---
title: "Polymers"
book: "School Chemistry — Grades 1 to 12"
subject: chemistry
language: en
chapter: 49
exercises: 15
source: https://one-course.com/books/chemistry/1/en/chapter/49-polymers
license: CC-BY-NC-SA-4.0
credit: "One Chemistry Book, One Course (one-course.com)"
---

# Chapter 49 — Polymers

A fleece jacket can be spun from melted drink bottles; a climbing rope is made of nylon; this page is cellulose, made by trees. All three are [polymers](#def-g12-polymers-polymer): [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) thousands of [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) long, built by repeating one small unit again and again. Their properties — soft or rigid, elastic or brittle, meltable or not — come less from the [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) they contain than from the way their long chains are arranged.

**You already know.**

[Plastics](https://one-course.com/books/chemistry/1/en/chapter/15-plastics-and-synthetic-materials#def-g8-plastics-plastic) are made of very long [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule); [thermoplastics](https://one-course.com/books/chemistry/1/en/chapter/15-plastics-and-synthetic-materials#def-g8-plastics-thermoplastic) soften on heating, [thermosets](https://one-course.com/books/chemistry/1/en/chapter/15-plastics-and-synthetic-materials#def-g8-plastics-thermoplastic) do not ([Chapter 15](https://one-course.com/books/chemistry/1/en/chapter/15-plastics-and-synthetic-materials#ch-g8-plastics)). [Alcohol](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-alcohol), acid, [ester](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-carboxylic-acid), [amine](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-amine) and [amide](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-amine) groups ([Chapter 33](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#ch-g11-functional-groups)). An addition joins two [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) into one without losing any [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) ([Chapter 40](https://one-course.com/books/chemistry/1/en/chapter/40-reaction-mechanisms-curly-arrows#ch-g12-curly-arrows)).

![Used drink bottles and a fleece jacket: the same polymer, in two shapes.](https://one-course.com/images/onecourse/chapters/chemistry-1/g12-polymers/img-8e3b8191ea06.jpg)

*Used drink bottles and a fleece jacket: the same [polymer](#def-g12-polymers-polymer), in two shapes.*

## 49.1 Monomers, polymers, repeat units

**Definition 49.1 (Polymer, monomer, polymerisation).**

A *polymer* is a substance made of very large [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule), the macromolecules, each built from many small units bonded one after the other. The small [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) from which it is made are its *monomers*; the reaction that joins them is a *polymerisation*.

**Definition 49.2 (Repeat unit, degree of polymerisation).**

The *repeat unit* of a [polymer](#def-g12-polymers-polymer) is the smallest group of [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) whose repetition gives the chain; it is written between brackets with an index $n$. The number $n$ of repeat units in a chain is its *degree of polymerisation*.

![The polymerisation of ethene: n molecules of monomer give one chain of polyethene, whose repeat unit -CH2-CH2- is written between brackets.](https://one-course.com/images/onecourse/chapters/chemistry-1/g12-polymers/fig-7d7102a104f8.svg)

*The [polymerisation](#def-g12-polymers-polymer) of ethene: $n$ [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) of [monomer](#def-g12-polymers-polymer) give one chain of polyethene, whose [repeat unit](#def-g12-polymers-repeat-unit) $\ce{-CH2-CH2-}$ is written between brackets.*

**Proposition 49.3 (Molar mass of a polymer).**

A chain of [degree of polymerisation](#def-g12-polymers-repeat-unit) $n$ has a [molar mass](https://one-course.com/books/chemistry/1/en/chapter/25-the-mole-and-molar-mass#def-g10-the-mole-molar-mass)

$$
M(\text{polymer}) \approx n \times M(\text{repeat unit}) ,
$$

the two ends of the chain being negligible for large $n$.

**Proof.** The chain is $n$ [repeat units](#def-g12-polymers-repeat-unit) plus two end groups of a few [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) each; for $n$ in the thousands, the end groups weigh less than a thousandth of the whole. ∎

**Example 49.4 (A polyethene chain).**

A chain of polyethene of [molar mass](https://one-course.com/books/chemistry/1/en/chapter/25-the-mole-and-molar-mass#def-g10-the-mole-molar-mass) $280\,000\,\mathrm{g}/\mathrm{mol}$ has $n = 280000/28.0 = 10000$ [repeat units](#def-g12-polymers-repeat-unit), so twenty thousand carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) in a row. In a real sample the chains do not all have the same length: $n$ is an average.

