Polymer

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 identical 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, without the use of specific catalysts, when a polymer chain stops growing during polymerization can be effectively random.

Proteins are polymers of amino acids. Some 20 different monomers are involved in the chain, the sequence of monomers determining the shape and activity of the final protein. 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.

Intermolecular forces

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. Polyesters 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 forces. 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.

Branching

During the propagation of polymer chains, branching can occur. In radical polymerization, this is when a chain curls back and bonds to an earlier part of the chain. When this curl breaks, it leaves small chains sprouting from the main carbon backbone. Branched carbon chains cannot line up as close to each other as unbranched chains can. This causes less contact between atoms of different chains, and fewer opportunities for induced or permanent dipoles to occur. A low density results from the chains being further apart. Lower melting points and tensile strengths are evident, because the intermolecular bonds are weaker and require less energy to break.

Stereoregularity

Stereoregularity or tacticity describes the isomeric arrangement of functional groups on the backbone of carbon chains. Isotactic chains are defined as having methyl groups aligned in one direction. This enables them to line up close to each other, creating crystalline areas and resulting in highly rigid polymers.

In contrast, atactic chains have randomly aligned methyl groups. The chains do not fit together well and the intermolecular forces are low. This leads to a low density and tensile strength, but a high degree of flexibility.

Syndiotactic methyl groups alternate regularly in opposite directions. because of this regularity, syndiotactic chains can position themselves close to each, though not as close as isotactic polymers. Syndiotactic polymers have better impact strength than isotactic polymers because of the higher flexibility resulting from their weaker intermolecular forces.

Copolymerization

Copolymerization is polymerization with two or more different monomers. Already mentioned are the 20 amino acid monomers that make up protein chains. Copolymerization of different monomers can result in varied properties of polymers, just as different amino acids result in different shapes of proteins. For example, copolymerising ethene with small amounts of hex-1-ene forms linear lowidensity polyethene (LLDPE). The short branches resulting from hex-1-ene lower the density and enable the formation of crystalline regions within the polymer. This means that LLDPE can withstand strong tearing forces whilst remaining flexible.

Polymer characterization

A variety of laboratory techniques are used to determine the properties of polymers. Techniques such as wide angle xray scattering, small angle xray scattering, and small angle neutron scattering are used to determine the crystalline structure of polymers. Gel permeation chromatography is used to determine the number average molecular weight, weight average molecular weight, and polydispersity. FTIR is used to determine composition. Thermal properties such as the glass transition temperature and melting point can be determined by differential scanning calorimetry and Dynamic Mechanical Analysis. Thermal degradation followed by analysis of the fragments is one more technique for determining the possible structure of the polymer.

See also: Polymerization -- Biopolymer -- Condensation polymer -- Synthetic polymer