1,721,140 research outputs found
Stress-induced phase transformations in propylene-based copolymers: the trigonal form of isotactic polypropylene in propylene-butene copolymers prepared with metallocene catalysts.
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Structure and mechanical properties of copolymers of syndiotactic polypropylene with branched monomers.
No full textCrystallization and elastic properties of syndiotactic propene-branched 1-olefins copolymers.
No full textNew polymerization catalysts discovered by high throughput screening techniques: a further challenge for computational methods
No full textIn the last ten years, the more relevant catalysts for stereoselective olefin polymerization have been discovered by high throughput screening techniques (HTS) [1,2]. The catalyst discovery workflow using the HTS infrastructure reached such a level [3] that the “familiar” concept of “catalyst ligand design” by basic principles [4] seems to play a marginal role at the present stage. By using this approach a greater number of pyridyl-amine ligands with enhanced structural diversity were synthesized (see Figure 1). In designing the expanded ligand array, three convenient positions for structural amplification were identified, labeled in the Figure as R1, R2 and the presence of unusual reactivity of metal-aryl bond (see Hf-CAryl and Hf-CAlkyl bonds in system 3 of Figure 1). However, the stereoselectivity mechanism of propene polymerization promoted by such systems is still unknown [1,2]. In this communication, by means of computational methods, we investigated the enantioselectivity of propene insertion for systems 1-3 reported in Figure 1. Surprisingly, the generally accepted model of "chiral growing chain orientation" [5] was ineffective for this class of catalysts and a new model for the propene enantioselectivity has been sorted out.
Despite the computational results tuned by DFT methods combined with dispersion and solvent corrections [6], there are two aspects that we would like to stress: a) our model is able to explain the subtle interplay between the steric and electronic effects as well as the reactivity of metal-aryl bond; b) it appears a general model, able after 35 years to reconnect the Ziegler-Natta catalysis to the more general asymmetric catalysis [7]. Let us make a final comment: we are well aware that discoveries of such systems were possible thanks to experimental HTS technologies because of the huge number of variables involved; nevertheless, we are still convinced that a rational catalysts design is not over the game as soon as new models are developed
New models to explain the stereoselectivity of propene polymerization by group 4 metal catalysts
No full textThe concept of “catalyst ligand design” for olefin polymerization has been deeply modified by the use of high
throughput screening techniques in the last ten years. A special case is the new class of Hf(IV)-pyridylamido-based
olefin polymerization catalysts discovered by a Dow/Symyx collaboration. These systems have attracted a lot of
attention because of several unique features: a) they are industrially relevant producing highly isotactic polypropylenes
with a solution process technology at temperature > 100 °C; b) in combination with a second catalyst they undergo
reversible trans-alkylation, performing novel olefin block copolymers via ‘chain shuttling’.
The stereoselectivity mechanism of propene polymerization promoted by such systems is still unknown .
In this communication by means of computational methods we investigated the enantioselectivity of propene insertion
for systems I-III reported in Chart 1.
Surprisingly, the generally accepted model of "chiral growing chain orientation" was ineffective for this class of
catalysts and a new model for the propene enantioselectivity has been sorted out
Block copolymers-based approach for nanoporous substrates with defined geometry for enzyme immobilization
No full textCrystal engineering by covalent bonding of propylene and butene in isotactic copolymers prepared with metallocene catalysts.
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