## 49.2 Addition polymers

**Definition 49.5 (Addition polymer, condensation polymer).**

An *addition polymer* forms by additions of [monomers](#def-g12-polymers-polymer) carrying a $\ce{C=C}$ [double bond](https://one-course.com/books/chemistry/1/en/chapter/24-lewis-structures-and-the-shape-of-molecules#def-g10-lewis-and-shape-multiple-bond), with no other product: its [repeat unit](#def-g12-polymers-repeat-unit) has the same [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) as the [monomer](#def-g12-polymers-polymer). A *condensation polymer* forms by reactions between two [functional groups](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-functional-group) that join two [monomers](#def-g12-polymers-polymer) and release a small [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule), usually water, at each link.

| [monomer](#def-g12-polymers-polymer) | [polymer](#def-g12-polymers-polymer) | [repeat unit](#def-g12-polymers-repeat-unit) | uses |
| --- | --- | --- | --- |
| ethene $\ce{CH2=CH2}$ | polyethene (PE) | $\ce{-CH2-CH2-}$ | bags, bottles |
| chloroethene $\ce{CH2=CHCl}$ | poly(vinyl chloride) (PVC) | $\ce{-CH2-CHCl-}$ | pipes, frames |
| phenylethene $\ce{CH2=CH-C6H5}$ | polystyrene (PS) | $\ce{-CH2-CH(C6H5)-}$ | cups, foam |
| tetrafluoroethene $\ce{CF2=CF2}$ | PTFE | $\ce{-CF2-CF2-}$ | non-stick pans |

*Four [addition polymers](#def-g12-polymers-addition-polymer). In each, the [double bond](https://one-course.com/books/chemistry/1/en/chapter/24-lewis-structures-and-the-shape-of-molecules#def-g10-lewis-and-shape-multiple-bond) of the [monomer](#def-g12-polymers-polymer) opens, and its two carbons join the chain.*

**Method 49.6 (Finding the monomer from a polymer).**

1. Find the [repeat unit](#def-g12-polymers-repeat-unit) : the smallest group whose repetition gives the chain.
2. If the main chain holds only carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) , it is an [addition polymer](#def-g12-polymers-addition-polymer) : put back a [double bond](https://one-course.com/books/chemistry/1/en/chapter/24-lewis-structures-and-the-shape-of-molecules#def-g10-lewis-and-shape-multiple-bond) between the two carbons of the [repeat unit](#def-g12-polymers-repeat-unit) ’s main chain. That is the [monomer](#def-g12-polymers-polymer) .
3. If the main chain holds [ester](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-carboxylic-acid) or [amide](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-amine) links, it is a [condensation polymer](#def-g12-polymers-addition-polymer) : cut each link, and give back to each side the [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) of the small [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) released ( $\ce{-OH}$ to the $\ce{C=O}$ , $\ce{-H}$ to the $\ce{O}$ or $\ce{N}$ ). Those are the [monomers](#def-g12-polymers-polymer) .

**Example 49.7 (The monomer of PVC).**

The chain $\ce{-CH2-CHCl-CH2-CHCl-CH2-CHCl-}$ repeats $\ce{-CH2-CHCl-}$; it holds only carbons in its main chain, so the [monomer](#def-g12-polymers-polymer) is $\ce{CH2=CHCl}$, chloroethene.

## 49.3 Condensation polymers

A [monomer](#def-g12-polymers-polymer) with one reactive group can bond only once: it ends a chain. To build a chain by condensation, each [monomer](#def-g12-polymers-polymer) needs two groups, one at each end.

**Example 49.8 (PET, a polyester).**

Ethane-1,2-diol, $\ce{HO-CH2-CH2-OH}$, carries two [alcohol](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-alcohol) groups; benzene-1,4-dicarboxylic acid, $\ce{HOOC-C6H4-COOH}$, two acid groups. Each acid group forms an [ester](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-carboxylic-acid) with an [alcohol](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-alcohol) group and releases water: the chain grows from both ends. The [polymer](#def-g12-polymers-polymer) is poly(ethylene terephthalate), PET, the material of drink bottles and of polyester fibres. Its [repeat unit](#def-g12-polymers-repeat-unit), $\ce{C10H8O4}$, has a [molar mass](https://one-course.com/books/chemistry/1/en/chapter/25-the-mole-and-molar-mass#def-g10-the-mole-molar-mass) of $192.0\,\mathrm{g}/\mathrm{mol}$, that is the two [monomers](#def-g12-polymers-polymer) ($62.0 + 166.0$) minus two [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) of water ($36.0$).

