  In modern applications of heterogeneous liquid-phase nanocatalysis, the
catalysts (e.g., metal nanoparticles) need to be typically affixed to a
colloidal carrier system for stability and easy handling. "Passive carriers"
(e.g., simple polyelectrolytes) serve for a controlled synthesis of the
nanoparticles and prevent coagulation during catalysis. Recently, however,
hybrid conjugates of nanoparticles and synthetic thermosensitive polymers have
been developed that enable to change the catalytic activity of the
nanoparticles by external triggers. In particular, nanoparticles embedded in a
stimuli-responsive network made from poly(N-isopropylacrylamide) (PNIPAM) have
become the most-studied examples of such hybrids. It has been demonstrated that
the permeability of the polymer network and thus the reactant flux can be
switched and controlled by external stimuli. Such "active carriers" may thus be
viewed as true nanoreactors that open up new design routes in nano-catalysis
and elevate synthesis to create highly selective, programmable "colloidal
enzymes". However, only a comprehensive understanding of these materials on all
time and length scales can lead to a rational design of future, highly
functional materials. Here we review the current state of the theoretical and
multi-scale simulation approaches, aiming at a fundamental understanding of
these nanoreactors. In particular, we summarize a theoretical approach for
reaction rates of surface-catalyzed bimolecular reactions in responsive
nanoreactors in terms of the key material parameters, the polymer shell
permeability P and the reactant partition ratio K. We discuss recent computer
simulation studies of both atomistic and coarse-grained polymer models in which
these quantities have been characterized in some detail. We conclude with an
outlook on selected open questions and future theoretical challenges in
nanoreactor modeling.
