1,721,026 research outputs found
Looking 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
Buried Volume Analysis for Propene Polymerization Catalysis Promoted by Group 4 Metals: A Tool for Molecular Mass Prediction
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The role of noncovalent interactions in olefin polymerization catalysis: a further look to the fluorinated ligand effect
No full textThe noncovalent interactions and remote ligand effects in catalysis have attracted a large attention in the last years. The rationalization of such interactions is tricky and often their role can be elucidated only with a detailed mechanistic study. Here we report the case of fluorinated ligand systems showing a very peculiar behavior in the catalysis of olefin polymerization. The results reported aim to clarify the origin of such behavior underlying the nature of the control of the fluorinated groups on the key steps of the polymerization. These insights can offer new cues to rationalize similar evidences in other reaction fields
Increasing project control and guidance efficiency through a time-frame simulation approach
Full text linkNowadays projects dynamicity and complexity make the control process highly critical. The existing planning and control techniques have frequently proved inadequacy to manage the present challenge. The paper proposes a simulative approach to managing with more efficiency projects life cycles. The appositely built simulation model is populated with both deterministic and stochastic elements: the formers come from the project plan; the stochastic elements have been introduced in order to consider the probabilistic nature of activities duration. In the planning phase the model generates a “baseline pencil” that gives a more confident estimation of the time to complete the project. During the execution phase the model is able to store the data related to the ongoing activities and updates in real-time the estimation of the project completion. Contemporary, it allows the calculation of specific performance indexes which permit to signal in real-time possible occurring “warnings” to users and suggest potential solutions
A structured approach to managing technical process reengineering projects
No full textThe emerging knowledge-based economy has pointed out theimportance of competence and its interface in the conception of
any economic process. There has been a natural evolution in industrial innovation processes: from a sequence of separated phases
(sequential approach) to a multidisciplinary network of competences (interactive approach). These changes have brought about the
increase in the information to be managed and coordinated especially if the project regards processes such as reengineering ones in which
the technical aspects are predominant and critical. The aim of this paper is to propose an integrated methodology based on a logical
framework which guarantees effective analysis and management of data generated during a project of Product/Process Reengineering
(PR). The framework is able to coordinate these information flows whose complexity is related to the different nature of the data
involved, by supporting design and operation processes during the evaluation of all the feasible alternatives
Constructing Bridges between Computational Tools in Heterogeneous and Homogeneous Catalysis
No full textBetter catalysts are needed to address numerous challenges faced by humanity. In this Perspective, we review concepts and tools in theoretical and computational chemistry that can help to accelerate the rational design of homogeneous and heterogeneous catalysts. In particular, we focus on the following three topics: (1) identification of key intermediates and transition states in a reaction using the energetic span model, (2) disentanglement of factors influencing the relative stability of the key species using energy decomposition analysis and the activation strain model, and (3) discovery of new catalysts using volcano relationships. To facilitate wider use of these techniques across different areas, we illustrate their potentials and pitfalls when applied to the study of homogeneous and heterogeneous catalysts
Bi-functional and mono-component organocatalysts for the ring-opening alternating co-polymerisation of anhydride and epoxide
No full textBi-functional organocatalysts constituted by a (thio-)urea moiety and an iminophosphorane moiety were synthesized and optimised for the ring-opening alternating co-polymerisation of phthalic anhydride with three different epoxides: cyclohexene oxide, propylene oxide and butylene oxide. The most effective catalyst featured a cyclohexyl urea moiety, an iminophosphorane moiety with three 2,4-dimethyl-3-methoxy phenyl substituents, and a short spacer length of two carbon atoms between them. All tested epoxides reached quantitative conversion within 24 hours with ester-selectivities up to >97%. NMR and DFT experiments reveal that the catalysts exist in solution as dimers that dissociate during the initiation of the polymerisation. During the polymerisation, the catalyst is coordinated to the growing chain and further modulates its reactivity through reversible protonation/deprotonation suppressing transesterification side reactions even at prolonged polymerisation times without the need for a co-catalyst. The rate-determining step of the polymerisation is the ring-opening of the epoxide by the carboxylate chain end, and accordingly, higher temperatures (up to 150 degrees C) and higher concentrations of epoxide and catalyst increase polymerisation rates
The Impact of Charge in a Ni(II) Polymerization Catalyst
No full textA direct comparison between neutral active sites and their corresponding cationic analogues is enabled by protonation of neutral bis(imino)phenoxy complexes, active for ethylene polymerization. The additional imine motif compared to parent salicylaldiminato catalysts does not influence the microstructure of the products in ethylene polymerization, but allows for the incorporation of a proton right next to the active center in an N···H+···O bridge yielding cationic complexes. These show an increased Ni-O bond length and a drastically reduced electron density on the Ni atom. In pressure reactor experiments, two different catalysts that produce linear HDPE or undergo extensive chain walking, respectively, in their neutral version, both produce short chain oligomers when the catalyst is charged cationically. A mechanistic analysis by DFT methods reveals an increased propensity for β-hydride elimination compared to ethylene insertion chain growth for the cationic complexes. This results from a higher relative stability of β-agostic species vs olefin-coordinated species
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