Sequence-controlled copolymers have recently attracted great interest in a variety of applications, including antimicrobial materials. However, owing to the nature of radical polymerization, targeting multiblocks with low degree of polymerization is complicated due to the possibility of defective chains, significantly affecting the purity of the targeted materials. In addition, the effect of optimum DP and defective chains on the antimicrobial properties of sequence-controlled copolymers remains elusive. Herein, we report the quantitative synthesis of low molecular weight copolymers via photo-induced ATRP aiming to identify the influence of degree of polymerization, block order and the defects on antimicrobial properties of sequence-controlled materials. We demonstrate that sequence-controlled copolymers with shorter amine blocks increase the antimicrobial efficacy of the resulting material towards Gram-negative bacteria while shorter hydrophobic blocks improve the efficacy towards Gram-positive bacteria. Importantly, we also demonstrate that sequence-controlled materials with very low degree of polymerization (DP = 3) exhibit the highest antimicrobial activity, despite the presence of defective chains. This work offers new insights into the structure/property relationship and highlights the promise of low DP, sequence-controlled block copolymers prepared by controlled polymerizations.