![PET from its two monomers: each ester link (the -CO-O- groups) releases one molecule of water.](https://one-course.com/images/onecourse/chapters/chemistry-1/g12-polymers/fig-6b3320aa3005.svg)

*PET from its two [monomers](#def-g12-polymers-polymer): each [ester](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-carboxylic-acid) link (the $\ce{-CO-O-}$ groups) releases one [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) of water.*

**Example 49.9 (Nylon-6,6, a polyamide).**

Hexane-1,6-diamine, $\ce{H2N-(CH2)6-NH2}$, and hexanedioic acid, $\ce{HOOC-(CH2)4-COOH}$, join by [amide](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-amine) links, each releasing one [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) of water:

$$
\cdots\ce{-NH-(CH2)6-NH-CO-(CH2)4-CO-}\cdots
$$

The two sixes in the name count the carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) of each [monomer](#def-g12-polymers-polymer). The [amide](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-amine) links of neighbouring chains attract each other by [hydrogen bonds](https://one-course.com/books/chemistry/1/en/chapter/30-electronegativity-polarity-and-intermolecular-forces#def-g11-polarity-and-cohesion-hydrogen-bond), which makes nylon fibres strong.

**History — Carothers and nylon.**

The chemist Wallace Carothers, who led a research laboratory of a chemical company, set out to prove that [polymers](#def-g12-polymers-polymer) were genuine giant [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) and to make them on purpose. His team first made polyesters, which melted too easily to be useful; in 1934 they turned to [amines](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-amine) and made polyamides, among them the fibre later named nylon. Nylon went into production in 1939, first as stockings that caused a sensation at a World’s Fair, then as parachutes, ropes and toothbrush bristles.

![Wallace Carothers in his laboratory.](https://one-course.com/images/onecourse/chapters/chemistry-1/g12-polymers/img-56ea3e715b45.jpg)

*Wallace Carothers in his laboratory.*

## 49.4 Structure and properties

Two samples of the same [polymer](#def-g12-polymers-polymer) can behave very differently: what counts is the length of the chains, their shape, and how they are packed.

**Proposition 49.10 (Chains and properties).**

- Longer chains tangle more: the material is stronger and melts at a higher temperature.
- Linear chains can line up side by side in ordered, *crystalline* regions; branched chains cannot, and stay disordered, *amorphous* . Crystalline regions make a [polymer](#def-g12-polymers-polymer) denser, stiffer and less transparent.
- Chains held side by side only by intermolecular forces slide past each other when heated: the [polymer](#def-g12-polymers-polymer) is a [thermoplastic](https://one-course.com/books/chemistry/1/en/chapter/15-plastics-and-synthetic-materials#def-g8-plastics-thermoplastic) . Chains joined by covalent *cross-links* cannot: the [polymer](#def-g12-polymers-polymer) is a [thermoset](https://one-course.com/books/chemistry/1/en/chapter/15-plastics-and-synthetic-materials#def-g8-plastics-thermoplastic) , or, with few cross-links, a rubber-like elastic solid.

**Proof.** Heating gives the chains enough energy to overcome the intermolecular forces between them ([Chapter 30](https://one-course.com/books/chemistry/1/en/chapter/30-electronegativity-polarity-and-intermolecular-forces#ch-g11-polarity-and-cohesion)), which are weaker than [covalent bonds](https://one-course.com/books/chemistry/1/en/chapter/24-lewis-structures-and-the-shape-of-molecules#def-g10-lewis-and-shape-covalent-bond); the more contact between chains, the more energy this takes. Cross-links are covalent: breaking them destroys the material instead of melting it. ∎

![Four arrangements of polymer chains. The cross-links (orange) are covalent bonds between chains.](https://one-course.com/images/onecourse/chapters/chemistry-1/g12-polymers/fig-be5e765fd8d2.svg)

