# Precipitation Polymerization: A Powerful Tool for Preparation of Uniform Polymer Particles
## 2 Categories of Precipitation Polymerization
### Self-Stable Precipitation Polymerization  
In recent years, the [PERSON] group has developed a novel heterogeneous polymerization technique, termed self-stabilized precipitation polymerization (2 SPP), beyond emulsion/dispersion/suspension. Compared with the TRPP method, this reaction system has the following characteristics: (1) High monomer concentration and the choice of the reaction medium is crucial; (2) Quiescent polymerization, a stable colloid composed of uniform polymer particles and reaction medium was formed through a self-nucleation and surface deposition process in the absence of any stabilizers; (3) The polymer particles can be easily separated from the solvent, and the supernatant liquid can be recycled, making this one of the most efficient, green and easily scaled-up strategies [113]. A possible mechanism was proposed for this 2 SPP process that the nucleation of particles occurred homogeneously only at the early stage in the solution phase; polymer chains will aggregate and gradually grow and nucleate out of the solution. Then polymer chains are continuously deposited on the particles that were precipitated from the solution and eventually grow into balls [114]. [PERSON] et al. further used a fluorescence self-imaging method based on aggregation-induced  
Figure 7: Three Proposed Particle Growth Mechanism in RAFTTPP of MAA. (**a**) By capturing oligomeric radicals with particle surface vinyl groups; (**b**) By adsorbing oligomeric polymer chains from the continuous phase; (**c**) By the surface-initiated RAFT polymerization through the surface dithioester groups. Reprinted with permission from Ref. [48]. Copyright 2019 American Chemical Society.  
luminescence to monitor the process of particle generation and growth during 2 SPP in a real-time and disturb-free manner [115].  
A range of common olefinic monomers, including mono-olefins, di-olefins, and aromatic olefins, can undergo a 2 SPP reaction in a specific solvent environment with maleic anhydride (MAH) or its derivatives. The copolymerization equipped with highly enhanced reactivity of MAH with vinyl acetate (VAc), styrene, \(\alpha\)-methyl styrene, and DVB [116; 117; 118] has been achieved, and the obtained polymers could support further preparing bio-based, thermally stable, or fluorescent materials [119; 120; 121; 122; 123; 124; 125]. The 2 SPP method also provides a new way to transform huge amounts of olefinic compounds in the C4\(\sim\)C9 fraction of the petrochemical industry and prepare polyolefin products containing acid anhydride functional groups. This convenient and efficient process can make up for deficiencies of the heterogeneous catalyst (Ziegler-Natta, Metallocenes, etc.) in the preparation of polar copolymers and getting well-defined nano- or micron-sized polymeric particles [38].  
Recently, such heterogenization strategies have been explored in the field of ethylene polymerization [126] and ethylene-polar monomer copolymerization [127] to obtain polyolefin spherical particles with improved mechanical properties and great product morphology control. Through modulating polymerization parameters, the particles could be well-tuned, causing the precipitation and separation of polymers from reactive medium avoiding reactor fouling (see Figure 8). [PERSON] et al. designed an ionic cluster strategy to synthesize polar-functionalized polyolefins via PP, which greatly enhances the catalyst's thermal stability (90-150 \({}^{\circ}\)C) and enables the homopolymerization of both terminal and internal polar-functionalized olefins [127].