# Highly Flowable Nano TiO2/Porous Organic Polymer (POP) Supports for Efficient Metallocene Catalysts
## 1 Introduction  
Polyolefins are undoubtedly one of the most robust fields in polymer production and consumption globally [1; 2; 3]. Along with continuously growing demand worldwide, there is the remarkable progress of polyolefin catalysts and polymerization process technologies [4; 5; 6; 7; 8; 9; 10]. The single-site metallocene catalysts have attracted extensive interest, both from academia and industry, due to their abilities to precisely control the polymer microstructure and stereospecificity by simply tailoring the ligands used [11; 12; 13; 14; 15].  
Heterogeneous catalysis is required in the gas-phase or the slurry process for industrial applications to avoid reactor fouling and to control the polymer morphology or the forming granule reactor by immobilization of the homogeneous catalytic sites into a carrier [16; 17; 18; 19; 20]. Inorganic supports, such as silica gel, magnesium chloride, aluminum oxide, zeolite, molecular sieves, etc., have been reported as metallocene catalysts carriers in numerous literatures. Some of the inorganic supports used as metallocene catalysts were successfully commercialized. However, they often need fastidious treatment to remove the surface acidic groups, which leads to active sites deactivation of the supported metallocene catalysts, and the inorganic residual may affect the produced polymer properties [21; 22; 23; 24].  
The porous organic polymers (POPs) are a potential alternative for commercial immobilized metallocene catalysts, and they have attracted ever-increasing attention in a wide range of applications for the last two decades due to their unique properties, such as high surface area, varied synthesis strategy, easy functionality, excellent thermal stability, etc. [25; 26; 27; 28; 29; 30]. They also provide a more homogeneous environment for active sites during the olefin polymerization, and they need no complex pre-treatment for immobilization and suffer from no severe catalytic deactivation due to the surface acidic group. A great deal of POPs with different functional groups were prepared for polyolefin catalysts support, and they exhibited excellent ethylene polymerization activity. We designed and synthesized a series of POPs with different functional comonomers, such as HEMA, 2-hydroxypropyl methacrylate (HPMA), and 4-vinylbenzyl chloride (VBC), and we also discussed the feasibility for synthesis of different pore structure, bulk density, and morphology via a dispersion polymerization method, and the highly porous polymers showed excellent catalytic performance [31; 32; 33; 34].  
However, the particle size, bulk density, and surface morphology of the prepared POPs are hard to control and generally have bad flowability as a catalysts support, due to the thermodynamic incompatibility between solvent, comonomers, and the prepared polymers, which limited their commercial applications as catalyst support. A small number of nano metal oxides [32], such as Fe\({}_{3}\)O\({}_{4}\), Fe\({}_{2}\)O\({}_{3}\), and CuO, were introduced in the preparation of POPs to tune pore structure, bulk density, morphology, etc., and then the added metal oxides were removed by acid etching for olefin polymerization catalysts.  
In this paper, nano TiO\({}_{2}\) beads were selected for an effective template agent to modulate the pore structure and particle morphology of the prepared TiO\({}_{2}\)/POP composites without needing to remove the metal oxide, and highly free-flowing TiO\({}_{2}\)/POP composites with higher bulk density, narrow particle size distribution, and high ethylene catalytic activity were obtained.