Character of the backbone
In a way the character of the backbone chain depends on the type of polymerization: in step-growth polymerization the monomer moiety becomes the backbone, and thus the backbone is typically functional, like in polythiophenes or low band gap polymers in organic semiconductors. In chain-growth polymerization, typically applied for alkenes, the backbone is not functional, but is bearing the functional side chains or pendant groups. However, in polypeptides, the backbone is as important for the functionality of the polymer as the side chains. The backbone in polypeptides consists of carbon and nitrogen atoms of the constituent amino acids and does not include the side chains.
The character of the backbone, i.e. its flexibility, determines the thermal properties of the polymer (such as the glass transition temperature), e.g. in polisiloxanes the backbone chain is very flexible, resultin in very low glass transition temperature of -123 °C. The polymers with a rigid backbone are prone to crystallization (e.g. polythiophenes) in thin films and in solution. Crystallization in its turn affects the optical properties of the polymers, its optical band gap and electronic levels.
The configuration of a polypeptide relies on the flexibility of the torsion angles of the backbone.  The flexibility of the peptide backbone determines its ability to stretch which is important for proteins under mechanical stress.
Types of backbone
- saturated alkane (typical for vinyl polymers);
- step-growth polymers (polyaniline, polythiophene, PEDOT) backbone. These often have derivatized heterocycles as monomers, such as thiophenes, diazoles or pyrroles.
- polypeptide backbone;
- fullerene backbone;
- polysaccharide backbone. Although, by the way it is formed it's similar to the peptide backbone, but has different properties due to the type of monomers linked into the backbone.
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