Monomers, Polymers and Addition Polymerisation

1Twenty thousand ethene molecules in one chain

At almost every water point in Rwanda you will see yellow jerrycans. Most are blow-moulded from high-density poly(ethene). Each molecule in that plastic is a chain of many thousands of carbon atoms, built by joining small ethene molecules, ${\mathrm{C}}_2{\mathrm{H}}_4\backslash mathrm\{C\_2H\_4\}$, end to end. A gas that boils at −104 °C becomes a tough solid that holds 20 litres of water and survives years in the sun. This unit is about how such giant molecules are made, why they behave as they do, and what happens to them when we throw them away.

2Addition polymerisation

In addition polymerisation, monomers that contain a carbon–carbon double bond join together and nothing else is formed. One of the two bonds in each C=C (the π bond) opens, and the electrons are used to link each monomer to the next. So:

• the monomer must be unsaturated (an alkene or a substituted alkene); ethane cannot polymerise;
• the polymer is the only product, so the atom economy is 100 %, and the polymer has the same empirical formula as the monomer;
• the relative molecular mass of a chain is $nn$ times that of the monomer: $M_r(\text{polymer})=n\times M_r(\text{monomer})M\_r(\backslash text\{polymer\})\; =\; n\; \backslash times\; M\_r(\backslash text\{monomer\})$;
• the main chain of the polymer contains only carbon atoms, joined by C–C single bonds that are hard to break. This is why addition polymers are not biodegradable (Lesson 4).

The equation for ethene is written with $nn$ in front of the monomer and the repeat unit in brackets:

$$n\mathrm{C}{\mathrm{H}}_2=\mathrm{C}{\mathrm{H}}_2⟶[-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2-{]}_{n}n\backslash ,\backslash mathrm\{CH\_2\{=\}CH\_2\; \backslash longrightarrow\; \backslash bigl[\{-\}CH\_2\{-\}CH\_2\{-\}\backslash bigr]\_\{\backslash mathit\{n\}\}\}$$

Figure 1.1 The equation for making poly(ethene), drawn in full. Three rules for a repeat unit: the double bond has become a single bond, the two bonds at the ends pass THROUGH the brackets (they join to the next units), and a subscript $nn$ sits outside the closing bracket.

3Drawing the repeat unit of any addition polymer

Every monomer used in addition polymerisation can be written as $\mathrm{C}{\mathrm{H}}_2=\mathrm{C}\mathrm{H}\mathrm{X}\backslash mathrm\{CH\_2\{=\}CHX\}$ or $\mathrm{C}\mathrm{X}\mathrm{Y}=\mathrm{C}\mathrm{Z}\mathrm{W}\backslash mathrm\{CXY\{=\}CZW\}$. To draw its repeat unit:

1. draw the two carbons of the C=C side by side, joined by a single bond;
2. put the groups that were on those carbons above and below them, in the same places;
3. add a bond out to each side, draw brackets across those bonds, and write $nn$.

However big the side group is (a benzene ring, a nitrile group or an ester), the repeat unit has only two carbon atoms in the main chain. The side groups hang off the chain. Reading a chain works the other way: find two main-chain carbons that repeat, box them, and put the double bond back between them to get the monomer (Figure 1.2).

Figure 1.2 A section of poly(propene). The methyl groups are side groups on every second carbon. The dashed box is one repeat unit, $-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}\mathrm{H}(\mathrm{C}{\mathrm{H}}_3)-\backslash mathrm\{-CH\_2\{-\}CH(CH\_3)-\}$; putting the double bond back gives the monomer, propene, $\mathrm{C}{\mathrm{H}}_2=\mathrm{C}\mathrm{H}\mathrm{C}{\mathrm{H}}_3\backslash mathrm\{CH\_2\{=\}CHCH\_3\}$. Note that the main chain is NOT a chain of three carbons.

Table 1.1: Some important addition polymers

4The mechanism: free-radical addition

Low-density poly(ethene) is made by free-radical polymerisation: ethene is heated to about 200 °C at a very high pressure (roughly 1000 to 3000 atm) with a trace of oxygen or an organic peroxide, $\mathrm{R}\mathrm{O}-\mathrm{O}\mathrm{R}\backslash mathrm\{RO\{-\}OR\}$, as the initiator. The reaction is a chain reaction in three stages.

Initiation. The weak O–O bond breaks homolytically (each oxygen keeps one electron), and the radical adds to a monomer:

$$\mathrm{R}\mathrm{O}-\mathrm{O}\mathrm{R}\stackrel{\text{heat}}{\to }2\mathrm{R}{\mathrm{O}}^{\bullet }\mathrm{R}{\mathrm{O}}^{\bullet }+\mathrm{C}{\mathrm{H}}_2=\mathrm{C}{\mathrm{H}}_2⟶\mathrm{R}\mathrm{O}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2{}^{\bullet }\backslash mathrm\{RO\{-\}OR\; \backslash xrightarrow\{\backslash text\{heat\}\}\; 2\backslash ,RO^\{\backslash bullet\}\}\; \backslash qquad\; \backslash mathrm\{RO^\{\backslash bullet\}\; +\; CH\_2\{=\}CH\_2\; \backslash longrightarrow\; RO\{-\}CH\_2\{-\}CH\_2\{\}^\{\backslash bullet\}\}$$

Propagation. The carbon radical adds to another ethene molecule, and a new radical forms at the end of a chain two carbons longer (Figure 1.3). This step repeats thousands of times in a fraction of a second:

$$\mathrm{R}\mathrm{O}-\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2{}^{\bullet }+\mathrm{C}{\mathrm{H}}_2=\mathrm{C}{\mathrm{H}}_2⟶\mathrm{R}\mathrm{O}-\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2{}^{\bullet }\backslash mathrm\{RO\{-\}CH\_2CH\_2\{\}^\{\backslash bullet\}\; +\; CH\_2\{=\}CH\_2\; \backslash longrightarrow\; RO\{-\}CH\_2CH\_2CH\_2CH\_2\{\}^\{\backslash bullet\}\}$$

Termination. Two radicals meet and pair their electrons, so the chain stops growing:

$${\mathrm{R}}^{\mathrm{\prime }\bullet }+{}^{\bullet }{\mathrm{R}}^{\mathrm{\prime }\mathrm{\prime }}⟶{\mathrm{R}}^{\mathrm{\prime }}-{\mathrm{R}}^{\mathrm{\prime }\mathrm{\prime }}\backslash mathrm\{R\text{'}^\{\backslash bullet\}\; +\; \{\}^\{\backslash bullet\}R\text{'}\text{'}\; \backslash longrightarrow\; R\text{'}\{-\}R\text{'}\text{'}\}$$

Termination happens at random, which is why the chains in a sample have a spread of lengths. A radical can also pull a hydrogen atom from the middle of another chain, and a branch then grows from that point. Branched chains cannot pack closely, which gives low-density poly(ethene) (LDPE). High-density poly(ethene) (HDPE) is made at about 60 °C and only a few atmospheres with a Ziegler–Natta catalyst (titanium(IV) chloride with triethylaluminium). Its chains are almost unbranched, pack closely and give a stiffer, stronger plastic: the jerrycan.

Figure 1.3 The propagation step, drawn with fish-hook (half-headed) arrows. The radical's unpaired electron and one π electron form the new C–C bond; the other π electron is left on the far carbon, which becomes the new radical end.