聚合

In addition polymerisation 加成聚合, many small molecules join into one very long chain, with no other product made.

Each small molecule is a monomer 单体. It must be unsaturated — it has a C=C double bond. The double bond opens up so that the monomers can link together. The long chain that forms is the polymer 聚合物.

Repeat units

The repeat unit 重复单元 is the small part that is copied again and again along the chain. To find it, take the monomer, change the C=C to a single C–C, and draw bonds going out at each end.

• poly(ethene) 聚乙烯 is made from ethene:

• poly(chloroethene) 聚氯乙烯 (PVC) is made from chloroethene:

Finding the monomer

To go the other way, look at one repeat unit of the polymer, and put the C=C double bond back in. That gives you the monomer.

Worked example. A polymer has the repeat unit $-(\text{CH}_2{-}\text{CH}(\text{CH}_3))_n-$. Identify the monomer and name the polymer. Reverse the rule you used to build it: rub out the bonds sticking out at each end, and turn the single C-C in the backbone back into a C=C. That gives $\text{CH}_2{=}\text{CH}(\text{CH}_3)$, which is propene - so the polymer is poly(propene). Two checks catch most errors: the monomer must have the same molecular formula as the repeat unit (addition polymerisation adds nothing and loses nothing), and the double bond goes back into the backbone, never into the side group.

Poly(alkene)s are very hard to get rid of:

• they are non-biodegradable 不可生物降解 — microbes cannot break them down, so they stay in the ground for a very long time.
• their combustion 燃烧 (burning) can release harmful gases. For example, burning PVC gives off toxic hydrogen chloride.

Common mistakes.

• Putting the side group into the backbone. In poly(propene) the backbone is $\text{–CH}_2\text{–CH–}$ and the $\text{CH}_3$ hangs off it: only the two carbons of the old C=C join the chain.
• Leaving a C=C inside the repeat unit. The double bond opens to make the chain, so the repeat unit has only single bonds.
• Drawing the bonds at the ends of the repeat unit stopping inside the brackets. The two continuation bonds must cross the brackets, with $n$ outside.
• Calling ethene's polymer "polyethane". Name it from the monomer: poly(ethene), poly(propene), poly(chloroethene).
• Saying addition polymers are reactive because the monomer was. The polymer has only strong, non-polar C–C and C–H bonds, so it is inert.

• Draw the repeat unit in brackets with the two bonds crossing them and $n$ outside; keep the backbone carbons.
• Deduce the monomer from a polymer (and vice versa) — a very common question.
• Addition polymers are inert and non-biodegradable — link to landfill, recycling and incineration issues.
• Do not confuse with condensation (no small molecule is lost in addition polymerisation).