Synthetic Polymers
8 marks · HardPolymers can be made using different methods, giving materials with different environmental impacts. Use your knowledge of both addition polymerisation and biodegradable polyesters to answer the following.
Draw the displayed formula for the repeat unit of poly(propene), an addition polymer.
Lactic acid (CH3CH(OH)COOH) can be used to make a biodegradable polyester called PLA (polylactic acid).
Describe how lactic acid monomers join to form PLA. Include the type of reaction and the functional groups involved.
Draw the displayed formula for the repeat unit of polylactic acid (PLA).
State one advantage of using biodegradable polyesters like PLA instead of addition polymers such as poly(propene) for disposable items.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Displayed formula for poly(propene) repeat unit: [—CH2—CH(CH3)—]n or drawn with correct bonds and two-carbon backbone with CH3 on second carbon. · must show —CH2—CH(CH3)— repeat unit with correct linkage and brackets, 'n' subscript
(b(i)) Lactic acid monomers join by condensation polymerisation, where the –OH of the carboxylic acid group of one molecule reacts with the –H of the hydroxyl group of another, releasing water and forming an ester linkage. · must show condensation polymerisation, ester linkage formed between —OH and —COOH groups, reference to water removal
(b(ii)) Displayed formula of PLA repeat unit: —[O—CH(CH3)—CO]— with correct linkage and brackets, repeated unit (n). · must show displayed formula with ester linkage, side-chain CH3, correct repeat unit structure
(c) They biodegrade more easily, reducing long-term pollution and problems with disposal. · must show break down faster, less environmental impact, biodegradable
Show mark scheme and worked solution
Marking points
A1 (a) · 2 marks · Correct displayed formula for poly(propene) repeat unit; structure and labels accurate; brackets and subscript 'n' shown
M1 (b(i)) · 1 mark · Identifies that lactic acid molecules join by condensation polymerisation and forms an ester linkage
M2 (b(i)) · 1 mark · Describes functional groups: –OH and –COOH are involved, and water is released
A2 (b(ii)) · 2 marks · Correct displayed formula for PLA (polylactic acid) repeat unit with all necessary linkages, side groups, and correct orientation; brackets and 'n'
B1 (c) · 2 marks · Clearly states a valid advantage of biodegradable polyesters over addition polymers and explains why
Worked solution
a) Poly(propene) repeat unit (displayed formula):
b) (i) Lactic acid contains both –OH and –COOH groups in each molecule. During condensation polymerisation, the –OH on the carboxylic acid of one lactic acid reacts with the –H from the hydroxyl group of another, releasing water and joining the monomers by an ester linkage.
b) (ii) The repeat unit for polylactic acid (PLA) can be represented as:
c) One advantage of using biodegradable polyesters such as PLA is that they can break down more easily in the environment, avoiding problems caused by the non-biodegradable nature of addition polymers like poly(propene), which persist in landfill and create pollution.