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Ulrich S Schubert - One of the best experts on this subject based on the ideXlab platform.
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single mode microwave ovens as new reaction devices accelerating the Living Polymerization of 2 ethyl 2 oxazoline
Macromolecular Rapid Communications, 2004Co-Authors: Frank Wiesbrock, Richard Hoogenboom, C Caroline H Abeln, Ulrich S SchubertAbstract:The ring-opening cationic Polymerization of 2-ethyl-2-oxazoline was performed in a single-mode microwave reactor as the first example of a microwave-assisted Living Polymerization. The observed increase in reaction rates by a factor of 350 (6 h → 1 min) in the range from 80 to 190°C could be attributed solely to a temperature effect as was clearly shown by control experiments and the determined activation energy. Because of the homogenous microwave irradation, the Polymerization could be performed in bulk or with drastically reduced solvent ratios (green chemistry).
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single mode microwave ovens as new reaction devices accelerating the Living Polymerization of 2 ethyl 2 oxazoline
Macromolecular Rapid Communications, 2004Co-Authors: Frank Wiesbrock, Richard Hoogenboom, C Caroline H Abeln, Ulrich S SchubertAbstract:The ring-opening cationic Polymerization of 2-ethyl-2-oxazoline was performed in a single-mode microwave reactor as the first example of a microwave-assisted Living Polymerization. The observed increase in reaction rates by a factor of 350 (6 h 1 min) in the range from 80 to 190 °C could be attributed solely to a temperature effect as was clearly shown by control experiments and the determined activation energy. Because of the homogenous microwave irradiation, the Polymerization could be performed in bulk or with drastically reduced solvent ratios
Taelim Choi - One of the best experts on this subject based on the ideXlab platform.
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Living Polymerization Caught in the Act: Direct Observation of an Arrested Intermediate in Metathesis Polymerization
AMER CHEMICAL SOC, 2019Co-Authors: Jung-ah Song, Robert H Grubbs, Bohyun Park, Soohyung Kim, Cheol Kang, Dongwhan Lee, Mu-hyun Baik, Taelim ChoiAbstract:© 2019 American Chemical Society.Understanding the stability and reactivity of the propagating species is critical in Living Polymerization. Therefore, most Living olefin metathesis Polymerizations require the stabilization of the catalyst by coordination of external ligands containing Lewis basic heteroatoms, e.g., phosphines and pyridines. However, in some cases, chemists postulated that the propagating metal carbene could also be stabilized by olefin chelation. Here, we disclose that stable 16-electron olefin-chelated Ru carbenes play a key role in previously reported Living/controlled ring-opening metathesis Polymerization of endo-tricyclo[4.2.2.02,5]deca-3,9-diene and cycloPolymerization of 1,8-nonadiynes using Grubbs catalysts. We successfully isolated these propagating species during Polymerization and confirmed their olefin-chelated structures using X-ray crystallography and NMR analysis. DFT calculations and van 't Hoff plots from the equilibrium between olefin-chelated Ru carbenes and 3-chloropyridine (Py)-coordinated carbenes revealed that entropically favored olefin chelation overwhelmed enthalpically more stable Py-coordinated Ru carbenes at room temperature. Therefore, olefin chelation stabilized the propagating species and slowed down the propagation relative to initiation, thereby lowering polydispersity. This finding provides a deeper understanding of the olefin metathesis Polymerization mechanism using Grubbs catalysts and offers clues for designing new controlled/Living Polymerization
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fast Living Polymerization of challenging aryl isocyanides using an air stable bisphosphine chelated nickel ii initiator
Macromolecules, 2018Co-Authors: Jaeho Lee, Suyong Shin, Taelim ChoiAbstract:Here we report a highly efficient Living Polymerization of challenging electron-rich or sterically hindered aryl isocyanides using an air-stable, but highly active, bisphosphine-chelated nickel(II) complex. Initially, the Living character was examined by screening various Ni(II) complexes, and we identified o-Tol(dppe)NiCl as an excellent initiator for the Living Polymerization of aryl isocyanides. On the basis of chain extension experiments and in situ 31P NMR spectroscopy, we concluded that the high stability of the propagating species due to the tightly bound chelating ligand was crucial for successful Living Polymerizations. Not only reactive electron-poor aryl isocyanides but also more challenging electron-rich or sterically hindered aryl isocyanides underwent fast Living Polymerizations to give polymers having controlled Mn with narrow dispersity. In addition, we confirmed that the electronic character of the monomer significantly affected the Polymerization efficiency by comparing the polymerizatio...
