Paraffin-Inert Atmospheric Solid Analysis Probe: A Fast and Easy Approach To Characterize Extremely Air-Sensitive Organometallic Complexes by Mass Spectrometry.

Rational characterization of most organometallic compounds is hampered by their high reactivity, in particular, toward oxygen and water. Mass spectrometry experiments require physical introduction of the sample in the ionization source. So, the main challenge is to transfer air-sensitive organometallic compounds from inert atmosphere to the ionization source. In this aim, we have developed an easy technique that allows the analysis of air-sensitive compounds using the atmospheric solid analysis probe (ASAP). This method consists of a glass capillary filled with the sample (solid or liquid) and sealed by a paraffin plug to maintain the inert sample until the ionization process. It is illustrated through the structural characterization of a new highly air-sensitive dinuclear zirconium complex supported by an original switchable stilbene platform.

A Theoretical Outlook on the Stereoselectivity Origins of Isoselective Zirconocene Propylene Polymerization Catalysts.

The first three insertion steps of propylene for isoselective metallocenes from the one-carbon-bridged cyclopentadienyl-fluorenyl {Cp/Flu} and silicon-bridged ansa-bis(indenyl) {SBI} families were computed by using a theoretical method implementing the B3PW91 functional in combination with solvent corrections incorporated with the Solvation Model based on Density (SMD) continuum model. For C1 -symmetric {Cp/Flu}-type metallocenes, two mechanisms of stereocontrol were validated theoretically: more facile and more stereoselective chain "stationary" insertion (or site epimerization backskip) and less stereoselective alternating mechanisms. For the C2 -symmetric {SBI}-type system, the computation results were in complete agreement with the sole operating chain migratory insertion mechanism. The thermochemical data obtained through the study were used to predict microstructures of polypropylenes by using three-parameter and one-parameter statistical models for the two metallocene systems, respectively. The calculated meso/rac pentad distributions were found to be in good agreement with those determined experimentally for isotactic polypropylene samples obtained at different polymerization temperatures.

Real-time Analysis of a Working Triethylaluminium-Modified Cr/Ti/SiO2 Ethylene Polymerization Catalyst with In†Situ Infrared Spectroscopy.

A diffuse reflectance infrared Fourier-transform (DRIFT) study has been conducted at 373 K and 1 bar on an industrial Cr/Ti/SiO2 Phillips-type catalyst modified with, and without, triethylaluminium (TEAl) as co-catalyst. The reaction rate of the polymerization of ethylene, as monitored by the increase in the methylene stretching band of the growing polyethylene (PE), has been investigated as a function of the titanium content. After an initial period of mixed kinetics, with the reaction rate significantly higher for the TEAl-modified catalysts compared with the non-modified catalysts, the polymerization proceeded as a pseudo-zero-order reaction with a reaction rate that increased as a function of titanium loading. Furthermore, it was found that the higher Ti loading caused the appearance of more acidic hydroxyl groups and modified the Cr sites by making them more Lewis acidic, ultimately shortening the induction time and increasing the initial polymerization rate.

Polyethylene with Reverse Co-monomer Incorporation: From an Industrial Serendipitous Discovery to Fundamental Understanding.

A triethylaluminium(TEAl)-modified Phillips ethylene polymerisation Cr/Ti/SiO2 catalyst has been developed with two distinct active regions positioned respectively in the inner core and outer shell of the catalyst particle. DRIFTS, EPR, UV-Vis-NIR DRS, STXM, SEM-EDX and GPC-IR studies revealed that the catalyst produces simultaneously two different polymers, i.e., low molecular weight linear-chain polyethylene in the Ti-abundant catalyst particle shell and high molecular weight short-chain branched polyethylene in the Ti-scarce catalyst particle core. Co-monomers for the short-chain branched polymer were generated in situ within the TEAl-impregnated confined space of the Ti-scarce catalyst particle core in close proximity to the active sites that produced the high molecular weight polymer. These results demonstrate that the catalyst particle architecture directly affects polymer composition, offering the perspective of making high-performance polyethylene from a single reactor system using this modified Phillips catalyst.