Opening the double bond

Section 2 of 4

Ethene's double bond is what lets it join into a chain. A double bond is two shared pairs of electrons between the same two carbons, and one of the two can open.

When one bond of the C=C opens, each carbon has a free hand. It uses it to bond to a carbon of the next ethene molecule, whose double bond opens in turn, and the chain keeps growing one ethene at a time. Every atom of every ethene ends up in the chain, and nothing is given off. This is addition polymerisation: the addition across a C=C double bond, the reaction you met earlier, repeated thousands of times.

Part of a poly(ethene) chain, with one repeat unit marked Six carbon atoms in a row joined by single bonds, each carrying one hydrogen atom above and one below. The chain carries on past both edges. Two neighbouring carbon atoms and their four hydrogen atoms are shaded and labelled repeat unit: the chain is this unit again and again. CHHCHHCHHCHHCHHCHH repeat unit
The two-carbon unit from one ethene, –⁠CH₂–⁠CH₂–⁠, repeats along the whole chain: it is the repeat unit.

Drawing 10,000 carbons is out of the question, so chemists draw one repeat unit: the part of the chain that comes from one monomer. It goes in square brackets, with the bonds to its neighbours crossing the brackets, and a small n after them. The n stands for a very large number.

Ethene molecules join to make poly(ethene) Left: n ethene molecules, n meaning a very large number, each drawn with two carbon atoms joined by a double bond and two hydrogen atoms on each carbon. One line of the double bond is amber. An arrow points to the repeat unit of poly(ethene): two carbon atoms joined by a single bond, each with two hydrogen atoms, inside square brackets with a small n after them. A bond leaves each side of the brackets, in amber, to join the next unit. CCHHHHCCHHHH n n ethene poly(ethene)
Many ethene molecules, n in all, make poly(ethene). One line of each double bond (amber) opens, and the amber bonds crossing the brackets join each repeat unit to the next.

Other monomers, other plastics

Other molecules with a C=C double bond join in the same way. Swap one hydrogen of ethene for a methyl group, -⁠CH₃, and you have propene, which makes poly(propene). Swap it for a chlorine atom and you have chloroethene, which makes poly(chloroethene), better known as PVC.

Propene and chloroethene make poly(propene) and PVC Two equations. Top: n propene molecules, CH2=CH-CH3, join to make poly(propene), whose repeat unit is two carbon atoms in brackets with a CH3 group hanging below the second carbon. Bottom: n chloroethene molecules, CH2=CHCl, join to make poly(chloroethene), PVC, whose repeat unit carries a chlorine atom on the second carbon. In both, one line of the monomer's double bond is amber, and the bonds leaving the brackets are amber. CCHHHCH3CCHHHCH3 n n propene poly(propene) CCHHHClCCHHHCl n n chloroethene poly(chloroethene), PVC
Propene and chloroethene join just like ethene. The double bond opens, and the -⁠CH₃ group or chlorine atom ends up on every second carbon of the chain.
Addition polymers in this section
MonomerPolymerUsed for
ethenepoly(ethene)bags, bottles, food wrap
propenepoly(propene)food tubs, bottle tops, ropes
chloro­ethenePVCwindow frames, pipes, cable coating

Quick check

Tetrafluoroethene, CF₂=CF₂, joins into PTFE, the non-stick coating on many frying pans. What repeat unit does each CF₂=CF₂ molecule give the chain?