Polymers
Giant molecules from small repeating units — how monomers link into the plastics, fibres and rubbers that shape the modern world
- What a polymer, a monomer and the degree of polymerisation are.
- How polymers are classified by source, structure, molecular forces and mode of polymerisation.
- The mechanism of addition (chain-growth) polymerisation and how it differs from condensation (step-growth).
- A working directory of monomers and polymers — polythene, PVC, Teflon, nylon, Terylene, Bakelite, Buna-S and more.
- Why natural rubber is improved by vulcanisation.
- Number- and weight-average molecular mass, the PDI, and what makes a polymer biodegradable.
What Is a Polymer?
A polymer (Greek poly = many, meros = parts) is a very large molecule built by joining many small repeating units. Each small unit is a monomer, and the reaction that links them is polymerisation. The number of monomer units in a single chain is the degree of polymerisation (\(n\)); for useful materials \(n\) runs from hundreds to many thousands.
The mass of the polymer is essentially \(n\) times the mass of the repeating unit. Because a sample contains chains of different lengths, a polymer has no single sharp molecular mass — only an average, a point we return to in Section 31-8.
Classifying Polymers
The same word "polymer" covers silk and Styrofoam, so chemists classify them along four independent axes. A given polymer carries a label on each axis at once.
| Basis | Classes | Examples |
|---|---|---|
| Source | natural · semi-synthetic · synthetic | starch · cellulose acetate · nylon |
| Structure | linear · branched · cross-linked | HDPE · LDPE · Bakelite |
| Molecular forces | elastomers · fibres · thermoplastics · thermosetting | Buna-S · nylon · polythene · Bakelite |
| Mode of synthesis | addition (chain-growth) · condensation (step-growth) | polythene · nylon-6,6 |
Addition Polymerisation
In addition (chain-growth) polymerisation, monomers containing a \(\ce{C=C}\) double bond add to one another with no loss of any atom. The repeating unit therefore has the same empirical formula as the monomer. Most run by a free-radical chain mechanism in three stages.
Initiation: a peroxide initiator breaks into radicals, \(\ce{R-O-O-R -> 2 RO^.}\), which add to a monomer. Propagation: the new radical adds to another monomer, again and again, growing the chain. Termination: two growing radicals combine (or disproportionate), ending growth. Ethene → polythene, vinyl chloride → PVC, styrene → polystyrene all follow this route.
Condensation Polymerisation
In condensation (step-growth) polymerisation, monomers each bearing two reactive functional groups join with the loss of a small molecule — usually water. Two families dominate: polyamides (amide links, like the nylons) and polyesters (ester links, like Terylene).
Copolymers
A copolymer is built from two or more different monomers, in contrast to a homopolymer made from one. Mixing monomers lets chemists tune properties: Buna-S (styrene + butadiene) is a tougher synthetic rubber than either monomer alone would give, and Buna-N (acrylonitrile + butadiene) resists oils and solvents. Copolymerisation can proceed by either an addition or a condensation route.
Monomer–Polymer Directory
This is the single most examinable table in the chapter. Learn each monomer, its polymer, the synthesis type, and one use. Read it as a map of "what makes what."
| Polymer | Monomer(s) | Type | Use |
|---|---|---|---|
| Polythene | ethene | addition | bags, bottles, pipes |
| Polypropene | propene | addition | ropes, containers |
| PVC | vinyl chloride | addition | pipes, raincoats, insulation |
| Teflon (PTFE) | tetrafluoroethene | addition | non-stick coatings, gaskets |
| Polystyrene | styrene | addition | packaging, cups, insulation |
| Orlon / Acrilan (PAN) | acrylonitrile | addition | synthetic wool, fibres |
| Nylon-6,6 | hexamethylenediamine + adipic acid | condensation | textiles, ropes, tyre cord |
| Nylon-6 | caprolactam | condensation | tyre cord, fabrics |
| Terylene / Dacron (PET) | ethylene glycol + terephthalic acid | condensation | fibres, bottles, film |
| Bakelite | phenol + formaldehyde | condensation | switches, handles, plugs |
| Buna-S (SBR) | 1,3-butadiene + styrene | addition (copolymer) | tyres, soles |
| Buna-N | 1,3-butadiene + acrylonitrile | addition (copolymer) | oil-resistant rubber, hoses |
| Neoprene | chloroprene | addition | conveyor belts, gaskets |
Rubber & Vulcanisation
Natural rubber is cis-1,4-polyisoprene — a long, coiled addition polymer of isoprene (2-methyl-1,3-butadiene). The cis geometry keeps the chains from packing, giving the springy, elastic coil. Raw rubber is soft, sticky when warm and brittle when cold, so it is rarely used as found.
