative example. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The detailed content and technology of the present invention are described below with accompanying drawings:  
      The present invention provides an oxidizing agent in the form of a complex that is useful for the synthesis of conductive polymer.  
      The implementation of the present invention is further depicted in the examples. First the reaction formula of the present invention is illustrated as follows:  
                 
          M—Dm—Np Organic metal complex, where 
            M Fe (III) salt, Cu (II) salt, Ce (IV) salt, Cr (VI) ion, etc. p-toluenesulfonate, naphthalene sulfonate, dodecylbenzenesulfonate,     D organic sulfonate, perchlorate, and chlorine ion.     N five or six-membered cyclic, nitrogen-containing compound having the functional group of  
                 
 
 or nitrogen-containing compound having the functional group of  
                 
   
            m, P&gt;=1        

      Commonly seen commercial conductive polymers include polyaniline, polypyrrole, polythiophene, polyphenylvinylene, and derivatives of those polymers, e.g. poly(3,4-ethylenedioxythiophene).  
      Metal ion salts used as oxidizing agent in the oxidative polymerization of conductive polymer include Fe (III) salt, Cu (II) salt, Ce (IV) salt, and Cr (VI) salt. In the example of poly(3,4-ethylenedioxythiophene), the oxidative polymerization using ferric p-toluenesulfonate as oxidizing agent can be expressed in formula (I), where p-toluene sulfonic acid formed by ferric p-toluenesulfonate after oxidation is the dopant for the polymer.  
      Aside from oxidizing agents such as p-toluenesulfonate, naphthalene sulfonate, dodecylbenzenesulfonate, organic sulfonate, perchlorate, and ferric chloride, their reductants can also be used as dopant without the need of adding separate dopant. The use of other oxidizing agents requires the addition of dopant in order for poly(3,4-ethylenedioxythiophene) to exhibit high conductivity. Aside from the aforementioned p-toluenesulfonate and ferric chloride, commonly seen dopants include AsF 6   − , BF 4   − , PF 6   − , I, and monoprotic or polyprotic acid, such as sulfuric acid, phosphoric acid, hydrochloric acid, perchloric acid, and their salts and derivatives, e.g. polystylene sulfonic acid (PSS) and salts.  
      The oxidative polymerization of monomers to obtain conductive polymer is affected by the oxidation potential level of monomer as well as the oxidizing capability of oxidizing agent. In the example of Fe (III) oxidizing agent, the Fe (III) ion would form a complex with a specific compound having lone pair electrons, which alters the oxidizing strength of Fe (III) ion. The stronger the complex bonding, the lower the oxidizing capability of the oxidizing agent. However, excessive complex bonding would cause the oxidizing agent to lose its oxidizing strength and strip its ability to synthesize long-chain conductive polymer.  
      The present invention uses nitrogen-containing compound having lone pair electrons with partial π-electron character to form proper complex bonding with metal ions with oxidizing capability so as to weaken the oxidizing strength of Fe (III) ion at room temperature. As such, the mixture of monomers and oxidizing agent can have long-term stability under room temperature in a system with low solvent content, while undergoes polymerization at a high temperature.  
      Nitrogen-containing compound having lone pair electrons with partial π-electron character includes five or six-membered cyclic compound having the functional group of  
                 
 
 straight-chained or cyclic compound of amide, imide, urethane or urea having the functional group of  
                 
 
 This kind of nitrogen-containing compound is far more powerful than oxygen-containing compound in forming a complex with Fe (III) ion. Thus its ability to retard the oxidizing capability of Fe (III) ion at room temperature is also far greater than that of oxygen-containing compounds. But the metal ion in the complex still retains good oxidizing capability at high temperature and is able to synthesize conjugated polymers with excellent conductivity. 
 
      The present invention uses a five-membered cyclic compound having the functional group of  
                 
 
 e.g. imidazole, 1-methylimidazole, pyrazole, triazole, pyridine, pyridazine, and derivatives thereof that can properly slow down the oxidizing capability of metal ion at room temperature or a little higher to form a complex with Fe (III) salt. The present invention also uses six-membered cyclic compound having the functional group of  
                 
 
 that has greater complexing capacity, e.g. pyridine, pyridazine or 1,2-diazine and derivatives thereof; compounds having amide functional group, e.g. formamide, N-methyl formamide, N,N-dimethylformamide, acetamide, N-methyl acetamide, and N,N-dimethylacetamide; compounds having imide functional group, e.g. succinimide, and phthalimide; compounds having urethane functional group, e.g. urethane; and compounds having urea functional group, e.g. urea, N,N′-dimethylurea, and tetramethylurea. Thus the mixture of monomers and oxidizing agent in the structure of a metal complex formed of metal ion salts having oxidizing capability and nitrogen-containing compound having lone pair electrons with partial π-electron character have long-term stability under room temperature, and can synthesize polymer having excellent conductivity at higher temperature. 
 