*Four arrangements of [polymer](#def-g12-polymers-polymer) chains. The cross-links (orange) are [covalent bonds](https://one-course.com/books/chemistry/1/en/chapter/24-lewis-structures-and-the-shape-of-molecules#def-g10-lewis-and-shape-covalent-bond) between chains.*

**Example 49.11 (Two polyethenes).**

Polyethene made under very high pressure has many short branches: its chains pack badly, and it gives soft, transparent films for bags. Made with a [catalyst](https://one-course.com/books/chemistry/1/en/chapter/42-catalysis#def-g12-catalysis-catalyst) at low pressure, its chains are nearly linear and pack into crystalline regions: it is denser and stiffer, and gives milk bottles and crates. Same [monomer](#def-g12-polymers-polymer), same [repeat unit](#def-g12-polymers-repeat-unit), different architecture.

## 49.5 Natural polymers

Living things made [polymers](#def-g12-polymers-polymer) long before chemists did.

- **Cellulose** and **starch** are both chains of glucose units, $\ce{C6H10O5}$ per unit, joined differently: cellulose forms straight, strong fibres (wood, cotton, paper); starch forms coils that the body digests.
- **Proteins** are chains of amino acids joined by [amide](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-amine) links, the peptide bonds of the dipeptide of [Chapter 48](https://one-course.com/books/chemistry/1/en/chapter/48-synthesis-strategy-and-green-chemistry#ch-g12-synthesis-strategy) .
- **DNA** is a chain of nucleotides; the order of its four kinds of unit stores genetic information.
- **Natural rubber** is an [addition polymer](#def-g12-polymers-addition-polymer) of isoprene, $\ce{CH2=C(CH3)-CH=CH2}$ , collected as a milky liquid, latex, from the bark of the rubber tree. Heated with sulfur, its chains are linked by sulfur bridges: the rubber becomes tougher and keeps its shape, as in a tyre.

![Tapping a rubber tree: latex flows from a cut in the bark into a cup.](https://one-course.com/images/onecourse/chapters/chemistry-1/g12-polymers/img-46e724f1ae33.jpg)

*Tapping a rubber tree: latex flows from a cut in the bark into a cup.*

**Remark 49.12 (Making and unmaking).**

A [thermoplastic](https://one-course.com/books/chemistry/1/en/chapter/15-plastics-and-synthetic-materials#def-g8-plastics-thermoplastic) can be melted and shaped again: mechanical [recycling](https://one-course.com/books/chemistry/1/en/chapter/7-from-raw-materials-to-recycling#def-g5-raw-materials-and-recycling-recycling). A [condensation polymer](#def-g12-polymers-addition-polymer) can also be broken down to its [monomers](#def-g12-polymers-polymer) by the reverse reaction, a hydrolysis, and the [monomers](#def-g12-polymers-polymer) made into new [polymer](#def-g12-polymers-polymer): chemical [recycling](https://one-course.com/books/chemistry/1/en/chapter/7-from-raw-materials-to-recycling#def-g5-raw-materials-and-recycling-recycling). A [thermoset](https://one-course.com/books/chemistry/1/en/chapter/15-plastics-and-synthetic-materials#def-g8-plastics-thermoplastic) can do neither easily.

## 49.6 Exercises

**Exercise 49.1 ★.**

Give the [monomer](#def-g12-polymers-polymer) of polypropene, whose [repeat unit](#def-g12-polymers-repeat-unit) is $\ce{-CH2-CH(CH3)-}$.

**Solution of Exercise 49.1.**

Propene, $\ce{CH2=CH-CH3}$.

**Exercise 49.2 ★.**

Write the [repeat unit](#def-g12-polymers-repeat-unit) of the [polymer](#def-g12-polymers-polymer) made from tetrafluoroethene, $\ce{CF2=CF2}$. Is it an addition or a [condensation polymer](#def-g12-polymers-addition-polymer)?

**Solution of Exercise 49.2.**

$\ce{-CF2-CF2-}$; an [addition polymer](#def-g12-polymers-addition-polymer): the [monomer](#def-g12-polymers-polymer) has a $\ce{C=C}$ bond and no other product forms.

**Exercise 49.3 ★.**

Classify as addition or [condensation polymers](#def-g12-polymers-addition-polymer): polyethene, PET, nylon, PVC, polystyrene.

**Solution of Exercise 49.3.**

Addition: polyethene, PVC, polystyrene. Condensation: PET (a polyester), nylon (a polyamide).

