Ch 23 Addition reaction

• Plastics are polymers which are made up of very large molecules

• They are replacing traditional materials such as metal, wood, paper and glass

Properties of plastic
• Usually strong but light
• Usually good insulators of heat and electricity
• Usually flexible but some are very rigid
• Con be molded easily into any shape
• Transparent but can be easily dyed
• Chemical inert, no reaction with water, acids, alkalis and most other chemicals since saturated hydrocarbons are unreactive
Thermoplastic Thermosetting plastics
Apply heat Can be melted or soften by heat at relatively low T Don’t melt or soften on heating, high thermal stability
Reactivity Unreactive Unreactive
Physical Flammable, flexible or hard and don’t conduct Non-flammable, hard, resistant to deformation, don’t
properties electricity conduct electricity
Examples Addition polymers and condensation polymers

Polymers
• A polymer is a compound which consist of very large molecules formed by joining many small monomer molecules together
repeatedly
• Polymerization is the process of repeatedly joining many small molecules together to form very large molecules under high
temperatures, high pressure and with catalyst
• The compounds whose molecules join together repeatedly in polymerization are called monomers
• Monomer molecules can be obtained and separated by cracking of petroleum followed by fractional distillation
• Polymer molecules have no fixed size. Each polymer chain has macromolecular structure, which consists of at least several hundred
monomer molecules
• All plastic are polymers but not all polymers are plastics

• Polymers can be natural or man-made

• Plastics are man made polymers are made by modifying natural polymers
• Synthetic polymers are made artificially from the corresponding monomers
Addition polymerization and addition polymers
• It is a reaction in which monomer molecules (small unsaturated compounds containing C=C or C≡C, except benzene compound) join
together repeatedly to form polymer molecules with out the elimination of small molecules (H2O HCl) under high T, high pressure
and with catalyst
• Polymers that are made by addition polymerization are called addition polymers
• In the reaction, the monomer molecules don’t lose atoms, and the polymer is the only product
• The monomer molecules most contain C=C double bonds and they undergo repeated addition reaction among themselves to form
an addition polymer
• All addition polymers are thermoplastics, which can be melted or softened by heat of relatively low T
• Addition polymers are no longer alkenes
• They can represent by the general formula:

• By opening the C=C double bond, a monomer molecule will have 2 free electrons which can join to other molecules

• By repeating this addition process many times, long polymer chains form and each polymer chain is a macromolecule
• Each consist of at least several hundred monomer molecules joined together

• A Repeating unit is the smallest part of a polymer molecule, which repeats to give the whole polymer structure e.g.
• The general equation for addition polymerization is:
Polyethene (Polythene) (PE)

• The monomers ethene polymerizes to a long chain molecule of polyethene

• Is the repeating unit and n is the number of units present in the polyethene chain

1. Low -density polyethene (LDPE)

Properties Uses
• It has many branched chains, which packed irregularly • It is mainly used to make plastic bags, wrapping film
and so the van der Waals’ forces between molecules for food and flexible cold-water pipes
are weaker
• It has a lower MP of 105, flexible and softens in
boiling water

2. High-density polyethene (HDPE)

Properties Uses
• It is nearly no branched chains, which packed more • It is mainly used to make bottles for certain chemicals,
regularly and so the can der Waals’ forces between buckets and boxes
molecules is stronger
• It has a higher MP of 135 and hard

Q: Why don’t polyethene react with Con. HCl and Acidified potassium permanganate solution?
A: This is because polythene is a saturated hydrocarbon which is unreactive. It can resist attacks of acid and oxidizing agents
Q: Is polyethene combustible in air?
A: Like all other hydrocarbons, burning of polyethene produces CO2 and H2O. CO and carbon particles (soot) mat also be produced during
combustion of polyethene. This is because hydrocarbons with long chain length tend to burn less completely in air

Polypropene (Polypropylene) (PP)

• The monomers propene polymerizes to a long chain molecule of polypropene
 Properties Uses
• It has a higher MP of 165 • It is mainly used to make ropes, fibers, baskets, luggage
• Its density falls between low-density polyethene and high- cases, food containers and packaging film
density polyethene
• It is stronger than polyethene
• It is resistant to many chemical solvents, acids and alkalis

Polyvinyl chloride (PVC)

• The monomers chloroethene polymerizes to a long chain molecule of polyvinyl chloride

Properties Uses
PVC • It is stiff and brittle • Making floor tiles and pipes
PVC with plasticizers • Softer and more flexible • Make shower curtains, tablecloths,
(When mixed with plasticizer) • Water resistant raincoats and artificial leather
• Good insulator of electricity • Make insulating coverings for electrical
wires
• PVC isn’t used to make food containers because it is poisonous

Polystyrene (PS)
• The monomers styrene polymerizes to a long chain molecule of polyvinyl chloride
 Properties Uses
Polystyrene • Transparent • Sample bottle, cups, measuring cups
• Hard but brittle and CD cases
Expanded polystyrene • Very low density • Packaging material
(By heating granular polystyrene with a • Quite rigid • Lightweight ceiling tiles in buildings
foaming agent) • Excellent heat insulator • Disposable foam cups and food boxes
(it is a while solid foam) • Good shock absorbent
• Non-toxic

Perspex (polymethyl methacrylate or acrylic)

Properties Uses
• Hard, tough and rigid • Glass substitute (Aeroplane windows, advertising sign
• Highly transparent boxes, safety spectacles, illuminated signs and plastic
• Not easily scratched optical fibers)
Preparing polystyrene in the lab
1) Put equal volume of styrene and kerosene in a flask. The kerosene is used as a solvent of the
styrene and as a catalyst.
2) To prevent the loss of the reaction mixture, heat the mixture under reflux at about 100 C for 60
minutes.
3) After cooling, pour the reaction mixture into a beaker of 100 cm3 of methanol.
4) A white wax-like solid polystyrene forms on the surface of the methanol which is insoluble in
methanol.

Uses of some common addition polymers

• Most plastics are blended with additives.
• The additives can make plastics easier to process and modify their properties.
• Some common additives include :
Antioxidants To prevent reaction with oxygen
Pigments To give colour
Stabilizers To inhibit undesirable degradation in use and prevent colors from
fading
Plasticizers To make the plastics softer and easier to mould
Blowing agents To produce foamed materials
Fillers To make the products stronger and opaque