9-1 Chemistry/Polymers

Addition Polymerisation

Alkenes can be used to make polymers by addition polymerisation. Here, unsaturated monomer molecules (alkenes) can react together, under pressure and with a catalyst to form a chain of repeating (poly) units with single carbon bonds.

Addition of alkene monomers to form a polymer.

Notice that the hydrogen atoms in the ethene compound are directly upwards or downwards. Also, we write a large n to show that we mean many (but an unspecified quantity of many). These form a long chain, which has bonds attaching either side, showing this.The polymer has exactly the same number of atoms as all of the monomers since that is the only product produced. So ethene monomers would make poly(ethene). For larger alkenes we cheat and write the same structure of ethene with an extra compound added on. For example with propene:

Chemical structure of propylene which is used in addition polymerisation

The polymer would look exactly the same as the above example with CH₃ as opposed to a hydrogen.

Condensation polymerisation

Condensation polymerisation involves monomers with 2 different functional groups reacting together. This process often results in small molecules such as water being formed (thus they are called condensation reactions). The simplest polymers are produced from 2 different monomers with two of the same functional groups on each monomer:

${\text{Diol + Dicarboxylic acid}}\rightarrow {\text{Condensation polymer + Water}}$

The square in the diagram represents the carbon chains. The important thing are the endings, this is the chemical equation showing the important bits:

${\ce {HO-OH + COOH-COOH -> (COO-COO)_n + 2n(H2O)}}$

Notice that all of the hydrogen bonds with oxygen from the OH group of the dicarboxylic acid, we can get a functional group pattern:

${\text{(di)-alcohol + (di)-carboxylic acid}}\rightarrow {\text{poly(-ester) + water}}$

Seems very similar?...

Amino acids

Amino acids have 2 different functional groups in a molecule - a basic amino group (${\ce {-NH2}}$) and a carboxyl group (${\ce {-COOH}}$). The two groups can react in a condensation reaction to form a dipeptide:

Notice that the -OH and H form H2O and the rest remains the same (but goes upright) to form the polymer and water. Here is the chemical formula:

${\ce {H2NCH2COOH -> (-HNCH2COO-) + H2O}}$

Polypeptides are proteins (chains of amino acids). Proteins contain different amino acids which give the protein different properties and instructions.

DNA

DNA double helix structure.

DNA (Deoxyribonucleic acid) is a large molecule essential for life. DNA encodes genetic instructions for the development and functioning of living organisms and viruses. Most DNA molecules are 2 polymer chains, made from four different monomers called nucleotides (A, T, C, G), in the form of a double helix. Other naturally occurring polymers include proteins, starch and cellulose.