  Extensive coarse grained molecular dynamics simulations are performed to
investigate the conformational phase diagram of a neutral polymer in the
presence of attractive crowders. We show that, for low crowded densities, the
polymer predominantly shows three phases as a function of both intra polymer
and polymer-crowder interactions: (1) weak intra polymer and weak
polymer-crowder attractive interactions induce extended or coil polymer
conformations (phase E) (2) strong intra polymer and relatively weak
polymer-crowder attractive interactions induce collapsed or globular
conformations (phase CI) and (3) strong polymer-crowder attractive
interactions, regardless of intra polymer interactions, induce a second
collapsed or globular conformation that encloses bridging crowders (phase CB).
The detailed phase diagram is obtained by determining the phase boundaries
delineating the different phases based on an analysis of the radius of gyration
as well as bridging crowders. The dependence of the phase diagram on strength
of crowder-crowder attractive interactions and crowder density is clarified. We
also show that when the crowder density is increased, a third collapsed phase
of the polymer emerges for weak intra polymer attractive interactions. This
crowder density induced compaction is shown to be enhanced by stronger
crowder-crowder attraction and is different from the depletion induced collapse
mechanism which is primarily driven by repulsive interactions. We also provide
a unified explanation of the observed reentrant swollen/extended conformations
of earlier simulations of weak and strongly self interacting polymers in terms
of crowder-crowder attractive interactions.