**Exercise 49.4 ★.**

Which of these [polymers](#def-g12-polymers-polymer) are natural: cellulose, nylon, starch, PVC, rubber from latex, proteins?

**Solution of Exercise 49.4.**

Cellulose, starch, rubber from latex, proteins. Nylon and PVC are synthetic.

**Exercise 49.5 ★.**

Why must each [monomer](#def-g12-polymers-polymer) of a [condensation polymer](#def-g12-polymers-addition-polymer) carry two reactive groups?

**Solution of Exercise 49.5.**

Each link uses one group of each [monomer](#def-g12-polymers-polymer); with two groups, a [monomer](#def-g12-polymers-polymer) can bond on both sides and the chain keeps growing. A [monomer](#def-g12-polymers-polymer) with one group would stop the chain.

**Exercise 49.6 ★★.**

A sample of PVC has an average [molar mass](https://one-course.com/books/chemistry/1/en/chapter/25-the-mole-and-molar-mass#def-g10-the-mole-molar-mass) of $125\,000\,\mathrm{g}/\mathrm{mol}$. Compute its [degree of polymerisation](#def-g12-polymers-repeat-unit).

**Solution of Exercise 49.6.**

$M(\ce{C2H3Cl}) = 24.0 + 3.0 + 35.5 = 62.5\,\mathrm{g}/\mathrm{mol}$; $n = 125000/62.5 = 2000$.

**Exercise 49.7 ★★.**

A chain of polystyrene has a [degree of polymerisation](#def-g12-polymers-repeat-unit) of 2500. Compute its [molar mass](https://one-course.com/books/chemistry/1/en/chapter/25-the-mole-and-molar-mass#def-g10-the-mole-molar-mass).

**Solution of Exercise 49.7.**

$M(\ce{C8H8}) = 104.0\,\mathrm{g}/\mathrm{mol}$; $M = 2500 \times 104.0 =
2.60 \times 10^{5}\,\mathrm{g}/\mathrm{mol}$.

**Exercise 49.8 ★★.**

Lactic acid, $\ce{CH3-CH(OH)-COOH}$, carries an [alcohol](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-alcohol) group and an acid group. Show that it can form a polyester on its own, write its [repeat unit](#def-g12-polymers-repeat-unit), and name the small [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) released.

**Solution of Exercise 49.8.**

The acid group of one [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) forms an [ester](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-carboxylic-acid) with the [alcohol](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-alcohol) group of the next, which still has a free acid group: the chain grows. [Repeat unit](#def-g12-polymers-repeat-unit) $\ce{-O-CH(CH3)-CO-}$; water is released at each link.

**Exercise 49.9 ★★.**

Explain, with the figure of chain arrangements, why bag films are soft and transparent while milk bottles are stiff and opaque.

**Solution of Exercise 49.9.**

Bag films are made of branched chains, which cannot pack: the material is amorphous, soft and transparent. Milk bottles are made of nearly linear chains, which pack into crystalline regions: denser, stiffer, and the crystalline regions scatter light.

**Exercise 49.10 ★★.**

The handle of a pan is a [thermoset](https://one-course.com/books/chemistry/1/en/chapter/15-plastics-and-synthetic-materials#def-g8-plastics-thermoplastic), its [plastic](https://one-course.com/books/chemistry/1/en/chapter/15-plastics-and-synthetic-materials#def-g8-plastics-plastic) lid knob too. Describe their chains. Why can they not be melted and recycled?

**Solution of Exercise 49.10.**

Their chains are joined by many covalent cross-links. Heating cannot make the chains slide; it breaks bonds and destroys the material instead of melting it.

**Exercise 49.11 ★★.**

A chain of nylon-6,6 has a [molar mass](https://one-course.com/books/chemistry/1/en/chapter/25-the-mole-and-molar-mass#def-g10-the-mole-molar-mass) of $22\,600\,\mathrm{g}/\mathrm{mol}$. Compute the [molar mass](https://one-course.com/books/chemistry/1/en/chapter/25-the-mole-and-molar-mass#def-g10-the-mole-molar-mass) of its [repeat unit](#def-g12-polymers-repeat-unit) $\ce{C12H22N2O2}$, then its [degree of polymerisation](#def-g12-polymers-repeat-unit).

