Promote Exchange Reactions Research Articles

Soft, fully bio‐based poly‐hydroxyl thermosets based on catalyst‐free transesterification with decent re‐processability

AbstractMost of the vitrimers based on transesterification (TE) highly rely on the usage of large amounts of catalysts to promote exchange reactions to obtain re‐processability, which brings additional concerns on human health or environmental issues. Herein, we demonstrate that TE vitrimers with soft networks (low Tg), though free of catalysts nor any other special design, can be still dynamic enough to be easily re‐processed. In this paper, fully bio‐based and catalyst‐free thermosets from epoxidized soybean oil (ESO) and bio‐dicarboxylic acids (DA) were synthesized and found to possess decent re‐processability. By means of compressive remolding (10 MPa, 160°C), the ESO‐DA thermosets heal visible defects within 30 min and restore the mechanical strength within 3 h. Upon 3 times of mechanical recycling, the mechanical strengths remain nearly unchanged compared with those of pristine thermosets. Stress relaxation experiments confirm the network rearrangement at high temperatures (≥150°C) as well as vitrimeric flow of the materials. The materials are also capable of being welded without solvents under a small pressure. Lastly, a facile fusion method was employed to tune the mechanical strength of the recycled ESO vitrimers.

Electron‐Irradiation Promoted Exchange Reaction as a Tool for Surface Engineering and Chemical Lithography

AbstractSelf‐assembled monolayers (SAMs) can serve as versatile resist/template materials for surface engineering and electron beam lithography (EBL), making possible a new type of lateral patterning: chemical lithography (CL). Whereas CL has been well established for aromatic SAMs, it is hardly possible for aliphatic monolayers, because of extensive irradiation‐induced damage excluding selective modification of specific chemical groups. Turning this drawback into an advantage, the irradiation‐promoted exchange reaction approach is developed, which is described in detail in the present review. The key idea of the approach is tuning the extent of the exchange reaction between a primary aliphatic SAM covering the substrate and a potential molecular substituent, which is capable of building a SAM on the same support, by electron irradiation. The major advantages of the approach are low irradiation doses (≤1 mC cm−2) and flexible choice of SAM‐forming molecules, with a broad pool available commercially. Consequently, a large variety of binary SAMs with controlled compositions can be prepared and, in combination with EBL, complex chemical patterns can be fabricated, serving in particular as templates for subsequent area‐selective chemical reactions, surface‐initiated polymerization, attachment of nanoparticles, non‐specific and specific proteins adsorption, and growth of 3D DNA nanostructures.

Reprocessability of dynamic polydioxaborolane networks activated by heat, moisture and mechanical force

Covalent adaptive networks have shown great potential in sustainable crosslinked polymer materials. The incorporation of reversible covalent bonds also brings many possibilities for the material processing and recycling. Herein, we designed a polydioxaborolane network (PDOB) with abundant boronic-ester linkages. PDOBs showed tunable mechanical properties and viscoelasticity from highly stretchable elastomer to rigid thermoset by varying the composition. The dynamic dioxaborolane metathesis and reversible hydrolysis/rebinding of boronic-esters endowed PDOBs with remarkable reprocessability with the assistance of heat or moisture. Furthermore, the theoretical topology freezing transition temperature (Tv) of PDOBs was calculated to be below their glass transition temperature (Tg). We investigated the reprocessability of PDOBs at different temperature and found that PDOBs could be remolded at a temperature window between Tv and Tg even without the participation of water. The compressive force could break the reversible linkages into reactive sites, as well as promote exchange reactions, which subsequently facilitated interfacial integration.

Promoted Exchange Reaction between Alkanethiolate Self-Assembled Monolayers and an Azide-Bearing Substituent

The possibility of ultraviolet (UV) light promoted exchange reaction (UVPER) between the primary alkanethiolate (AT) self-assembled monolayers (SAMs) and an azide-functionalized substituent (12-azido-1-dodecanethiol, C12N3), capable of click reaction with ethynyl-bearing species, is demonstrated. This reaction resulted in the mixed AT/C12N3 films, with the portion of C12N3 precisely controlled by selection of a suitable UV dose. As the primary matrix, either nonsubstituted or oligo(ethylene glycol) (OEG)-substituted AT SAMs were used, targeting mixed SAMs of chemical and biological significance. To demonstrate the flexibility of the approach, UV light with two different wavelengths (254 and 375 nm) was used, applied to the nonsubstituted and OEG-substituted AT SAMs, respectively. The surface density of the chemically active azide groups embedded in the nonreactive primary matrix could be varied according to the composition of the mixed SAMs, as demonstrated by the subsequent click reaction between these S...

