Molecular weight distribution modeling in low-density polyethylene polymerization; impact of scission mechanisms in the case of CSTR

Abstract

The role of scission reactions in low-density polyethylene (ldPE) polymerization has recently been the subject of various modeling studies. The linear scission approximation leading to a uniform fragment length distribution, seems to describe the scission mechanism well. For linear scission the predicted bimodal molecular weight distribution (MWD) in the case of a CSTR agrees with that found by size exclusion chromatography (SEC). However, scission of branched polymers does not follow the linear scission model, since it leads to predominately long and short fragments. Therefore, we systematically compared the linear scission model with the newly proposed ‘topological scission’ model. We found a pronounced difference in MWD between linear scission, yielding a bimodality and topological scission, leading to a long tail in MWD. In addition we examined mechanical scission that could occur in stirred autoclave reactors due to mechanical action. Mechanical scission also turns out to generate bimodality in MWD. The molecular architecture of ldPE is also discussed in connection with scission kinetics and the topological scission model.