**2. Synthesis of polyanilines**

Polyaniline have different chemical structures which is attributed to the oxidation state of the polymer backbone, hence it exists in different states [12, 13]. The general representation of the polyaniline structure can be described by the following structural formula (**Figure 1**):

**Figure 1.** *The general structure of polyaniline.*

were y = 1, 0.5 and 0 correspond to fully reduced polyaniline (leucoemeraldine), the half oxidised polyaniline (emeraldine) and fully oxidised polyaniline (pernigraniline), respectively [13]. Upon doping, PANI can be interconverted from one oxidation state to another [13]. The interconversions can be clearly deduced as presented in **Figure 2**.

There are two general methods which are employed to synthesise conducting polymers through electrochemical oxidation of the monomers or chemical oxidation of the monomers, namely: electrochemical polymerisation and chemical polymerisation.

### **2.1 Electrochemical polymerisation**

Electrochemical polymerisation or method can be carried out by employing one of the three techniques: (i) applying a constant current (galvanostatic), or (ii) applying a constant potential (potentiostatic), and lastly (iii) by applying a potential scanning/cycling to the aqueous solution of aniline [14]. Polymerisation process is performed in strongly acidic aqueous electrolyte using a radical polymerisation mechanism which allows a formation of anilinium radical cation

**149**

*Polyaniline-Based Nanocomposites for Environmental Remediation*

*Oxidation of aniline monomer during polymerisation of aniline [21, 22].*

by aniline oxidation on the electrode[15, 16]. Electrochemical polymerisation

a.low pH which is needed for preparation of conductive polymeric materials,

b.the dopant anion incorporated into polymer to determine the morphology, conductivity, rate of polymerisation growth and influences degradation

The electrochemical process is more advantageous since film properties such as thickness and conductivity can be controlled by the synthesis parameters, including the current density, substrate, pH, nature and concentration of

Like electrochemical polymerisation, chemical polymerisation is also carried out in an acidic medium such as hydrochloric acid (HCl) and formic acid which helps in yielding primary polymer [19]. For this process to occur, a dopant or oxidant is required. The dopant reagents used during this process are the oxidising agents such as ammonium persulfate (APS), ferric chloride (FeCl3), hydrogen peroxide (H2O2) and ceric nitrate (Ce(NO3)3). The principal function of the oxidant is to withdraw a proton from an aniline molecule, without forming a strong coordination bond

The general mechanism involved during polymerisation of aniline proceeds dominantly via radical mechanisms. Radical mechanisms can be subdivided into initiation, chain propagation and termination steps, which results in stable intermediate resonance structures. The three different stages of polymerisation are

The initial step of aniline oxidative polymerisation is the generation of the aniline cation radical in the oxidation of aniline with an oxidant as shown in **Figure 2** [21, 22]. The aniline cation radical undergoes resonance to attain the most stable and reactive radical cation which is free from steric hindrances [23]. This step is the slowest step in the reaction, hence it's deemed as the rate determining step in aniline polymerisation [24].

Head to tail coupling of the *N*- and *para*- radical cations takes place (**Figure 3**), yielding a dicationic dimer species. This dimer further undergoes the process of re-aromatisation which causes it to revert to its neutral state, yielding an

either with the substrate intermediate or with the final product [20].

*2.2.1 Step 1: initiation step (oxidation of aniline monomer)*

*2.2.2 Step 2: radical coupling and re-aromatisation*

*DOI: http://dx.doi.org/10.5772/intechopen.82384*

strongly depends on the following factors:

c.inert electrode such as Pt, Au and graphite [17].

process and

**Figure 2.**

electrolyte [18].

**2.2 Chemical polymerisation**

illustrated from **Figures 2**–**4**.

*Polyaniline-Based Nanocomposites for Environmental Remediation DOI: http://dx.doi.org/10.5772/intechopen.82384*

*Trace Metals in the Environment - New Approaches and Recent Advances*

and were studied for different applications.

**2. Synthesis of polyanilines**

ing structural formula (**Figure 1**):

presented in **Figure 2**.

*The general structure of polyaniline.*

**2.1 Electrochemical polymerisation**

polymerisation.

