Mechanism and Kinetics of Catalyzed Chain Growth

Abstract

In this work, the activation and polymerization mechanism of Catalyzed Chain Growth (CCG) polymerization of ethylene and styrene was studied. On the basis of two metallocene catalysts (Cp* 2 ZrCl 2 and Cp* 2 NdCl 2 Li(OEt 2 ) 2 ) and dibenzyl magnesium (BzMgBz) and n-butyloctyl magnesium (BuMgOct) as co-catalysts the mechanism was determined via a model reaction and afterwards transferred to a typical CCG system. The model system consisting of Cp* 2 ZrCl 2 and BzMgBz cannot perform β-hydride elimination and was first studied via Nuclear Magnetic Resonance (NMR). A mechanistic scheme was proposed and kinetic coefficients were successfully derived via modeling in PREDICI TM . Data analysis was simplified due to direct calculation of the coefficients from NMR data. The analytical toolbox was extended to Ultra-Violet/Visible (UV/Vis) spectroscopic studies. Based on this method the influence of di-n-butyl ether (DBE) on the catalyst/co-catalyst system was studied and a strong lowering of rate coefficients was found. The co-catalyst was switched to BuMgOct, where β-hydride elimination of the alkyl chains was found to be fast at 70 • C and dominating the activation mechanism. This hinders the system to perform ethylene polymerization. The UV/Vis method was applied to the reaction of Cp* 2 NdCl 2 Li(OEt 2 ) 2 with BzMgBz. Equilibrium constants between a monoalkylated complex and the benzyl brigded bimetallic Nd-Mg complex were determined in dependence of temperature. The reaction turned out to be strongly dependent on the presence of other coordinating molecules like DBE. The activation mechanism of Cp* 2 ZrCl 2 and BzMgBz was successfully utilized for modeling the polymerization of styrene-d8 by NMR studies. It was found that the basic steps of the exchange reactions are similar to those of activation. The model was successfully extended to the introduction of polymeric species, and kinetic parameters were derived. The process of monomer addition was found to be chain-length dependent and the influence of the single rate coefficients on the reaction were screened. It was found that a low catalyst precursor concentration can lead to a rate enhancement. Addition of tetrahydrofurane was found to be disadvantageous for the polymerization reaction. BuMgOct as co-catalyst leads to a process where the termination reaction of the alkyl Scheme 2.2: Transfer reaction scheme for magnesium derivatives. Adapted with permission from S. Bogaert et al. [21] Copyright 2000 WILEY-VCH Verlag GmbH, D-69451 WeinheimIn contrast to neodymium and iron-based catalysts, the chosen Cp * 2 ZrCl 2 (A)/ BzMgBz model system is non-paramagnetic. Initialization is a critical phase during polymerization in CCG. If the chains from the CTA are not transferred fast and the first monomer addition is slow the MMD will be broadened or the process goes over into a CCTP like polymerization. Results of this section were already published in literature [269] .

Temporal Variability of Microplastic Concentrations in Inland Waters: An Automated, Semicontinuous Sampling of Microplastics ≥11 μm in a Stream in Southern Germany

“…The results presented in this study do not comprise all polymers from the standard 25 and adapted database. 22 Polyoxymethylene and rubber are constituents of SAM and were excluded from all results due to observed contamination.…”

“…All samples from the recovery experiment (U-SS, T-SS and T-BS) were analyzed as described in section 2.4. However, because the recovery experiments focused on particles and 5 specific polymer types, the standard database for automated IR analysis (version 1.0.1) 25 and APA pipeline (APAv1.1.1) 23 were used. Because the database identifies PET particles (as used in the recovery experiments) as polyester (PES), the corresponding results are reported as PET/PES.…”