<P> Polymer Chemistry: </P> <P> 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 . These include polythiophenes or low band gap polymers in organic semiconductors . In chain - growth polymerization, typically applied for alkenes, the backbone is not functional, but bears the functional side chains or pendant groups . </P> <P> The character of the backbone, i.e. its flexibility, determines the thermal properties of the polymer (such as the glass transition temperature). For example, in polisiloxanes, the backbone chain is very flexible, which results in a very low glass transition temperature of - 123 ° C. The polymers with rigid backbones 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 . </P> <P> There are some similarities and many differences inherent in the character of biopolymer backbones . The backbone of each of the three biological polymers; proteins, carbohydrates, and nucleic acids, is formed through a net condensation reaction . In a condensation reaction, monomers are covalently connected along with the loss of some small molecule, most commonly water . Because they are polymerized through complex enzymatic mechanisms, none of the biopolymers' backbones are formed through the elimination of water but through the elimination of other small biological molecules . Each of these biopolymers can be characterized as either a heteropolymer, meaning it consists of more than one monomer ordered in the backbone chain, or a homopolymer, which consists of just one repeating monomer . Polypeptides and nucleic acids are very commonly heteropolymers whereas common carbohydrate macromolecules such as glycogen can be homopolymers . This is because the chemical differences of peptide and nucleotide monomers determines the biological function of their polymers whereas common carbohydrate monomers have one general function such as for energy storage and delivery . </P>

Group of atoms that are added to carbon backbones