**Figure 1.**

these conducting polymers, PANI has been widely studied due to its low cost, ease of synthesis, good environmental stability, unique doping/de-doping property and relatively high conductivity [5]. The discovery of PANI dates back to about 180 years ago to the experiments made by Runge [6]. From his work reported in 1834, he discovered that a dark green PANI changed to black when the mixture of copper(II) chloride and aniline nitrate is heated on a porcelain plate to 100°C [6, 7]. Anciently, PANI was known as 'aniline black', after forming an undesirable black powder deposit on the anode during oxidation of aniline [8, 9] and is the most stable CP that can be easily protonated (with an acid) to increase conductivity or deprotonated (with a base) to reduce its conductivity [10]. In 1862, Letheby prepared it through oxidation of aniline under mild conditions [9, 11]. Attempt to control the synthesis conditions of polyaniline grew until in the 1910s when Green and Woodhead managed successfully to control the conditions, which led to the discovery of its four oxidation states [8]. This was followed by Jozefowic's group in the 1960s and 1970s for better understanding of the material [6, 8]. After this, the study of polyaniline with other (intrinsic conducting polymers) ICPs increased tremendously worldwide

Polyaniline have different chemical structures which is attributed to the oxidation state of the polymer backbone, hence it exists in different states [12, 13]. The general representation of the polyaniline structure can be described by the follow-

were y = 1, 0.5 and 0 correspond to fully reduced polyaniline (leucoemeraldine), the half oxidised polyaniline (emeraldine) and fully oxidised polyaniline (pernigraniline), respectively [13]. Upon doping, PANI can be interconverted from one oxidation state to another [13]. The interconversions can be clearly deduced as

There are two general methods which are employed to synthesise conducting polymers through electrochemical oxidation of the monomers or chemical oxidation of the monomers, namely: electrochemical polymerisation and chemical

Electrochemical polymerisation or method can be carried out by employing one of the three techniques: (i) applying a constant current (galvanostatic), or (ii) applying a constant potential (potentiostatic), and lastly (iii) by applying a potential scanning/cycling to the aqueous solution of aniline [14]. Polymerisation

process is performed in strongly acidic aqueous electrolyte using a radical polymerisation mechanism which allows a formation of anilinium radical cation

**148**

**Figure 2.** *Oxidation of aniline monomer during polymerisation of aniline [21, 22].*

by aniline oxidation on the electrode[15, 16]. Electrochemical polymerisation strongly depends on the following factors:


The electrochemical process is more advantageous since film properties such as thickness and conductivity can be controlled by the synthesis parameters, including the current density, substrate, pH, nature and concentration of electrolyte [18].

#### **2.2 Chemical polymerisation**

Like electrochemical polymerisation, chemical polymerisation is also carried out in an acidic medium such as hydrochloric acid (HCl) and formic acid which helps in yielding primary polymer [19]. For this process to occur, a dopant or oxidant is required. The dopant reagents used during this process are the oxidising agents such as ammonium persulfate (APS), ferric chloride (FeCl3), hydrogen peroxide (H2O2) and ceric nitrate (Ce(NO3)3). The principal function of the oxidant is to withdraw a proton from an aniline molecule, without forming a strong coordination bond either with the substrate intermediate or with the final product [20].

The general mechanism involved during polymerisation of aniline proceeds dominantly via radical mechanisms. Radical mechanisms can be subdivided into initiation, chain propagation and termination steps, which results in stable intermediate resonance structures. The three different stages of polymerisation are illustrated from **Figures 2**–**4**.

#### *2.2.1 Step 1: initiation step (oxidation of aniline monomer)*

The initial step of aniline oxidative polymerisation is the generation of the aniline cation radical in the oxidation of aniline with an oxidant as shown in **Figure 2** [21, 22]. The aniline cation radical undergoes resonance to attain the most stable and reactive radical cation which is free from steric hindrances [23]. This step is the slowest step in the reaction, hence it's deemed as the rate determining step in aniline polymerisation [24].

#### *2.2.2 Step 2: radical coupling and re-aromatisation*

Head to tail coupling of the *N*- and *para*- radical cations takes place (**Figure 3**), yielding a dicationic dimer species. This dimer further undergoes the process of re-aromatisation which causes it to revert to its neutral state, yielding an

**Figure 4.** *Formation of a trimer and polymer formation [27].*

intermediate referred to as *p*-aminodiphenylamine (PADPA) [25, 26]. These processes are also accompanied by the elimination of two protons.

#### *2.2.3 Step 3: chain propagation*

**Figure 3.**

The dimers are immediately oxidised and then react with a stable aniline cation radical via an electrophilic aromatic substitution, followed by deprotonation and rearrangements to afford the trimer as seen in **Figure 4** [27]. The trimer further undergoes oxidation and reacts with aniline cation radical to form a tetramer and so on.
