1,720,967 research outputs found
Endowing Polythioester Vitrimer with Intrinsic Crystallinity and Chemical Recyclability
No full textTechnologically important thermosets face a long-standing end-of-life (EoL) problem of non-reprocessability, a more sustainable solution of which has resolved to nascent vitrimers that can merge the robust material properties of thermosets and the reprocessability of thermoplastics. However, the lifecycle of vitrimers is still finite, as they often suffer from significant deterioration of mechanical performance following multiple reprocessing cycles, analogous to mechanical recycling, and they often show undesired creep under working conditions. To address these two key limitations, we have developed a cross-linked semi-crystalline polythioester with both dynamic covalent bonds and intrinsic crystallinity and chemical recyclability, affording a vitrimeric system that exhibits not only reprocessability and crystallinity-restricted creep but also complete chemical recyclability to initial monomer by catalyzed depolymerization in solution or bulk. Therefore, reported herein is an "infinite" vitrimer system that is empowered with a facile closed-loop EoL option once serial reprocessing deteriorates performance and the material can no longer meet the application requirements. Specifically, the polythioester vitrimer was constructed by copolymerization of a bicyclic thioester with a bis-dithiolane, producing dynamically cross-linked polythioesters with excellent property tunability, from amorphous to semi-crystalline states and melting transition temperatures from 91 to 178 °C
Hybrid monomer design for unifying conflicting polymerizability, recyclability, and performance properties
Full text linkIntrinsically recyclable polymers represent a circular economy approach to address plastics problems. However, the design of such circular polymers is challenged by unyielding trade-offs between the monomer's polymerizability and the polymer's depoly-merizability and performance properties. Here, we introduce a hybrid monomer design strategy that synergistically couples a high ceiling temperature (HCT) sub-structure for high polymerizability and performance properties with a low ceiling temperature (LCT) sub-structure for high depolymerizability and recyclability within the same monomer structure. Thus, structural hybridization between HCT e-caprolactone and LCT g-butyrolactone led to an offspring [3.2.1]bicyclic lactone, which exhibits both high polymerizability and depolymerizability, otherwise conflicting properties in a typical monomer. The resulting polymer becomes a high-performance material, and thermal transition temperatures are similar to 200 degrees C higher and tensile modulus 103 higher than its parent polymers. These results demonstrate that the HCT/LCT hybrid monomer strategy is a powerful approach for designing circular polymers where conflicting properties must be exploited and unified
Catalyst-Site-Controlled Coordination Polymerization of Polar Vinyl Monomers to Highly Syndiotactic Polymers
No full textAbstract: This contribution reports a combined synthetic, kinetic, mechanistic, and theoretical/computational
study of the recently discovered catalyst-site-controlled coordination polymerization of polar vinyl monomers
[such as methyl methacrylate (MMA) and N,N-dimethylacrylamide (DMAA)] into highly syndiotactic polymers.
Among the 12 Cs-ligated ansa-cyclopentadienyl (Cp)-R2E(C,Si)-fluorenyl (Flu) group 4 metallocene catalyst
systems examinedswhich varied in metal center, anion structure, bridging atom and substituents, and
ligand substitution patternscationic ansa-metallocene ester enolate catalyst 6+[B(C6F5)4]-, derived from
the activation of the precatalyst [Ph2C(Cp)(2,7-tBu2-Flu)]Zr[OC(OiPr)dCMe2]2 with [Ph3C][B(C6F5)4], stood
out as the best catalyst in all aspects of the MMA polymerization at room temperature, including the highest
activity (1554 h-1 TOF), efficiency (98% I*), syndiotacticity (94% rr), and control (predicted number-average
molecular weight and 1.14 molecular weight distribution). Kinetic and mechanistic results are consistent
with a catalyst-site-controlled, monometallic coordination-addition mechanism, involving fast intramolecular
addition within the catalyst-monomer complex leading to the resting eight-membered ester enolate chelate,
followed by the rate-limiting ring-opening of the chelate to regenerate the active species. This work has
also uncovered several unique features of this polymerization system that are in marked contrast to the
propylene polymerization by analogous Cs-ligated cationic alkyl catalysts: a constant syndiotacticity of PMMA
produced over a wide polymerization temperature range (i.e., from 0 C, 94% rr to 25 C, 94% rr to 50 C,
93% rr); insensitivity of its high activity, degree of control, and stereoselectivity to solvent polarity and structure
of weakly coordinating anions; and deviation from a pure site-control mechanism at high [MMA]/[catalyst]
ratios. Computational results provide theoretical support for the proposed monomer-assisted, catalyst-site
epimerization, after an enantiofacial mistake, to a thermodynamically more stable resting state, which
accounts for the observed higher than expected [mr] contents based on a pure site-controlled mechanism.
