Effect of Morphology of Titanium-Magnesium Catalysts and Polymer Particles Produced on the Molecular Mass of Polyethylene
Effect of texture of Ti-Mg catalysts (TMC) and formed particles of polymer on molecular weight of polyethylene (PE) is investigated. ТМCs with the close average particle size (≈ 5 microns), but different porosity, crystal structure and packing of micro- and subparticles, and also the different content of the titanium are used for research. Composition, porous and crystal structure of these catalysts, and also interrelation of properties of the catalyst with formation of polymer with various porosity it is established in our previous works1,2. Superhighly active "low-percentage" ТМC (≤ 0.1 wt % of Ti), according to our data3, irrespective of support type, contain high concentration of the active sites. These ТМC are used as model systems for revealing of effect of morphology of the catalyst on properties of the polymer produced. It is found that catalysts which have a high surface, small diameter of pores (≤4 nm) and highly disordered crystal structure, allow to produce polymer with lower molecular weight under various polymerization conditions.
Effect of hydrogen concentration at ethylene polymerization on ТМC with the various content of titanium and a various structure was studied, and constants of transfer of a polymeric chain with hydrogen have been evaluated. It is established that regulation of molecular weight of PE by hydrogen is defined by morphology of the catalyst and does not depend on the content of the titanium in the catalyst. Catalysts which produce the particles of polymer consisting from densely packed micro - and subparticles, regulate molecular weight of polyethylene more effectively.
On the basis of the data received in this work and results of our previous researches 1-3, the possible reasons of the effect of structural features of catalysts and polymer particles on the molecular weight of PE are discussed.
1. Tatiana B. Mikenas, Evgeny I. Koshevoy, Svetlana V. Cherepanova, Vladimir A. Zakharov. Study of the Composition and Morphology of New Modifications of Titanium-Magnesium Catalysts with Improved Properties in Ethylene Polymerization. J. Polym. Sci., Part A: Polym. Chem., 2016, 54, 2545–255.
2. Tatiana B. Mikenas, Evgeny I. Koshevoy, Vladimir A. Zakharov. Effect of the Structure of Titanium–Magnesium Catalysts on the Morphology of Polyethylene Produced, J. Polym. Sci., Part A: Polym. Chem., 2017, 55, 2298–2308.
3. Tatiana B. Mikenas, Evgeny I. Koshevoy, Vladimir A. Zakharov, Marina I. Nikolaeva. Formation of Isolated Titanium (III) Ions as Active Sites of Supported Titanium–Magnesium Catalysts for Polymerization of Olefins. Macromol. Chem. Phys. 2014, V. 215. Nо. 18. P. 1707-1720.