# Application of Polymers as a Tool in Crystallization--A Review
## 2 Basic Research
### Discussion and Conclusions  
The first paragraph of this paper indicates that polymers offer a promising approach for the rational design of materials for controlling nucleation from solution (Figure 1). The confined crystallization with the application of polymeric matrixes becomes more and more popular since it allows one, for example, to mimic biological systems. The influence of gels on crystal growth depends on the type of gel being used. A decrease in solubility of any solute in the liquid may occur if the solvent interacts extensively with the polymers. Hence, the nucleation in gels in these cases occurs at relatively low supersaturations. The pores of gels are often referred as microreactors. Due to the specific conditions within such \"vessels\" a control over polymorphism may be achieved. This fact opens totally new routes of drug synthesis. However, the results obtained with the use of polymer microgels indicated that systematic studies are necessary in order to quantitatively clarify factors influencing, for example, surface mineral scaling crystallization. Other experimental results on polymer-solute interactions provided insights into nucleation at interfaces and enabled a rational designing of the nucleation of molecular crystals from solution (Figure 1). The lack of understanding the influence of surface properties on nucleation is an important problem in the design of crystallization processes. Unfortunately, current approaches are often empirical and lack any predictive capability. Usually, each system has to be treated as a separate case. However, a highly novel approach, PETI (Predicting Efficacy Through Intermolecular Interactions), presented in some papers, could be a tool in the rational design of polymer surfaces and the control of heterogeneous nucleation. This approach utilizes the Cambridge Structural Database (CSD) to determine the possibility of forming an intermolecular interaction between solute chemical moieties and polymer surfaces [44]. Moreover, one may also find other studies dedicated to the simulation and mathematical modeling of the crystallization mentioned above, e.g., a coarse-grained model which was successfully used for analysis of melt crystallization or crystallization with additives. Simultaneously, knowledge of the relationship between crystallization and phase separation is important to optimize solution processing performance of polymer blends. Crystal nucleation and crystal growth are coupled and it is sometimes difficult to distinguish their effects on phase separation. Therefore, many experimental works are devoted to these issues. Concluding, it has often been repeated that crystal growth is an art. Direct control of nucleation and crystal growth in a crystallization process is difficult to achieve but the use of assisting polymer additives offers many potential benefits to the food, chemical, and pharmaceutical industries (Figure 1). Figure 1: The classification of polymers’ usage as a tool in the crystallization process. Present and Future Perspectives of Applications of Crystallization Phenomena in the Presence of Polymers  
Although crystallization, for decades, was considered an empirical science, now, based on the advantages of research on nucleation and growth theory at the more precise molecular level, it turns to theory-based science and practice. The pharmaceutical industry is one of the examples where crystallization technology is the most important issue since it governs solid-state phase transformation and stability of pharmaceuticals. Therefore, vast publications related to this field concern groundbreaking discoveries about the effect of the polymer presence on co-crystals, polymorphs, and solvates of drugs.