聚合

In condensation polymerisation 缩合聚合, monomers join into a long chain 链 and a small molecule 小分子 (such as water or $\text{HCl}$) is lost each time a new bond forms. Each monomer needs two reactive groups 反应基团, so the chain can grow at both ends.

Polyesters

A polyester 聚酯 has many ester links 酯键 along its chain. You can make one from:

• a diol 二醇 (two $\text{–OH}$ groups) and a dicarboxylic acid 二羧酸 (two $\text{–COOH}$ groups), or a dioyl chloride 二酰氯.
• a single hydroxycarboxylic acid 羟基羧酸, which has both an $\text{–OH}$ and a $\text{–COOH}$.

Polyamides

A polyamide 聚酰胺 has many amide links 酰胺键. You can make one from:

• a diamine 二胺 (two $\text{–NH}_2$ groups) and a dicarboxylic acid or a dioyl chloride.
• a single aminocarboxylic acid 氨基羧酸, or from amino acids 氨基酸 joining together (proteins 蛋白质 are natural polyamides).

Repeat units and monomers

The repeat unit 重复单元 of a condensation polymer contains parts of both monomers, minus the atoms lost as the small molecule. To find the monomers 单体 from a section of polymer, break the chain at each ester or amide link, then add back $\text{–OH}$ and $\text{–H}$ (or $\text{–Cl}$).

Worked example. A polymer chain contains repeating $\text{–CONH–}$ links. Name the type of polymer, identify the monomers, and give the small molecule lost. A $\text{–CONH–}$ link is an amide, so this is a polyamide made by condensation polymerisation. To find the monomers, break the chain at each amide link and give the cut ends their atoms back: the carbon side takes $\text{–OH}$, making a dicarboxylic acid, and the nitrogen side takes $\text{–H}$, making a diamine. The small molecule lost at each link is water (or $\text{HCl}$, if an acyl dichloride was used in place of the acid). Each monomer must have two functional groups, or the chain could never keep growing - if the monomer you propose has only one, you have broken the chain in the wrong place.

• poly(alkene)s 聚烯烃 are chemically inert 惰性的 — they have only strong, non-polar 非极性的 C–C and C–H bonds, so they are hard to biodegrade 生物降解 and last a long time.
• some polymers are made so that light can break them down (they are photodegradable 可光降解的).
• polyesters and polyamides are biodegradable 可生物降解, because their ester and amide links can be broken by acidic or alkaline 碱性的 hydrolysis 水解.

Common mistakes.

• Drawing a condensation polymer's repeat unit with the full monomers. Remove the atoms lost as water or $\text{HCl}$ at each link, and show the bonds at both ends as continuation bonds across the brackets.
• Losing water when a dioyl chloride is used. A dioyl chloride loses $\text{HCl}$, not water.
• Breaking the chain in the wrong place to find the monomers. Cut through the C–O of the ester or the C–N of the amide (between C=O and O/N), not next to it.
• Calling a polymer with a C=C monomer a condensation polymer because the backbone looks long. If nothing is lost, it is addition polymerisation.
• Saying poly(alkene)s are biodegradable because they are "organic". Their non-polar C–C and C–H bonds give no site for hydrolysis, so they persist; only the ester and amide links of condensation polymers hydrolyse.

• Condensation polymers (polyesters, polyamides) form with loss of a small molecule ($\text{H}_2\text{O}$ or HCl) — draw the repeat unit and the lost molecule.
• Identify the monomers from the polymer by breaking the ester or amide link — a common question.
• Predict the type from the monomers: a $\text{C}=\text{C}$ gives addition; two functional groups give condensation.
• Polyesters and polyamides are hydrolysable (more degradable); addition polymers are not.