Poly(n-hexylisocyanate) (PHIC-NH) as a rod-like polymer having a -NH group at one end of the polymer chain was found to instantly (less than a few seconds) be completely decomposed by CH(3)O(-)Na(+) in tetrahydrofuran/CH(3)OH under a mild experimental condition ([CH(3)O(-)Na(+)] < 1 mol L(-1) at room temperature). The mechanism of the decomposition for the PHIC-NH consists of two steps: the first is abstraction of the proton for PHIC-NH by Na(+) with a slow reaction rate, and the second is the consequent depolymerization of PHIC-N(+) based on the equilibrium polymerization with a rapid depolymerization rate. The decomposition rate constants (k(d)) depend on M(w) of PHIC-NH, namely k(d) similar to M(w) (-1.0). The decomposition of the PHIC-NR having an end-capped -NR group was completely depressed. Using an "all or nothing" mechanism for the decomposition, the (PHIC)(3.43) comb-shaped polymer and polystyrene (PSt)-graft-(PHIC-NH)(4.28) graft copolymer were, respectively, decomposed to produce (PHIC-NH)(3.43-1.03) and PSt-graft-(PHIC-NH)(4.28-0) in a series with different numbers of PHIC-NH combs and PHIC-NH grafts by regulating the amount of CH(3)O(-)Na(+) and the decomposition time. Molecular structure of (PHIC-NH)(3.43-1.03) and PSt-graft-(PHIC-NH)(4.28-0) was discussed from a viewpoint of PSt-reduced chain dimension per molar mass. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 3939-3950, 2011