PhotoCAN silicone elastomers, based on the thiol–ene reaction, exhibit rapid and reversible changes in dynamic modulus at room temperature when illuminated by UV. By combining results from magic angle spinning solid-state NMR as well as EPR and rheometry measurements, both under UV, it is concluded that the mechanical response can be attributed to a combination of dissociative, associative, and oxidation reactions. The cleavage of the C–S bonds under UV in the presence of an excess of thiyl radicals is identified as the reversible dissociative reaction responsible for abrupt drops in the storage modulus. A slower but concurrent reaction is a termination process involving thiyl radicals to form disulfide bonds. A kinetic model is developed that successfully relates the rates of the underlying reaction mechanisms to changes in the storage modulus. The results provide a basis for designing new, ambient temperature photoresponsive covalently adaptive network materials.
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Expanding the thiol–X toolbox: photoinitiation and materials application of the acid-catalyzed thiol–ene (ACT) reaction
The acid-catalyzed thiol–ene reaction (ACT) is a unique thiol–X conjugation strategy that produces S,X-acetal conjugates. Unlike the well-known radical-mediated thiol–ene and anion-mediated thiol-Michael reactions that produce static thioether bonds, acetals provide unique function for various fields such as drug delivery and protecting group chemistries; however, this reaction is relatively underutilized for creating new and unique materials owing to the unexplored reactivity over a broad set of substrates and potential side reactions. Solution-phase studies using a range of thiol and alkene substrates were conducted to evaluate the ACT reaction as a conjugation strategy. Substrates that efficiently undergo cationic polymerizations, such as those containing vinyl functional groups, were found to be highly reactive to thiols in the presence of catalytic amounts of acid. Additionally, sequential initiation of three separate thiol–X reactions (thiol-Michael, ACT, and thiol–ene) was achieved in a one-pot scheme simply by the addition of the appropriate catalyst demonstrating substrate selectivity. Furthermore, photoinitiation of the ACT reaction was achieved for the first time under 470 nm blue light using a novel photochromic photoacid. Finally, using multifunctional monomers, solid-state polymer networks were formed using the ACT reaction producing acetal crosslinks. The presence of S,X-acetal bonds results in an increased glass transition temperature of 20 °C as compared with the same polymeric film polymerized through the radical thiol–ene mechanism. This investigation demonstrates the broad impact of the ACT reaction and expands upon the diverse thiol–X library of conjugation strategies towards the development of novel materials systems.
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- Award ID(s):
- 1757353
- NSF-PAR ID:
- 10249240
- Date Published:
- Journal Name:
- Polymer Chemistry
- Volume:
- 12
- Issue:
- 10
- ISSN:
- 1759-9954
- Page Range / eLocation ID:
- 1562 to 1570
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0PY01593H
