35. Polymerisation

A diol and a dicarboxylic acid (or dioyl chloride) can link repeatedly through ester bonds, releasing water or HCl. A hydroxycarboxylic acid can self-condense in the same way.

Each monomer must have two reactive groups to extend the chain. The repeating unit contains the ester linkage –CO–O– and the polymer’s end groups depend on stoichiometric balance.

Ethane-1,2-diol plus benzene-1,4-dicarboxylic acid gives a chain containing –OCH₂CH₂OCOC₆H₄CO– repeats.

A single alcohol plus a single acid makes a small ester, not a polyester; chain growth requires at least two functional groups on each linking unit.

A diamine and a dicarboxylic acid (or dioyl chloride) can condense to form a polyamide. Aminocarboxylic acids and amino acids can also self-condense when each molecule supplies both groups.

Each amide link forms between –NH₂ and –COOH/–COCl, releasing water or HCl. Two functional groups on each linking unit allow chains to continue.

Hexane-1,6-diamine plus hexanedioic acid gives a nylon-like chain containing –NH(CH₂)₆NHCO(CH₂)₄CO– repeats.

A mixture of a monoamine and monoacid makes a small amide, not a high polymer; chain growth requires bifunctionality.

A condensation-polymer repeat unit is the smallest section that repeats after the monomers lose a small molecule such as water or HCl. Draw the new covalent link, then show the unreacted atoms at each end.

For a diol/diacid polymer, the repeat contains –O–CO–; for a diamine/diacid polymer, it contains –NH–CO–. Include the correct carbon count from each monomer.

Ethane-1,2-diol + benzene-1,4-dicarboxylic acid gives –OCH₂CH₂OCOC₆H₄CO– as the repeating section.

Do not remove atoms from the carbon skeleton or draw a terminal molecule instead of a repeating unit.

To identify condensation monomers, cut each ester or amide link at the bond formed during condensation and restore the corresponding –OH/–COOH or –NH₂ groups.

An ester link points to a diol plus dicarboxylic acid (or a hydroxycarboxylic acid); an amide link points to a diamine plus dicarboxylic acid or amino acid family.

From –O–CH₂CH₂–O–CO–C₆H₄–CO–, restore ethane-1,2-diol and benzene-1,4-dicarboxylic acid.

Do not interpret every oxygen as an alcohol group; inspect whether it sits in –CO–O– or another linkage.

An alkene monomer with a C=C bond normally undergoes addition polymerisation: the π bond opens and no small molecule is lost. Monomers with two functional groups undergo condensation polymerisation and release a small molecule.

Inspect the starting structures before looking at the product. C=C suggests poly(alkene); –OH/–COOH/–NH₂ pairs suggest polyester or polyamide.

Ethene → poly(ethene) by addition; hexane-1,6-diamine + hexanedioic acid → polyamide by condensation.

A polymer being “formed from two monomers” does not automatically make it condensation; the deciding evidence is the new link and small-molecule loss.

A polymer section made by addition retains the carbon backbone from opened C=C bonds and has no alternating small-molecule loss. A condensation section contains ester or amide links formed with elimination of water or HCl.

Work backwards: locate the repeating linkage, restore monomer functional groups, and ask whether a small molecule must have been removed.

A chain with –CH₂–CH₂– repeats comes from addition of ethene; a chain with –CO–NH– links is a condensation polyamide.

Do not classify from the polymer name alone; the repeat structure is the evidence.

In a poly(alkene), the original C=C bonds have become strong C–C single bonds in a saturated carbon backbone. With no easily attacked functional group, the material is relatively chemically inert.

This stability gives useful durability but also makes many poly(alkenes) difficult for organisms and ordinary conditions to biodegrade.

Poly(ethene) packaging can remain for long periods because hydrolysis has no ester or amide link to attack; fragmentation by sunlight is not the same as complete biodegradation.

“Inert” does not mean indestructible or non-reactive under every extreme condition; it means resistant under ordinary biological/environmental conditions.

Ultraviolet light can supply enough energy to break polymer bonds or create reactive radicals, causing chains to fragment. This is photodegradation, not necessarily complete biodegradation.

The rate depends on the polymer, light intensity, oxygen and stabilising additives. Fragmentation may produce smaller pieces without converting them fully to CO₂ and water.

A poly(alkene) exposed to sunlight can become brittle as chains break, yet the resulting microfragments can persist.

“Breaks into pieces” does not mean “biodegradable”; physical fragmentation and biological mineralisation are different endpoints.

Polyesters contain ester bonds and polyamides contain amide bonds. Water, acid or alkali can hydrolyse these polar links, so such polymers can be more biodegradable than poly(alkenes) under suitable conditions.

Hydrolysis rate depends on accessibility, temperature, pH and polymer structure. “Biodegradable” is a conditions-dependent claim, not an instant disappearance label.

An ester link can split into an alcohol and carboxylic acid (or carboxylate in alkali); an amide link gives an amine/ammonium product plus acid/carboxylate.

The presence of an ester or amide makes hydrolysis possible, not automatically rapid in every environment.