Comparative NMR study on the reactions of Hf(IV) organometallic complexes with Al/Zn alkyls

NMR spectroscopy has been exploited to investigate the reactions of Hf(IV) organometallic complexes with trialkylaluminum and dialkylzinc, with the aim of obtaining insights into the elementary steps of coordinative chain transfer polymerization (CCTP). Bis(cyclopentadienyl)hafnium dimethyl (Cp2HfMe2, 1Me2) and [N-(2,6-diisopropylphenyl)-α-(2-isopropylphenyl)-6-(1-naphthalenyl)-2-pyridinemethanaminato]hafnium dimethyl (2Me2) complexes have been chosen as case studies for understanding the differences between poorly performing and highly active CCTP catalysts, in an attempt to assess the effect of the ancillary ligand on the transalkylation rate. 2Me2 was found to react much more quickly with both AlEt3 and ZnEt2 in comparison to 1Me2, mainly due to a remarkably lower activation enthalpy. In addition, while the ethylation rate was found to depend on the nature of the alkylating agent for 1Me2, it does not for 2Me2. This difference in reactivity was observed also in the case of the ion pairs obtained by reacting 1Me2 and 2Me2 with [CPh3][B(C6F5)4]. For the latter species, NMR indicated that two main deactivation pathways, namely anion decomposition and σ-bond methatesis of Hf–alkyl groups, occur

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

Structural Analysis of Copolymers of Syndiotactic Polypropylene with 13C-Enriched Ethylene

A structural analysis of copolymers of syndiotactic polypropylene with small amounts of 13 C-enriched ethylene, in the range 0.4-2.6 mol %, is reported. X-ray diffraction and solid-state C-13 NMR CPMAS data indicate that the as-prepared copolymer samples are crystallized in the conformationally disordered modification of form II of sPP containing kink bands. The disorder corresponds to the presence of portions of chains in the trans-planar conformation in chains having a prevailing 2-fold helical conformation. A direct evidence of the partial inclusion of the ethylene units in the crystalline regions of the copolymers is provided

Beyond Traditional Morphological Characterization of Lung Neuroendocrine Neoplasms: In Silico Study of Next-Generation Sequencing Mutations Analysis across the Four World Health Organization Defined Groups

Lung neuroendocrine neoplasms (LNENs) classes, as proposed by the World Health Organization 2015, do not provide properly prognostic and therapeutic indications. In fact, high-throughput molecular analysis, based on next-generation sequencing, identified novel molecular subgroups, associated with different genomic signatures, that could pave the way for alternative therapeutic approaches. The present review, coupled with in silico molecular analysis, could show the current genomic alterations state in actual LNENS groups. Interestingly our manuscript suggests that the molecular novelties could improve the LNENs therapeutics efficacy. In more detail, we reported the differences of gene alterations and mutational rate between LNENS, confirming the central pathogenetic role given by a different mutational rate in chromatin remodeling genes and tumor suppressors TP53-RB1. In conclusion, our results underlined that a further molecular layer is needed to improve the efficacy of LNENs medical treatment.Lung neuroendocrine neoplasms (LNENs) represent a rare and heterogeneous population of lung tumors. LNENs incidence rate has increased dramatically over the past 30 years. The current World Health Organization LNENs classification (WHO 2015), distinguished four LNENs prognostic categories, according to their morphology, necrosis amount and mitotic count: typical carcinoid (TC), atypical-carcinoid (AC), large cell neuroendocrine carcinoma (LCNEC) and small cell lung cancer (SCLC). At present, due to their rarity and biological heterogeneity there is still no consensus on the best therapeutic approach. Next-generation-sequencing analysis showed that WHO 2015 LNENs classes, could be characterized also by specific molecular alterations: frequently mutated genes involving chromatin remodeling and generally characterized by low mutational burden (MB) are frequently detected in both TC and AC; otherwise, TP53 and RB1 tumor suppressor genes alterations and high MB are usually detected in LCNEC and SCLC. We provide an overview concerning gene mutations in each WHO 2015 LNENs class in order to report the current LNENs mutational status as potential tool to better understand their clinical outcome and to drive medical treatment