Strong activation of MgCl₂‐supported Ziegler‐Natta catalysts by treatments with BCl₃: Evidence and application of the “cluster” model of active sites

Abstract

Heterogeneous Ziegler‐Natta precatalysts (with phthalate as internal donor) were modified by treatments with BCl3 (2 h in heptane; T = 20–90 °C; B/Ti = 0.1–5) before their use in the polymerization of propylene to modify the active sites distribution. If performed on previously “detitanated” precatalysts, the treatment leads to a strong increase of productivity (up to one order of magnitude) without drastic modifications of polypropylenes properties (tacticity, molecular weight distribution). In addition, these findings are in good agreement with the hypothesis of a “cluster” organization of active sites allowing to rationalize activation by BCl3 by formation of heteronuclear B‐Ti clusters. The activation method was also applied to unmodified precatalysts and gave a significant gain of productivity. The simple and versatile activation process can also be performed under mild conditions (low T and low [BCl3]). © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5784–5791, 2009

Modifications of the active sites distribution in the Ziegler‐Natta polymerization of propylene using Lewis acids

“…All other precatalysts were found to be active for the polymerization of propylene. As previously reported,13 a drastic increase of productivity was evidenced after treatment with BCl 3 of the “detitanated” precatalyst and the activity of initial precatalyst is surpassed (27,300 kg/molTi/h for T 0 + B 30 ; 2700 kg/molTi/h for T 0 ; and 9700 kg/molTi/h for ref. ; Table 1).…”

“…In summary for the first time, the distribution of active sites has been slightly modified by introduction of Lewis acid such as SiCl 4 and SnCl 4 with an increase of the selectivity of propylene insertion (tacticity) what was not observed with BCl 3 . A variable gain of activity is also observed following the order: BCl 3 >> SiCl 4 > SnCl 4 without a significant increase of polypropylenes molecular weight ( M n , M w ), which indicates an increase of the number of active sites as already observed with BCl 3 13. On the other hand, the distribution of active sites regenerated by SiCl 4 and SnCl 4 after controlled “detitanation” by thermal treatment is not identical to the original one (as observed with BCl 3 ): the relative amount of “isotactic” active sites is increased corresponding either to an increase of the number of intrinsically “isotactic” preactive sites and/or to an increase of the number of sites, which are converted by External Lewis Base and AlEt 3 in “isotactic” active sites.…”

“…Lewis acids were selected among (metallic or not) halogenated derivates according to the two principal following criteria: (1) solubility in hydrocarbons (or chlorinated hydrocarbons) solvents without introducing any polar solvents; (2) inability to lead by itself to polymerization active species when AlEt 3 is added. Based on these criteria GaCl 3 , SnCl 4 , SiCl 4 , and SbCl 5 , varying by the oxidation state and the Lewis acidity of the central ion (M) in the chloride molecule MCl n , were selected and for comparison point of view precatalysts were also modified with TiCl 4 and BCl 3 according to previous work 13…”

“…1). 13 This simple treatment with BCl 3 increased the activity up to one order of magnitude without any significant change of polymer properties and thus of the distribution of active centers. The simple and versatile activation method was also applied to unmodified conventional precatalysts and gave a significant gain of productivity 13, 14…”

Highly efficient FeCl3 doped Mg(OEt)2/TiCl4-based Ziegler–Natta catalysts for ethylene polymerization

“…This finding was in line with other researchers results which were published before. [35][36][37] Furthermore, it was observed that the activity increase in the presence of FeCl 3 -SiCl 4 mixture was higher than it in the presence of FeCl 3 alone. It is possible that besides halogen-donating ability of SiCl 4 which acts as promoter in ethylene polymerization, the synergistic effect of double metal halide and metalloid compounds generated more active sites, which were suitable for ethylene polymerization.…”