1.8 Polymers The General Structure of Polymers A polymer is a very large molecule that is built from monomers. A monomer is one of the repeating units that make up a polymer. Many biological molecules, such as silk, are natural polymers that are built inside living organisms. In the last couple of centuries, humans began manufacturing synthetic polymers for a wide range of applications. Polyethene (more commonly known as polyethylene) is an example of a homopolymer: a polymer formed by reactions involving a single type of monomer. Substituents on the chain of carbon atoms give the polymer different properties. Other polymers that form by addition reactions include polypropene (polypropylene) and polystyrene. A copolymer has different types of monomers combined to form the polymer chain.
formation of silk A polymerization reaction may continue until thousands of base molecules have joined the chain. Natural and Synthetic Polymers Natural polymers are made by all living things, from bacteria to mammals. Those manufactured in the cells of plants include starch and cellulose, which are homopolymers of the monomer glucose. Other natural polymers include the molecules RNA and DNA that are produced within cell nuclei. Synthetic polymers are usually made from monomers sourced from plants or from petrochemicals. Examples of synthetic polymers include polyester and polyamide fabrics, containers made of polyethene or polypropene, fluoropolymers used as non-stick coatings on cookware, the rubber of automobile tires, and the super-strong Kevlar fibres used in body armour. Synthetic Addition Polymers An addition polymer is the result of the reaction between monomers with unsaturated carbon carbon bonds, similar to the addition reactions for alkenes.
Teflon polypropene p.85 of text Plastics A plastic is a synthetic polymer that can be moulded into shape (often under heat and pressure) and will then retain its shape when cooled. There are many kinds of plastics, including polymers of substituted ethene monomers: polystyrene, polyvinyl chloride, nylon, polyesters, rubber, polyethene, and polypropene. Plastics are usually manufactured from petrochemicals. Not all polymers are plastic, but all plastics are polymers. Plasticizers In addition to the polymer itself, many plastic products include plasticizers. Plasticizers are substances added to increase flexibility, making the plastics easier to produce and shape into useful objects.
Properties of Addition Polymers They are chemically unreactive (ideal for containers) Generally they are flexible and mouldable Some monomers have two double bonds. These monomers can therefore link to two separate polymer chains at the same time. As well as forming their regular polymer chain, they form strong covalent bonds, called crosslinks, between adjacent polymer chains. The more crosslinks there are, the more rigid the plastic is.
Synthetic Condensation Polymers Carboxylic acids react with alcohols to form esters, and with amines to form amides. These reactions are called condensation reactions. When monomers can join, end to end, in ester or amide linkages, polymers called polyesters and polyamides are produced. Because polyesters and polyamides result from condensation reactions, these polymers are called condensation polymers. To form a polyester or a polyamide, the monomer molecule must have two functional groups, one at each end of the molecule. The functional groups that meet end to end must be a carboxyl group ( COOH) and either a hydroxyl ( OH) group or an amine group ( NH2). A polyester is a polymer formed by a series of esterification reactions between monomers.
Dacron A polyamide is a polymer formed by a condensation reaction resulting in amide linkages between monomers.
Properties of Condensation Polymers Crosslinks play an important part in the properties of condensation polymers. Polyamide chains, such as nylon, have amine groups that can hydrogen-bond with the C=O groups on other chains. As a result, polyamide chains form exceptionally strong fibres. Similarly, the strong attractive forces between polar groups in polyesters, such as Dacron, hold the separate polymer chains together, giving them considerable strength. Worksheet 1.8: Polymers p.93 Q. 2, 3, 4, 8 p.99 Q. 1, 2, 3, 5, 6 Kevlar