Polymer-controlled crystallization
is becoming an increasingly
important approach to achieve functional materials precipitated from
the solution phase. Nevertheless, there exist multiple pathways under
the control of yet unpredictable kinetic factors, which significantly
hinder the mechanistic understanding. Herein, the mechanistic study
of polymer-controlled precipitation of a typical drug compound, indomethacin,
was performed. The presence of a nonionic additive poly(vinylpyrrolidone)
induces the heterogeneous nucleation of networks of dumbbell-shaped
crystalline microspheres on amorphous precursor domains, which is
an emerging pathway in polymer-controlled crystallization. This pathway
is also verified in the precipitation of l-histidine in the
presence of poly(acrylic acid) in the current study. As a comparison,
the presence of a cationic polymeric additive, branched polyethylenimine,
promotes the formation of aggregates of amorphous nanoparticles and
the subsequent crystallization of a spherical-shaped microsphere on
each aggregate, a pathway in line with those in numerous studies of
polymer-controlled crystallization of inorganic compounds. Our results
suggest that the heterogeneous crystallization of microspheres on
the amorphous precursor domains could be a specific pathway occurring
in polymer-controlled precipitation of organic compounds mainly due
to the fast kinetics of the precipitation process. In short, our study
broadens our understanding of polymer-controlled crystallization of
functional organic crystals.