6.2.12 - Addition Polymers from Alkenes

• addition polymerisation

  addition reaction where monomers are joined together as double bonds open up to form long chains called polymers
• how is low density polyethene made?

  heating at high pressure and temperature with initiators

  O-O bond breaks homolytically forming free radicals then polymerisation happens at a series of propagation steps

  addition polymerisation
• structure of low-density polyethene

  branched due to complex collisions between radicals so prevents molecules from packing together too closely hence the low density
• properties of low density polyethene

  low melting point

  highly flexible and ductile

  soft

  lightweight

  good electrical insulator

  high chemical resistance
• manufacture of high density polyethene

  ethene at low temperature and pressure with a catalyst (Ziegler-Natta catalyst or other metal compounds)

  addition polymerisation
• structure of high density polyethene

  extra long chains with little branching so chains pack closer hence high density
• properties of high density polyethene

  high melting point

  flexible

  durable

  easy to process

  cheap

  good chemical resistance

  lightweight
• how can polymers be disposed of?

  buried in landfills

  reused after sorting into different types

  burning
• burying polymers in landfills

  only when plastic is hard to separate from other waste , not enough to make separation worthwhile or too difficult to recycle

  problem: amount of waste keeps increasing
• burning polymers

  used as heat to generate electricity

  can release toxic gases like HCl - to remove these gases are passed through scrubbers that neutralise toxic gases by reacting them with a base
• reusing polymers after sorting them into different types

  recycled by melting or remoulding them

  cracked into monomers to be used as organic feedstock to make more plastics or other compounds
• making polymers more sustainable

  use reactants that are safe and environmentally friendly

  use as few raw materials as possible

  use renewable raw materials

  energy use kept at a minimum using catalysts

  limit amount of waste products formed

  lifespan of polymer should be appropriate for its use
• biodegradable polymers

  polymers that naturally decompose - organisms digest them
• what can renewable polymers be made from?

  renewable raw materials like starch or from oil fractions like isoprene (a hydrocarbon)
• benefits of using renewable raw materials

  they won't run out

  when they biodegrade CO2 is released but if plant based this is carbon neutral

  plant based polymers save energy over their lifetime
• drawbacks of biodegradable polymers

  they need the right conditions to decompose so need to be separated from non-biodegradable plastics

  more expensive then non-biodegradable plastics
• properties of polyethene

  light

  flexible

  easily moulded

  transparent

  good insulator

  resistant to water, acids and alkalis
• uses of polyethene

  plastic bags and bottles

  beakers

  insulation for cables

  joint replacements
• properties of polypropene

  tough

  easily moulded

  transparent

  good insulator

  very resistant to water, acids and alkalis
• uses of polypropene

  fibre for ropes and carpets

  crates

  toys
• properties of polychloroethene (PVC)

  tough

  rigid or flexible

  very resistant to water, acids and alkalis
• uses of polychloroethene (PVC)

  guttering and window frames

  insulation for cables

  waterproof clothing

  flooring

  clingfilm