**Solution of Exercise 49.11.**

$M(\ce{C12H22N2O2}) = 144.0 + 22.0 + 28.0 + 32.0 = 226.0\,\mathrm{g}/\mathrm{mol}$; $n = 22600/226.0 = 100$.

**Exercise 49.12 ★★★.**

Write the reaction between one [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) of hexane-1,6-diamine and one of hexanedioic acid, giving the first [amide](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-amine) link. Why can the product go on reacting at both ends?

**Solution of Exercise 49.12.**

$$
\ce{H2N(CH2)6NH2 + HOOC(CH2)4COOH -> H2N(CH2)6NHCO(CH2)4COOH + H2O} .
$$

The product still carries a free [amine](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-amine) group at one end and a free acid group at the other: each can react with another [monomer](#def-g12-polymers-polymer).

**Exercise 49.13 ★★★.**

What mass of water is released when $1.0\,\mathrm{kg}$ of nylon-6,6 is made?

**Solution of Exercise 49.13.**

Two [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) of water per [repeat unit](#def-g12-polymers-repeat-unit). $n = 1000/226.0 =
4.42\,\mathrm{mol}$ of [repeat units](#def-g12-polymers-repeat-unit), so $8.85\,\mathrm{mol}$ of water: $8.85 \times 18.0 = 159\,\mathrm{g}$.

**Exercise 49.14 ★★★.**

Natural rubber is soft and sticky when warm; heated with sulfur, it becomes the elastic, tough rubber of tyres. Explain with the chains. Why is a tyre so hard to recycle?

**Solution of Exercise 49.14.**

Warm natural rubber is made of chains that slide past one another. Sulfur bridges link the chains: they stretch but spring back, and no longer flow. Being cross-linked, a tyre cannot be melted and reshaped.

**Exercise 49.15 ★★★.**

Starch and cellulose have the same formula per unit, $\ce{C6H10O5}$. A cellulose chain has a [molar mass](https://one-course.com/books/chemistry/1/en/chapter/25-the-mole-and-molar-mass#def-g10-the-mole-molar-mass) of $1.6 \times 10^{6}\,\mathrm{g}/\mathrm{mol}$. How many glucose units does it hold? How long is it, if each unit adds $0.5\,\mathrm{nm}$ to its length?

**Solution of Exercise 49.15.**

$M(\ce{C6H10O5}) = 162.0\,\mathrm{g}/\mathrm{mol}$; $n = 1.6 \times 10^{6}/162.0 = 9900$ units; length $9900 \times 0.5 = 4900\,\mathrm{nm}$, about $5\,\text{µ}\mathrm{m}$.

## 49.7 Problem: From Bottle to Fleece

**Problem 49.1.**

Weekend problem — how many 25 g bottles go into one fleece jacket?

A fleece jacket is made of $300\,\mathrm{g}$ of PET fibre. Used bottles of $25\,\mathrm{g}$ each are collected, sorted, washed, shredded, melted and spun; in this process $80\,\%$ of the mass of the bottles ends up as fibre in the jacket.

**Part I — The [polymer](#def-g12-polymers-polymer).**

1. Name the two [monomers](#def-g12-polymers-polymer) of PET and the [functional groups](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-functional-group) they carry.
2. Which link forms between them? What small [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) is released?
3. Is PET an addition or a [condensation polymer](#def-g12-polymers-addition-polymer) ?
4. Compute the molar masses of the two [monomers](#def-g12-polymers-polymer) .
5. Deduce the [molar mass](https://one-course.com/books/chemistry/1/en/chapter/25-the-mole-and-molar-mass#def-g10-the-mole-molar-mass) of the [repeat unit](#def-g12-polymers-repeat-unit) $\ce{C10H8O4}$ , and check it with its formula.

**Part II — The chains.**

6. The PET of a bottle has chains of average [molar mass](https://one-course.com/books/chemistry/1/en/chapter/25-the-mole-and-molar-mass#def-g10-the-mole-molar-mass) $38\,400\,\mathrm{g}/\mathrm{mol}$ . Compute the [degree of polymerisation](#def-g12-polymers-repeat-unit) .
7. How many [ester](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-carboxylic-acid) links does one chain contain, about?
8. PET of bottles is partly crystalline. What does that say about its chains?
9. Why can PET be melted and spun into a fibre?

