(494e) Modeling the Oxygen Initiated High-Pressure Polymerization of Ethylene in Tubular Reactors

For the industrial high-pressure polymerization of ethylene in tubular reactors organic peroxides, oxygen or a combination of both are used as initiators. Oxygen acts furthermore as an inhibitor in a very complex way.[1] Previous publications dealing with oxygen initiated ethylene polymerizations trace back to simplified mechanisms or regression methods.[2,3] In the last case, the resulting phenomenologically fitted reaction rate coefficients can only be used in terms of a coupled dataset. They cannot be extracted and applied detachedly from each other. In this connection, our goal was to derive arrhenius parameters, which can be used in combination with independently measured coefficients from laboratory scale experiments (e.g. coefficients given by Busch).[4] Furthermore, we extended the existing mechanism of Brandolin et al. and supplemented an additional reaction step.[3] The resulting approach allows a good agreement of experimental and simulated temperature profiles as well as microstructural homo  and co polymer properties.

Within the inhibition reaction a growing macroradical reacts with an oxygen molecule yielding an oxygen terminated chain with a radical functionality.[2] At this point we assume a subsequent inhibition reaction of the oxygen terminated chain and a second growing macroradical. The resulting macroperoxide slowly decomposes at higher temperatures and re initiates the polymerization. This reaction sequence causes smoothly curved temperature profiles with very broad temperature maxima, which are characteristic for the oxygen initiated process.

A hybrid simulation method is used to model the complex polymerization network. The approach combines the advantages of stochastic and deterministic modeling. It offers a deep insight into the polymeric microstructure of individual macromolecules. In this connection, the exact position of incorporated oxygen or co monomer and the unique branching structure is accessible. Furthermore, the topologic information is used to handle the β-scission in a topological correct way and to determine contraction factors as well as seniority priority distributions.

[1] R. C. M. Zabisky, W. M. Chan, P. E. Gloor, A. E. Hamielec, Polymer 1992, 33, 2243.
[2] Y. Tatsukami, T. Takahashi, H. Yoshioka, Makromol. Chem. 1980, 181, 1107.
[3] A. Brandolin, M. H. Lacunza, P. E. Ugrin, N. J. Capiati, Polym. React. Eng. 1996, 4, 193.
[4] M. Busch, Macromol. Theory. Simul. 2001, 10, 262.