Polymer Bulletin 43, 231–238 (1999) * Corresponding author Polymer Bulletin  Springer-Verlag 1999 Study of the MgCl 2 recrystallization conditions on Ziegler-Natta catalyst properties A. Parada * , T. Rajmankina, J. Chirinos Laboratorio de Polímeros, Dpto. de Química, Facultad de Ciencias, La Universidad del Zulia, Maracaibo, Venezuela Received: 14 December 1998/Revised version: 17 August 1999/Accepted: 17 August 1999 Summary Magnesium dichloride supported titanium catalysts were prepared by dissolving anhydrous MgCl 2 in 1-hexanol/isooctane. Then recrystallization took place through different techniques such as evaporation of the solvent, quick cooling, precipitation with silicon tetrachloride and precipitation with titanium tetrachloride. The FT-IR analysis showed that several degrees of dealcoholation took place as a result of the adducts of MgCl 2 • XROH. Moreover, it was found that the most active catalysts in the polymerization of ethylene were those obtained recrystallizing MgCl 2 by precipitation with SiCl 4 . On the other hand, those directly recrystallized with TiCl 4 were the least reactive and the ones that produced polyethylenes with the highest molecular weights and the lowest degrees of crystallinity. Introduction Since the excellent properties of MgCl 2 supported Ziegler-Natta catalysts for production of polyolefins were discovered, many studies on treating methods of MgCl 2 have been carried out in order to improve the yield of catalysts through the increase of the surface area and through a better interaction between MgCl 2 and TiCl 4 . These treatments have been the ball-milling (1-5), the chemical conversion (6) and the recrystallization methods (7-15). The ball-milling method allows to reduce the size of MgCl 2 crystallites increasing, as a consequence, the surface area of the catalyst and its activity. Nevertheless, its major disadvantage is a prevalent difficulty in controlling the polymer morphology (9-14). On the other hand, the recrystallization method, starting from MgCl 2 solutions, is the most recent and offers the possibility of controlling the polymer morphology through that of the catalyst (12-14). Frequently, alcohols which react with MgCl 2 to give adducts of different stoichiometry, depending on the lenght of alkyl group of alcohol, are used to dissolve the MgCl 2 (16). In the case of the adducts of MgCl 2 • XROH, it is convenient to remove the alcohol since, during the impregnation phase, this reacts with TiCl 4 and produces titanium chloride alcoxides that are inactive to polymerization. Therefore, the removal of the alcohol is very important to achieve high activities. In previously published works, the recrystallization of the support has been made by the evaporation of the solvent (11, 12, 14,15) or by cooling (7-10, 13), followed by the washing of the substratum before the impregnation phase; however, only a partial removal of the alcohol occurs through these techniques.