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Fast Living Polymerization of Challenging Aryl Isocyanides Using an Air-Stable Bisphosphine-Chelated Nickel(II) Initiator
2018Co-Authors: Jaeho Lee, Suyong Shin, Taelim ChoiAbstract:Here we report a highly efficient Living Polymerization of challenging electron-rich or sterically hindered aryl isocyanides using an air-stable, but highly active, bisphosphine-chelated nickel(II) complex. Initially, the Living character was examined by screening various Ni(II) complexes, and we identified o-Tol(dppe)NiCl as an excellent initiator for the Living Polymerization of aryl isocyanides. On the basis of chain extension experiments and in situ 31P NMR spectroscopy, we concluded that the high stability of the propagating species due to the tightly bound chelating ligand was crucial for successful Living Polymerizations. Not only reactive electron-poor aryl isocyanides but also more challenging electron-rich or sterically hindered aryl isocyanides underwent fast Living Polymerizations to give polymers having controlled Mn with narrow dispersity. In addition, we confirmed that the electronic character of the monomer significantly affected the Polymerization efficiency by comparing the Polymerization of 4-octyloxyphenyl isocyanide and 3-octyloxyphenyl isocyanide, which have the same substituents at different positions on the phenyl ring. Furthermore, ABCDE pentablock copolymer containing various substituents was efficiently synthesized in only 1 min
Akira Hirao - One of the best experts on this subject based on the ideXlab platform.
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Living anionic Polymerization of 1 4 diisopropenylbenzene
Macromolecules, 2015Co-Authors: Raita Goseki, Suguru Onuki, Shunsuke Tanaka, Takashi Ishizone, Akira HiraoAbstract:The anionic Polymerization of diisopropenylbenzene (DIPB) derivatives was conducted in THF at −78 °C with a specially designed initiator system prepared from oligo(α-methylstyryl)lithium and an excess potassium tert-butoxide (KOBut) (2.7–5.0 equiv to the Li salt). Among the ortho-, meta-, and para-isomers of DIPB derivatives, it was found that the para-isomer (p-DIPB) successfully underwent the Living Polymerization in a selective manner through one of the two isopropenyl groups under the above stated conditions. With this Living Polymerization system, soluble polymers with controllable Mn values ranging from 7620 to 31 500 g/mol and near monodisperse distributions (Mw/Mn ≤ 1.03) were obtained for the first time. The obtained Living polymers were stable at −78 °C even after 168 h and at −40 °C after 6 h, in which the intermolecular addition reaction of the chain-end anion to the pendant isopropenyl group could be completely suppressed. In contrast, the Living Polymerization of either the ortho- or meta-is...
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Living Anionic Polymerization of 1,4-Diisopropenylbenzene
2015Co-Authors: Raita Goseki, Suguru Onuki, Shunsuke Tanaka, Takashi Ishizone, Akira HiraoAbstract:The anionic Polymerization of diisopropenylbenzene (DIPB) derivatives was conducted in THF at −78 °C with a specially designed initiator system prepared from oligo(α-methylstyryl)lithium and an excess potassium tert-butoxide (KOBut) (2.7–5.0 equiv to the Li salt). Among the ortho-, meta-, and para-isomers of DIPB derivatives, it was found that the para-isomer (p-DIPB) successfully underwent the Living Polymerization in a selective manner through one of the two isopropenyl groups under the above stated conditions. With this Living Polymerization system, soluble polymers with controllable Mn values ranging from 7620 to 31 500 g/mol and near monodisperse distributions (Mw/Mn ≤ 1.03) were obtained for the first time. The obtained Living polymers were stable at −78 °C even after 168 h and at −40 °C after 6 h, in which the intermolecular addition reaction of the chain-end anion to the pendant isopropenyl group could be completely suppressed. In contrast, the Living Polymerization of either the ortho- or meta-isomer was not successful under the same conditions. The block coPolymerization of p-DIPB with either styrene (S), 2-vinylpyridine (2VP), or tert-butyl methacrylate (tBMA) by the sequential addition of such monomers was conducted. Four new PS-block-P(p-DIPB), P2VP-block-P(p-DIPB), P(p-DIPB)-block-P2VP, and P(p-DIPB)-block-PtBMA containing reactive P(p-DIPB) segments were synthesized. On the basis of the block coPolymerization results, it is understood that p-DIPB is comparable to 2VP and located between S and tBMA in monomer reactivity. The reactivity increases as follows: S < 2VP ∼ p-DIPB < tBMA, while the nucleophilicity of the Living chain-end anion decreases in the following order: PS– > P2VP– ∼ P(p-DIPB) – > P(tBMA) –
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synthesis of well defined functionalized polystyrenes with a definite number of chloromethylphenyl groups at chain ends or in chains by means of anionic Living Polymerization in conjunction with functional group transformation
Macromolecules, 1999Co-Authors: Akira Hirao, Mayumi HayashiAbstract:The syntheses of well-defined polystyrenes functionalized with a definite number (from one to four) of chloromethylphenyl groups at the chain ends or in the chains by means of anionic Living Polymerization in conjunction with functional group transformation are described. The synthetic method involves the introduction of anion-stable methoxymethylphenyl or tert-butyldimethylsilyloxymethylphenyl groups at the chain ends or in the chains and subsequent transformation reactions of these groups into chloromethylphenyl groups with BCl3. It is also possible to transform the tert-butyldimethylsilyloxymethylphenyl group into bromomethylphenyl and iodomethylphenyl groups by treatment with Me3SiCl−LiBr, and Me3SiCl−NaI, respectively. Furthermore, the synthesis of well-defined polystyrenes having two, four, and six methoxymethylphenyl and six chloromethylphenyl termini via a new iterative approach using 3-(tert-butyldimethylsilyloxy)-1-propyllithium is described. It involves repeated chemical transformations at the ...
