Synthetic Polymers

8 marks · Hard

This question is about the structures of polymers made by different processes.

(a)

Draw the displayed formula for the repeat unit of poly(ethene), an addition polymer.

[2 marks]

(b)

Lactic acid, CH3CH(OH)COOH, can polymerise to form a biodegradable polyester (polylactic acid, PLA).

(i)

Draw the structure of a lactic acid monomer, showing all atoms and bonds.

[2 marks]

(ii)

Draw the displayed formula for the repeat unit of polylactic acid (PLA), as formed from lactic acid.

[2 marks]

(c)

Explain why PLA is biodegradable, but poly(ethene) is not.

[2 marks]

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.

Show answer

(a) Displayed formula for poly(ethene) repeat unit: [—CH2—CH2—]n, drawn with bonds, brackets and the correct number of atoms. · must show —CH2—CH2— repeat unit, correct structure with brackets and n

(b(i)) Displayed formula for lactic acid monomer: CH3–CH(OH)–COOH; each atom and bond shown clearly. · must show all atoms and bonds correctly shown, CH3, OH on central carbon, COOH group

(b(ii)) Displayed formula for the repeat unit of PLA: —[O—CH(CH3)—CO]—n, with correct ester linkage and all atoms shown. · must show ester linkage, side chain CH3, full displayed formula, repeat unit with n

(c) PLA contains ester linkages that can be broken down by hydrolysis (by enzymes or water), allowing microorganisms to degrade it. Poly(ethene) has only strong carbon-carbon single bonds, which are not readily broken down by natural processes. · must show ester linkages in PLA, hydrolysis by enzymes or water, poly(ethene) has only C–C bonds, not broken by microorganisms

Show mark scheme and worked solution

Marking points

A1 (a) · 2 marks · Correct displayed formula for poly(ethene) repeat unit (with bonds, brackets, and n subscript)

A2 (b(i)) · 2 marks · Draws accurate structure of lactic acid, showing CH3, central carbon with —OH, and —COOH group

A3 (b(ii)) · 2 marks · Draws accurate displayed formula for PLA repeat unit, showing ester linkage and correct side chain

B1 (c) · 2 marks · Explains with reference to ester linkage and hydrolysis (PLA), and lack of reactivity of C–C bonds (poly(ethene))

Worked solution

a) The displayed formula for poly(ethene) repeat unit:

b) (i) The structure of a lactic acid monomer:

b) (ii) The displayed formula for the repeat unit of PLA is:

c) PLA is biodegradable because the ester linkages in its polymer chain can be hydrolysed by water or enzymes, enabling it to be broken down by microorganisms. Poly(ethene) consists only of C–C and C–H bonds, which are very stable and not easily broken by environmental processes, so it does not biodegrade.