Organic - Alkenes (Polymers) Flashcards
What are polymers?
Polymers are very large molecules that are built up from small molecules, called monomers. The process in which monomers join together is called polymerisation. There are two types of polymers: addition (no other products formed) and condensation (a small molecule such as water is lost). They occur naturally everywhere: starch, proteins, cellulose and DNA are all polymers.
One way of classifying polymers is by the type of reaction by which they are made.
What are addition polymers?
Addition polymers are made from a monomer or monomers with a carbon-carbon double bond (alkenes). The monomer has the general formula CH2CHR.
What happens when monomers polymerise?
When the monomers polymerise, the double bond opens and the monomers bond together to form a backbone of carbon atoms.
How do you identify the addition polymer formed from the monomer?
An addition polymer is formed from monomers with carbon-carbon double bonds.
There is usually only one monomer (though it is possible to have more), and the double bond opens to form a single bond. This will give the repeat unit for the polymer.
How do you identify the monomer used to make an addition polymer?
An addition polymer must have a backbone of carbon atoms and the monomer must contain at least two carbons, so that there can be a carbon-carbon double bond.
How are plastics modified?
The properties of polymers materials can be considerably modified by the use of additives such as plasticisers. These are small molecules that get between the polymer chains forcing them apart and allowing them to slide across each other.
For example, PVC is rigid enough for use as drainpipes, but with addition of plasticise, it becomes flexible enough for making aprons.
Why are polyalkenes chemically inert?
Addition polymers are polyalkenes. The molecules are saturated, as all of the bonds holding the carbon chain together are single. They contain multiple strong, non-polar covalent bonds. This makes them generally unreactive and chemically inert.
Why are polyalkenes not biodegradable?
Polyalkenes, in spite of their name, have a backbone which is a long chain saturated alkane molecule. Alkanes have strong non-polar C-C and C-H bonds. So, they are very unreactive molecules, which is a useful property in many ways. However, this does mean that they are not attacked by biological agents - like enzymes - and so they are not biodegradable. This is an increasing problem in today’s world, where waste disposal is becoming more and more difficult.
What is low density polythene?
Low density poly(ethene) is made by polymerising ethene at high pressure and high temperature via a free-radical mechanism. This produces a polymer with a certain amount of chain branching. This is a consequence of the rather random nature of free-radical reactions. The branched chains do not pack together particularly well and the product is quite flexible, stretches well and has a fairly low density. These properties make it suitable for packaging (plastic bags), sheeting and insulation for electrical cables.
What is high density polythene?
High density polythene is made at temperatures and pressures little greater than room conditions and uses a Ziegler-Natta catalyst, named after its developers. This results in a polymer with much less chain branching (around one branch for every 200 carbons on the main chain). The chains can pack together well. This makes the density of the plastic greater and its melting temperature higher. Typical uses are milk crates, buckets and bottles for which low density polythene would be insufficiently rigid.
What are plasticisers?
The properties of polymers can be tailored to make them suitable for a variety of application by the use of various additives. Plasticisers are small molecules that get in between the polymer chains, pushing them apart and making them more flexible. This reduces the strength of the intermolecular forces, making it easier for the chains to slide over each other. This is how poly(chlorethene) also called polyvinyl chloride (PVC) or just vinyl can be made rigid enough for use as drainpipes and flexible enough for plastic aprons.
What are the properties of addition polymers?
- Polymers are saturated (single bonds between carbon atoms in the main chain).
- The carbon chain back bone is usually non-polar.
- This makes them unreactive (chemically inert), so they are very hard to break down.
- Strong, covalent bonds within the polymer molecules, weak intermolecular forces (usually van der Waals) between the polymer chains.
- The stronger the intermolecular forces, the higher the melting point.
- Most polymers contain a mixture of chains of various lengths so melt over a range of temperatures.
What are features of polythene?
- Has no branches (side chains).
- Polymer chains pack closely together, attracted by van der Waals forces.
- Makes a strong (high density polythene) and rigid material,
What are the features of PVC?
- Covalent bonds between carbon and chlorine are polar.
- Permanent dipole-dipole forces between polymer chains.
- PVC is hard but brittle (used to make drain pipes and window frames).
- It has waterproof properties. It gains these properties by the addition of plasticisers during the reaction.
What are the uses of polythene?
polymer name: poly(ethene)
uses: carrier bags, bottles, washing up bowls
