Due to their advantageous properties such as lower density, high specific surface and high loading capacity, polymer-based hollow microspheres show promising potential of applications in the areas of white pigment, carriers for drugs, bioactive and catalytically active substances, micro-reactor, etc. Thus, it is a worthwhile and challenging work to prepare polymer-based hollow microspheres by exploring new methods and new technologies. Here, the development of the preparation methods and applications of polymer-based hollow microspheres are reviewed. Firstly, the traditional methods, such as osmotic swelling method, hard template synthesis method, layer-by-layer assembly, and etc. are introduced. Then, the new methods presented in recent years, including self-assembly method, soft template synthesis method, Pickering emulsion polymerization and self-removing template method, are emphatically described. Furthermore, the applications of polymer-based hollow microspheres are introduced briefly. At last, the current problems as well as the corresponding research directions are discussed.

Contents
1 Introduction
2 Traditional methods to prepare polymer-based hollow microspheres
2.1 Osmotic swelling method
2.2 Seed swelling polymerization
2.3 Hard template synthesis method
2.4 Layer-by-layer assembly
2.5 Miniemulsion or microemulsion polymerization
2.6 W/O/W method
3 New methods to prepare polymer-based hollow microspheres in recent years
3.1 Solvent-evaporation method
3.2 Self-assembly method
3.3 Soft template synthesis method
3.4 Cross-linked micelles method
3.5 Pickering emulsion polymerization
3.6 Self-removing template method
4 Applications of polymer-based hollow microspheres
4.1 Covering pigment
4.2 Microreactors
4.3 Wastewater treatment
4.4 Drug carrier and controlled release
4.5 Encapsulation and immobilization of bioactive and catalytically active substances
5 Conclusion and outlook