Conjugated Polymers with Substituents

4 SOLUBLE CONJUGATED POLYMERS 4-1 Conjugated polymers with substituents. 

Since the discovery that conjugated polymers can exhibit high conductivity when doped, a tremendous amount of work has been done on these materids. Solid conducting polymers form inhomogeneous porous media, with an ill-defined structure. In many case, complex structures are obtained, with coexisting amorphous and crystalline, more or less fibrillar, 

The discovery that conjugated polymers can be solubilized with appropriate substituents has given access to single chain properties. The volume of work devoted to the synthesis of new soluble materials has may be reached or even overpassed that devoted to the control of the solid phase through Ae polymerization. In this part we focus the attention on substituted polyacetylene, poly o diene and polydiacetylene, which have been the most investigated. 

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Table 5.2. Conformation of conjugated polymers and saturated polymers with substituent side groups. ( ) Measured with small-angle neutron scattering,...

Li, L., K.E. Counts, S. Kurosawa, A.S. Teja, and D.M. Collard. 2004. Tuning the electronic structure and solubility of conjugated polymers with perfluoroalkyl substituents Poly(3-per-fluorooctylthiophene), the first supercritical-COa-soluble conjugated polymer. Adv Mater 16 (2) 180-183. 

The predominant 7t-conjugated polymers with organosilicon substituents in the side chains are polyacetylenes [143-148], polythiophenevinylenes (PTV) [149], and poly(l,4-phenylenevinylene)s (PPV) [150-175]. 

Figure 1 Example of conjugated polymers with side-groups. Most of the substituents are aromatic or alkyl groups (a) Polydiacetylene, (b) polyacetylene, (c) polythiophene. Side-groups of some soluble polydiacetylene are given in table 7-1.

Since the discovery that Poly-n-alkylthiophenes can be solubilized (15), they have been the subject Of intensive studies. The stability of the polymer together with the possibility to study the doped polymer in solution are die origins of the interest Besides, poly-n alkylthiophene films exhibit drastic change as shown with the thermochromism which varies widi the size of the substituent (Salaneck et al, this book). Moreover poly-n alkylthiophene provides an unique oppcntunity to investigate the chain conformation of a conjugal polymer with a conjugated backbone maiicedly different from the substituted polydiacetylene or polyacetylene. 

Incorporation of chiral units into polymers generates optically active polymers.27 Two types of optically active polymers could be obtained according to where the chiral units reside optically active polymers with chirality derived from chiral side chains and optically active polymers with chirality derived from tire chiral main chain. The circular dichroism (CD) measurement of 32, an optically active polymer with chiral side chains, showed that the chiral substituents have induced main-chain chirality. The induced main-chain chirality disappeared at higher temperature and appeared upon cooling. This type of chiral conjugated polymer is potentially useful in reversing optical recording28 ... 

In the area of ion sensing, cation recognition by electrodes containing functionalized redox-active polymers has been an area of considerable interest. Fabre and co-workers have reported the development of a boronate-functionalized polypyrrole as a fluoride anion-responsive electroactive polymer film. The electropolymerizable polypyrrole precursor (11) (Fig. 11) was synthesized by the hydroboration reaction of l-(phenylsulfonyl)-3-vinylpyrrole with diisopinocampheylborane followed by treatment with pinacol and the deprotection of the pyrrole ring.33 The same methodology was utilized for the production of several electropolymerizable aromatic compounds (of pyrrole (12) (Fig. 11), thiophene (13 and 14) (Fig. 11), and aniline) bearing boronic acid and boronate substituents as precursors of fluoride- and/or chloride-responsive conjugated polymer.34... 

Many substituents stabilize the monomer but have no appreciable effect on polymer stability, since resonance is only possible with the former. The net effect is to decrease the exothermicity of the polymerization. Thus hyperconjugation of alkyl groups with the C=C lowers AH for propylene and 1-butene polymerizations. Conjugation of the C=C with substituents such as the benzene ring (styrene and a-methylstyrene), and alkene double bond (butadiene and isoprene), the carbonyl linkage (acrylic acid, methyl acrylate, methyl methacrylate), and the nitrile group (acrylonitrile) similarly leads to stabilization of the monomer and decreases enthalpies of polymerization. When the substituent is poorly conjugating as in vinyl acetate, the AH is close to the value for ethylene. 

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