Vulcanisation cures these faults. Heating rubber with a few percent of sulphur forms sulphur cross-links between chains. These bridges stop the chains sliding past one another, making the rubber harder, stronger, more elastic over a wide temperature range, and no longer tacky.
Molecular Mass of Polymers
Because a polymer sample is a mixture of chains of different lengths, its molecular mass is an average, reported two ways.
| Average | Definition | Weighted by |
|---|---|---|
| Number average \( \overline{M}_n \) | \( \dfrac{\sum N_i M_i}{\sum N_i} \) | number of molecules |
| Weight average \( \overline{M}_w \) | \( \dfrac{\sum N_i M_i^{2}}{\sum N_i M_i} \) | mass of molecules |
The PDI measures how spread out the chain lengths are. A perfectly uniform polymer would have \( \overline{M}_w = \overline{M}_n \) and \( \text{PDI}=1 \); for real synthetic polymers \( \overline{M}_w > \overline{M}_n \), so \( \text{PDI}>1 \). Natural polymers tend toward 1; step-growth synthetics run higher.
Biodegradable Polymers
Most synthetic polymers resist microbial attack and persist as waste. Biodegradable polymers are designed to break down in the environment, usually because their backbones contain ester or amide links that micro-organisms and water can hydrolyse.
| Polymer | Built from | Note |
|---|---|---|
| PHBV | 3-hydroxybutanoic + 3-hydroxypentanoic acid | a polyester; used in controlled drug release and packaging |
| Nylon-2-nylon-6 | glycine + aminocaproic acid | biodegradable polyamide |
| PGA / PLA | glycolic / lactic acid | dissolvable surgical sutures |
Putting It to Work
Problem. Identify the monomers of Teflon and of PVC.
Solution. Both are addition polymers of a substituted ethene:
Problem. Classify polythene, nylon-6,6 and Terylene by mode of polymerisation.
Solution. A double bond means addition; loss of water means condensation:
Problem. Which two monomers form nylon-6,6, and what small molecule is lost?
Solution. A 6-carbon diamine and a 6-carbon diacid, losing water:
Problem. What does heating natural rubber with sulphur achieve, and how?
Solution. Sulphur forms cross-links between chains:
Problem. A sample has 2 mol of chains of mass \(20{,}000\) and 3 mol of chains of mass \(30{,}000\). Find \( \overline{M}_n \).
Solution. Weight each mass by the number of molecules:
Problem. Which of polythene, Buna-S and PVC is a copolymer, and from what?
Solution. A copolymer needs two different monomers:
Chapter Summary
Large molecules of many repeating monomer units; size set by the degree of polymerisation \(n\).
By source, structure, molecular forces (elastomer/fibre/thermoplastic/thermoset) and synthesis.
\(\ce{C=C}\) monomers, no atom lost; free-radical chain: initiation, propagation, termination.
Difunctional monomers lose water; polyamides (nylon) and polyesters (Terylene).
cis-polyisoprene; vulcanisation adds sulphur cross-links to toughen it.
\( \overline{M}_n \), \( \overline{M}_w \), \( \text{PDI}\ge1 \); biodegradable polymers carry hydrolysable links.
Problems
For each item, first place the polymer on the four classification axes, then reason from structure to property. Difficulty rises down the list.
- Define polymer, monomer and degree of polymerisation with one example each.
- Classify polymers by molecular forces and give one example of each class.
- Distinguish addition from condensation polymerisation with an example of each.
- Outline the free-radical mechanism for the polymerisation of ethene.
- Name the monomers of PVC, Teflon, polystyrene and Orlon.
- Which two monomers form (a) nylon-6,6 and (b) Terylene? What is lost in each case?
- Why does nylon-6 have a single number while nylon-6,6 has two?
- What is a copolymer? Identify the monomers of Buna-S and Buna-N.
- Describe vulcanisation and explain how it improves natural rubber.
- Distinguish HDPE from LDPE in terms of structure and use.
- Define number- and weight-average molecular mass and the PDI. Why is PDI ≥ 1?
- What makes a polymer biodegradable? Name two biodegradable polymers.