Polymers

Plastics belong to a group of substances called polymers: large molecules made by linking together many smaller molecules, much like paper clips in a long chain. Different types of small molecules form links in different ways, by either addition (double bond becomes single bond) or condensation (two molecules form one large molecule, water as byproduct) reactions. The types of small units and linkages can be manipulated to produce materials with desired properties such as strength, flexibility, high or low density, transparency, and chemical stability. As consumer needs change, new polymers are designed and manufactured. There are also natural polymers, like starch with the monomer glucose.

The small subunits in these long chains of molecules are called monomers, and the long chains of many monomers (generally 10 or more) are called polymers. The chemical process by which monomers are joined to form polymers is called polymerization. The monomers in any polymer might be identical, or they may occur in a repeating pattern. The monomers of an organic polymer may be linked together by C-C, C-O or C-N bonds.

The name of a polymer is usually written with the prefix poly- (meaning “many”) before the name of the monomer. Often the common name of the monomer is used, rather than the IUPAC name, like polyethylene using the common name of the monomer ethene.

Addition Polymers

Addition polymerization is a reaction in which monomers with double bonds are joined together through multiple addition reactions to form a polymer.

There are three stages in the synthesis of addition polymers: initiation, propagation, and termination. In the first stage, an initiating molecule (such as a peroxide) with an unpaired electron forms a bond to one of the carbon atoms in the double bond of a molecule. The electrons shift in the newly bonded molecule, which now has an unpaired electron at the other end of its original double bond. This unpaired electron is now available to form another covalent bond with another atom or group. The addition reaction continues and the chain “propagates.” The chain growth is terminated when any two unpaired electron ends combine, forming a covalent bond that links two growing chains together.

Polyethylene

Polyethylene (IUPAC polyethene) is made by connecting many ethene (CH2=CH2) molecules end to end. It is used for insulating electrical wires and for making plastic containers. Alkenes undergo addition reactions with other molecules that add to the single carbon bond. Under certain conditions, alkenes can also undergo addition reactions with other alkenes; the double bond in each alkene monomer breaks to form a single bond, freeing up an unbonded electron pair that then forms a single carbon–carbon bond with another monomer.

Polypropene

Made up of propene (CH2=CHCH3) molecules linked end to end. Forming similar to polyethene, many propene monomers come together and their double bonds are broken to form single bonds with each other. The only difference is that in the chain, every monomer’s methyl group will remain single bonded to the carbon chain, but the double bond broken for each allows single bonding with the rest of the monomers.

Polyvinyl Chloride

Polyvinyl chloride, commonly known as PVC, is an addition polymer of chloroethene (Cl-CH=CH2). A common name for chloroethene is vinyl chloride. PVC is used as insulation for electrical wires and as a coating on fabrics used for raincoats and upholstery materials. Like polypropene, the functional group is on every other carbon atom while the double bond breaks to form two bonds with two other monomers.

Polystyrene

When a benzene ring (CHCH2CH2CH2CH2CH2) is attached to an ethene molecule (CH2=CH2), the molecule is vinyl benzene, commonly called styrene. An addition polymer of styrene is polystyrene. Now the entire benzene ring is the functional group left on every other carbon as the double bond breaks.

Condensation Polymers

When monomers without a double bond want to form long polymer chains, they achieve this through condensation polymerization. It is a reaction in which monomers are joined together by the formation of ester or amide bonds. A second smaller product, usually water, is produced by this reaction. For condensation polymerization to occur, each monomer must have two functional groups (usually one at each end of the molecule).

If a molecule has one functional group, it can react and link with one other molecule to form a dimer. To form a chain, a molecule must link to one other molecule at each end; that is, it needs to have two reactive functional groups, one at each end of the molecule. For example, carboxylic acids react with alcohols to form esters, and with amines to form amides; these are both condensation reactions. When monomers join end to end in ester or amide linkages, we get polyesters and polyamides. Many of these synthetic condensation polymers are widely used for fabrics and fibres.

Which statement describes condensation polymers? All of these:

Polyesters

When ester (RCOOR’) molecules are formed via esterification, the two reactant molecules are linked together into a single ester molecule. This condensation reaction can be repeated to form not just one ester molecule, but many esters joined in a long chain, a polyester. This is accomplished using a dicarboxylic acid (an acid with a carboxyl group at each end of the molecule), and a diol (an alcohol with a hydroxyl group at each end of the molecule). Ester linkages can then be formed end to end between alternating acid molecules and alcohol molecules.