1,721,135 research outputs found
LIVING PROPENE POLYMERIZATION WITH BIS(PHENOXY-IMINE) GROUP 4 METAL CATALYSTS: A THEORETICAL STUDY
No full textPeriodic and high-temperature disordered conformation of polytetrafluoroethylene chains: an ab-initio modeling
No full textWe report here the main results of a successful attempt to predict some macroscopic properties
of representative polymers of technological relevance both in regular and disordered forms by using first
principle quantum mechanical approaches at microscopic level. Until now, the prediction of the structural
and thermal properties of those polymers has been mostly a domain of molecular mechanics methods. To
overcome the limits of those classical computational tools whenever physical properties are significantly
influenced by stereoelectronic effects (e.g., electron rich substituents), we employed methods rooted in
the Density Functional Theory (DFT). A general computational strategy including the proper choice of periodic
boundary conditions (PBC), functional, basis set, and model system size, has been tested and validated
for saturated polymers such as polyethylene and isotactic/syndiotactic polypropylenes. On the basis of
these results, a comprehensive study of poly(tetrafluoroethylene) (PTFE) chains in both regular periodic
and disordered conformations has been performed. A statistical approach has been next applied to obtain
the thermal concentration of defects and to reproduce the thermal behavior of the investigated polymer. At
the end, a very good agreement with experimental X-ray diffraction and IR results has been achieved,
definitely reaching a good understanding of the widely studied disorder phenomenon determining the main
technological properties of poly(tetrafluoroethylene) (the trade Teflon) and, at the same time, identifying
the proper computational tools to investigate perfluoro-compounds and other complex polymeric systems
Origin of the Regiochemistry of Propene Insertion at Octahedral Column 4 Polymerization Catalysts: Design or Serendipity ?
No full textLooking into the Quest for Stereoselective Ring-Opening Polymerization of Racemic Lactide with Chiral Organocatalysts
No full textA model for stereoselective ring-opening polymerization (ROP) of racemic lactide (rac-LA) catalyzed by chiral thiourea-based organocatalysts is presented based on density functional theory (DFT) calculations. The model reveals the complex mechanism of the process, highlighting (a) multiple mechanistic pathways, (b) enantioselective activation of monomer reactive faces (re for RR-LA and si for SS-LA), and (c) shifts in the rate-determining steps from the nucleophilic addition to the ring opening step depending on monomer chirality. The interplay between enantiomorphic site control and chain-end control is also sorted out, demonstrating good agreement with experimental stereoselectivity. Furthermore, the model is extended to chiral thiourea-based bifunctional catalysts, providing insights into the enhanced stereochemical outcomes observed in the ROP of rac-LA
Mechanistic insights on 1-butene polymerization catalyzed by homogeneous single-site catalysts: a DFT computational study
No full textIsotactic poly (1-butene) (iPB) is an interesting semi-crystalline thermoplastic material characterized by notable physical and mechanical attributes encompassing superior creep and stress resistance, elevated toughness, stiffness, and thermal endurance. These distinctive features position iPB as a viable candidate for specific applications; however, its widespread utilization is hindered by certain inherent limitations. Indeed, iPB manifests an intricate polymorphic behavior, and the gradual and spontaneous transition of the kinetically favored form II to the thermodynamically favored form I during aging introduces alterations to the material’s properties. Despite its potential, the attainment of iPB with an exceedingly high molecular mass remains elusive, particularly when employing homogeneous catalysts renowned for their efficacy in propene polymerization. In this study we analyze the mechanistic aspects governing 1-butene polymerization by using DFT calculations modelling the regioselectivity of 1-butene insertions and the termination reactions occurring after primary (1,2) and secondary (2,1) insertions. Finally, the isomerization pathways leading to the formation of 4,1 units in iPB samples synthesized by homogenous catalysts is also discussed. All these aspects, furnish a mechanistic picture of the main drawbacks of an “old” but still interesting material
Origin of the Regiochemistry of Propene Insertion at Octahedral Column 4 Polymerization Catalysts: Design or Serendipity ?
No full textLa variazione delle tensioni residue nello spessore dei tubi in acciaio: determinazione sperimentale e conseguenze sulla capacità portante.
No full textX Congresso C.T.A., Montecatini, October 198
A Computational Evaluation of the Steric and Electronic Contributions in Stereoselective Olefin Polymerization with Pyridylamido-Type Catalysts
No full textA density functional theory (DFT) study combined with the steric maps of buried volume (%VBur) as molecular descriptors and an energy decomposition analysis through the ASM (activation strain model)–NEDA (natural energy decomposition analysis) approach were applied to investigate the origins of stereoselectivity for propene polymerization promoted by pyridylamido-type nonmetallocene systems. The relationships between the fine tuning of the ligand and the propene stereoregularity were rationalized (e.g., the metallacycle size, chemical nature of the bridge, and substituents at the ortho-position on the aniline moieties). The DFT calculations and %VBur steric maps reproduced the experimental trend: substituents on the bridge and on the ortho-positions of aniline fragments enhance the stereoselectivity. The ASM–NEDA analysis enabled the separation of the steric and electronic effects and revealed how subtle ligand modification may affect the stereoselectivity of the process
Regiochemistry of propene insertion with group 4 polymerization catalysts from a theoretical perspective
No full textMOLECULAR MODELING OF THE REGIOCHEMISTRY OF OLEFIN INSERTION WITH SINGLE-SITE POLYMERIZATION CATALYSTS
No full text- …
