Oxidative polymerization, polyheterocycles

Diaminothiophene could be used for the construction of condensed hetero bi- and tricycles, employing standard reactions like condensation with diketones, Skraup reaction, etc. <83BSF(2)159>. Condensation with the a-diketone (506) leads to 2,3-dihexylthieno[3,4-i ]pyrazine (507) (Equation (48)). Oxidative polymerization of this (FeCl3) has resulted in a new low band-gap polyheterocycle <92CC1672>. 

Polyheterocycles. Heterocychc monomers such as pyrrole and thiophene form hiUy conjugated polymers (4) with the potential for doped conductivity when polymerization occurs in the 2, 5 positions as shown in equation 6. The heterocycle monomers can be polymerized by an oxidative coupling mechanism, which can be initiated by either chemical or electrochemical means. Similar methods have been used to synthesize poly(p-phenylenes). 

A similar mechanism has been found in the case of other polyheterocycles. For example, the deposition of polycarbazole has been shown to involve the initial formation of cation radicals. The electrooxidation of carbazole in the nonaqueous medium was thoroughly studied by Ambrose and Nelson [10], who established that a dicarbazyl radical resulting from the coupling of carbazole cations at the 3-position is formed. These authors did not report polymerization of carbazole. However, others who have investigated the oxidation of carbazole [11] reported polymerization. An iodinated, chemically polymerized material was also reported [12]. The mechanism for the formation of polycarbazole in dimethyl formamide (DMF) or aqueous medium is shown in Scheme 2 [11]. 

Polyheterocycles. Heterocychc monomers constitute a third class of monomers that can he polymerized to form fully conjugated polymers the most common of these monomers, shown in equation 7, are p5urole (X = NH), thiophene (X = S), and furan (X = O). They can he doped to give electrical conductivity when pol5unerization occurs in the 2,5-positions. These monomers can be polymerized both by electrochemical and chemical methods. The polyheterocycles have received considerable attention for their electron-rich nature, which leads to materials that are easily oxidized and therefore more stable in the oxidized state. Additionally, the increased structural complexity of polyheterocycles relative to polyacetylenes makes structural modifications possible for improved processability. 

At the start of the 1960s, priorities centred on postulating and testing new structural principles for polymers. One approach, other than the classical polymerization methods, was that of oxidative coupling. By this technique, copper chloride and aluminium chloride were used to make an oligobenzene from benzene [Ic]. This reaction was systematically extended and became established as the general aufbau method of synthesizing polyaro-matics and polyheterocycles [la]. 

Currently, there are several classes of EAPs that have been shown to exhibit conductivities of metals and semiconductors in the doped state. They are the polyacetylenes, poly(para-phenylene)s, polyheterocycles, poly(phenylene vinylenels, polyanilines, and conjugated ladder polymers. Conducting polymers are made by both chemical and electrochemical polymerization. In the neutral (uncharged) state, EAPs are insulators or semiconductors. Oxidized (p-doped) EAPs have had electrons removed from the backbone, resulting in delocalized polymer cation formation. Reduced (n-doped) EAPs have had electrons added to the backbone, resulting in polymer anion formation. 

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