1,721,173 research outputs found
NMR applications in polyolefins: beyond resolution
No full text13C NMR spectroscopy is the elective technique of polyolefin microstructure analysis, and the progress in this spectroscopy parallels that in our stereochemical understanding of the polymers (which is quite obvious) and the reactions to make them.[1]
In the latter respect, one should realize that each macromolecule is like a tape where the story of the polymerization is faithfully and sequencially recorded. In order to know that story, of course, one must be able to read the tape. In the 1990s, our group demonstrated that the use of high-field 13C NMR can lead the resolution of polypropylene spectra to an unprecedented level of detail, with substantial advances in the determination of polymer configuration.[1] Some years later, we achieved similar advances with olefin copolymers.[2]
On the other hand, the intrinsically poor sensitivity of natural abundance 13C NMR spectroscopy remained an obstacle preventing evaluations of many important microstructural features such as e.g. chain ends, regiodefects, long-chain branches etc. It was only recently that the introduction of high-temperature cryoprobes improved the picture dramatically.[3] With a signal-to-noise (S/N) ratio higher by roughly 1 order of magnitude compared with conventional ones, the new probes can be applied to speed-up spectral acquisition (by roughly 100 times to achieve a given S/N) and/or lower the threshold for detectability. The former application is especially valuable in combination with High Throughput Experimentation (HTE); the latter, in turn, makes it possible to ‘see the invisible’. This talk will highlight both aspects for a number of representative case histories
Advances in 13C NMR and other analytical tools in the rapid screening of Ziegler-Natta catalysts
No full textZiegler-Natta (ZN) catalysts for isotactic polypropylene (iPP): a glimpse inside the black box
No full textClassical ZN catalysts still have the monopoly of industrial iPP production. With their complex formulation, including an ‘activated’ MgCl2 support, a Ti precursor, an Al-alkyl activator, and in most cases a properly chosen pair of electron donor modifiers, these systems are rightfully considered, in many respects, ‘black box’ ones, and so far no true cases of active surface design have been reported. On the other hand, it is indeed possible to tailor application based on trial-and-error, primarily by means of a smart selection of the electron donors, recently also under the guidance of ‘black-box’ QSAR models. In the latter respect, a collaboration between SABIC and this laboratory, making intensive use of High Throughput Experimentation (HTE) screening methods, gave important results, including the identification of electron donors with improved performance. In parallel with the ‘black-box’ QSAR, a fundamental study of the catalysts is also in progress; moving from the HTE database, added with statistical analyses of 13C NMR polymer microstructure, quenched-flow kinetic experiments, and periodic and cluster DFT calculations, quantitative hypotheses on the interactions between the various catalyst system components and the local structure of the active surfaces are being formulated, and it happens more and more that partial views of what hides in the ‘black-box’ are obtained. In this presentation, we will describe what we saw, along with the actions that, in our opinion, are necessary to fully remove the cover and highlight the box
Applications of High-Throughput Screening and High-Temperature Cryoprobe NMR to in-Depth Polyolefin Microstruc ture Analysis
No full textHigh Throughput Screening Approach to Electron Donor Exchanges in MgCl2-supported ZN Catalysts
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Quantitative Screening of Ziegler-Natta: Catalyst Regioselectivity
No full textHigh Throughput Experimentation (HTE) has gained importance in the screening of olefin polymerization catalysts. In particular, its recent integration with ‘fast’ 13C NMR spectrometers represents a most powerful tool for mechanistic investigations.
This presentation introduces a quantitative HTE approach to the regioselectivity of Ziegler-Natta (ZN) catalysts for isotactic polypropylene (iPP), and its relationship with the so-called ‘hydrogen response’. The proposed protocol makes use of a state-of-the art HTE platform (Freeslate PPR48) and a Bruker Avance III 400 spectrometer featuring a 5 mm high-temperature cryoprobe.
Quite surprisingly we found that, at odds with stereoselectivity, regioselectivity of these catalysts is primarily dictated by the lnternal Donor (ID), and only marginally affected by the External Donor (ED). ID’s belonging in the same chemical class (e.g. 1,3-dimethoxypropanes) end up with similar regioselectivities, almost irrespectively of their steric demand. The mechanistic interpretation of these findings is not obvious.
On the other hand, the crucial relationship between regioselectivity and molecular weight capability of a given catalyst in the presence of H2 was confirmed; in most cases, the number average polymerization degree of the polymers obtained at practical p(H2)/p(C3H6) ratios coincides with the reciprocal of 2,1 unit concentration, fully supporting the ‘dormant site’ concept
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