Dielectric behavior was examined for nanodomain-forming styrene isoprene (SI) diblock and SIIS triblock copolymers, the former having type-A dipoles aligned along its I block, whereas the latter being the head-to-head dimer of the former and having the dipoles inverted at the midpoint of the middle II block. The slow dielectric relaxation at low temperatures ( cylinder (Cyl) > lamella (La). This order appears to naturally reflect the domain continuity that helps large-scale motion of the I block without violating the osmotic constraint. In the Sph and/or Cyl domains, all II blocks of the SIIS copolymer have their ends fixed on the surface of the same domain and are geometrically classified as loop-type blocks. These loop-type blocks are naively expected to behave similarly, in a dynamic sense, to the tail-type I blocks of SI. Nevertheless, comparison of the dielectric data of the SITS and SI copolymers suggested that some of the II blocks are classified as dynamically pseudobridge-like blocks behaving differently from the tail-type I blocks. This behavior of the II blocks in the Sph/Cyl domains was speculated to reflect their end-to-end distance: This distance governs the direction of II block tension that acts on the midpoint of this block thereby affecting the dielectrically detected midpoint motion. The II block in the Sph/Cyl domain appears to exhibit the pseudobridge-like behavior when it has a large end-to-end distance and the two half-fragments of this block pull the midpoint in the opposite directions. This argument is crude but seems to be physically acceptable, as suggested from an approximate analysis of the distribution of the end-to-end distance in those domains.