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synthesis of star branched polymers by means of anionic Living Polymerization coupled with functional group transformation
Macromolecules, 1999Co-Authors: Mayumi Hayashi, Katsuhiro Kojima, Akira HiraoAbstract:To synthesize well-defined regular and heteroarmed star-branched polymers, we have developed a new and versatile methodology based on anionic Living Polymerization coupled with functional group transformation. The synthetic procedure employed in our methodology involves the following three reaction stages: The first stage is to prepare the precursory polymers with a defined number of methoxymethylphenyl groups by reacting polystyryllithium with specially designed reagents. The methoxymethylphenyl groups thus introduced are quantitatively transformed into chloromethylphenyl groups in the second stage reaction. At the third stage, anionic Living polymers are reacted to couple with the chlorinated precursory polymers to afford the desired star-branched polymers. In fact, well-controlled three-, four-, and six-armed star polystyrenes were successfully synthesized by using the above-mentioned procedure. More interestingly, well-defined hetero four-armed ABC2 and five-armed AB4 star polymers have been readily ...
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Protection and Polymerization of Functional Monomers. 28. Anionic Living Polymerization of Styrene Derivatives Containing Acetal-Protected Monosaccharide Residues
Macromolecules, 1998Co-Authors: Surapich Loykulnant, Mayumi Hayashi, Akira HiraoAbstract:Six styrene derivatives meta-substituted with acetal-protected glucofuranoses (1) and (2), galactopyranose (3), fructopyranose (4), and sorbofuranose (5) and para-substituted with acetal-protected glucofuranose (6) were synthesized by the Williamson reactions of m- or p-(chloromethyl)styrene with the corresponding protected monosaccharides in DMF and were anionically polymerized. The anionic Polymerizations were carried out with s-BuLi in THF at −78 °C for 30 min. Among the monomers, 1−5, were found to undergo anionic Living Polymerization to afford quantitatively the polymers of predictable molecular weights and narrow molecular weight distributions (Mw/Mn < 1.13). Novel well-defined block copolymers, polystyrene-block-poly(1) starting either from Living polystyrene or the Living polymer of 1, were successfully synthesized. By contrast, no appreciable Polymerization of 6 occurred under identical conditions.
James L Hedrick - One of the best experts on this subject based on the ideXlab platform.
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thiourea based bifunctional organocatalysis supramolecular recognition for Living Polymerization
Journal of the American Chemical Society, 2005Co-Authors: Andrew P Dove, Russell C Pratt, Bas G G Lohmeijer, Robert M Waymouth, James L HedrickAbstract:A versatile, metal-free, organocatalytic approach to the Living ring-opening Polymerization of lactide using a bifunctional thiourea−tertiary amine catalyst is described. Mild and highly selective Polymerization conditions produced poly(lactides) with predictable molecular weights and extremely narrow polydispersities (∼1.05), characteristic of a Living Polymerization. The extraordinary selectivity of this catalyst system for Polymerization relative to transesterification is remarkably unusual. The low polydispersities and exceptional control observed are a consequence of selective transesterification of lactide relative to the open chain esters. Presumably, the ring strain of lactide provides both a driving force for the Polymerization and a kinetic preference for Polymerization relative to transesterification with catalyst. We postulate that the initiating/propagating alcohol is activated by acid−base interaction with the tertiary amine moiety and the carbonyl of the lactide monomer is simultaneously ac...
