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Same Ligand, Different Metals: Diiodo-Salan Complexes of the Group 4 Triad in Isospecific Polymerization of 1-Hexene and Propylene
No full textIn this work we describe the synthesis of a Salan ligand featuring diiodo-phenolate rings, the dibenzyl complexes of this ligand with the group 4 triad metals, and the polymerization of 1-hexene and propylene by these complexes. The polymerization of propylene by titanium- Salan complexes was not reported previously, and the activity of such a catalyst, revealed herein, is quite remarkable
Stereogradient Poly(Lactic Acid) from meso‐Lactide/L‐Lactide Mixtures
No full textTheproductionof L-lactidefromL-lacticacidinvolvesa substantialformationofmeso-lactideas animpurity,and,uponpolymerizationwiththe industrialcatalysttin octanoate,resultsin poly(L-lacticacid)ofreducedcrystallinitydueto stereoerrorsrandomlydistributedalongthe polymerchains.We describea newapproachwherein,insteadof avoidingstereoerrorsbyremovingthemeso-lactidepriorto polymerization,thestereoerrorsin the polymerare tolerated,by crowdingthemin a stereogradientcopolymer.A zirconiumcomplexof an aminetris(phenolate)ligandis foundtoexhibitveryhighsyndioselectivityin the ringopeningpolymerizationcatalysisofmeso-lactideat roomtemper-ature,and givesrise to stereogradientcopolymersin thepolymerizationof mixturesofmeso-lactide/L-lactideinthemeltat 180°C. Relativeto thestereo-randomcopolymersobtainedwithtin octanoate,thestereo-gradientcopolymersexhibitenhancedcrystallinitiesmanifestedin lowersolubilitiesandhighermeltingtemperaturesand enthalpies
Group 4 Metal Complexes of PhenyleneâSalalen Ligands in rac-Lactide Polymerization Giving High Molecular Weight Stereoblock Poly(lactic acid)
No full textPhenyleneâsalalensâsequential tetradentate dianionic ONNO-type ligands that include an ortho-phenylene group bridging between an imine and amine internal donors bound to phenol arms with a broad variety of substitution patterns are described. Zirconium and hafnium complexes of the type [ONNOM(O-tBu)2] were formed as single diastereomers, which, according to crystallographic structures, featured the fac(around the amine)-mer(around the imine) wrapping mode. The reactivity and stereoselectivity in rac-lactide (rac-LA) polymerizations were found to depend on the substitution pattern: complexes featuring small groups on the imine-side phenol and bulky groups on the amine-side phenol exhibited the favorable combination of high activity and high isoselectivity (Pmâ¤0.91). Isotactic stereoblock copolymers of high molecular weights were prepared. The polymers crystallized in the stereocomplex phase according to thermal (differential scanning calorimetry) and crystallographic analysis
Divergent [ONNNMg–Cl] complexes in highly active and living lactide polymerization
No full textChloro-magnesium complexes of divergent tetradentate-monoanionic {ONNN}-type ligands which formed as either mononuclear or dinuclear complexes are reported. These complexes catalyze the ring-opening polymerization of lactide with high activity and in a living manner, and enable the synthesis of well-defined stereo-diblock copolymers with Mn > 200 kDa, as well as stereo-n-block copolymers (n = tri, tetra) with high molecular weights by sequential monomer addition. Thermal and crystallographic characterizations revealed that even the very high molecular weight PLA copolymers crystallized in a stereocomplex phase, and that their degradation temperature was as high as 354 °C
Tailor-Made Stereoblock Copolymers of Poly(lactic acid) by a Truly Living Polymerization Catalyst
No full textPoly(lactic acid) (PLA) is a biodegradable polymer prepared by the catalyzed ring opening polymerization of lactide. An ideal catalyst should enable a sequential polymerization of the lactide enantiomers to afford stereoblock copolymers with predetermined number and lengths of blocks. We describe a magnesium based catalyst that combines very high activity with a true-living nature, which gives access to PLA materials of unprecedented microstructures. Full consumption of thousands of equivalents of L-LA within minutes gave PLLA of expected molecular weights and narrow molecular weight distributions. Precise PLLA-b-PDLA diblock copolymers having block lengths of up to 500 repeat units were readily prepared within 30 min, and their thermal characterization revealed a stereocomplex phase only with very high melting transitions and melting enthalpies. The one pot sequential polymerization was extended up to precise hexablocks having "dialed-in" block lengths
Block–Stereoblock Copolymers of Poly(ε-Caprolactone) and Poly(Lactic Acid)
No full textA magnesium complex of the type ONNNMg-HMDS wherein ONNN is a sequential tetradentate monoanionic ligand is introduced. In the presence of an alcohol initiator this complex catalyzes the living and immortal homopolymerization of the lactide enantiomers and ε-caprolactone at room-temperature with exceptionally high activities, as well as the precise block copolymerization of these monomers in a one-pot synthesis by sequential monomer addition. Copolymers of unprecedented microstructures such as the PCL-b-PLLA-b-PDLA and PDLA-b-PLLA-b-PCL-b-PLLA-b-PDLA block–stereoblock microstructures that feature unique thermal properties are readily accessed
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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