:For other meanings see Substitution.
Polystyrene has a chain where the substituent R is a phenyl group (C6H5), corresponding to a benzene ring minus one hydrogen: -CH2-CH(C6H5)n-, so the repeat unit is -CH2-CH(C6H5)n-.
Starting with aspartic acid β esters undergoing acylation to differentiate the first substituent, linked to carbon-2, followed by Dakin-West conversion to keto-amide to introduce the second substituent, and ending with the Robinson-Gabriel cyclodehydration at 90°C for 30 minutes with either phosphorus oxychloride in DMF or catalytic sulfuric acid in acetic anhydride.