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(h) Synthetic polymers

Syllabus
2024
Topic
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Level
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Learning objectives

4.44Addition polymer formationKnow that an addition polymer is formed by joining up many small molecules called monomers4.45Addition polymer repeat unitsUnderstand how to draw the repeat unit of an addition polymer, including poly(ethene), poly(propene), poly(chloroethene) and (poly)tetrafluoroethene4.46Monomers and repeat unitsUnderstand how to deduce the structure of a monomer from the repeat unit of an addition polymer and vice versa4.47Addition polymer disposalExplain problems in the disposal of addition polymers, including:• their inertness and inability to biodegrade• the production of toxic gases when they are burned.448C Polyester formationKnow that condensation polymerisation, in which a dicarboxylic acid reacts with a diol, produces a polyester and water449C Polyester formulaeWrite the structural and displayed formula of a polyester repeat unit from its monomers, including ethanedioic acid reacting with ethanediol.450C Biodegradable polyestersKnow that some polyesters, known as biopolyesters, are biodegradable

Build an addition polymer from monomers

An addition polymer forms when many small molecules called monomers join to make one long-chain molecule.

For an alkene monomer, the carbon-carbon double bond opens: the two carbon atoms form a single bond in the polymer backbone and each can bond to the next repeat unit.

n\ce{CH2=CH2 -> [-CH2-CH2-]_{n}}

Monomer Addition polymer
small molecule long-chain molecule
contains a reactive C=C\ce{C=C} bond backbone contains C−C\ce{C-C} single bonds
many molecules join no small molecule is eliminated

Addition polymerisation does not produce water or another small-molecule by-product. Do not describe the monomer as a repeat unit until its double bond has been changed to the polymer backbone.

Draw addition-polymer repeat units

To draw an addition-polymer repeat unit, change the monomer's C=C\ce{C=C} bond to C−C\ce{C-C}, keep every group attached to the same carbon, and show continuation bonds through brackets with nn outside.

Monomer Polymer repeat unit Polymer
CHX2=CHX2\ce{CH2=CH2} [−CHX2−CHX2X−]Xn\ce{[-CH2-CH2-]_{n}} poly(ethene)
CHX2=CHCHX3\ce{CH2=CHCH3} [−CHX2−CH(CHX3)X−]Xn\ce{[-CH2-CH(CH3)-]_{n}} poly(propene)
CHX2=CHCl\ce{CH2=CHCl} [−CHX2−CHClX−]Xn\ce{[-CH2-CHCl-]_{n}} poly(chloroethene), PVC
CFX2=CFX2\ce{CF2=CF2} [−CFX2−CFX2X−]Xn\ce{[-CF2-CF2-]_{n}} poly(tetrafluoroethene), PTFE

Check four features: no C=C\ce{C=C} remains; both backbone carbons are present; all H atoms and substituents are retained; and each end has a continuation bond crossing the bracket.

Brackets alone are not enough. A repeat unit with a double bond, a missing substituent, or no continuation bonds does not represent the addition polymer correctly.

Convert between a monomer and its repeat unit

The two carbon atoms joined in an alkene monomer become the two-carbon backbone of its addition-polymer repeat unit. The conversion is reversible on paper.

Direction Method
monomer →\rightarrow repeat unit change C=C\ce{C=C} to C−C\ce{C-C}, retain attached groups, add two continuation bonds and brackets
repeat unit →\rightarrow monomer remove brackets and continuation bonds, then change the backbone C−C\ce{C-C} between the two repeat carbons to C=C\ce{C=C}

\ce{CH2=CHCH3 <=> [-CH2-CH(CH3)-]_{n}}

After restoring the double bond, each carbon must still have four bonds. Groups such as CHX3\ce{CH3}, Cl\ce{Cl} or F\ce{F} stay on the carbon where they appeared in the repeat unit.

Do not insert a double bond between repeat units or remove side groups. The monomer is one alkene molecule, so it has no brackets, nn, or continuation bonds.

Explain addition-polymer disposal problems

Many addition polymers are chemically inert and cannot be broken down by microorganisms, so they are non-biodegradable and persist after disposal.

Disposal route Environmental problem Explanation
landfill limited space is occupied for a long time inert, non-biodegradable polymers decompose extremely slowly
burning harmful gases may be released combustion can produce toxic gases; burning poly(chloroethene) can produce hydrogen chloride

A complete explanation links a property to its consequence: inert →\rightarrow not biodegraded →\rightarrow long-term landfill accumulation; burning →\rightarrow toxic gases →\rightarrow harm to organisms or air quality.

Do not say that inert polymers react with soil or that burning always makes them harmless. Name the disposal method and its distinct problem rather than giving the same vague pollution claim twice.

Form a polyester by condensation polymerisation

In condensation polymerisation, a dicarboxylic acid reacts with a diol to form a polyester and water.

Substance Two functional groups Role
dicarboxylic acid two −COOH\ce{-COOH} groups supplies carbonyl-containing parts of ester links
diol two −OH\ce{-OH} groups supplies oxygen-containing parts of ester links
polyester many −C(=O)−OX−\ce{-C(=O)-O-} links long-chain condensation polymer

n\ce{HOOC-R-COOH} + n\ce{HO-R'-OH -> [-(C(=O)-R-C(=O)-O-R'-O)-]_{n}} + \ce{H2O}

Each new ester linkage forms when the reacting groups lose the elements of water. Because both monomers have a functional group at each end, the reaction can continue to build a chain.

A diol plus a dicarboxylic acid is not addition polymerisation: a small molecule is eliminated and the backbone contains ester linkages. The precise water coefficient depends on how polymer end groups are represented.

Construct a polyester repeat unit from its monomers

To construct a polyester repeat unit, remove OH\ce{OH} from each carboxyl group and H\ce{H} from each alcohol group, then join the remaining fragments through −C(=O)−OX−\ce{-C(=O)-O-} ester links.

Reactant Structural formula Retained chain fragment
ethanedioic acid HOOC−COOH\ce{HOOC-COOH} −C(=O)−C(=O)X−\ce{-C(=O)-C(=O)-}
ethanediol HO−CHX2−CHX2−OH\ce{HO-CH2-CH2-OH} −O−CHX2−CHX2−OX−\ce{-O-CH2-CH2-O-}

n\ce{HOOC-COOH} + n\ce{HO-CH2-CH2-OH -> [-C(=O)-C(=O)-O-CH2-CH2-O-]_{n}} + 2n\ce{H2O}

Draw both retained fragments, join them through an ester linkage, place brackets around one complete acid-plus-diol repeat, and put continuation bonds through the bracket. Finally check that the repeat contains both carbonyl carbons and both CHX2\ce{CH2} groups.

Do not change a carbonyl C=O\ce{C=O} to C−O\ce{C-O}, omit an oxygen, or copy the addition-polymer method. A polyester repeat unit must contain the −C(=O)−OX−\ce{-C(=O)-O-} linkage and fragments from both monomers.

Recognise biodegradable polyesters

A biopolyester is a polyester that is biodegradable: microorganisms can break it down into simpler substances.

Polymer After disposal
biodegradable polyester (biopolyester) can be decomposed by microorganisms
non-biodegradable polymer persists because microorganisms cannot break it down

Biodegradability can reduce long-term persistence and accumulation compared with a non-biodegradable polymer when suitable biological conditions are present.

Some polyesters are biodegradable; this does not mean that every polyester or every polymer is biodegradable. 'Bio' here identifies the ability to undergo biological decomposition.