13.5 Polymerisation in Alkenes

13.5 Polymerisation in Alkenes

1. Polymer

   • Large molecules formed from many thousands of smaller, individual repeat units known as monomers

2. Unsaturated Alkenes go through ADDITION POLYMERISATION

   • produces long, saturated chains
   • Addition polymers have high molecular masses
   • synthetic polymers are usually named after the monomer, prefixed by 'poly'
   • Repeat unit is the specific arrangement of atoms in the polymer molecule

3. Enviromental Concerns

   • Most alkene-based polymers are non-biodegradeable!
   • Waste plastics can be BURIED
     • However the sheer amount of waste we produce is becoming a problem so land fill needs to be reduced
     • When they are difficult to separate from other plastics
     • When they are not in sufficient quantities to make separation worth while
     • When it is too technically difficult to recycle
   • Waste plastics can be REUSED
     • Many plastics are made from non-renewable oil-fractions = reuse as much as possible
     • After SORTING:
       • Some plastics (e.g polypropene) can be recycled by melting and remoulding them
       • Some plastics can be cracked back into their monomers and used as ORGANIC FEEDSTOCK to make more plastics and chemicals
         • Feedstock recycling is good as it is able to handle unsorted and unwashed polymers
   • Waste plastics can be BURNED
     • Heat can be used to generate electricity
     • Process needs to be carefully controlled to reduce TOXIC GASES.
       • For example, when a polymer that contains Cl is burned, it releases HCl, which has to be removed
         • Waste gases pass through scrubbers which can neutralise gases such as HCl by allowing them to react with a base

4. Biodegradable and photodegradable polymers

   • Biodegradable polymers decompose quickly in certain conditions as organisms can digest them
   • Can be made from renewable sources such as starch, cellulose, plant oils, proteins or oil fractions, such as from the hydrocarbon isoprene
     • However these are more expensive then their non-biodegradable equivalents
   • Require right conditions - compost heap
   • Need collecting and seperating from non-biodegradable polymers
   • Potential uses - e.g/ plastic sheeting used to protect plants from frost - contains starch grains embedded into it so that the starch is broken down by microorganisms and remaining polyethene crumbles to dust over time
   • Photodegradable polymers degrade when exposed to sunlight

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