Polymer





A polymer is a long, repeating organic chain, formed through the linkage of
many identical smaller molecules called monomers.

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.

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) 20 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 it 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.

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 low density 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.