Properties of the Linear Chain Polymers (CH)_χ and (SN)_χ

Abstract

It is almost obvious that a linear-chain polymer should show an electronic one-dimensionality provided it is a good conductor. Most polymers, however, have a quite high resistivity; for example, polyethylene is used as a good electrical insulator. The reason for this is the lack of conduction electrons in these polymers. A carbon atom in polyethylene, (CH₂)_x, has four electrons in its outer shell; two of them are used to form bonds with hydrogen atoms and the remaining two form the σ-bonds between carbon atoms as shown in Fig. 6.1a. These electrons cannot move freely. One may expect that the polymer (CH)_X, in contrast to (CH₂)_x, has a free electron on each carbon atom. Moreover a polymer consisting of carbon atoms only, C_x, will have two excess electrons which are not used in the σ-bonding.¹ These excess electrons enter π-orbitals making a double bond between carbon atoms as shown in Fig. 6.1b and c. Another drawing, Fig. 6.1d, is also possible for the structure of (CH)_x. It is well known that the two states (Fig. 6.1b, d) form a resonance. In other words, the π-orbitals overlap along the chain giving rise to a π-band. In principle, any polymer can become metallic as long as it has π-electrons.