  Synthetic methods to control the structure of materials at sub-micron scales
are typically based on the self-assembly of structural building blocks with
precise size and morphology. On the other hand, many living systems can
generate structure across a broad range of length scales in one step directly
from macromolecules, using phase separation. Here, we introduce and control
structure at the nano- and microscales through polymerization in the solid
state, which has the unusual capability of both triggering and arresting phase
separation. In particular, we show that atom transfer radical polymerization
(ATRP) enables control of nucleation, growth, and stabilization of
phase-separated poly-methylmethacrylate (PMMA) domains in a solid polystyrene
(PS) matrix. ATRP yields durable nanostructures with low size dispersity and
high degrees of structural correlations. Furthermore, we demonstrate that the
length scale of these materials is controlled by the synthesis parameters.
