Well-defined polystyrene (PSt) with terminal carboxylic acid groups can be prepared via atom transfer radical polymerization (ATRP). One method involves the use of protected alpha-halocarboxylic acid initiators. Unprotected a-halocarboxylic acid initiators, such as 2-bromobutyric acid (BBA), had low initiator efficiency of 0.1-0.2 and were not effective for the ATRP of styrene. Protection of the carboxylic acid group of BBA by a trimethylsilyl, tert-butyldimethylsilyl, or tert-butyl group led to improved initiator efficiencies of ca. 0.6, 0.8, and 1, respectively, for the ATRP of styrene. Subsequent hydrolysis of the protecting groups can afford well-defined PSt with terminal carboxylic acid groups. Another method involves the use of carboxylic acid initiators with remote halogens, such as 4-(1-bromoethyl)benzoic acid and 4-(2-(2-bromopropionyloxy)ethoxy)benzoic acid. Well-defined PSt with terminal carboxylic acid groups and low polydispersities were prepared with initiator efficiencies close to 0.7.