Thiol–Ene Chemistry: A Greener Approach to Making Chemically and Thermally Stable Fibers

Abstract

Fibers of micrometer and submicrometer diameters have been of significant interest in recent years owing to their advanced applications in diverse fields such as optoelectronics, regenerative medicine, piezoelectrics, ceramic materials, etc. There are a number of processes to make thin fibers including electrospinning, melt blowing, and recently developed Forcespinning. However, use of solvents or heat to lower viscosity for processing is common to all existing polymer fiber manufacturing methods, and a greener approach to making fibers remains a challenge. Interestingly, nature has engineered spiders and silkworms with a benign way of making mechanically strong and tough fibers through an intricate self-assembly of protein constituents during the fiber formation process. Comprehending the biosynthetic process and precisely replicating it has been a challenging task. However, we find that extruding small functional segments into solid fibrillar structures, through mediation of chemical interactions between the subunits, is a design approach that can be broadly adapted from nature to realize a greener fiber manufacturing process. Using the robust chemistry of thiol–ene photopolymerization, we demonstrate here that a photocurable mixture of a multifunctional acrylate, a tetrafunctional thiol, and a photoinitiator can be processed into continuous fibers by in situ photopolymerization during electrospinning under ambient conditions. The fibers are mechanically robust and have excellent chemical and thermal stability. While electrospinning has been used to demonstrate this concept, the chemistry could be broadly adapted into other fiber manufacturing methods to produce fibers without using solvents or heat.

Two tandem multicomponent reactions for the synthesis of sequence-defined polymers

“…The thiolate anion is generally a strong nucleophile, and it readily adds to the electron-deficient carbon-carbon double bond of methacrylate, yielding a carbon-centered anionic intermediate. The latter can abstract a hydrogen to yield a thioether as the product, as shown in Scheme 1 [36]. The signals due to the methylene unit of methacrylate (at 5.7, 6.2 ppm) are very weak in 48 h, as shown in Figure 2.…”

“…However, primary amines are able to ring-open a cyclic thiolactone, thereby releasing a thiol, whereas secondary and tertiary amines do not perform this reaction [25,31,35]. On the other hand, the thiol can react with methacrylate very efficiently in the presence of a nucleophile such as a tertiary amine [35,36]. Therefore, N,N-bis(2-oxotetrahydrothiophen-3-yl) adipamide and N,N-dimethyl-1,3-propanediamine (DMP-DA) were added to the completed Passerini three-component reaction system.…”

“…The thiol unit is very reactive toward the carbon-carbon double bond of the methacrylate unit [36,37]. In the Michael addition based thiol-ene click reaction, a proton is abstracted from the thiol unit to give a thiolate anion.…”

“…The sequence-defined polymer with clickable alkene side unit obtained via two tandem multicomponent reactions has a molecular weight of 49.8 kDa and polydispersity of 2.03 after a reaction time of 48 h, as shown in Figure 3. It should be noted that, although the alkene units can readily react with thiols via a radical-mediated thiol-ene click reaction, here the thiol-intermediate does not react with the alkene side units because there is no radical source in the threecomponent polymerization system [33,36]. As shown in Figure 4, it is very clear that the signals for the alkene unit at 5.8 and 5.2 ppm survive after reaction for 48 h. These clickable alkene side units can be used as reacting units for the modification of the sequence-defined polymer via thiolene click reaction.…”

Manipulating characteristic timescales and fiber morphology in simultaneous centrifugal spinning and photopolymerization

“…Recently, our group demonstrated a new method for fabricating nonwoven fibers directly from non-volatile liquid monomer mixtures instead of from polymer solutions or polymer melts [19], thereby foregoing the use of solvent and heat altogether. In this method, a mixture containing monomers and a photoinitiator was simultaneously electrospun and photopolymerized via UV initiated thiol-ene chemistry to produce solid fibers in ambient conditions.…”