Polymerisation in alkenes

• Unsaturated alkene molecules undergo addition polymeration to produce long saturated chains
• The properties of a polymer depends on the monomer used
• Industrial polymerisation is carried out at high temperature and pressure using catalysts
• Addition polymers have high molecular masses
• synthetic polymers are usually named after the monomer that reacts to form their giant molecules
• A repeat unit is the specific arrangement of atoms in the polymer molecule that repeats over and over again
• The repeat unit is always written in square brackets. After the bracket you place a letter n to show that there is a large number of repeats
• Poly(ethene) is one of the most commonly used polymers:

  supermarket bags

  shampoo bottles

  children's toys
• High density poly(ethene) (HDPE) has linear chains giving the plastic some strength. children's toys, detergent bottles, water pipes
• Low density poly(ethene) (LDPE) has branched chains resulting in little strength but is flexible. Plastic films and plastic bags
• Poly(chloroethene) aka poly(vinyl chloride) or PVC can be prepared to make a polymer that is flexible or rigid
• The lack of reactivity that makes polymers suitable for storing food and chemicals safely also presents chemists with a challenge in their disposal
• Many alkene-based polymers are non-biodegradeable. The growing amount of polymer waset has serious environmental effects e.g. killing marine life
• Recycling polymers reduces their environmental impact by conserving finite fossil fuels as well as reducing the amount of waste going to landfill.

  Discarded polymers have to be sorted by type. The recycling process is undermined if polymers are mixed as this renders the product unusable.

  Once sorted, the polymers are chopped into flakes, washed, dried, and melted. The recycled polymer is cut into pellets and used by manufacturers to make new products
• The disposal and recycling of PVC is hazardous due to the high chlorine content and the range of additives present in the polymer.

  Dumping PVC in landfill is not sustainable

  When burnt, PVC releases hydrogen chloride, a corrosive gas, and other pollutants like toxic dioxins

  New technology uses solvents to dissolve the polymer. High grade PVC is then recovered by precipitation from the solvent, and the solvent is used again
• As polymers are derived from petroleum or natural gas, they have a high stored energy value. Waste polymers can be incinerated to produce heat, generating steam to drive a turbine producing electricity.
• Feedstock recycling describes the chemical and thermal processes that reclaim monomers, gases, or oil from waste polymers. The products from feedstock resemble those produced from crude oil in refineries. These materials can be used as raw materials for the production of new polymers. A major advantage of feedstock recycling is that it is able to handle unsorted and unwashed polymers
• Biodegradable polymers are broken down by microorganisms into water, carbon dioxide, and biological compounds. These polymers are usually made from starch, cellulose, or contain additives that alter the structure of traditional polymers so that microorganisms can break them down
• compostable polymers degrade and leave no visible or toxic residues.
• Where the use of plant-based polymers is not possible, photodegradable oil-based polymers are being developed. These polymers contain bonds that are weakened by absorbing light to start the degradation