Abstract
•Pseudo-homogeneous versus in-situ heterogeneous polymerization offered a novel approach to study metallocene-catalyzed ethylene polymerization.•Boundary layer mass transfer, methylaluminoxane anion design, and comonomer steric hindrance variedly affected ethylene polymerization.•The measured catalyst productivity and polyethylene properties duly reflected locus of polymerization, comonomer effect, chain transfer process, and micromixing effect.•Mathematical correlations of selected thermal properties to polyethylene microstructure offered new insights.•Flory model and Gibbs–Thompson equation well described comonomer-mediated polyethylene crystal lattice disruption.
This study reports a novel conceptual framework that can be easily experimented to evaluate the effects of hydrodynamic boundary layer mass transfer, methylaluminoxane (MAO) anion design, and comonomer steric hindrance on metallocene-catalyzed ethylene polymerization. This approach was illustrated by conducting homo- and isomeric copolymerization of ethylene with 1-hexene and 4-methyl-1-pentene in the presence of bis(n-butylcyclopentadienyl) zirconium dichloride (nBuCp)2ZrCl2, using (i) MAO anion 1 (unsupported [MAOCl2]−) and pseudo-homogeneous reference polymerization, and (ii) MAO anion 2 (supported Si−O−[MAOCl2]−) and in-situ heterogeneous polymerization. The measured polymer morphology, catalyst productivity, molecular weight distribution, and inter-chain composition distribution were related to the locus of polymerization, comonomer effect, in-situ chain transfer process, and micromixing effect, respectively. The peak melting and crystallization temperatures and %crystallinity were mathematically correlated to the parameters of microstructural composition distributions, melt fractionation temperatures, and average lamellar thickness. These relations showed to be insightful. The comonomer-induced enchainment defects and the eventual partial disruption of the crystal lattice were successfully modeled using Flory and Gibbs–Thompson equations. The present methodology can also be applied to study ethylene−α-olefin copolymerization, performed using MAO-activated non-metallocene precatalysts.
[Display omitted] The %crystallinity Xc of the as-synthesized polyethylenes increased linearly as a function of weight average lamellar thickness Lwav DSC GT, despite variation in MAO (methyl aluminoxane) anion and comonomer types and the modes of polymerization (MAO anion 1: pseudo-homogeneous versus MAO anion 2: in-situ heterogeneous). Therefore, the homo- and the copolymers experienced similar chain folding behavior. The iso- and n-butyl branches refrained from chain folding, and the Gibbs–Thompson (GT) equation successfully modeled the combined influence of comonomer-induced inter- and intra-chain copolymer composition distributions on Xc.