Polymers — Study Notes

1. Introduction

Polymers are high molecular mass compounds formed by the combination of many small repeating units called monomers. The word polymer comes from the Greek roots poly (many) and meros (units). Polymers are everywhere in modern life: plastics, fibres, rubber, adhesives, and biological macromolecules such as proteins and cellulose are all polymers. The chapter introduces the classification of polymers, the mechanisms of their formation, and their important applications.

Polymers are classified in several ways. On the basis of their source they are natural (proteins, cellulose, natural rubber), synthetic (nylon, PVC, Teflon), or semi-synthetic (rayon, cellulose nitrate). On the basis of their structure, they are linear, branched, or cross-linked polymers. On the basis of their mode of polymerisation, they are addition or condensation polymers, and on the basis of their response to heat, they are thermoplastics or thermosetting polymers.

The chapter deals with the two major mechanisms of polymerisation: addition polymerisation, which proceeds through free radicals, cations, or anions, and condensation polymerisation, which involves the elimination of small molecules. Important polymers such as polyethylene, PVC, Teflon, nylon-6,6, polyester, and the rubbers are described in detail, along with their monomers and uses. Understanding how polymers are made and why their properties differ allows chemists to design materials for specific applications.

2. Classification of Polymers

Based on Source

• Natural polymers: Found in nature, e.g., proteins, starch, cellulose, natural rubber, silk, and wool.
• Synthetic polymers: Man-made, e.g., nylon, polythene, PVC, Teflon, and polyester.
• Semi-synthetic polymers: Made by chemically modifying natural polymers, e.g., cellulose nitrate (rayon), cellulose acetate.

Based on Structure

• Linear polymers: Monomers join in a linear chain, e.g., polythene, nylon, PVC.
• Branched chain polymers: Linear chains with branches, e.g., low-density polythene.
• Cross-linked polymers: Chains joined by cross-links, e.g., bakelite, melamine.

Based on Mode of Polymerisation

• Addition polymers: Formed by repeated addition of monomers without elimination of any by-product; monomers contain a double bond.
• Condensation polymers: Formed by repeated condensation reactions with elimination of small molecules like water, ammonia, or HCl; monomers contain two functional groups.

Based on Thermal Response

• Thermoplastics: Soften on heating and can be remoulded, e.g., polythene, PVC, nylon.
• Thermosetting polymers: Harden irreversibly on heating and cannot be remoulded, e.g., bakelite, urea-formaldehyde resin.

3. Polymerisation Mechanisms

Addition Polymerisation

Addition polymerisation involves the linking of monomers containing double bonds. It is initiated by free radicals, cations, or anions. The free radical mechanism has three steps:

1. Initiation: A free radical is generated from an initiator such as benzoyl peroxide: $$(C_6H_5COO)_2 \rightarrow 2C_6H_5COO^\bullet \rightarrow 2C_6H_5^\bullet + 2CO_2$$
2. Propagation: The free radical adds to the alkene monomer, generating a new radical that continues the chain.
3. Termination: Two growing chains combine or disproportionate to stop the reaction.

Cationic and anionic polymerisations use acid or base initiators respectively and are favoured for monomers with electron-donating or electron-withdrawing groups.

Condensation Polymerisation

Condensation polymerisation requires monomers with two or more functional groups. The polymer forms with the elimination of small molecules. Examples include the formation of nylon-6,6 from hexamethylenediamine and adipic acid, and the formation of polyester (terephthalic acid and ethylene glycol). The molecular mass of condensation polymers is generally lower than that of addition polymers.

Copolymerisation

Copolymerisation involves two different monomers; the resulting polymer contains both monomers in the chain. For example, butadiene and styrene copolymerise to form Buna-S synthetic rubber.

4. Important Polymers and Their Uses

Addition Polymers

• Polythene (polyethylene): Made by the polymerisation of ethylene. Low-density polythene (LDPE) is used for carry bags and squeeze bottles; high-density polythene (HDPE) is used for pipes and containers. Ziegler-Natta catalysts give high-density linear polyethylene.
• PVC (polyvinyl chloride): Made from vinyl chloride; used for pipes, raincoats, and floor coverings.
• Teflon (PTFE): Made from tetrafluoroethene; used for non-stick cookware and as a lubricant.
• Polystyrene: Made from styrene; used for insulation and packaging.
• PAN (polyacrylonitrile): Made from acrylonitrile; used as the precursor for acrylic fibres and carbon fibres.

