RAFT polymerization of acrylamide monomers with long alkyl sidechains was studied for application to block copolymer synthesis and nanoscale assembly using the Langmuir–Blodgett technique. Controlled radical polymerization was demonstrated by optimizing the chain transfer agent (CTA) concentration: amphiphilic acrylamide block copolymer, poly(N-dodecylacrylamide)-block-poly(N-tert-pentylacrylamide) (pDDA-b-ptPA) was obtained with high molecular weight (1.3 × 10⁴) and narrow molecular distribution (M_w/M_n = 1.4). The diblock copolymer formed a stable monolayer at the air–water interface under a surface pressure of 30 mN m⁻¹, but the presence of the junction between pDDA and ptPA affected the monolayer mechanical properties at higher surface pressures. Results provide insight into the two-dimensional self-assembly of amphiphilic block copolymers consisting of molecular-level amphiphilic block components.