Molecular Weight and Branching Distribution Modeling in Radical Polymerization with Transfer to Polymer and Scission Under Gel Conditions and Allowing for Multiradicals

Abstract

A population balance model for the prediction of molecular weight distribution (MWD) in a continuous stirred tank reactor (CSTR) has been developed accounting for multiradicals and gel formation in the framework of Galerkin-FEM. In the absence of recombination, gel does not form, but accounting for multiradicals leads to a better prediction of the long MWD tail. Results of the multiradical model with topological scission are well in line with Monte Carlo (MC) simulations. For the case of recombination without scission the multiradical model leads to perfect agreement with MC simulations as regards prediction of the gel fraction and chain length distribution. The classical monoradical model fails to describe the gel regime. We account for gel fragmentation in systems with gelation and scission. Results for this case are in agreement with MC simulations. A nongel assuming variant allows properly detecting the gelpoint and the associated distribution. The scission model adopted, linear or topological scission, turns out to be of extreme importance for the gel regime prediction.

Three-dimensional chain-length-branching-combination points distribution modeling of low density polyethylene in a continuous stirred tank reactor allowing for gelation

“…As it was already shown in Yaghini and Iedema (2014b) strong combination leads to gel formation very easily, even for relatively weak branching -the gelpoint is at around twice the present branching level. The full 2D solution is shown in Figure 4.…”

“…Note that the 1D-chain length distribution is not very broad and with uni-modal distribution form (Yaghini and Iedema, 2014b). The 2D solution has been obtained using a fairly coarse grid of maximum chain length 3 A check of the overall chain length distribution obtained from the 2D solution showed perfect agreement with the 1D multiradical model (Yaghini and Iedema, 2014b) and with Monte Carlo simulations. Figure 4 shows that for the 2D solution there is a good agreement with the Monte Carlo simulations, especially for smaller chains.…”

“…This problem will be treated in the context of the 2D problem in the same manner as it was realized in the 1D multiradical model (Yaghini and Iedema, 2014b). It was considered that the simplest manner of addressing gel is assuming that its reactive groups in the gel behave in an identical way as these groups in the sol molecules.…”

“…according to either the Previously, the 1-dimensional multiradical model allowed treating the cases where gel formation was combined with random scission (Yaghini and Iedema, 2014b). In principle, the 2D model permits for a similar approach.…”

“…However in view of the necessary simplification concerning the redistribution of branch points, which will have expectedly a strong impact on the results, it was decided to restrict the scission calculations to the systems without gelation. Confining kinetic parameter space to the non-gel systems is influenced by the random scission model adopted (Yaghini and Iedema, 2014b). It was found that by assuming linear scission, the range of possible chain transfer to polymer rate coefficients (determining branch formation) is much larger than when assuming topological scission.…”