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in situ generation of carbenes a general and versatile platform for organocatalytic Living Polymerization
Journal of the American Chemical Society, 2003Co-Authors: Gregory W Nyce, Robert M Waymouth, Eric F Connor, Thierry Glauser, Andreas Mock, James L HedrickAbstract:A metal-free, organocatalytic approach to Living Polymerization using N-heterocyclic carbenes as nucleophilic catalysts generated and used in situ in a single-pot process is detailed. The N-heterocyclic carbene catalyst platform is extremely versatile, as the nature of the substituents has a pronounced effect of catalyst stability and activity toward different substrates. The generation of imidazolium- and thiazaolium-based carbenes was accomplished from the reaction of the corresponding salts with the appropriate bases. This allowed the rapid screening of libraries of catalysts that provided a basic understanding of catalyst structure (sterics, electronics, etc.) with the Polymerization rate, control, substrate, and range of molecular weights. The imidazole-based catalysts were significantly more active toward ROP than the thiazolium-based analogues. No appreciable differences between imidazol-2-ylidene and imidazolin-2-ylidene catalysts were observed. Less sterically demanding carbenes were found to be more active toward ring-opening Polymerization (ROP) than their sterically encumbered analogues for lactone Polymerization. These data prompted the investigation of ionic liquid as a precatalyst reservoir in a phase-transfer Polymerization with an immiscible THF solution of monomer and initiator. In situ activation of the ionic liquid generates carbene that migrates to the organic phase effecting Living ROP. Precatalyst (ionic liquid) regeneration terminates Polymerization. This simple reaction/recycle protocol readily allows repetitive ROPs from the ionic liquid using commercially available materials.
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first example of n heterocyclic carbenes as catalysts for Living Polymerization organocatalytic ring opening Polymerization of cyclic esters
Journal of the American Chemical Society, 2002Co-Authors: Eric F Connor, Gregory W Nyce, Matthew Myers, And Andreas Mock, James L HedrickAbstract:A novel metal-free, organocatalytic approach to Living Polymerization is presented. N-heterocyclic carbenes were employed as nucleophilic catalysts for the ring-opening Polymerization (ROP) of cyclic ester monomers. The catalysts is used in combination with an initiator, such as an alcohol, which generates an α-end group bearing the ester from the initiating alcohol upon ring-opening and a hydroxyl functional ω-chain end that propagates the chain. This class of catalyst proved to be more reactive than tertiary amine and phosphine nucleophiles, producing narrowly dispersed polymers of predictable molecular weights at room temperature in 1−2 h. Catalysis with respect to both initiating alcohol and monomer was observed. Control of the α and ω end-groups was demonstrated with a pyrene-labeled initiator, allowing the preparation of well-defined macromolecular architectures. Analogous to the ROP of cyclic esters using biocatalysts, the polymeriztion pathway using the N-heterocyclic carbenes is believed to ensue...
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first example of n heterocyclic carbenes as catalysts for Living Polymerization organocatalytic ring opening Polymerization of cyclic esters
Journal of the American Chemical Society, 2002Co-Authors: Eric F Connor, Gregory W Nyce, Matthew Myers, And Andreas Mock, James L HedrickAbstract:A novel metal-free, organocatalytic approach to Living Polymerization is presented. N-heterocyclic carbenes were employed as nucleophilic catalysts for the ring-opening Polymerization (ROP) of cyclic ester monomers. The catalysts is used in combination with an initiator, such as an alcohol, which generates an alpha-end group bearing the ester from the initiating alcohol upon ring-opening and a hydroxyl functional omega-chain end that propagates the chain. This class of catalyst proved to be more reactive than tertiary amine and phosphine nucleophiles, producing narrowly dispersed polymers of predictable molecular weights at room temperature in 1-2 h. Catalysis with respect to both initiating alcohol and monomer was observed. Control of the alpha and omega end-groups was demonstrated with a pyrene-labeled initiator, allowing the preparation of well-defined macromolecular architectures. Analogous to the ROP of cyclic esters using biocatalysts, the polymeriztion pathway using the N-heterocyclic carbenes is believed to ensue through a monomer-activated mechanism.
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new paradigms for organic catalysts the first organocatalytic Living Polymerization
Angewandte Chemie, 2001Co-Authors: Fredrik Nederberg, Thierry Glauser, Eric F Connor, Michael Moller, James L HedrickAbstract:A metal-free approach to the Living ring-opening Polymerization (ROP, shown schematically) of lactide has been developed using strongly basic amines such as 4-(dimethylamino)pyridine as transesterification catalysts. These organic catalysts must be used in combination with a nucleophile such as an alcohol, which is the actual initiating species.
Tsutomu Yokozawa - One of the best experts on this subject based on the ideXlab platform.
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chain growth polycondensation the Living Polymerization process in polycondensation
Progress in Polymer Science, 2007Co-Authors: Tsutomu Yokozawa, Akihiro YokoyamaAbstract:Abstract The historical development of research on the Living Polymerization process in polycondensation is reviewed. Classical polycondensation is a step-growth process, but a Living Polymerization polycondensation must proceed by a chain-growth rather than a step growth mechanism. Early work demonstrated that some polycondensations do not obey Flory's statistical treatment: for example, high molecular weight polymer may be obtained, even at low conversion. This means that a chain-growth mechanism must be involved, with or without a step-growth mechanism. Recent years have seen dramatic development in understanding of polycondensations that proceed only by chain-growth (chain-growth polycondensation). Several possible mechanisms are: (1) activation of the polymer end group by changed substituent effects between the monomer and the polymer, as with aromatic polyamides, polyesters, polyethers, poly(ether sulfone)s and poly(ether ketone)s; (2) activation of the polymer end group by transfer to it of the catalyst, as with polythiophenes; (3) transfer of the reactive species, derived from the initiator, to the polymer end group, as with polymethylenes and polyphosphazenes; and (4) phase-transfer Polymerization in a biphase composed of a monomer storage phase and a Polymerization phase, as with aliphatic polyesters. These chain-growth polycondensations have been applied to the synthesis of condensation polymers with various architectures: block copolymers, star polymers, graft copolymers, etc.
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Chain-Growth Polycondensation: Living Polymerization Nature in Polycondensation and Approach to Condensation Polymer Architecture
Polymer Journal, 2004Co-Authors: Tsutomu Yokozawa, Akihiro YokoyamaAbstract:In this review article, polycondensation that proceeds in a chain-growth Polymerization manner (“chain-growth polycondensation”) for well-defined condensation polymers are described. Our approach to chain-growth polycondensation is (1) activation of polymer end group by substituent effects changed between monomer and polymer and (2) phase-transfer Polymerization in biphase composed of monomer store phase and Polymerization phase. In the approach (1), a variety of condensation polymers such as aromatic polyamides, aromatic polyesters, aromatic polyethers, poly(ether sulfone), and polythiophene with defined molecular weights and low polydispersities were obtained. Their polycondensations had all of the characteristics of Living Polymerization: a linear correlation between molecular weights and monomer conversion maintaining low polydispersities, and control over molecular weights by the feed ratio of monomer to initiator. Taking advantage of the nature of Living Polymerization in this polycondensation, we synthesized diblock copolymers of different kinds of aromatic polyamides and of aromatic polyamide and conventional polymers such as poly(ethylene glycol), polystyrene, and poly(tetrahydrofuran), as well as triblock copolymers and star polymers containing aromatic polyamide units. Some copolymers were arranged in a supramolecular self-assembly. In the approach (2), the polycondensation of solid monomer dispersed in organic solvent with a phase transfer catalyst (PTC) was carried out, where solid monomer did not react with each other, and the monomer transferred to organic solvent with PTC reacted with an initiator and the polymer end group selectively in organic solvent, to yield well-defined polyesters.
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chain growth polycondensation for aromatic polyethers with low polydispersities Living Polymerization nature in polycondensation
Macromolecules, 2003Co-Authors: Yukimitsu Suzuki, Shuichi Hiraoka, And Akihiro Yokoyama, Tsutomu YokozawaAbstract:Polycondensation normally proceeds in a step-growth reaction manner to give polymers with a wide range of molecular weights. However, the polycondensation of potassium 5-cyano-4-fluoro-2-propylphenolate (1) proceeded at 150 °C in a chain Polymerization manner from an initiator, 4-fluoro-4‘-trifluoromethylbenzophenone (2a), to give aromatic polyethers having controlled molecular weights and low polydispersities (Mw/Mn ≤ 1.1). The resulting polycondensation of 1 had all of the characteristics of Living Polymerization and displayed a linear correlation between molecular weight and monomer conversion, maintaining low polydispersities. The MALDI−TOF mass spectrum of poly1 revealed that this polycondensation did not include conventional step-growth polycondensation which gave the polymer without initiation unit and macrocycles. The poly1 with low polydispersity showed higher crystallinity than that with broad molecular weight distribution, obtained by the conventional polycondensation of 1 without 2a.