CHEMISTRY A LEVEL(FORM SIX) NOTES – ORGANIC CHEMISTRY 1.1 -POLYMERS

ORGANIC CHEMISTRY 1.1 – POLYMERS

It is obtained from 2 words:

• Poly – many
• Merons – units

· Polymers are gigantic molecules which form chains of molecules.

· A polymer is a giant chain-like molecule obtained by intermolecular combination of a large number of small molecules of the same or different types known as monomers.

E.g: Nylon 6,6

· Monomers are small molecules (simple molecules having low molecular weight) which are capable of combining almost infinitely together to form polymers.

Structure of polymers

The primary structure of polymers is given by the types of monomers and their arrangement in molecules in the polymer.

If the polymer is made up of one type of monomer, it is known as a Homopolymer or simple polymer.

E.g:

• i.
• ii. PVC i.e

If the polymer is made up of two or more different types of monomers, it is known as a copolymer.

There are 3 types of copolymer depending on the order of polymer units.

TYPES OF POLYMERS

• Synthetic Polymers.
• Natural / Synthetic rubber.
• Vulcanisation.

Examples of copolymer include:

• (a) Nylon 6,6
• (b) Polyester

Polymer can be classified into 2 classes on the basis of their structures:

• (a) Linear polymer
• (b) Branched chain polymer

Linear chain polymer

Branched chain polymer

Properties:

• Linear chain polymers have higher melting and boiling points than branched polymers.
• Reason: Van der Waals forces are stronger in the linear chain polymer.
• Linear polymers have higher density.
• Linear polymers also have higher tensile strength.

i) Classification based on molecular forces

There are 2 major classes:

• 1. Thermosetting polymer
• 2. Thermoplastic polymer

THERMOSETTING POLYMER

• These are polymers which cannot be moulded on heating.
• When heated they become hard.
• Reason: On heating they undergo further reaction which increases cross-linked chains. They cannot be fused, i.e., infusible.
• E.g: Bakelite

THERMOPLASTIC POLYMER

• These are polymers which can be moulded on heating.
• They become soft when heated and become stiff when cooled.
• E.g: PVC
• They are linear structures with no cross linkage. Van der Waals forces exist between molecules.

ii) Classification based on the nature of polymer

There are 2 major classes:

• 1. Natural
• 2. Artificial or synthetic polymer

Natural polymers

Example:

• i. Cotton made up of cellulose, starch (made of glucose)
• ii. Bamboo stick
• iii. Paper
• iv. Proteins
• v. DNA, monomer nucleic acid

Artificial polymers

Example:

iii) Classification based on the mechanism of polymerization

There are 2 major classes of polymer:

• a. Addition polymer
• b. Condensation polymer

Addition polymerization

This is due to combination of unsaturated monomers (contain double or triple bonds).

Ethene polyethene

Addition polymerization proceeds via 3 possible mechanisms:

• 1. Free radical polymerization
• 2. Cationic polymerization
• 3. Anionic polymerization

Free radical polymerization

Free radical polymerization is one in which the reactions are catalyzed by free radicals obtained by organic peroxide (R–O–O–R’).

Step 01: Chain initiation step

Step 02: Chain propagation step

Step 03: Chain termination step

The reaction stops when two free radicals combine or when the radicals undergo disproportionation.

R + R → Polymer

Other polymers can be made from:

Cationic polymerization

In this polymerization, the initiator is an electrophile. The electrophile adds to the alkene causing it to become a cation.

Example of initiators:

• Electrophiles H⁺, NO₂^–
• Lewis acid e.g. , , H⁺

Mechanism

1. Chain initiation step

Here we use a strong base to remove the hydrogen.

2. Chain propagation step

3. Chain termination step

Anionic polymerization

Mechanism:

Initiated by nucleophile that reacts with alkenes to form anionic propagating site.

Example of nucleophiles:

• NaNH₂
• Butyl lithium (BuLi)
• R–O^– (Alkoxide)
• Grignard reagent

This reaction occurs if the carbon is stabilized by a suitable electron-withdrawing group.

Uses of polymers:

• Used in making pipes, bags, containers, and unbreakable bottles.

Polyethene

Used in making pipes, containers, bottles, electrical insulation.

Styrene

Uses:

• Household goods
• Electrical insulation
• Optical lenses

Polychloroethane

Typical chain section

Uses:

• Cable insulation
• Pipes
• Hoses
• Fabrics
• Flooring

Polytetrafluoroethane

Uses:

• Surface coating of fans (non-stick)
• Pipes
• Apparatus for chemical plants

Polypropanonitrile

Monomer:

Uses:

• Textiles

Polyethanylethanoate (vinyl acetate)

Uses:

• Paints
• Adhesives
• Water repellent coating

Perspex (polymethyl-2-methyl(propanoate))

Uses:

• Optical lenses
• Glasses
• Used as glue and paint

CONDENSATION POLYMERIZATION

It takes place through condensation reaction between two bifunctional or trifunctional monomers with simultaneous loss of small molecules such as H₂O, HCl, NH₃, etc.

Example:

• i. Polyesters
• ii. Polyamides or polypeptides
• iii. Terylene

It is made up of two monomers.

Polyester e.g. Terylene.

Polyamides e.g. Nylon 6,6 (polyamide)

It is made up of two monomers.