# Precipitation Polymerization: A Powerful Tool for Preparation of Uniform Polymer Particles
## 1 Introduction  
Functional materials are the core of the new materials field and have been developed very rapidly in recent years [1]. The polymer microspheres with optimized characteristics, such as uniform size and shapes, the functionality of the base polymer, the morphology of the polymer particles, and the degree of crosslinking, are attractive for a wide number of applications, including supporting phases for separation science [2, 3], biomedical devices [4], chromatography separation [5, 6, 7], casting additives [8], and controlled release reservoirs [9, 10].  
Polymer microspheres of a particular size and uniformity are generally obtained with one of the heterogeneous polymerizations, including suspension, emulsion, dispersion, and seeded polymerizations. All the methods mentioned above require the presence of a suitable stabilizer or surfactant, usually in large quantities, to induce the formation of particles and prevent the aggregation of the colloidal particles. For instance, in dispersion polymerization, polymeric particles form and precipitate from the reaction medium, and stabilizers such as poly(vinyl pyrrolidone) must be utilized to stabilize polymeric particles. Precipitation polymerization (PP), first reported by [PERSON] et al. [11], was a very impressive method for the preparation of surface-clean microspheres without adding any stabilizer/surfactant or other additives, in which polymeric microspheres form by self-crosslinking of monomers/crosslinkers and precipitate out of the homogeneous solution due to the newly formed polymers with low solubility in the selected reaction media (\(\Theta\) solvent) [12].  
PP starts from a homogeneous mixture of monomer, initiator, and optional solvent [11, 13, 14]. [PERSON] et al. proposed that precipitation polymerization of divinylbenzene (DVB) in near-\(\Theta\) solvents is an entropic precipitation in cases where cross-linking prevents the polymer and solvent from freely mixing [15], and they further proposed the transient solvent-swollen gel layer mechanism to explain the formation, stabilization,and growth of microspheres [13; 14]. Uniform spherical polymer particles are formed through the particle nucleation and growth stages, where particle nuclei are generated by the aggregation of oligomers at the beginning of the polymerization, and their growth occurs mainly through capturing oligomeric radicals from the reaction medium by their reacting with the residual vinyl groups on the surfaces of the existing particles [14]. Further examinations showed that the presence of bifunctional monomers, suitable reaction medium, and moderate shaking are the keys to the formation of highly cross-linked monodisperse microspheres and various functionalized, porous, and core-shell microspheres by PP have been well prepared [16; 17]. Figure 1 shows the polydivinylbenzene (PDVB) microspheres with porous shells prepared in a toluene/acetonitrile mixture.  
Recent years have witnessed the rapid development of precipitation polymerization, and again, the method could offer an efficient way to tune the size and porosity of the spherical particles through control of the polymerization condition, as well as avoiding environmental pollution and increasing costs of additives, thus opening up numerous potential applications of functional materials [6; 18]. More researches have been focused on such techniques, such as molecularly imprinted polymers (MIPs) [19; 20; 21; 22; 23] or liquid crystalline (LC) polymer particles [24; 25], which have been attractive in recent years. Figure 2 shows the typical crosslinkers and monomers (including copolymer monomers) adopted for PP, along with 2,2-azobisisobutyronitrile (AIBN) as the frequently used initiator.  
Figure 1: SEM image of poly(DVB-55) microspheres with porous shells prepared in a toluene/acetonitrile (40/60) mixture. Reprinted with permission from Ref. [16]. Copyright 2000 American Chemical Society.