Most
of the vitrimers based on ester linkages reported so far contain
a Lewis acid or a strong organic base as the transesterification catalyst.
The recyclability and reusability of these vitrimers are dependent
on the catalyst retention, stability, and sintering issues. Herewith,
a set of β-activated ester-based vitrimers are reported that
can be thermally reprocessed at ∼150 °C under catalyst-free
conditions. The relaxation temperature decreases to 110 °C in
the presence of Sn(Oct)2. Importantly, the precursor of
these vitrimers, malonic ester, is a cost-effective naturally occurring
ester and can be extracted from various fruit juices. As a proof of
concept, poly(hydroxyethyl methacrylate) is used as the hydroxyl precursor
for the synthesis of vitrimers. These vitrimers display an adequate
tensile strength (11.3–33.0 MPa), elongation (80–290%),
and resilience. The materials can be effectively self-healed and reprocessed
in the presence of heat without sacrificing the tensile properties.
The vitrimers based on Sn(Oct)2 exhibit mechanical properties
similar to that of the catalyst-free analogues and reprocess at ∼110
°C. These vitrimers may potentially be utilized for the development
of coatings, hydrogels, biomaterials, adhesives, and commodity plastics
in the future.
Though conventional polyurethane foams are among the most widely used polymer materials currently, the lack of reprocessability and creep resistance ability restricts their further commercial growth. Imparting recyclability, self-healing, and creep resilience ability to segmented PU networks (PUNs) is desirable to further enhance their commercial utility and address the concern related to polymer waste accumulation. In this article, we utilize the strategy of incorporating a dynamic carboxylate linkage in the main chain of polytetramethylene oxide (PTMO)-based segmented PUNs to achieve the above. The resulting PUNs displayed adequate tensile properties (tensile strength ≈ 8−33 MPa, elongation at break ≈ 345−680%, and Young's modulus ≈ 19−270 MPa). The samples displayed typical vitrimer behavior and stress relaxed in the temperature range of 140 to 180 °C. The PUNs, thermally reprocessed at 150 °C, recovered up to 70−85% of the original tensile strength. Importantly, these vitrimers displayed improved creep resistant behavior at elevated temperature, which has been a concern with the conventional vitrimer materials. The PUNs displayed triple shape memory abilities at inflection temperatures of 60 and 130 °C. The samples also exhibited effective self-healing and self-welding ability. Overall, these PUNs with versatile properties are potential candidates for a range of applications.
Preparation and studies of ion exchangeable epoxy resins, stimuli responsive hydrogels, and polymer-dye conjugates have been accomplished through hydrazide based click reactions using polyacryloyl hydrazide (PAH) as the precursor. A convenient synthesis of PAH with quantitative functionality was achieved by treatment of polymethyl acrylate with hydrazine hydrate in the presence of tetra-n-butyl ammonium bromide. PAH was cured with bisphenol A diglycidyl ether (BADGE) at 60 °C to form transparent resins with superior mechanical properties (tensile strength = 2-40 MPa, Young's modulus = 3.3-1043 MPa, and ultimate elongation = 9-75%) compared to the conventional resins prepared using triethylene tetramine. The resins exhibited higher ion exchange capacities (1.2-6.3 mmol/g) compared to the commercial AHA ammonium-type (Tokuyama Co., Japan) membranes. An azo dye with aldehyde functionality was covalently attached to PAH through hydrazone linkage, and the dye labeled PAH exhibited colorimetric sensing ability for base and acids up to micromolar concentration. The swelling of the PAH based hydrogel varied in the range 4-450% depending on the pH and temperature of the medium. The hydrogels gradually released 30% of the original encapsulated dye in a period of 200 h. PAH-hydroxy naphthaldehyde conjugate released 75% of the original loading in ∼11 days at 37 °C and pH 5.0 through cleavage of the -CONHN═C- linkage. The study depicts the versatility of PAH as a precursor and inspires synthesis of a range of new materials based on PAH in the future.
The syntheses of end-functional polyisobutylenes (PIBs) including hydroxy, amino, carboxy, azide, propargyl, methoxy, and thymine end groups have been accomplished using nucleophilic substitution reactions. S N 2 reactions on PIB-Allyl-X (X ) Cl or Br), the trans-1,4 addition product of the capping reaction of living PIB with 1,3-butadiene, with different nucleophiles successfully yielded the corresponding PIB-Allyl-OH, PIB-Allyl-OMe, PIB-Allyl-NH 2 , PIB-Allyl-OCH 2 CtCH, PIB-Allyl-N 3 , and PIB-Allyl-CH 2 COOH with quantitative functionalization as determined by 1 H and 13 C NMR, FT-IR, and matrix-assisted time-of-flight mass spectroscopy. As expected, the rate of substitution was faster with PIB-Allyl-Br compared to PIB-Allyl-Cl. GPC analysis of the precursor PIB-Allyl-X and the products indicated that the polymer chain is unaffected by the substitution reactions. The synthesis of hydroxy telechelic PIBs was also achieved using X-Allyl-PIB-Allyl-X at reaction conditions similar to that employed for the preparation of PIB-Allyl-OH. The methodology was extended to the synthesis of PIB block copolymers by employing polymeric nucleophiles. PIB-b-poly(ethylene oxide) (PEO) was synthesized by the nucleophilic substitution of PIB-Allyl-Cl with PEO-O -Na + .
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