Anionic polymerization of n-hexyl isocyanate (HIC) in THF at -98 degrees C under 10(-6) Torr for the kinetic study was performed by the initiation of sodium diphenyl-methane (NaDPM) in either the absence or presence of sodium tetraphenylborate (NaBPh4; [NaBPh4](0)/[NaDPM](0) = 0 or 5). The common-ion effect of NaBPh4 to suppress the formation of unstable free amidate anions led to termination-free propagation by amidate ion pairs. In [HIC](0)/[NaDPM](0) = 50.8/101/201, the initiation of NaDPM early reached similar to 100% efficiencies during propagation, which led to the yield of poly(n-hexyl isocyanate)s (PHICs) with predictable molecular weights (M-n,(theo) = 6.50/12.7/24.7 kDa; M-n= 6.50/12.7/26.1 kDa) and low dispersities (D = 1.06/1.07/1.15). Within the conditions, the rate of propagation accorded with a first-order dependence on [NaDPM](0), indicating that the propagating amidate ion pairs are intrinsically unimeric (nonassociated). Kinetics of anionic copolymerization of HIC and allyl isocyanate (AIC) exhibited a monomer distribution toward a tapered block sequence.