DFT calculations rationalize why the Ph2C< bridged catalyst 6 exhibits higher stereoselectivity than other
catalysts with the Me2C< or Me2Si< bridge: the bridge rigidity pushes the η3-bound Flu ligand closer to the
growing chain and the monomer, thereby increasing ΔEstereo between the competing transition states for
the addition of a monomer molecule to the opposite (correct and wrong) enantiofaces of the enolate growing
chain. The relative polymerization activity of this catalyst series is shown to correlate with the relative
energetics of the back-biting of the penultimate unit and ion-pair formation
Closed-loop polymer upcycling by installing property-enhancing comonomer sequences and recyclability
No full textThe concept of upcycling postconsumer plastics into higher-value products is attractive, but the challenges remain to develop a cost-effective upcycling scheme, discover property-enhancing structures, and, most importantly, install recyclability into upcycled plastics to enable a circular lifecycle. Reported herein is a convenient and effective strategy to upcycle polyester, exemplified by poly(glycolic acid) (PGA), via transesterification (TEster) in bioderived, commercially available γ-butyrolactone (BL) that serves as both the solvent and comonomer, which generates sequence-defined copolymer poly(GA-co-BL). Owing to the isolated glycolic sequence present in the copolymer created uniquely by TEster, it exhibits much-enhanced thermal stability (≥44 °C) over both homopolymers or copolymers without such sequences. This upconverted copolymer is chemically recyclable, enabling a complete recovery of pure glycolic acid and BL feedstocks
Biodegradable Polyhydroxyalkanoates by Stereoselective Copolymerization of Racemic Diolides: Stereocontrol and Polyolefin-Like Properties
No full textBacterial polyhydroxyalkanoates (PHAs) are a unique class of biodegradable polymers because of their biodegradability in ambient environments and structural diversity enabled by side-chain groups. However, the biosynthesis of PHAs is slow and expensive, limiting their broader applications as commodity plastics. To overcome such limitation, the catalyzed chemical synthesis of bacterial PHAs has been developed, using the metal-catalyzed stereoselective ring-opening (co)polymerization of racemic cyclic diolides (rac-8DLR, R=alkyl group). In this combined experimental and computational study, polymerization kinetics, stereocontrol, copolymerization characteristics, and the properties of the resulting PHAs have been examined. Most notably, stereoselective copolymerizations of rac-8DLMe with rac-8DLR (R=Et, Bu) have yielded high-molecular-weight, crystalline isotactic PHA copolymers that are hard, ductile, and tough plastics, and exhibit polyolefin-like thermal and mechanical properties
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Chemically circular, mechanically tough, and melt-processable polyhydroxyalkanoates
No full textPolyhydroxyalkanoates (PHAs) have attracted increasing interest as sustainable plastics because of their biorenewability and biodegradability in the ambient environment. However, current semicrystalline PHAs face three long-standing challenges to broad commercial implementation and application: lack of melt processability, mechanical brittleness, and unrealized recyclability, the last of which is essential for achieving a circular plastics economy. Here we report a synthetic PHA platform that addresses the origin of thermal instability by eliminating α-hydrogens in the PHA repeat units and thus precluding facile cis-elimination during thermal degradation. This simple α,α-disubstitution in PHAs enhances the thermal stability so substantially that the PHAs become melt-processable. Synergistically, this structural modification also endows the PHAs with the mechanical toughness, intrinsic crystallinity, and closed-loop chemical recyclability
- …
