  Conformation-dependent design of polymer sequences can be considered as a
tool to control macromolecular self-assembly. We consider the monomer unit
sequences created via the modification of polymers in a homogeneous melt in
accordance with the spatial positions of the monomer units. The geometrical
patterns of lamellae, hexagonally packed cylinders, and balls arranged in a
body-centered cubic lattice are considered as typical microphase-separated
morphologies of block copolymers. Random trajectories of polymer chains are
described by the diffusion-type equations and, in parallel, simulated in the
computer modeling. The probability distributions of block length $k$, which are
analogous to the first-passage probabilities, are calculated analytically and
determined from the computer simulations. In any domain, the probability
distribution can be described by the asymptote $~k^{-3/2}$ at moderate values
of $k$ if the spatial size of the block is less than the smallest
characteristic size of the domain. For large blocks, the exponential asymptote
$exp(-const \, k a^2/d_{as}^2)$ is valid, $d_{as}$ being the asymptotic domain
length (a is the monomer unit size). The number average block lengths and their
dispersities change linearly with the block length for lamellae, cylinders, and
balls, when the domain is characterized by a single characteristic size. If the
domain is described by more than one size, the number average block length can
grow nonlinearly with the domain sizes and the length das can depend on all of
them.
