Abstract
The kinetics and terminations of ethylene polymerization, mediated by five bisarylimine pyridine (BIP) iron dichloride precatalysts, and activated by large amounts of methyl aluminoxane (MAO) was studied. Narrow distributed paraffins from initially formed aluminum polymeryls and broader distributed 1-polyolefins and (bimodal) mixtures, thereof, were obtained after acidic workup. The main pathway of olefin formation is beta-hydrogen transfer to ethylene. The rate of polymerization in the initial phase is inversely proportional to the co-catalyst concentration for all pre-catalysts; a first-order dependence was found on ethylene and catalyst concentrations. The inhibition by aluminum alkyls is released to some extent in a second phase, which arises after the original methyl groups are transformed into n-alkyl entities and the aluminum polymeryls partly precipitate in the toluene medium. The catalysis is interpretable in a mechanism, wherein, the relative rate of chain shuttling, beta-hydrogen transfer and insertion of ethylene are determining the outcome. Beta-hydrogen transfer enables catalyst mobility, which leads to a (degenerate) chain growth of already precipitated aluminum alkyls. Stronger Lewis acidic centers of the single site catalysts, and those with smaller ligands, are more prone to yield 1-olefins and to undergo a faster reversible alkyl exchange between aluminum and iron.
Highlights
Plastics are an essential part of everyday life, in their preparation, and in part, due to the diverse options of recycling profit from effective catalysts
The inhibition by aluminum alkyls is released to some extent in a second phase, which arises after the original methyl groups are transformed into n-alkyl entities and the aluminum polymeryls partly precipitate in the toluene medium
The catalyst system is built by the injection of the bisarylimine pyridine (BIP) iron dichloride pre-catalyst as toluene suspension into the reaction medium containing the co-catalyst and after saturation with ethylene
Summary
Plastics are an essential part of everyday life, in their preparation, and in part, due to the diverse options of recycling profit from effective catalysts. The overall property profiles of the various types of polyethylene make it indispensable for a manifold of short and long term applications. It is robust, electrically and thermal isolating, easy and cheap to process. Due to its high social, and commercial relevance, even 60 years after the start of industrial PE-production, there are still efforts underway to optimize olefin polymerization [3] These comprise the development and investigation of new catalysts in the context of morphology (powder) control, and the improvement of existing processes and technologies. The main targets are an extended control of the product and particle properties resulting in a reduction of the production costs and extension of the application profiles [4,5]
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