Polyesters and polyamides — OCR A-Level Chemistry

Condensation polymers form when monomers each carry two functional groups and join repeatedly, releasing a small molecule such as water or hydrogen chloride at every linkage. A polyester forms from a dicarboxylic acid and a diol; a polyamide forms from a dicarboxylic acid and a diamine. Because each monomer is difunctional, the chain extends in two directions and no atoms are wasted beyond the small molecule expelled. Contrast addition polymers, where alkene monomers join with no by-product and the repeat unit keeps the same atoms. Recognise a repeat unit by its two ester or amide linkages and identify the monomers by breaking those linkages and adding back the lost groups.

Condensation polymerisation builds a polymer when two difunctional monomers react repeatedly, expelling a small molecule at each new linkage. To form a polyester, a dicarboxylic acid reacts with a diol and water is eliminated as each ester bond forms. To form a polyamide, a dicarboxylic acid reacts with a diamine and water (or hydrogen chloride if diacyl chlorides are used) is eliminated as each amide bond forms. The repeat unit therefore contains two new linkages and lacks the atoms of the expelled molecule. Draw the monomers, remove the eliminated groups, and join the ends to build the repeat unit; reverse the process to recover the monomers.

Polyesters are condensation polymers formed when a dicarboxylic acid reacts with a diol. Each monomer has two functional groups, so the molecules link repeatedly and release a small molecule, usually water, at every new ester linkage. The repeat unit contains the ester group –COO–, with the carbonyl carbon coming from the acid and the oxygen from the alcohol. A named example is poly(ethylene terephthalate), made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol. To answer a multiple-choice question, identify the ester linkage in each option, check that the repeat unit has the correct atoms and that the two monomer units alternate, and confirm the small molecule eliminated. Polyesters can be hydrolysed, which matters for recycling and biodegradability.

Polyamides are condensation polymers formed when a dicarboxylic acid reacts with a diamine, or when an amino acid polymerises with itself. Each new amide linkage –CONH– releases a small molecule, usually water. The carbonyl carbon comes from the acid and the nitrogen from the amine. A named example is nylon-6,6, made from hexanedioic acid and 1,6-diaminohexane. In a multiple-choice question, find the amide group, check that the repeat unit alternates acid-derived and amine-derived fragments, and confirm the eliminated molecule. Polyamides have strong hydrogen bonding between chains through the N–H and C=O groups, giving high melting points, and they can be hydrolysed.

This statement introduces the hydrolysis of the ester and amide linkages found in condensation polymers. Acid hydrolysis of a polyester breaks each ester group by adding water, giving the parent dicarboxylic acid and the parent diol; heating with aqueous acid speeds this up. Base hydrolysis uses aqueous alkali such as sodium hydroxide: the ester is cleaved to the carboxylate salt and the alcohol, and the salt forms because the acid is neutralised in alkali. For a polyamide, acid hydrolysis gives the dicarboxylic acid and the diammonium salt of the diamine, while base hydrolysis gives the dicarboxylate salt and the free diamine. Recognise that hydrolysis is the reverse of condensation polymerisation and that the products depend on whether the medium is acidic or alkaline.

Polyesters such as PET contain many ester linkages, –COO–, formed by condensation between a dicarboxylic acid and a diol. The specification asks you to know how these ester groups react. Under acid hydrolysis, each ester link is broken by water with acid catalysis, giving the parent dicarboxylic acid and the diol; heating with dilute acid and water drives the reaction. Under base hydrolysis, hydroxide ions attack the ester link irreversibly, giving the dicarboxylate salt (for example the sodium salt of the diacid) and the diol, because the carboxylic acid formed is deprotonated to the carboxylate. So acid hydrolysis returns acid plus alcohol, while base hydrolysis returns carboxylate salt plus alcohol. Recognise that the polymer backbone is cleaved at every ester group, so the products are the monomers (or their salts), not a single small molecule.

Polyamides are condensation polymers in which the repeat unit is joined by amide linkages, written as –CO–NH–. Each amide group forms when a carboxylic acid group on one monomer reacts with an amine group on another, releasing a small molecule such as water; the acid carbon and the amine nitrogen stay in the chain, so the linkage is a genuine part of the backbone rather than a side group. A familiar example is nylon-6,6, made from hexanedioic acid and 1,6-diaminohexane, whose repeat unit contains two amide groups. When you draw or read a repeat unit, locate the –CO–NH– unit, check the carbonyl carbon is bonded to the nitrogen, and confirm the chain continues from the carbonyl carbon on one side and from the nitrogen on the other. Recognising these groups lets you predict hydrolysis products and distinguish polyamides from polyesters, whose linkage is –CO–O–.

This criterion asks you to predict the polymer formed from given monomers by deciding whether the process is addition or condensation. In addition polymerisation an alkene with a C=C double bond opens that bond and joins with others, so the repeat unit has the same atoms as the monomer and no small molecule is lost; poly(ethene) from ethene is the model case. In condensation polymerisation two functional groups react, for example a carboxylic acid with an alcohol to give a polyester with –CO–O– linkages, or a carboxylic acid with an amine to give a polyamide with –CO–NH– linkages, and a small molecule such as water is eliminated each time a link forms. To predict, identify the functional groups, decide which type of polymerisation applies, then draw the repeat unit showing the new linkage and the continuation bonds, and name any small molecule released.

A repeat unit is the smallest section of a polymer chain that, repeated many times, reproduces the whole structure. For a polyester, take a dicarboxylic acid and a diol: the acid contributes a –COO– link and the diol contributes the other half of that ester link, so the repeat unit contains one residue of each monomer joined by two ester groups. For a polyamide, a dicarboxylic acid and a diamine give a repeat unit with –CONH– amide links. Draw the two monomer residues joined, then place the two free bonds at the outer ends of the repeat unit, shown crossing the brackets. Remove the –OH from the acid and the –H from the diol (or amine) at each joining point, and never leave the eliminated atoms inside the unit. Check that the repeat unit has the same atoms as the two residues minus the small molecules lost.

Working backwards from a polymer section means locating the repeating link and splitting the chain there. In a polyester, find each –COO– ester group: the carbonyl carbon and its oxygen come from the dicarboxylic acid, and the oxygen on the other side comes from the diol. Break the C–O single bond of the ester link, add –OH to the carbonyl side to rebuild the acid and add –H to the other side to rebuild the diol. In a polyamide, find each –CONH– amide link: the carbonyl side rebuilds the dicarboxylic acid and the nitrogen side rebuilds the diamine, again adding –OH and –H respectively. Repeat for every link in the section so each monomer is identified, and check that the two monomers together account for all the atoms shown.

When you identify the type of polymerisation that forms a polyester or a polyamide, decide between condensation and addition by looking at what happens to the monomers. In condensation polymerisation two different monomers, each with two functional groups, join repeatedly and a small molecule such as water or hydrogen chloride is eliminated at every linkage. A diol plus a dicarboxylic acid gives a polyester with ester links; a diamine plus a dicarboxylic acid gives a polyamide with amide links. In addition polymerisation an alkene monomer with a C=C double bond opens that bond and adds to the growing chain with no small molecule lost. So the presence of two functional groups per monomer and the loss of a small molecule each time identify condensation; an unsaturated monomer adding with nothing eliminated identifies addition.