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The term polymer covers a large, diverse group of molecules, including substances from proteins to high-strength kevlar fibres. A key feature that distinguishes polymers from other large molecules is the repetition of units of atoms (monomers) in their chains. This occurs during polymerization, in which many monomer molecules link to each other. For example, the formation of polyethene involves thousands of ethene molecules bonding together to form a chain of repeating -CH2- units:
Polymers are often named in terms of their monomer units, for example polyethylene is represented by:
Because polymers are distinguished by their constituent monomers, polymer chains within a substance are often not of equal length. This is unlike other molecules in which every atom is acounted for, each molecule having a set molecular mass. Differing chain lengths occur because polymer chains terminate during polymerization after random intervals of chain lengthening (propagation).
Proteins are polymers of amino acids. From a dozen to some hundred of the (about) twenty different monomers form the chain, the sequence of monomers determining the shape and activity of the final protein. But there are active regions, surrounded by, as is believed now (Aug 2003), structural regions, whose sole role is to expose the active region(s) (there may be more than one on a given protein). So the absolute sequence of amino acids is not important, as long as the active regions are expressed (being accessible from the outside) properly. Also, whereas the formation of polyethylene occurs spontaneously given the right conditions, the manufacture of biopolymers such as proteins and nucleic acids requires the help of catalysts (substances that facilitate or accelerate reactions.) Since the 1950s, catalysts have also revolutionised the development of synthetic polymers. By allowing more careful control over polymerization reactions, polymers with new properties, such as the ability to emit coloured light, have been manufactured.
The attractive forces between polymer chains play a large part in determining a polymer's properties. Because polymer chains are so long, these interchain forces are amplified far beyond the attractions between conventional molecules. Also, longer chains are more amorphous (randomly oriented). Polymers can be visualised as tangled spaghetti chains - pulling any one spaghetti strand out is a lot harder the more tangled the chains are. These stronger forces typically result in high tensile strength and melting points.
The intermolecular forces in polymers are determined by dipoles in the monomer units. Polymers containing amide groups can form hydrogen bonds between adjacent chains; the positive hydrogen atoms in N-H groups of one chain are strongly attracted to the oxygen atoms in C=O groups on another. These strong hydrogen bonds result in, for example, the high tensile strength and melting point of kevlar. PolyesterSEM picture of a bend in a high surface area polyester fiber with a seven-lobed cross section Polyester is a category of polymers which contain the ester functional group in their main chain. Although polyesters do exist in nature, polyester generally refs have dipole-dipole bonding between the oxygen atoms in C=O groups and the hydrogens in H-C groups. Dipole bonding is not as strong as hydrogen bonding, so ethene's melting point and strength are lower than kevlar's, but polyesters have greater flexibility.
Ethene, however, has no permanent dipole. The attractive forces between polyethylene chains arise from weak van der Waals forceIn chemistry, the term van der Waals force originally referred to all forms of intermolecular forces; however, in modern usage it tends to refer only to London forces: those forces which arise from induced rather than permanent dipoles. The forces are nams. Molecules can be thought of as being surrounded by a cloud of negative electrons. As two polymer chains approach, their electron clouds repel one another. This has the effect of lowering the electron density on one side of a polymer chain, creating a slight positive dipole on this side. This charge is enough to actually attract the second polymer chain. Van der Waals forces are quite weak, however, so polyethene melts at low temperatures.
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