267 research outputs found
Erratum: "Random and block copolymers based on 4-methyl-1-pentene and 1-pentene" (Polymer (2012) 53 (3096-3106))
No full textExploration of the effect of 2,6-(t-Bu)2-4-Me-C6H2OH (BHT) in chain shuttling polymerization
No full textThe interactions of a sterically hindered phenol [2,¿6-¿(t-¿Bu)¿2-¿4-¿Me-¿C6H2OH] (BHT) with the scavenger MAO (AlR3) and ZnR2 during Hf¿/Zr-¿based chain shuttling polymn. in a semi-¿batch reactor have been investigated. NMR model studies indicated preferential binding of BHT to aluminum under these conditions. Subsequently, reproducible polymn. runs gave rise to copolymers that were thoroughly characterized by HT-¿SEC, HT-¿HPLC, DSC, thermal fractionation (SSA)¿, 13C NMR, d. measurements, CRYSTAF and optical microscopy to unravel their complex microstructures. The obtained materials differ from a simple soln. blend of materials, sep. produced by single catalysts, but also from multi-¿block copolymers as obtained by DOW's continuous process, although a blocky structure can be rationalized
Synthesis and characterization of iPP-sPP stereoblock produced by a binary metallocene system
No full textStereoblock polypropylenes comprising of iPP and sPP segments are synthesized by polymerization of the following binary system of metallocenes: the Cs-symmetric [2,7-t-Bu2(Flu)2Ph2C(Cp)ZrCl2] and the C2-symmetric rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2. Blends of samples made either by each catalyst individually (solution blend) with materials obtained with the mixed catalyst system (reactor blend) are compared. The simultaneous presence of MAO and DEZ, enhancing fast and reversible transfer of the growing chains between the two active centers, leads to the formation of a stereoblock microstructure. In this case, low molecular weight polymers are obtained. The junction between the blocks is qualitatively observed in 13C NMR. When made in toluene, the stereoblock material consists of a majority of syndiotactic sequences, whereas the ratio is more equilibrated when the polymerization was conducted in the more polar chlorobenzene. This is confirmed by the results obtained with 13C NMR, CRYSTAF, HT HPLC, DSC, SSA, WAXD, and optical microscopy
Ancillary ligand effects in organoyttrium chemistry
No full textAlthough the development of organometallic chemistry and homogeneous catalysiss tarted seperately in the early fifties, these fields of research have become tirmly intertwined during
the last decade. This stems lrom the fact that transition metalc arbon and hydrido bonds play a key role in many C-X (X = C, H, hetero atom) transformation.
Zie: summary/samenvatting
Random and block copolymers based on 4-methyl-1-pentene and 1-pentene
No full textThe zirconium acetamidinate catalyst {Cp*Zr(Me)2[N(Et)C(Me)N(tBu)]} (Cp* = ŋ5-C5Me5) was used to synthesize both random and block copolymers based on 4-methyl-1-pentene (4M1P) and 1-pentene. The polymers have been characterized by NMR spectroscopy, SEC, DSC, high temperature HPLC and CRYSTAF. Unexpectedly, the yields and molecular weights decreased with increasing amounts of 1-pentene. The reason for this behavior is that 1-pentene occasionally undergoes 2,1-misinsertions trapping the catalyst in a dormant state. These 2,1-misinsertions do not seem to occur with the bulky 4M1P (branched α-olefin). Adding a small amount of ethylene reactivates the catalyst. Unlike most semi-crystalline polymers, the density of the crystalline phase of isotactic P4M1P can be lower than of the amorphous phase, when crystallized under very high pressures. To characterize this peculiar behavior of 4M1P-based polymers, various samples have been subjected to Pressure-Volume-Temperature (PVT) measurements. While the P4M1P homopolymers and block copolymers show the expected decrease in specific volume upon crystallization, the 4M1P-rich random copolymers proved not to vary in specific volume under the same conditions
Advanced applications of the flat surface science model of the Phillips ethylene polymerization catalyst
No full textIncompletely condensed silsesquioxanes: Versatile tools in developing silica-supported olefin polymerization catalysts
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Absence of pathogenic mutations in CD59 in chronic inflammatory demyelinating polyradiculoneuropathy
No full text\ua9 2019 Duchateau et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.Objective Mutations in CD59 cause CIDP-like polyneuropathy in children with inherited chronic hemolysis. We hypothesized that mutations in CD59 might be found in a subset of sporadic CIDP patients. Methods 35 patients from two centers, fulfilling the EFNS/PNS diagnostic criteria for CIDP were included. CD59 coding region was amplified by PCR and Sanger sequenced. Results One rare variant was detected in a patient which resulted in a synonymous change and predicted to be neutral. Pathogenic variants were absent in our cohort. Interpretation Our pilot study suggests that mutations in CD59 are absent in adult-onset sporadic CIDP
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