Polymerisation of alkenes

• Polymers are very large molecules made when hundreds of monomers join together to form long chains.

  The word 'polymer' comes from the Greek words poly (meaning many) and meros (meaning parts). Plastics are synthetic polymers that can be shaped by heat or pressure.
• What keeps the chain together?

  The monomers in a polymer are joned together by covalent bonds between atoms. In a covalent bond, each atom shares one or more electron with another atom. The bonds are sometimes shows as sticks.
• Polyethene (sometimes called polythene) is a polymer made from ethene. The process by which polyethene and other polymers is made is called polymerization (additional polymerization). This is because many monomers (ethene molecules) are added together.
• Plastics are polymers. Polymers are formed when lots of smaller molecules chemically react together to form a longer molecule.

  Nylon is formed in a 'condensation polymerisation' reaction between 1,6-diaminohexane (has basic groups at each end) and decanedioyl dichloride (has acidic groups at each end). These chemicals react together.

  Nylon has very different properties to polymers made from hydrocarbon monomers such as ethene.
• Polyalkenes


  Alkenes can undergo addition reactions with themselves to form a long chain polymer molecules. This reaction is called addition polymerization.

  The polymer can be represented by showing the repeating unit with square brackets around it. The n stands for an unspecified number of monomer units.
• Other polyalkenes


  Propene undergoes addition polymerization to form polypropene. Chloroethene undergoes addition polymerization to form polychloroethene.
• The properties of a polymer depend on:

  1. The monomer used to make it
  2. The conditions we choose to carry out the reaction
  3. Additives
• What factors might determine the properties of a plastic?

  • reaction conditions: temperature, pressure and catalysts affect the length and branching of the polymer chain
  • monomer: the type of monomer used affects the type of forces between polymer chains
  • additives: additives can 'lubricate' polymer chains, join them together with cross-links, or preserve them from decay.
• LDPE and HDPE

  The reaction conditions under which ethene polymerizes affect the structure and properties of the poly(e)thene.

  • Low-density polyethene (LDPE) is formed under a high pressure (1400atm) and a temperature of about 170 degress C. These conditions cause a high level of branching, meaning that the polymer chains cannot pack tightly together.
  • High-density polyethene (HDPE) is formed with a catalyst, a pressure of 2 atm and a temperature of about 70 degress C. Little branching occures under these conditions, resulting in chains that can pack tightly together to create a denser material.
• More about LDPE


  LDPE is a soft, flexible and stretchy plastic, with a melting point of about 120 degrees C. It is used to make:

  • plastic bags
  • squeezable bottles, and general purpose containers and trays.
  • other items that need to be soft and flexible, such as tubing.

  LDPE has the recycling symbol '4'.
• More about HDPE


  HDPE is a tough and flexible plastic, with a melting point of about 130 degrees C. It is used to make:

  • containers such as milk and detergent bottles
  • rigid items such as folding tables, chairs and pipes.

  HDPE has the recycling symbol '2'.
• HDPE is a linear molecule, LDPE is a branched molecule.
• More about polypropene

  Polypropene is a tough and flexible plastic, with a melting point of about 160 degrees C. It's used to make:

  • ropes, carpets, rugs and other textiles
  • medical, laboratory and kitchen items that need to withstand temperatures in autoclaves and dishwashers
  • certain bottles, buckets, containers and other items such as bottle tops and moulded fittings.

  Polypropene has the recycling symbol '3'.
• Addition polymerisation - Preparation

  Many are prepared by a free radical process involving high pressure, high temperature and a catalyst. The catalyst is usually a substance (e.g. an organic peroxide) which readily breaks up to form radicals which initiate a chain reaction. Another famous type of a catalyst is a Ziegler-Natta catalyst (named after the scientists who developed it). Such catalysts are based on the compound TiCl4.