Effect of internal lewis bases on recrystallized MgCl2-TiCl4 catalysts for polypropylene

The effect of different Lewis bases in TiCl4 catalysts supported on MgCl2, which was modified by the recrystallization method, has been investigated in terms of activity, selectivity, and morphology. Ethyl benzoate (EB) and 2,2,6,6‐tetramethylpiperidine (TMPip) were tested as the internal Lewis bases. Propylene polymerizations were performed at 50°C and 7 atm, using Al(C2H5)3 as co‐catalyst and TMPip as external modifier. Both supports and catalysts were characterized by BET, X‐ray, SEM, and other techniques. It was found that the electron donor plays an important role in the recrystallization step of MgCl2 from alcoholic MgCl2 solution. Ethyl benzoate was the more effective internal donor leading to the porous, free‐flowing, and stable high‐activity catalyst, while TMPip was not so effective as internal donor owing to its decomposition by SiCl4 during the support preparation. From short time polymerization kinetics, EB was found to increase both the values of the active site concentration ([C*]) and the propagation rate constant (kp). Comparison with the ball‐milled catalyst has been made in view of the morphology. The morphology of the polymers formed with the recrystallized catalysts was the globule‐like structure, while that with the milled catalyst was the broken structure due to its low surface area.