Polymerization is one of the most important concepts in IB Chemistry Topic 10 (Organic Chemistry) and Topic 20 (HL). It explains how small molecules (monomers) join together to form giant molecules (polymers) with unique physical and chemical properties. From plastics to proteins to DNA, polymerization is everywhere. Understanding how polymerization works is essential for analyzing materials, predicting products, and interpreting organic reactions on exams.
What Is Polymerization?
Polymerization is the chemical process in which small molecules called monomers join together to form long chains called polymers.
Key characteristics:
- Monomers are repeat units.
- Polymers are large molecules made of hundreds or thousands of monomers.
- Polymers have different properties from the monomers that form them.
Polymers may be natural (like proteins or cellulose) or synthetic (like nylon or polyethylene).
Types of Polymerization
IB Chemistry requires you to know two main types of polymerization:
- Addition Polymerization
- Condensation Polymerization
These differ in mechanism, monomers, and products.
1. Addition Polymerization
Addition polymerization occurs when alkene monomers join together by breaking their C=C double bonds.
Key features:
- Only one type of monomer is needed.
- Double bond opens to form single bonds.
- No by-products are formed.
This process produces many common plastics.
Example: Polyethene (polyethylene)
Monomer: ethene (CH₂=CH₂)
The double bond opens and monomers link:
CH₂=CH₂ → –(CH₂–CH₂)–ₙ
Polyethene is used in plastic bags and bottles.
Example: Polypropene
Monomer: propene
Forms a polymer with repeating CH₂–CH(CH₃) units.
Why addition polymerization occurs
Alkenes have a π bond that is:
- Weak
- Electron-rich
- Reactive
It readily breaks and links monomers together.
2. Condensation Polymerization
Condensation polymerization involves two different monomers, each containing functional groups that react together.
Key features:
- Monomers have two functional groups each.
- Polymers form by eliminating a small molecule, often water or HCl.
- Produces polyesters and polyamides.
This type is common in biological molecules.
Example: Polyester Formation
A typical reaction involves:
- A diol (two –OH groups)
- A dicarboxylic acid (two –COOH groups)
They react to form an ester linkage and release water:
–COOH + HO– → –COO– + H₂O
The polymer grows as these linkages continue forming.
Example polymer: PET (polyethylene terephthalate)
Used in water bottles and fabrics.
Example: Polyamide Formation (Nylon)
A diamine reacts with a dicarboxylic acid:
–COOH + H₂N– → –CONH– + H₂O
This forms amide linkages (peptide bonds).
Real-world example:
- Nylon-6,6
- Strong, durable, used in ropes and textiles
Condensation polymerization mimics biological systems—proteins form through the same type of bond formation.
Properties of Polymers
Polymers have unique properties due to their long chain length:
- High melting points
- High tensile strength
- Flexibility or rigidity depending on chain structure
- Resistance to chemicals
- Ability to form fibers and films
Small changes in monomer structure lead to major differences in polymer behavior.
Polymerization in the Real World
Polymers appear in:
- Plastics
- Clothing (polyester, nylon)
- DNA and proteins
- Rubber
- Adhesives
- Medical materials
Polymer chemistry is essential for materials science and biotechnology.
Common IB Misunderstandings
“Addition polymers produce water as a by-product.”
False. Only condensation polymers form water or small molecules.
“All polymers need two different monomers.”
Only condensation polymerization does—addition uses one monomer type.
“Alkanes can polymerize.”
No. Only alkenes (with double bonds) undergo addition polymerization.
“Condensation polymers are always synthetic.”
Incorrect. Proteins, cellulose, and starch are natural condensation polymers.
