POLYMER CHEMISTRY

Natural and Synthetic Polymers Polymers are made up of many (poly) repeating units (mer). Simple polymers are made up of single monomers joined together in either addition or condensation polymerisation.

Addition Polymers In addition polymers the monomers contain double bonds which are broken to join small molecules together in one long chain. Polythene: made from ethene H H H H H H H H H H H H C C + C C + C C ~ C C C C C C ~ ethene polyethene Other addition polymers: Polyvinyl chloride (PVC) and polystyrene are addition polymers, like polythene. The general equation for the formation of addition polymers is: H Z H Z H Z H Z H Z n C C ~ C C C C C C C C ~ H H H H H H H H H H monomer polymer in PVC Z = Cl, in polypropene Z = CH3 and in polystyrene Z =

What is a Polymer?

Condensation Polymers - Polyesters Esters are formed when an alcohol reacts with either a carboxylic acid or an acid chloride. Polyesters are made by reacting “double-ended” molecules such as a diol or dicarboxylic acid. HOOC X COOH + HO Y OH ~ OOC X COO Y OOC X COO Y~ dicarboxylic acid diol polyester + 2H2O The most important polyester is terylene, made from the esterification of ethane-1,2-diol and benzene-1,4-dicarboxylic acid.

Condensation Polymers - Polyamides Polyamides may be made by combining a diamine and a dicarboxylic acid or acid chloride. Nylon, the first synthetic fibre is a polyamide. It was developed in the 1930’s as a cheap alternative to silk.

Other Monomers – Kevlar and Polystyrene

Natural Polymers - Proteins glycine Proteins are polymers made from amino acid monomers. Each amino acid contains the NH2 group of an amine and the COOH group of an acid. The simplest amino acid is glycine. What is its formula? What is its IUPAC name? Apart from glycine, all amino acids have a chiral carbon (one with four different groups on it), and therefore form optical isomers. Normally only one optical isomer is useful in our bodies. alanine

Proteins - continued All the amino acids in proteins have the –NH2 group joined to the same carbon as the -COOH group. There are 20 amino acids involved in proteins required by humans. Eight of these cannot be made in our bodies and must be included in the diet. These are known as essential amino acids. In proteins, amino acids join together by condensation polymerisation. H

Protein continued Since each amino acid contains two active sites for the polymerisation reaction, they can be assembled in any order, providing an extremely large number of different protein molecules. Most common proteins contain about 100 amino acids. The bond that joins the amino acids together is called a peptide link. R-C-NH-R O the peptide link When the protein is digested, the polymer is hydrolysed in exactly the same way that amides are. The acid in the stomach breaks the peptide link.

Natural Polymers – Starch and Cellulose Glucose is the simplest sugar. It normally forms a 6-membered ring. There are two forms of the glucose ring,  and . In  glucose this OH group is below the plane of the ring and in  glucose it is above the plane. The ring can be broken to form a chain form of the molecule which shows that glucose is an aldehyde and hence gains its name of a ‘reducing sugar’. http://www.biotopics.co.uk/as/glucose2.html

Starch Starch is a condensation polymer made from glucose monomers. A molecule of water is lost at every join. When starch is hydrolysed, either by the action of enzymes or acid, water is added to reform the glucose monomers. Soluble starch (amylose) is formed from long straight chains of -glucose molecules, while the insoluble forms of starch e.g. amylopectin and glycogen are branched linear polymers of -glucose. Amylose structure

Glycogen and Amylopectin Glycogen has 1-6 branching, as does amylopectin, but glycogen has more branching

Cellulose In cellulose, the -glucose molecules are arranged in long, straight chains, but the arrangement is a little different from that of soluble starch, and it allows the chains to hydrogen bond with each other. The chains combine to form long fibres which resist most solvents and chemicals. Only a few specialised bacteria have the ability to hydrolyse cellulose. Without those bacteria in their stomachs grass-eating animals such as cows and sheep would starve. In cellulose the glucoside links alternate above and below the plane of the molecule. In starch they are all on the same side.

Cellulose

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