**Part III — [Recycling](https://one-course.com/books/chemistry/1/en/chapter/7-from-raw-materials-to-recycling#def-g5-raw-materials-and-recycling-recycling).**

10. What mass of bottles is needed for one jacket?
11. How many bottles is that?
12. What happens to the rest of the mass?
13. Which principle of [green chemistry](https://one-course.com/books/chemistry/1/en/chapter/48-synthesis-strategy-and-green-chemistry#def-g12-synthesis-strategy-green-chemistry) does this [recycling](https://one-course.com/books/chemistry/1/en/chapter/7-from-raw-materials-to-recycling#def-g5-raw-materials-and-recycling-recycling) apply?

**Part IV — Back to the [monomers](#def-g12-polymers-polymer).**

14. Write, for one [repeat unit](#def-g12-polymers-repeat-unit) , the hydrolysis of PET into its [monomers](#def-g12-polymers-polymer) .
15. What amount of [repeat units](#def-g12-polymers-repeat-unit) is there in $1.00\,\mathrm{kg}$ of PET?
16. What masses of each [monomer](#def-g12-polymers-polymer) does the full hydrolysis of $1.00\,\mathrm{kg}$ of PET give?
17. What mass of water does it consume? Check the [conservation of mass](https://one-course.com/books/chemistry/1/en/chapter/13-conservation-of-mass-and-balanced-equations#def-g8-balanced-equations-conservation) .
18. Give one advantage of this chemical [recycling](https://one-course.com/books/chemistry/1/en/chapter/7-from-raw-materials-to-recycling#def-g5-raw-materials-and-recycling-recycling) over melting.
19. State the final answer: how many $25\,\mathrm{g}$ bottles go into one fleece jacket?

**Solution of Problem 49.1.**

**1.** Ethane-1,2-diol (two [alcohol](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-alcohol) groups) and benzene-1,4-dicarboxylic acid (two [carboxylic acid](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-carboxylic-acid) groups).

**2.** An [ester](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-carboxylic-acid) link; water is released.

**3.** A [condensation polymer](#def-g12-polymers-addition-polymer).

**4.** $M(\ce{C2H6O2}) = 62.0\,\mathrm{g}/\mathrm{mol}$; $M(\ce{C8H6O4}) =
166.0\,\mathrm{g}/\mathrm{mol}$.

**5.** $62.0 + 166.0 - 2 \times 18.0 = 192.0\,\mathrm{g}/\mathrm{mol}$; and $10 \times 12.0 + 8 \times 1.0 + 4 \times 16.0 = 192.0$.

**6.** $n = 38400/192.0 = 200$.

**7.** Two [ester](https://one-course.com/books/chemistry/1/en/chapter/33-functional-groups-and-families#def-g11-functional-groups-carboxylic-acid) links per [repeat unit](#def-g12-polymers-repeat-unit): about 400.

**8.** Its chains are mostly linear, and parts of them line up in ordered regions.

**9.** It is a [thermoplastic](https://one-course.com/books/chemistry/1/en/chapter/15-plastics-and-synthetic-materials#def-g8-plastics-thermoplastic): its chains are not cross-linked and slide past each other when hot.

**10.** $300/0.80 = 375\,\mathrm{g}$.

**11.** $375/25 = 15$ bottles.

**12.** Caps, labels, dirt and scraps are removed or lost during sorting, washing and spinning.

**13.** Prevent waste (principle 1): a used object becomes [raw material](https://one-course.com/books/chemistry/1/en/chapter/7-from-raw-materials-to-recycling#def-g5-raw-materials-and-recycling-raw-material).

**14.** $\ce{C10H8O4 + 2H2O -> C2H6O2 + C8H6O4}$.

**15.** $1000/192.0 = 5.21\,\mathrm{mol}$.

**16.** Diol: $5.21 \times 62.0 = 323\,\mathrm{g}$; acid: $5.21 \times 166.0 = 865\,\mathrm{g}$.

**17.** $2 \times 5.21 \times 18.0 = 188\,\mathrm{g}$ of water; $1000 + 188 = 323 + 865 = 1188\,\mathrm{g}$: mass is conserved.

**18.** The [monomers](#def-g12-polymers-polymer) can be purified and give new PET as good as new, while melting shortens the chains a little each time.

**19.** One fleece jacket takes 15 bottles of $25\,\mathrm{g}$.