Making a polyester

5 marks · Hard

The polymerisation of propane-1,3-diol and benzene-1,4-dicarboxylic acid forms a polyester. Propane-1,3-diol has the formula and benzene-1,4-dicarboxylic acid has the formula .

(a)

Draw the structural formula of both monomers.

[2 marks]

(b)

Explain why water is eliminated when the two monomers react to form the polyester.

[1 mark]

(c)

State one everyday use of polyesters produced in this way.

[1 mark]

(d)

Name the type of polymerisation that occurs and give one reason for this classification.

[1 mark]

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) Structural formula for propane-1,3-diol: HOCH2CH2CH2OH; for benzene-1,4-dicarboxylic acid: HOOC–C6H4–COOH (ring with –COOH groups at para positions) · must show Correct structures for both monomers, showing –OH and –COOH groups

(b) Because the –OH from the carboxyl group of the acid and an H from the –OH group of the alcohol combine to form water each time an ester link forms. · must show Reference to removal of –OH and H (or water) from functional groups

(d) Condensation polymerisation; because a small molecule (usually water) is eliminated in the reaction. · must show Name (condensation polymerisation), reason (small molecule eliminated)

Common mistakes: Drawing incorrect structures (wrong position of functional groups); Saying no small molecule is lost or naming polymerisation type incorrectly

Show mark scheme and worked solution

Marking points

B1 (a) · 1 mark · Correct structure for propane-1,3-diol (HOCH2CH2CH2OH shown with both –OH groups at ends)

B2 (a) · 1 mark · Correct structure for benzene-1,4-dicarboxylic acid (benzene ring with –COOH at positions 1 and 4)

B3 (b) · 1 mark · Explains water is formed by removal of –OH from acid and H from alcohol (or equivalent reference to elimination from the two groups forming ester link)

B4 (c) · 1 mark · States any correct everyday use (e.g. textile, plastic bottle, clothing, fibres)

B5 (d) · 1 mark · Names condensation polymerisation and gives valid reason (small molecule eliminated)

Worked solution

a. The structural formula for propane-1,3-diol is HOCH2CH2CH2OH. The structural formula for benzene-1,4-dicarboxylic acid is a benzene ring with a –COOH group attached at carbon 1 and at carbon 4 (opposite sides of the ring).

b. Each time an ester bond forms between the acid and alcohol groups, the –OH from the acid and an H from the alcohol’s –OH combine and are eliminated as water.

c. An everyday use of polyesters like this is in the production of fibres for clothing or textiles.

d. The reaction type is condensation polymerisation because a small molecule, usually water, is produced each time a bond forms between the two monomers.