Polymerisation

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

Condensation links: a polyester forms ester links and a polyamide forms amide links; each new link releases a small molecule (here water)

Polyesters

A polyester 聚酯 has many ester links 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.
• a single hydroxycarboxylic acid, which has both an $\text{–OH}$ and a $\text{–COOH}$.

PET drinks bottles are made of a polyester — a condensation polymer, collected here for recycling

Polyamides

A polyamide 聚酰胺 has many amide links. 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, or from amino acids 氨基酸 joining together (proteins are natural polyamides).

The "nylon rope trick": nylon (a polyamide) forms where two reactant solutions meet, so a single long thread can be pulled out (it is pink here from an added indicator)

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 $\text{–Cl}$).

To find the monomers, break the chain at each link and add back –OH and –H to the cut ends — here giving the diol and the dicarboxylic acid

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.

The two kinds of polymerisation: addition opens a C=C and loses nothing; condensation joins two-group monomers and loses a small molecule

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

• 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.