In polymer science, the polymer chain or simply backbone of a polymer is the main chain of a polymer. Polymers are often classified according to the elements in the main chains. The character of the backbone, i.e. its flexibility, determines the properties of the polymer (such as the glass transition temperature). For example, in Silicone (silicone), the backbone chain is very flexible, which results in a very low glass transition temperature of .
Organic polymers
Common synthetic polymers have main chains composed of carbon, i.e. C-C-C-C.... Examples include
such as
polyethylene ((CH
2CH
2)
n) and many substituted derivative ((CH
2CH(R))
n) such as
polystyrene (R = C
6H
5),
polypropylene (R = CH
3), and
(R = CO
2R').
Other major classes of organic polymers are and . They have respectively -C(O)-O- and -C(O)-NH- groups in their backbones in addition to chains of carbon. Major commercial products are polyethyleneterephthalate ("PET"), ((C6H4CO2C2H4OC(O))n) and nylon-6 ((NH(CH2)5C(O))n).
Inorganic polymers
are a premier example of an inorganic polymer, even though they have extensive organic substituents. Their backbond is composed of alternating silicon and oxygen atoms, i.e. Si-O-Si-O... The silicon atoms bear two substituents, usually
methyl as in the case of polydimethylsiloxane. Some uncommon but illustrative inorganic polymers include
polythiazyl ((SN)x) with alternating S and N atoms, and polyphosphates ((PO
3−)
n).
Biopolymers
Major families of biopolymers are
(carbohydrates),
, and
. Many variants of each are known.
[V]
Proteins and peptides
Proteins are characterized by
Peptide bond (-N(H)-C(O)-) formed by the condensation of
. The sequence of the amino acids in the polypeptide backbone is known as the primary structure of the protein. Like almost all polymers, protein fold and twist, forming into the secondary structure, which is rigidified by
hydrogen bonding between the
Carbonyl group oxygens and amide hydrogens in the backbone, i.e. C=O---HN. Further interactions between residues of the individual amino acids form the protein's tertiary structure. For this reason, the primary structure of the amino acids in the polypeptide backbone is the map of the final structure of a protein, and it therefore indicates its biological function.
Spatial positions of backbone atoms can be reconstructed from the positions of alpha carbons using computational tools for the backbone reconstruction.
Carbohydrates
Carbohydrates arise by condensation of
such as
glucose. The polymers can be classified into
(up to 10 residues) and
(up to about 50,000 residues). The backbone chain is characterized by an ether bond between individual monosaccharides. This bond is called the
Glycosidic bond.
These backbone chains can be unbranched (containing one linear chain) or branched (containing multiple chains). The glycosidic linkages are designated as
Anomer depending on the relative
stereochemistry of the
(or most
oxidized) carbon. In a Fischer Projection, if the glycosidic linkage is on the same side or face as carbon 6 of a common biological saccharide, the carbohydrate is designated as
beta and if the linkage is on the opposite side it is designated as
alpha. In a traditional "chair structure" projection, if the linkage is on the same plane (equatorial or axial) as carbon 6 it is designated as
beta and on the opposite plane it is designated as
alpha. This is exemplified in
sucrose (table sugar) which contains a linkage that is
alpha to glucose and
beta to
fructose. Generally, carbohydrates which our bodies break down are
alpha-linked and those which have structural function are
beta-linked (example:
cellulose).
Nucleic acids
Deoxyribonucleic acid (DNA) and
RiboNucleic Acid (RNA) are the main examples of
. They arise by condensation of nucleotides. Their backbones form by the condensation of a hydroxy group on a
ribose with the
phosphate group on another ribose. This linkage is called a phosphodiester bond. The condensation is catalyzed by
called
. DNA and RNA can be millions of nucleotides long thus allowing for the genetic diversity of life. The bases project from the pentose-phosphate polymer backbone and are
in pairs to their complementary partners (A with T and G with C). This creates a double helix with pentose phosphate backbones on either side, thus forming a secondary structure.
See also
