A General Model Describing Molecular Weight Distribution and Branching Indices in Copolymerizations Demonstrated by the High-Pressure Free-Radical Copolymerization of Ethene and Methyl Acrylate

Abstract

A general copolymerization model is developed which incorporates various transfer reactions, like transfer to polymer and backbiting, and scission reactions. The potential of the model is investigated using the simulation package PREDICI® by performing a number of sensitivity analyses. The main focus of this work is directed toward the development of experimental strategies for deriving proper kinetic parameters based on simulation calculations. To provide a broad base for general application the model is designed for investigating fundamental aspects as well as providing the potential for use in technical production. This would mean dealing with complex reactor operation modes and dealing with multiple broad species distributions. At these simulations the high-pressure free-radical copolymerization of ethene (E) with methyl acrylate (MA) is used. This complex fluid-phase copolymerization system features all elementary reaction steps being discussed to occur at free-radical (co)polymerizations at the moment and is therefore self-suggesting for such investigations. The experimental strategy derived from the simulation study is verified by two experimental examples of E-MA copolymers both containing 15 mol-% methyl acrylate synthesized at 150°C and 2 000 bar featuring 22 mol-% and 32 mol-% acrylate conversion. It becomes obvious that the kinetic model is capable of well describing experimental molecular weight distributions and branching indices by one set of kinetic parameters. Successful experimental design and description of data demonstrate the usefulness of modeling for kinetic investigations. Moreover, they are the justification for a future application of models following strategies being proposed in this contribution for other copolymerization systems and more complex copolymerization applications. In this model the coupled implementation of the transfer to polymer and β-scission reaction is applied for the first time in copolymerizations (this description is close to the real process). Also the design of an experimental strategy showing singular sensitivities on the determination of rate coefficients for branching and scission in copolymerizations, is presented for the first time. An assumption that has still to be made within this model is the treatment of β-scission acting on a linear chain. However, as long as a macromolecule is not multiply long-chain branched there is no error introduced into modeling using this approximation.