1,721,041 research outputs found
Imagine polypropylene
No full textLinear polyethylene and isotactic polypropylene, the two largest-volume polymers on the market, were invented in the 1950s thanks to diverse mixes of serendipity, intuition and talent. After 70 years, a thoughtful revisitation of those ground-breaking discoveries can still be revealing and inspirational
Microstructure of Polypropylene
No full textA review, with 175 refs., on catalyst technologies for manuf. of polypropylene with well-controlled microstructure and properties for advanced applications. The development of transition metal catalysts with tunable structure and selectivity is discussed. Polypropylene products with novel and well-controlled microstructure are described. The use of high-field 13C NMR methods to study the stereochem. of polypropylene is also discusse
Metallocene-catalyzed propene polymerization: from microstructure to kinetics, 1 - C2-symmetric ansa-metallocenes and the "trigger" hypothesis
No full textThe First Molecularly Characterized (Isotactic Polypropylene)-block-Polyethylene Obtained via “Quasi-Living” Insertion Polymerization
No full textMonitoring the Kinetics of Internal Donor Clean-up from Ziegler-Natta Catalytic Surfaces: An Integrated Experimental and Computational Study
No full textSurface modification of MgCl2-supported Ziegler-Natta catalysts (ZNCs) by means of organic Lewis bases, either used as precatalyst components ("internal donors", ID) or in combination with the AlEt3 cocatalyst ("external donors", ED), is key for achieving a high stereoselectivity in propene polymerization. In fourth-generation systems, which are the working horses of this important catalyst class, the ID is an (ortho-)dialkyl phthalate; under polymerization conditions, this reacts with AlEt3 and must be replaced by an ED (typically an alkoxysilane) in order to obtain the desired performance. In a previous study, we investigated the molecular kinetics of the reaction between dibutyl phthalate and AlEt3 in solution by means of integrated experimental and computational protocols. A similar approach has now been applied to monitor the progress of the reaction for the complete catalyst system. Compared with solution (H# ≈ 15 kcal mol-1 ΔS# ≈ -28 cal mol-1 K-1), the activation parameters in heterogeneous phase (ΔH# ≈ 10 kcal mol-1 ΔS# ≈ -49 cal mol-1 K-1) indicate that phthalate reduction is less activated, but suffers from an augmented entropic penalty. This suggests that the ID reacts with AlEt3 while still on the MgCl2 surface, rather than in the liquid phase after desorption, and that the surface is not innocent. Whether or not an alkoxysilane ED was present in the system turned out to be immaterial on reaction kinetics. High-level DFT calculations on a well-established MgCl2/dibutyphthalate model cluster reproduced the experimental data with a remarkably good agreement (ΔH# ≈ 10 kcal mol-1 ΔS# ≈ -46 cal mol-1 K-1). Experiments and calculations agree on the first ethyl transfer to the reacting ester group representing the rate-determining step, in solution as well in heterogeneous phase. In both cases, a four-membered transition state (TS) appears to be involved; in heterogeneous phase, though, the ester carbonyl is extra-activated by the interaction with a Lewis-acidic surface Mg center, rather than a second AlEt3 molecule. In our opinion, the interest of the proposed approach goes beyond the present study; indeed, further applications can be proposed for the design and engineering of novel ZNCs with tailored behaviors
The First Molecularly Characterized (Isotactic Polypropylene)-block-Polyethylene Obtained via “Quasi-Living” Insertion Polymerization
No full textRegioirregular Monomeric Units in Ziegler-Natta Polypropylene: A Sensitive Probe of the Catalytic Sites
No full textIsotactic polypropylene (i-PP) is mainly produced with heterogeneous Ziegler-Natta (ZN) catalyst systems. Direct structural information on the active species is very difficult to achieve; as a matter of fact, the 13C NMR microstructure of the polymer has long been used as an indirect "fingerprint". For a long time, however, the intrinsically poor sensitivity of natural abundance 13C NMR spectroscopy hampered a complete evaluation of the microstructure, and many rare albeit important details escaped detection with standard tools and methods. The most prominent case is that of the regiodefects, which are specially important because they dictate ZN catalyst response to molecular hydrogen used as a chain transfer agent. In recent papers we have reported on the quantification of regioirregular 2,1 units in ZN i-PP by means of high-temperature cryoprobe 13C NMR. Here we shift the focus onto the stereochemical environment of such units, which was highlighted by comparing the spectra of suitable ZN PP fractions with those of model PP samples made with molecular catalysts of different structures and symmetries. The results lend support to our three-site model of ZN catalytic species, originally based on the stereochemistry of regioregular PP chains/blocks
Extraction of Reliable Molecular Information from Diffusion NMR Spectroscopy: Hydrodynamic Volume or Molecular Mass?
No full textMeasuring accurate translational self-diffusion coefficients (D-t) by NMR techniques with modern spectrometers has become rather routine. In contrast, the derivation of reliable molecular information therefrom still remains a nontrivial task. In this paper, two established approaches to estimating molecular size in terms of hydrodynamic volume (V-H) or molecular weight (M) are compared. Ad hoc designed experiments allowed the critical aspects of their application to be explored by translating relatively complex theoretical principles into practical take-home messages. For instance, comparing the D-t values of three isosteric Cp2MCl2 complexes (Cp=cyclopentadienyl, M=Ti, Zr, Hf), having significantly different molecular mass, provided an empirical demonstration that V-H is the critical molecular property affecting D-t. This central concept served to clarify the assumptions behind the derivation of D-t=integral(M) power laws from the Stokes-Einstein equation. Some pitfalls in establishing log (D-t) versus log (M) linear correlations for a set of species have been highlighted by further investigations of selected examples. The effectiveness of the Stokes-Einstein equation itself in describing the aggregation or polymerization of differently shaped species has been explored by comparing, for example, a ball-shaped silsesquioxane cage with its cigar-like dimeric form, or styrene with polystyrene macromolecules
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