Interfering Effects of Growing Chain Epimerization on Metallocene-Catalyzed Isotactic Propene Polymerization

The stereoregularity of polypropylene produced with C2-symmetric group 4 ansa-metallocene catalysts results from the interplay of two competing reactions, namely isotactic monomer polyinsertion and a side process of epimerization of the polymer chain at its active end; therefore, for this class of homogeneous catalysts, at variance with the “classical” heterogeneous Ziegler−Natta ones, enantioselectivity and stereoselectivity are not (necessarily) coincident. In this paper, possible methods for the separate determination of these two parameters are introduced and applied to propene polymerization in the presence of the prototypical catalyst rac-ethylene−bis(4,5,6,7-tetrahydro-1-indenyl)ZrCl2. The results prove that the relatively poor stereoselectivity of this catalyst above room temperature is consequent primarily to chain epimerization; monomer insertion indeed is highly enantioselective up to at least 80 °C. Preliminary evidence for the existence of more than one epimerization mechanism is also presented; this complicates the measurements of enantioselectivity based on 13C NMR characterizations of d-labeled poly(propene)

High-Field 13C NMR Characterization of Ethene-1-13C/Propene Copolymers Prepared with Cs-Symmetric ansa-Metallocene Catalysts: A Deeper Insight into the Regio- and Stereoselectivity of Syndiotactic Propene Polymerization

In this paper, we report the results of a 150 MHz 13C NMR characterization of ethene-1- 13C/propene copolymers at low (<5 mol %) ethene content prepared in the presence of the syndiotacticselective ansa-metallocene catalyst (Me)(Ph)C(cyclopentadienyl)(9-fluorenyl)ZrCl2 (cocatalyst, MAO). In particular, from the fine structure of the resonances of the ethene-1-13C units we conclude that the enantioselectivity of 1,2-propene insertion is substantially lower and the probability of chain back-skip substantially higher after an ethene insertion than after a propene one. Moreover, we find that the regioirregular 2,1-propene units (whose concentration is higher than claimed in the literature) are also substantially stereoirregular

Compatibilised polyolefin compositions

Compatibilised polyolefin compositions combining the positive properties of their respective components by using an olefinic di- or triblock copolymer as compatibiliser to generate a finely dispersed phase structure in the molten state and to improve adhesion between the blend components in the solid state, while not compromising processability of the polyolefin composition

NMR applications in polyolefins: beyond resolution

13C 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

Ziegler-Natta (ZN) catalysts for isotactic polypropylene (iPP): a glimpse inside the black box

Classical 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