Condensation Polymers

• Nylon-6,6: From hexamethylenediamine and adipic acid; used for textiles, ropes, and tyre cords.
• Nylon-6: From caprolactam; used for fabrics and gears.
• Terylene (Dacron): Polyester from terephthalic acid and ethylene glycol; used for clothing and packaging films.
• Bakelite: From phenol and formaldehyde; a thermosetting polymer used for electrical insulators and switchboards.
• Urea-formaldehyde resin: Used for adhesives and moulded articles.

Rubber

• Natural rubber: A polymer of isoprene (2-methyl-1,3-butadiene). Natural rubber is elastic but soft and sticky, and it is vulcanised by heating with sulphur, which forms cross-links and improves strength and elasticity.
• Synthetic rubbers: Include Buna-S (butadiene + styrene), Buna-N (butadiene + acrylonitrile), and neoprene (chloroprene).

5. Molecular Mass and Degree of Polymerisation

The degree of polymerisation is the number of monomer units in a polymer chain. The molecular mass of a polymer is an average, expressed either as number average molecular mass or weight average molecular mass. Polymers are polydisperse, meaning they contain chains of different lengths. The polydispersity index (PDI) is the ratio of the weight average to the number average molecular mass.

Important Concepts

• Number average molecular mass: Based on the number fraction of each chain.
• Weight average molecular mass: Based on the weight fraction of each chain.
• The degree of polymerisation, n, relates the polymer molecular mass to the monomer molecular mass.

Quick Revision Tables

Table 1: Polymer Classification

Table 2: Monomers of Common Polymers

Mind Map

Important Diagrams (SVG)

Diagram 1: Addition versus Condensation Polymerisation

Diagram 2: Vulcanisation of Natural Rubber

Common Mistakes

1. Believing that thermosetting polymers can be remoulded; they set irreversibly on heating and cannot be reshaped.
2. Confusing nylon-6 (from caprolactam) with nylon-6,6 (from hexamethylenediamine and adipic acid).
3. Writing condensation polymerisation without the elimination of a small molecule; the defining feature is the loss of H2O, NH3, or HCl.
4. Forgetting that natural rubber is a polymer of isoprene and that vulcanisation adds sulphur cross-links.
5. Believing that all plastics are thermoplastics; bakelite and melamine are thermosetting.
6. Mixing up the monomers of Terylene (terephthalic acid and ethylene glycol) with those of nylon.
7. Stating that addition polymerisation requires two functional groups; only condensation polymerisation requires bifunctional monomers.

Exam Tips

1. Memorise the monomer pairs for all common polymers; this is the most frequently asked direct question.
2. Learn the three categories of classification and be able to place a polymer in each category (e.g., nylon is synthetic, condensation, and thermoplastic).
3. Remember that natural rubber is cis-polyisoprene and vulcanisation introduces sulphur cross-links.
4. For addition polymerisation, know the free radical mechanism steps: initiation, propagation, and termination.
5. Associate the Ziegler-Natta catalyst with high-density polyethylene.
6. Know the distinguishing feature that condensation polymers have lower molecular masses than addition polymers.

Conclusion

Polymers illustrate how simple monomers can be assembled into materials with extraordinary diversity and utility. The chapter provides a systematic classification based on source, structure, polymerisation mechanism, and thermal behaviour, enabling chemists to predict and control polymer properties. The two major polymerisation mechanisms, addition and condensation, account for the formation of almost all synthetic polymers, from the ubiquitous polyethylene and PVC to the strong engineering polymers nylon and Terylene. Rubber chemistry, including vulcanisation, demonstrates how cross-linking transforms a weak natural material into a durable industrial one. Beyond synthetic materials, the same principles explain the structure of proteins, cellulose, and other biological polymers. This chapter connects molecular design with material applications, an essential theme in modern chemistry, materials science, and engineering.

Test Your Understanding

1. Define polymer, monomer, and degree of polymerisation.
2. Distinguish between addition and condensation polymers with examples.
3. Write the monomers of nylon-6,6, Terylene, and bakelite.
4. What is vulcanisation and why is it important for natural rubber?
5. Distinguish between thermoplastics and thermosetting polymers with examples.