High activity magnesium chloride supported catalysts for olefin polymerization. XXIX. Molecular basis of hydrogen activation of magnesium chloride supported Ziegler–Natta catalysts

Abstract

Hydrogen (pH2 = 72 torr) increases the rate of propylene polymerization by a MgCl2/ethyl benzoate/p‐cresol/AlEt3/TiCl4‐AlEt3/methyl‐p‐toluate catalyst (CW‐catalyst) by two‐to three‐fold which corresponds closely with the increase in the number of active sites as counted by radiolabeling with tritiated methanol. The oxidation states of titanium in decene polymerizations by the CW‐catalyst were determined as a function of time of polymerization (tp). In the absence of H2, all [Ti+n] for n = 2, 3, 4 remain constant during a batch polymerization. In the presence of H2 and within 5 min of tp, [Ti+2] decreases by an amount, corresponding to 15% of the total titanium and [Ti+3] increases by the same amount, while [Ti+4] is not changed. Therefore, three‐fourths of the H2 activation result from oxidative addition processes. The remaining one‐fourth of the H2 activation may be attributed to the activation of previously deactivated Ti+3 ions by hydrogenolysis. Monomer converts some of the EPR silent Ti+3 sites to EPR observable species resulting in their activation.

Promoting ethylene polymerization through cocatalyst modification

“…Hydrogen is used as a chain transfer agent to control the molecular weight of polyolefins in the coordination polymerization by decreasing the molecular weight [18,28,29]. Table 3 demonstrates the effect of hydrogen on the catalyst yield in the presence and absence of the promoter.…”

An Investigation on Polymerization of Ethylene by Ziegler-Natta Catalyst in the Presence of a Promoter: Polymerization Behavior and Polymer Microstructure

“…Polyethylene is used in a wide variety of applications due to its versatile physical and chemical properties. The heterogeneous Ziegler-Natta catalysts are the main systems among polyethylene catalysts, however, one of the disadvantages of these catalysts is that they show low activity in the presence of hydrogen as a chain transfer agent [1][2][3]. Hydrogen deactivates some potentially active sites, probably via irreversible adsorption on these sites, even before polymerization is initiated [4].…”

Hydrogen response in liquid propylene polymerization: Towards a generalized model

“…Many authors proposed different chemically‐based mechanisms. Chien and Nozaki31 explained the polymerization rate decay by the reaction of adjacent pairs of Ti +3 with monomer. Based on their observations, Guyot et al32 suggested that catalyst deactivation results from the formation of inactive Π‐allyl species with saturation of the polymer chain end.…”