Access to protein-inspired polyisocyanates with high molecular weights (MWs) from anionic polymerization of isocyanates is challenging as it requires exceptional livingness. For this purpose, a dimerically self-associated sodium diphenylamide (NaDPA) was introduced as a robust chain-end-protective initiator in the anionic polymerization of n-hexyl isocyanate (HIC) in the absence or presence of sodium tetraphenylborate (NaBPh4) additive. At [NaDPA](0)/[NaBPh4](0) = 5, unusual one-half initiation efficiency and one-half order of reaction kinetics were observed. Accordingly, initiator-transfer anionic polymerization (ITAP), a mechanism driven by the dimer of NaDPA ((NaDPA)(2)) in a dual role is proposed, in which one unimer initiates the polymerization and the other reversibly deactivates the propagating chain end through its repetitive cycles of 1:1 cross-association/dissociation. Living ITAP by NaDPA with NaBPh4 was proven by X-ray crystallography, density functional theory calculation, and quantitative yield of poly(n-hexyl isocyanate)s with an expanded range of controlled MWs (M-n = 6.09-47.8 kDa and D = 1.07-1.16).