A term in Chemistry, used to denote a large molecule formed by the repeated linking together of a number of smaller molecules. The individual small molecules are known as monomers. A very small polymer, formed from only a few monomers, is sometimes referred to as an oligomer. The process of forming a polymer is called Polymerisation. The formation of a polymer frequently requires the presence of another molecule that enables or speeds up the chemical reactions involved; such compounds are referred to as catalysts.
- proteins are irregular polymers of a variety of amino acids
- DNA and RNA are polymers of nucleotides. The enzymes used in their synthesis are called polymerases.
- Polysaccharides are carbohydrate polymers, including cellulose
Polymers are created by polymerization.
Structures and physical characteristics of polymers
The attractive forces in polymers result from instantaneous-dipole induced-dipole interactions. As two polymer chains approach their negative electron clouds repel, causing one chain's electrons to become less dense on one side creating a slight positive charge this attracts second chain.
Intermolecular forces allow polymer chains to slide past each other when heated, ending in different orientations without breaking down to ethene.
While propagating, radical exchange occurs when a C atom becomes a radical by losing a hydrogen atom but keeping the electron. Joining with another radical chain it creates side-branches.
The more branching that occurs, the further apart the chains, causing less contacts between atoms of different molecules and less opportunities for induced dipoles to operate, decreasing intermolecular forces.
Low density results from the chains being further apart. Also lower meltingpoints occur, because the intermolecular bonds are weaker and require less energy to break. These bonds can also easily be broken by work, so tensile strength is lower.
Stereoregularity or tacticity is how the functional groups are arranged on the backbone of carbon atoms.
Isotactic chains can line up closer to each other because all the methyl groups are on one side. This makes it crystalline (closely packed in a regular fashion) and highly rigid. Syndiotactic methyl groups alternate regularly, so the chain can position itself closer to another efficiently, though not as close as isotactic polymers. Syndiotactic polymers have better impact strength because of the higher flexibility due to weaker intermolecular forces. Crystallinity helps light to pass through it without being refracted, meaning a more transparent polymer.
Atactic chains do not fit together well because of the random methyl groups, so intermolecular forces are low, leading to low density and tensile strength, but high flexibility.
The longer the chain, the more points of contact with neighbouring chains. This increases intermolecular forces between the molecules. Also, longer chains are more tangled together which makes them harder to move about. These stronger forces result in higher tensile strength and melting/boiling points due to the molecules being harder to separate.
Copolymerization is the polymerization of two or more monomers. This can result in different properties for the final plastic.
For example, copolymerising ethene with small amounts of hex-1-ene forms linear low density polyethene. Although not as dense as HDPE, short branches enable the formation of crystalline regions which withstand tearing forces yet still remain flexible.
Serendipity in the development of polymers
Trying to manufacture a ketone, Fawcett and Gibson accidentally produced the radical benzoyl peroxide from benzaldehyde ? creating poly(ethene). Attempting to reproduce their results, the right amount of oxygen accidentally entered the chamber initiating addition polymerization.
Investigating organometallic compounds using the tri-methylaluminium compound, Ziegler had unexpected results. Tracking this down to nickel impurities, he experimented with many alkylaluminium compounds. Titanium compounds led to the production of long, unbranched, polymer chains. Without the chemical impurity, Ziegler would have never investigated these.
Wilkinson and Fisher were investigating organometallic compounds as catalysts. A student working on a modified Ziegler-Natta catalyst made of trimethyaluminium and titanocene produced a much higher yield of poly(ethene) than expected. Water vapour had accidently entered and reacted with the trimethyaluminium to form methyl alumoxane. When methyl alumoxane reacted with the metallocene it 'activated' it, increasing yield massively.