Thermal characterization of reactive blending of 70PC/30PET mixtures prepared in the presence/absence of samarium acetylacetonate as a transesterification catalyst

Abstract Poly(ethylene terephthalate) (PET)/polycarbonate (PC) blends were prepared in the presence and absence of samarium (III) acetylacetonate hydrate [Sm(acac)3] used as an ester-ester exchange reaction catalyst. Differential scanning calorimetry (DSC) and thermogravimetric analysis/differential thermogravimetry (TGA/DTG) were used to study the variations in the thermal properties of blends before and after reactive blending. Solubility measurements in methylene chloride (CH2Cl2) and infrared spectroscopy were also employed to highlight the structural changes that occurred during mixing in the presence of Sm(acac)3. The DSC results showed two distinct transition temperatures (T g ) for all the compositions. Also, the T g values of components were shifted compared to the parent polymers in both the blends prepared with and without catalyst. Further, the T g of the PET-rich phase was displaced to higher temperatures and that of the PC-rich phase showed a tendency to diminish as the catalyst’s concentration increased. At the same time, the melting temperature (T m ) of the PET-rich phase was shifted to lower temperatures. The evaluation of the compositions of the PC- and PET-rich phases using Wood’s equation showed a strong dependence on the catalyst concentration. Furthermore, the thermogravimetric thermograms showed that the thermal stability of the blends was between those of the homopolymers, and evidenced a net improvement relatively to the neat PET. The obtained results confirmed the aptitude of Sm(acac)3 to promote exchange reactions between PC and PET, and to achieve their compatibilization through an interfacial copolymer synthesis process.

Fabrication of ssDNA/Oligo(ethylene glycol) Monolayers by Promoted Exchange Reaction with Thiol and Disulfide Substituents

Biorepulsive oligo(ethylene glycol)-substituted alkanethiolate (OEG-AT) monolayers on gold can serve as primary templates for promoted (by electron irradiation) exchange reaction with thiolated ssDNA species, resulting in the formation of mixed OEG-AT/ssDNA monolayers of desired composition. Here we test the ability of alternative, disulfide precursors to serve as substituents in such a reaction. Two representative molecules, based on adenine-based homo-oligonucleotide (25-mer), were used, viz., asymmetric disulfide with a short second chain (A25SSOH) and symmetric disulfide (A25SSA25). The results were compared to the reference system of thiolated ssDNA (A25SH). Both disulfide precursors were found to be suitable for the reaction, further extending the types of commercially available compounds which can be used for this approach. A25SSOH exhibited quite high efficiency, similar to A25SH, while the efficiency of A25SSA25 was noticeably lower, especially at low irradiation doses (<0.6 mC/cm2). Also, the single component, A25SSA25-based ssDNA monolayer, was of lower quality as compared to the films prepared from the A25SH and A25SSOH precursors. The above observations were explained by the bulky character and conformational flexibility of A25SSA25, which hinder the proper assembly and efficient exchange reaction.

Irradiation Promoted Exchange Reaction with Disulfide Substituents

Exchange reaction between the primary self-assembled monolayer (SAM) on gold and potential molecular substituents capable of forming a SAM on the same substrate can be promoted by electron irradiation. Here we demonstrate that such a promoted reaction can be performed not only with thiols but with disulfides as substituents as well. This extends significantly the assortments of the suitable compounds, resulting in a broader variety of mixed SAMs and chemical patterns which can be fabricated by this technique. The kinetics of the promoted exchange reaction was studied in detail. The feasibility and practical usefulness of the approach were demonstrated by the experiments with a disulfide substituent bearing a tail group which can serve as an initiator for surface-initiated polymerization. A variety of complex polymer brush patterns was prepared using several representative polymers, relevant for biomedical research and applications, as test systems.

Fabrication of Mixed Self-Assembled Monolayers Designed for Avidin Immobilization by Irradiation Promoted Exchange Reaction

An applicability of irradiation-promoted exchange reaction (IPER) to the fabrication of mixed self-assembled monolayers (SAMs) composed of the protein-repelling matrix and the moieties bearing binding sites for specific attachment of a target protein is demonstrated. As test systems, we took mixed films of oligoethylene glycol (OEG)-substituted alkanethiols (OEG-ATs) and biotin-substituted alkanethiols (BATs) on Au{111}. Such SAMs are suitable for the specific immobilization of avidin protein and its variants. The composition of the mixed OEG-AT/BAT SAMs could be precisely controlled by varying the irradiation dose, which is important prerequisite for the fabrication of the respective patterns by electron-beam lithography. While the general trend in the immobilization of avidin onto the mixed OEG-AT/BAT SAMs prepared by IPER was found to be consistent with the earlier reports regarding the analogous films fabricated by the coassembly method, the concentration of the BAT component in the mixed SAMs needed for the maximum surface coverage of the specific protein was found to be somewhat lower, and the maximum avidin coverage somewhat higher in the case of IPER as compared to the coassembly method. We ascribe these differences to the lack of phase segregation and better separation of the individual BAT species in the OEG-AT matrix in the case of IPER.

Propagation of the hydrogen-halogen chain

Evidence exists that translationally-“hot” atoms can promote exchange reactions. It appears possible, therefore, that hot atoms formed in prior reactions may act as chain carriers in cases where chain propagation is by way of a reaction of appreciable activation energy. In the experiments reported here we have attempted to put this hypothesis to the test in a case where there is reason to believe that a rapid reaction may be producing translationally-hot atoms (D+X2→DX+X⋆, where X is Cl or Br), and a subsequent reaction is known to have relatively low collision efficiency with thermalized atoms (X+H2→HX+H, proceeding at ∼70°C). Despite these conditions, which appear to be favorable to the observation of chain propagation by hot atoms, the short chain which was observed gave evidence of being carried by thermalized atoms. It appears that, whatever the enhancement in collision efficiency for the hot-atom reaction, it is not sufficient to permit chemical reaction to compete with translational deactivation. The latter process occurs with a collision efficiency of 0.14, in this system.