3.3.12 Polymers

Condensation-polymer monomers need two reactive functional groups. A diol + dicarboxylic acid gives a polyester; a diamine + dicarboxylic acid gives a polyamide; an amino acid can self-condense through –NH₂ and –COOH.

When you draw the repeating unit, open the two functional groups that reacted and keep the link they formed in the backbone. Put brackets around the smallest repeating section and show continuation bonds through the brackets.

Find the link before naming the polymer type: –COO– is an ester link and –CONH– is an amide link. Terylene is a polyester, while nylon 6,6 and Kevlar are polyamides.

Then identify the smallest complete repeating unit without cutting through the functional link incorrectly. The repeat has to rebuild the exact chain when copied.

To link a condensation polymer to a use, choose a real structural feature first — strong intermolecular attractions, chain rigidity or high tensile strength, for example — and then show how that property helps the material do its job.

For a strong structure–property explanation, identify a feature that affects chain packing, rigidity or intermolecular attraction, then connect that property to the job the material has to do. The use should follow from the structure.

To draw a condensation polymer from its monomers, connect the correct functional groups to make ester or amide links, remove the small molecule lost in condensation and then bracket the smallest complete repeating unit.

Preserve each monomer carbon skeleton. A common error is losing a carbonyl carbon or an –O–/–NH– atom while trying to make the drawing look compact.

When a polymer chain is supplied, choose the smallest segment that reproduces it exactly by translation. For polymers made from two monomers, one repeat normally needs a contribution from both monomers.

Put bracket boundaries through backbone continuation bonds. Cutting through an ester or amide group can leave a fragment that does not reproduce the original chain.

Work backwards by hydrolysing the ester or amide links conceptually. An ester link regenerates carboxylic-acid and alcohol functionality; an amide link regenerates carboxylic-acid and amine functionality, subject to the conditions/ionic forms specified.

Check that every atom in the repeat appears in the recovered monomer set. Do not remove a carbonyl oxygen or invent an extra carbon when splitting the link.

When you explain attractions between condensation-polymer chains, look for polar groups and possible hydrogen-bond donors and acceptors. Stronger interchain attractions can raise strength and melting temperature.

Polyamides are the important hydrogen-bonding example because N–H groups can donate H bonds to carbonyl oxygens on neighbouring chains. Polyesters have polar ester groups but no N–H donor.

Compare a polyalkene with a polyester or polyamide: the polyalkene gives hydrolysis very little to attack. Its backbone is mainly non-polar C–C and C–H bonds with no readily hydrolysable links, so many reagents and enzymes cannot cleave the chain easily.

That chemical resistance also helps explain why many polyalkenes persist in the environment: without a hydrolysable functional group in the backbone, ordinary biological hydrolysis has no obvious cleavage point.

For biodegradability, look for a chemically cleavable link in the backbone. Polyesters and polyamides contain ester or amide groups that can be hydrolysed, whereas a polyalkene backbone does not provide that route.

Polyester and polyamide chains contain bonds that can be hydrolysed under suitable conditions, so the backbone can be chemically cleaved in a way a polyalkene C–C chain cannot.

Evaluate polymer disposal by considering separation/collection, energy and resource use, product quality, emissions and whether the polymer can be mechanically or chemically recycled.

Judge a disposal route with a short checklist: can the polymer be collected and separated, what energy/reagents are needed, what useful product is recovered, and what emissions or waste remain?

Hydrolyse a polyester or polyamide and you break the chain at ester or amide links. A polyalkene has an ordinary C–C backbone instead, so the same hydrolysis chemistry has no comparable bond to attack.

For a polyester or polyamide, cutting the functional-group link can give much smaller molecules under suitable hydrolysis conditions. A polyalkene offers no ester or amide link for that reaction, so the saturated carbon backbone persists.