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

Retrospective evaluation of whole exome and genome mutation calls in 746 cancer samples

Funder: NCI U24CA211006Abstract: The Cancer Genome Atlas (TCGA) and International Cancer Genome Consortium (ICGC) curated consensus somatic mutation calls using whole exome sequencing (WES) and whole genome sequencing (WGS), respectively. Here, as part of the ICGC/TCGA Pan-Cancer Analysis of Whole Genomes (PCAWG) Consortium, which aggregated whole genome sequencing data from 2,658 cancers across 38 tumour types, we compare WES and WGS side-by-side from 746 TCGA samples, finding that ~80% of mutations overlap in covered exonic regions. We estimate that low variant allele fraction (VAF < 15%) and clonal heterogeneity contribute up to 68% of private WGS mutations and 71% of private WES mutations. We observe that ~30% of private WGS mutations trace to mutations identified by a single variant caller in WES consensus efforts. WGS captures both ~50% more variation in exonic regions and un-observed mutations in loci with variable GC-content. Together, our analysis highlights technological divergences between two reproducible somatic variant detection efforts

Full assignment of the 13C NMR spectra of regioregular polypropylenes: methyl and methylene region

13C NMR spectroscopy is the main source of information on the stereochem. of Ziegler-Natta- and related transition meta-catalyzed propene polymns. In simple cases, like those of polypropylenes formed under pure enantiomorphic-site or chain-end control, the origin of the stereoselectivity can be easily recognized from the steric pentad distribution obtained from routine 13C NMR spectra. On the other hand, the variety of innovative polymers that can now be prepd. with "high-yield" heterogeneous and metallocene-based homogeneous catalysts under hybrid, multiple, or oscillating stereocontrol represent very complex systems, which are beyond the possibilities of configurational anal. by routine 13C NMR. In such cases, high-field 13C NMR can be highly advantageous. Indeed, in this paper we show that from the Me and methylene regions of 150 MHz 13C NMR spectra of polypropylenes of various tacticities, the stereosequence distribution can be detd. at a much finer level of detail, so as to obtain an adequate exptl. basis for the investigation of the many complicated mechanisms of stereocontrol presently encountered in Ziegler-Natta catalysis

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

We report the results of a 150 MHz 13C NMR characterization of ethene-1-13C/propene copolymers at low (<5 mol%) ethene content prepd. in the presence of the syndiotactic-selective 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 concn. is higher than claimed in the literature) are also substantially stereoirregular

C2-symmetric ansa-metallocene catalysts for propene polymerization: Stereoselectivity and enantioselectivity

A review with 71 refs. Important similarities between the new homogeneous catalysts for 1-alkene polymn. based on C2-sym. ansa-metallocenes of the 4th column and their heterogeneous Ziegler-Natta predecessors are unquestionable; in particular, the factors leading to the remarkable enantioselectivity of both catalyst classes have been proved to be strictly analogous. In the case of metallocene catalysts, however, an insidious side reaction of chain epimerization, unnoticed for a relatively long time, can undermine the stereoselectivity of the polymn. process; this recent finding and its mechanistic implications are discussed