Hydrogen Effects for Propylene Polymerization with Ultra Low TiCl₃ Loading MgCl₂‐Supported Catalyst

Abstract

Summary: Hydrogen effects for propylene polymerization were investigated with ultra low TiCl 3 loading MgCl 2 -supported catalysts in which the electric states of Ti species can be almost uniform. Hydrogen did not affect the catalyst activity, while the efficiency of hydrogen as a chain transfer agent was found to depend on the Ti content of the catalyst and the stereospecificity of the polymerization sites: Hydrogen was effective for isospecific sites independent of Ti contents, but inert for aspecific sites only at the extremely low Ti content. These results were explained within the island model, where isospecific sites may be located in the islands with other Ti species in their surroundings acting, as a steric hindrance for isospecific polymerization and as hydrogen dissociation sites after deactivation. Most of the aspecific sites should be isolated only at the extremely low Ti content. These isolated sites have no other Ti species in their surroundings, i.e. no hydrogen dissociation sites, and are inert to hydrogen.

Effects of titanium dispersion state on distribution and reactivity of active centers in propylene polymerization with MgCl2‐supported Ziegler‐Natta catalysts: A kinetic study based on active center counting

“…Cat-1 with ultralow Ti content is used as a model catalyst that mainly contains isolated mononuclear Ti species. [18][19][20][21][22][23][24][26][27][28][29][30] Cat-2 with higher Ti content and more clustered Ti species is similar with those supported ZÀ N catalysts used in polyolefin industry. Therefore, the catalysts used here can be taken as models for the two extremes of Ti dispersion states.…”

“…Similar positive correlation between Ti load of catalyst and isotacticity of the produced PP has been reported in literatures. [18][19][20][21][22][23][24][26][27][28][29][30] With proceeding of the polymerization, the content of aPP fraction did not change evidently in Cat-1 catalyzed polymerization in reaction period of 30-420 s, but sharply decreased in the later 180 s of reaction. In contrast, both the content of aPP and mPP fractions gradually decreased with proceeding of polymerization catalyzed by Cat-2.…”

“…Similar phenomena have been reported in literatures dealing with similar model catalysts of different Ti load. [18][19][20][21][22][23][24][26][27][28][29][30] Due to broad distribution of stereoregularity, two endothermic peaks were observed on DSC heating trace of each PP product (see Figure S1 of Supporting Information). The glass-transition temperature appeared at around À 9°C (see Figure S1).…”

“…By comparing olefin polymerization behaviors of different model ZÀ N catalysts with controlled Ti content, close correlations between the amount of clustered Ti species and stereoselectivity of TiCl 4 /MgCl 2 or TiCl 3 / MgCl 2 catalysts have been reported. [19][20][21][22][23][24] The model catalysts with ultralow Ti load were found to produce polypropylene with very low isotacticity, implying that clustered Ti species are the main active sites of isospecific polymerization. In our previous work, strong effects of Ti dispersion state on the responses of TiCl 4 / MgCl 2 type model catalysts to electron donor addition were reported, and mechanistic explanation was proposed based on the results of counting active centers in propylene polymerization.…”

Effect of AlEt3and an External Donor on the Distribution of Active Sites According to Their Stereospecificity in Propylene Polymerization over TiCl4/MgCl2Catalysts with Different Titanium Content

“…According to ref ,. the TiCl 3 ·3Py/MgCl 2 model catalysts with a low titanium content (≤0.1 wt%) produce PP with a higher activity, a narrower MWD, and a lower stereoregularity compared to similar TMCs having the high titanium content.…”