Biodegradable polyesters
2 marks · EasyLactic acid () is the monomer for the biodegradable polyester called PLA (polylactic acid).
Draw the displayed (structural) formula for a section of the polylactic acid (PLA) polymer, showing two repeat units joined together.
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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Must show: At least two lactic acid repeat units joined by an ester linkage, Removal of –OH and –H to show water loss, Displayed formula—show all bonds clearly
Also accept: Repeat unit drawn with bracket and subscript n, joined together in correct orientation
Common mistakes: Drawing lactic acid molecules simply linked without removal of water; Omitting the ester functional group
Show mark scheme and worked solution
Marking points
M1 · 1 mark · Displays two correct repeat units from lactic acid, connected by correct ester linkage (–COO–) between them
A1 · 1 mark · Shows all atoms and bonds (including side-chain –CH3 and –O– links), with correct removal of elements of water between units
Worked solution
The repeat unit of PLA from lactic acid is:
Displayed formula for two joined units: