| 1. |
- Hirschmann, Max, et al.
(författare)
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Bi-functional and mono-component organocatalysts for the ring-opening alternating co-polymerisation of anhydride and epoxide
- 2023
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Ingår i: Catalysis Science & Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 13:24, s. 7011-7021
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Tidskriftsartikel (refereegranskat)abstract
- Bi-functional organocatalysts constituted by a (thio-)urea moiety and an iminophosphorane moiety were synthesized and optimised for the ring-opening alternating co-polymerisation of phthalic anhydride with three different epoxides: cyclohexene oxide, propylene oxide and butylene oxide. The most effective catalyst featured a cyclohexyl urea moiety, an iminophosphorane moiety with three 2,4-dimethyl-3-methoxy phenyl substituents, and a short spacer length of two carbon atoms between them. All tested epoxides reached quantitative conversion within 24 hours with ester-selectivities up to >97%. NMR and DFT experiments reveal that the catalysts exist in solution as dimers that dissociate during the initiation of the polymerisation. During the polymerisation, the catalyst is coordinated to the growing chain and further modulates its reactivity through reversible protonation/deprotonation suppressing transesterification side reactions even at prolonged polymerisation times without the need for a co-catalyst. The rate-determining step of the polymerisation is the ring-opening of the epoxide by the carboxylate chain end, and accordingly, higher temperatures (up to 150 °C) and higher concentrations of epoxide and catalyst increase polymerisation rates.
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| 2. |
- Hirschmann, Max, et al.
(författare)
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Functional and degradable copolyesters by ring-opening copolymerization of and
- 2023
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Ingår i: European Polymer Journal. - : Elsevier BV. - 0014-3057 .- 1873-1945. ; 183
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Tidskriftsartikel (refereegranskat)abstract
- The ring-opening copolymerization (ROcoP) of epoxides and anhydrides is exploited to afford 4 structurally diverse and functional copolyesters. Mixtures of 2 epoxides (allyl glycidyl ether and butylene oxide) with 1 anhydride (succinic, glutaric, phthalic and homo phthalic anhydride) are copolymerized in the presence of bis (triphenylphosphine)iminium chloride (PPNCl) as organocatalyst. All monomer combinations yield vinyl-functionalized materials with alternating epoxide-anhydride units, statistical incorporation of both epoxides along the polymer chain and molar masses up to 28.3 kg/mol. The copolyesters are amorphous with a Tg between -39 degrees C and 38 degrees C. Together with the molar mass, the anhydride dictates the thermal stability of the copolyesters with glutaric anhydride resulting in a remarkably high thermal stability up to 310 degrees C. In a post-polymerization step, the pendant double bonds are radically crosslinked to gels with swelling ratios above 1500 % and com-parable to enhanced thermal stability with respect to the non-crosslinked, parent copolyesters. The degradation of the 4 copolyesters (before and after crosslinking) is tested in abiotic and enzymatic conditions: The highest degradation rates are observed for the non-crosslinked materials in enzymatic conditions with a mass loss of up to 60 % after 27 d. After crosslinking, the gels are more stable against degradation under both conditions, although a decrease in the gel content and a decrease in mass indicates that degradation still takes place.
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| 3. |
- Hirschmann, Max, et al.
(författare)
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Light-Controlled Lyotropic Liquid Crystallinity of Polyaspartates Exploited as Photo-Switchable Alignment Medium
- 2023
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 145:6, s. 3615-3623
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Tidskriftsartikel (refereegranskat)abstract
- Two polyaspartates bearing ortho-fluorinated azo-benzenes (pFAB) as photo-responsive groups in the side chain were synthesized: PpFABLA (1) and co-polyaspartate PpFABLA-co-PBLA [11, 75%(n/n) PpFABLA content]. As a consequence of the E/Z-isomerization of the side chain, PpFABLA (1) undergoes a visible-light-induced reversible coil-helix transition in solution: Green light (525 nm) affords the coil, and violet light (400 nm) affords the helix. pFAB significantly increases the thermal stability of the Z-isomer at 20 degrees C (t1/2 = 66 d for the Z-isomer) and effectively counters the favored back formation of the helix. At 20%(w/w) polymer concentration, the helical polymer forms a lyotropic liquid crystal (LLC) that further orients unidirectionally inside a magnetic field, while the coil polymer results in an isotropic solution. The high viscosity of the polymer solution stabilizes the coexistence of liquid crystalline and isotropic domains, which were obtained with spatial control by partial light irradiation. When used as an alignment medium, PpFABLA (1) enables (i) the measurement of dipolar couplings without the need for a separate isotropic reference and (ii) the differentiation of enantiomers. PpFABLA-co-PBLA (11) preserves the helical structure, by intention, independently of the E/Z-isomerization of the side chain: Both photo-isomers of PpFABLA-co-PBLA (11) form a helix that-at a concentration of 16%(w/w)-form an LLC. Despite the absence of a change in the secondary structure, the E/Z-isomerization of the side chain changes the morphology of the liquid crystal and leads to different sets of dipolar coupling for the same probe molecule.
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| 4. |
- Knoll, Kevin, et al.
(författare)
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A supramolecular and liquid crystalline water-based alignment medium based on azobenzene-substituted 1,3,5-benzenetricarboxamides
- 2022
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Ingår i: Magnetic Resonance in Chemistry. - : Wiley. - 0749-1581 .- 1097-458X. ; 60:6, s. 563-571
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Tidskriftsartikel (refereegranskat)abstract
- A supramolecular, lyotropic liquid crystalline alignment medium based on an azobenzene-containing 1,3,5-benzenetricarboxamide (BTA) building block is described and investigated. As we demonstrate, this water-based system is suitable for the investigation of various water-soluble analytes and allows for a scaling of alignment strength through variation of temperature. Additionally, alignment is shown to reversibly collapse above a certain temperature, yielding an isotropic solution. This collapse allows for isotropic reference measurements, which are typically needed in addition to those in an anisotropic environment, to be performed using the same sample just by varying the temperature. The medium described thus provides easy access to anisotropic NMR observables and simplifies structure elucidation techniques based thereon.
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| 5. |
- Rossi, Ruggero, et al.
(författare)
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Photoswitches in Order : One-Pot Synthesis of Azobenzene Main-Chain and Segmented Copolymers
- 2024
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Ingår i: ACS Applied Polymer Materials. - : American Chemical Society (ACS). - 2637-6105. ; 6:2, s. 1563-1572
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Tidskriftsartikel (refereegranskat)abstract
- Syntheses of multifunctional polymers aim to engineer a wide range of material properties by adjusting the composition and positioning of functional groups. While manifold syntheses of side-chain-functionalized polymers are known, synthetic protocols for main-chain-functionalized polymers are less common. This work describes a general one-pot strategy to prepare polymers containing multiple functional moieties in their main-chain, e.g., azobenzene units separated by variable oligomers. The polymerization proceeds in two steps, starting from a single azobenzene initiator and commercially available monomers (lactones and cyclic carbonates). Various main-chain-functionalized polymers were obtained with a predictable and adjustable ratio of monomer units (5-20) to photoswitchable azobenzene groups. The thermal properties of these polymers were analyzed and rationalized with regard to the parent polymers' properties and the peculiarities arising from their segmented microstructure. Furthermore, the azobenzenes' ability to undergo light-induced cis/trans-isomerization is confirmed. High isomerization yields of up to 90% were observed for the polymers in solution with a half-life of several days for the cis-isomers in solution. When irradiated as solid films, the azobenzenes still undergo isomerization, but the cis-isomers are less stable compared to the liquid state.
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