Self-assembling peptide materials are promising next-generation materials with applications that include tissue engineering scaffolds, drug delivery, bionanomedicine, and enviro-responsive materials. Despite these advances, synthetic methods to form peptides and peptide-polymer conjugates still largely rely on solid-phase peptide synthesis (SPPS) and N-carboxyanhydride ring-opening polymerization (NCA-ROP), while green methods remain largely undeveloped. This work demonstrates a protease-catalyzed peptide synthesis (PCPS) capable of directly grafting leucine ethyl ester (Leu-OEt) from the C-terminus of a methoxy poly(ethylene glycol)-phenylalanine ethyl ester macroinitiator in a one-pot, aqueous reaction. By using the natural tendency of the growing hydrophobic peptide segment to self-assemble, a large narrowing of the (Leu)(x) distributions for both mPEG(45)-b-Phe(Leu)(x) and oligo(Leu)(x) coproducts, relative to oligo(Leu)(x) synthesized in the absence of a macroinitiator (mPEG(45)-Phe-OEt)(x) was achieved. A mechanism is described where in situ beta-sheet coassembly of mPEG(45)-b-Phe(Leu)(x) and oligo(Leu)(x) coproducts during polymerization prevents peptide hydrolysis, providing a means to control the degree of polymerization (DP) and dispersity of diblock (Leu)(x) segments (matrix-assisted laser desorption time-of-flight (MALDI-TOF) x = 5.1, dispersity <= 1.02). The use of self-assembly to control the uniformity of peptides synthesized by PCPS paves the way for precise peptide block copolymer architectures with various polymer backbones and amino acid compositions synthesized by a green process.