13.2 Classification of Polymers | Polymers | Chemistry 12

Polymers can be classified on several bases: mode of synthesis, composition of monomers, thermal behaviour, and origin/source.

1. Classification Based on Mode of Synthesis

Addition Polymers

Formed when monomers containing C=C double bonds join together repeatedly without the loss of any small molecule. The monomer's molecular formula equals the repeating unit's formula.

General reaction: $n C H_{2} = C H_{2} catalyst [- C H_{2} - C H_{2} -]_{n}$

Polymer	Monomer	Repeating Unit
Polyethene	$C H_{2} = C H_{2}$	$- C H_{2} - C H_{2} -$
Polypropene	$C H_{3} C H = C H_{2}$	$- C H (C H_{3}) - C H_{2} -$
PVC	$C H_{2} = C H C l$	$- C H_{2} - C H C l -$
Polystyrene	$C_{6} H_{5} C H = C H_{2}$	$- C H_{2} - C H (C_{6} H_{5}) -$
Teflon (PTFE)	$C F_{2} = C F_{2}$	$- C F_{2} - C F_{2} -$

Condensation Polymers

Formed when monomers with two functional groups (e.g., $- C O O H$ , $- N H_{2}$ , $- O H$ ) react repeatedly with the elimination of a small molecule (usually $H_{2} O$ or $H C l$ ).

Polymer	Monomers	Eliminated molecule	Linkage
Nylon-6,6	Adipic acid + Hexamethylenediamine	$H_{2} O$	Amide ( $- C O - N H -$ )
Terylene (Dacron)	Terephthalic acid + Ethylene glycol	$H_{2} O$	Ester ( $- C O - O -$ )
Bakelite	Phenol + Formaldehyde	$H_{2} O$	C–C cross-links

Nylon-6,6 formation: $H O O C (C H_{2})_{4} C O O H + H_{2} N (C H_{2})_{6} N H_{2} \to [- C O (C H_{2})_{4} C O - N H (C H_{2})_{6} N H -]_{n} + n H_{2} O$

2. Classification Based on Composition of Monomers

Homopolymers

Made from only one type of monomer.

Example: PVC (from vinyl chloride), Polyethene (from ethene)

Copolymers

Made from two or more different monomers.

Example: SBR — Styrene-Butadiene Rubber (from styrene + 1,3-butadiene)
Example: Nylon-6,6 (from adipic acid + hexamethylenediamine)

3. Classification Based on Thermal Behaviour

Thermoplastics

Consist of linear or slightly branched chains held by weak intermolecular forces.
Soften on heating and can be remoulded repeatedly.
Example: Polyethene, PVC, Polystyrene

Thermosetting Plastics

Contain extensive cross-linking forming a rigid 3D network.
Do not soften on reheating — permanently set once formed.
Example: Bakelite, Melamine

4. Classification Based on Origin/Source

Type	Description	Examples
Natural	Occur in nature	Cellulose, Proteins, Natural Rubber, Starch
Synthetic	Man-made in laboratory/industry	Nylon, Terylene, PVC, Bakelite
Semi-synthetic	Natural polymers chemically modified	Rayon (cellulose acetate), Vulcanised rubber

5. Identifying Monomers from a Polymer Section

To deduce the repeating unit from a polymer:

Identify the smallest unit that repeats along the chain.
For addition polymers: the repeating unit = monomer (with double bond opened).
For condensation polymers: add back the eliminated molecule ( $H_{2} O$ ) to the repeating unit to recover the monomers.

Example: Given the repeating unit $[- N H (C H_{2})_{6} N H - C O (C H_{2})_{4} C O -]_{n}$ , the monomers are:

$H_{2} N (C H_{2})_{6} N H_{2}$ (hexamethylenediamine)
$H O O C (C H_{2})_{4} C O O H$ (adipic acid)

Polymers can be classified on several bases: mode of synthesis, composition of monomers, thermal behaviour, and origin/source.

1. Classification Based on Mode of Synthesis

Addition Polymers

Formed when monomers containing C=C double bonds join together repeatedly without the loss of any small molecule. The monomer's molecular formula equals the repeating unit's formula.

General reaction: $n C H_{2} = C H_{2} catalyst [- C H_{2} - C H_{2} -]_{n}$

Polymer	Monomer	Repeating Unit
Polyethene	$C H_{2} = C H_{2}$	$- C H_{2} - C H_{2} -$
Polypropene	$C H_{3} C H = C H_{2}$	$- C H (C H_{3}) - C H_{2} -$
PVC	$C H_{2} = C H C l$	$- C H_{2} - C H C l -$
Polystyrene	$C_{6} H_{5} C H = C H_{2}$	$- C H_{2} - C H (C_{6} H_{5}) -$
Teflon (PTFE)	$C F_{2} = C F_{2}$	$- C F_{2} - C F_{2} -$

Condensation Polymers

Formed when monomers with two functional groups (e.g., $- C O O H$ , $- N H_{2}$ , $- O H$ ) react repeatedly with the elimination of a small molecule (usually $H_{2} O$ or $H C l$ ).

Polymer	Monomers	Eliminated molecule	Linkage
Nylon-6,6	Adipic acid + Hexamethylenediamine	$H_{2} O$	Amide ( $- C O - N H -$ )
Terylene (Dacron)	Terephthalic acid + Ethylene glycol	$H_{2} O$	Ester ( $- C O - O -$ )
Bakelite	Phenol + Formaldehyde	$H_{2} O$	C–C cross-links

Nylon-6,6 formation: $H O O C (C H_{2})_{4} C O O H + H_{2} N (C H_{2})_{6} N H_{2} \to [- C O (C H_{2})_{4} C O - N H (C H_{2})_{6} N H -]_{n} + n H_{2} O$

2. Classification Based on Composition of Monomers

Homopolymers

Made from only one type of monomer.

Example: PVC (from vinyl chloride), Polyethene (from ethene)

Copolymers

Made from two or more different monomers.

Example: SBR — Styrene-Butadiene Rubber (from styrene + 1,3-butadiene)
Example: Nylon-6,6 (from adipic acid + hexamethylenediamine)

3. Classification Based on Thermal Behaviour

Thermoplastics

Consist of linear or slightly branched chains held by weak intermolecular forces.
Soften on heating and can be remoulded repeatedly.
Example: Polyethene, PVC, Polystyrene

Thermosetting Plastics

Contain extensive cross-linking forming a rigid 3D network.
Do not soften on reheating — permanently set once formed.
Example: Bakelite, Melamine

4. Classification Based on Origin/Source

Type	Description	Examples
Natural	Occur in nature	Cellulose, Proteins, Natural Rubber, Starch
Synthetic	Man-made in laboratory/industry	Nylon, Terylene, PVC, Bakelite
Semi-synthetic	Natural polymers chemically modified	Rayon (cellulose acetate), Vulcanised rubber

5. Identifying Monomers from a Polymer Section

To deduce the repeating unit from a polymer:

Identify the smallest unit that repeats along the chain.
For addition polymers: the repeating unit = monomer (with double bond opened).
For condensation polymers: add back the eliminated molecule ( $H_{2} O$ ) to the repeating unit to recover the monomers.

Example: Given the repeating unit $[- N H (C H_{2})_{6} N H - C O (C H_{2})_{4} C O -]_{n}$ , the monomers are:

$H_{2} N (C H_{2})_{6} N H_{2}$ (hexamethylenediamine)
$H O O C (C H_{2})_{4} C O O H$ (adipic acid)