      The technical means for achieving the objects of the present invention are illustrated in the examples below:  
     EXAMPLE 1  
      Dissolve ferric p-toluenesulfonate listed in  FIG. 1  with methanol and then mix it with a five or six-membered cyclic compound having the functional group of  
                 
 
 according to the six compositions given in the table to form a complex. Add 3,4-ethylenedioxythiophene (EDOT) monomers to the complex, and measure the change of viscosity of the six mixtures under room temperature (25° C.) versus time to compare the long-term stability of the mixtures made of different oxidizing agents. The results are as shown in  FIG. 2 . 
 
      As shown in  FIG. 2 , the specific viscosity of mixture of ferric p-toluenesulfonate and monomers without the addition of five or six-membered cyclic compound having the functional group of  
                 
 
 rose rapidly, suggesting the rapid progression of polymerization reaction. With such speed of viscosity variation, there won&#39;t be sufficient time to process or use this mixture. Although the ferric p-toluenesulfonate solution that contains tetrahydrofuran can slow down moderately the polymerization of monomers in the mixture, its mixture with monomers does not exhibit as good room-temperature stability as the other four mixtures containing metal complex formed of ferric p-toluenesulfonate with imidazole, 1-methylimidazole, triazole, or pyridine. 
 
     EXAMPLE 2  
      Dissolve ferric p-toluenesulfonate listed in  FIG. 1  with methanol and then mix it with a five or six-membered cyclic compound having the functional group of  
                 
 
 according to the six compositions given in the table to form a complex. Add 3,4-ethylenedioxythiophene (EDOT) monomers to the complex and allow it to react for three hours under 50° C., and then wash thoroughly with methanol, dry and made into pellet. Measure the conductivity of resulting PEDOT to compare the characteristics of PEDOT synthesized by different metal complex as shown in  FIG. 3 . 
 
      The results in  FIG. 3  show that polymer synthesized by the new oxidizing agent provided herein exhibits more superior conductivity.  
     EXAMPLE 3  
      Dissolve ferric p-toluenesulfonate listed in  FIG. 4  with methanol and then mix it with a compound having amide functional group according to the three compositions given in the table to form a complex. Add 3,4-ethylenedioxythiophene (EDOT) monomers to the complex, and measure the change of viscosity of the three mixtures under room temperature (25° C.) versus time to compare the long-term stability of the mixtures made of different oxidizing agents. The results are as shown in  FIG. 5 .  
      As shown in  FIG. 5 , the mixture of monomers and THF-containing ferric p-toluenesulfonate does not exhibit as good room-temperature stability as the other three mixtures containing metal complex formed of ferric p-toluenesulfonate with formamide, N-methylformamide, or N, N-dimethylformamide.  
     EXAMPLE 4  
      Dissolve ferric p-toluenesulfonate listed in  FIG. 4  with methanol and then mix it with a compound having amide functional group according to the six compositions given in the table to form a complex. Add 3,4-ethylenedioxythiophene (EDOT) monomers to the complex and allow it to react for three hours under 50° C., and then wash thoroughly with methanol, dry and made into pellet. Measure the conductivity of resulting PEDOT to compare the characteristics of PEDOT synthesized by different metal complex as shown in  FIG. 6 .  
      The results in  FIG. 6  show that polymer synthesized by the new oxidizing agent provided herein exhibits more superior conductivity.  
     EXAMPLE 5  
      Dissolve ferric p-toluenesulfonate listed in  FIG. 7  with methanol and then mix it with a compound having urea or urethane functional group according to the two compositions given in the table to form a complex. Add 3,4-ethylenedioxythiophene (EDOT) monomers to the complex, and measure the change of viscosity of the two mixtures under room temperature (25° C.) versus time to compare the long-term stability of the mixtures made of different oxidizing agents. The results are as shown in  FIG. 8 .  
      As shown in  FIG. 8 , the specific viscosity of mixture of monomers and ferric p-toluenesulfonate that is not added with any compound having urea or urethane functional group rose rapidly, suggesting the rapid progression of polymerization reaction. Although the ferric p-toluenesulfonate solution that contains tetrahydrofuran can slow down the polymerization of monomers in the mixture, its mixture with monomers does not exhibit as good room-temperature stability as the other two mixtures containing metal complex formed of ferric p-toluenesulfonate with 1,3-dimethylurea or 1,1,3,3-tetramethlyurea.  
     EXAMPLE 6  
      Dissolve ferric p-toluenesulfonate listed in  FIG. 7  with methanol and then mix it with a compound having urea or urethane functional group according to the two compositions given in the table to form a complex. Add 3,4-ethylenedioxythiophene (EDOT) monomers to the complex and allow it to react for three hours under 50° C., and then wash thoroughly with methanol, dry and made into pellet. Measure the conductivity of resulting PEDOT to compare the characteristics of PEDOT synthesized by different metal complex as shown in  FIG. 9 .  
      The results in  FIG. 9  show that polymer synthesized by the new oxidizing agent provided herein exhibits more superior conductivity.  
      The mixture of monomers with metal complex oxidizing agent disclosed herein not only has long-term stability at room temperature, polymer synthesized at a high temperature thereof also possess high conductivity.  
      The preferred embodiments of the present invention have been disclosed in the examples. However the examples should not be construed as a limitation on the actual applicable scope of the invention, and as such, all modifications and alterations without departing from the spirits of the invention and appended claims, including the other embodiments shall remain within the protected scope and claims of the invention.