Fragmentation of polymerization catalysts
Many industrial polymerization reactions are carried out with supported catalysts. Typically porous silica, MgCl2 or certain polymers are used as supports. For supported catalysts on initiation of polymerization, the active sites on the catalyst surface are rapidly fouled due to encapsulation by the polymer product. However, the catalyst may undergo fragmentation due to accumulation of polymers within the catalyst particles. This fragmentation results in exposure of new active catalytic sites and maintains the catalytic activity. The fragmentation process ensures access of the monomers to the active catalyst sites. The fragmentation of catalyst particles are typically observed for olefin polymerization reactions such as polyethylene and polypropylene productions with Ziegler–Natta catalysts. Fragmentation of catalyst particles results in higher polymer yield. Since recovery of the catalyst particles from polymer product is difficult and expensive, fragmentation of catalyst makes the catalyst particles small enough so that final product quality is not affected. In the final product, the size of the catalysts particles are in the range of ~ 100 nm which are embedded in large polymer particles of 200 -1000 µm diameter.
Fragmentation and polymer growth models
1. Core – shell model
According to this model, catalyst particles do not break up in the beginning of the polymerization process. Initially, polymerization occurs on the surface of the particle which acts as a core. Then, the polymer grows in the form of a shell around the core. After formation of accumulated polymer shell, the monomer has to diffuse through the polymer layer to reach the catalyst surface, where it reacts. The model is more applicable for catalysts with low porosity for which monomer diffusion is limited
Fig. 4. Core – shell model for polymer growth