Kinetic Modeling of Radical Thiol–Ene Chemistry for Macromolecular Design: Importance of Side Reactions and Diffusional Limitations

Abstract

The radical thiol–ene coupling of thiol-functionalized polystyrene (PS-SH) with dodecyl vinyl ether (DVE) and the polystyrene-b-poly(vinyl acetate) (PS-b-PVAc) polymer–polymer conjugation using 2,2-dimethoxy-2-phenylacetophenone (DMPA) as photoinitiator are modeled to assess the importance of diffusional limitations and side reactions. Intrinsic chemical rate coefficients are determined based on a kinetic study of the coupling of benzyl thiol (BT) and DVE. The addition and transfer reactions are chemically controlled, whereas diffusional limitations on termination slightly increase the coupling efficiency. Termination by recombination of carbon-centered radicals and addition of DMPA derived radicals to DVE are shown to be mainly responsible for the reduced coupling efficiency in case polymeric species are involved. The obtained results confirm the idea to disregard radical thiol–ene chemistry as a true member of the family of “click” chemistry techniques for polymer–polymer conjugation and show that the initial conditions have a significant impact on the coupling efficiency.

Novel thiol- amine- and amino acid functional xylan derivatives synthesized by thiol–ene reaction

“…Generally, using large excess of the thiol did not significantly affect the degree of functionalization. The thiol-ene adducts had higher molecular weights and polydispersity indices than the starting material, which may be an indication of some extent of crosslinking by radical side-reactions, a common limitation of the thiol-ene procedure (Derboven et al, 2013). The only exceptions were reactions employing cysteamine hydrochloride which gave significantly lower molecular weight products, which may indicate degradation of the xylan backbone, possibly by a mechanism similar to that reported for chitosan involving cationic amino groups (Hsu, Don, & Chiu, 2002).…”

“…For reactions made with thioacetic acid, the obtained degree of substitution (DS) values based on elemental analysis were 15-38% lower than the theoretical number of consumed double bonds, possibly due to side-reactions, such as homopropagation and termination by carbon-carbon coupling (Derboven et al, 2013). The formed acetylthiopropyl xylan had limited water solubility and produced clear solutions only in dimethyl sulfoxide and dimethylacetamide.…”

“…The reduced water solubility of the forming acetylthiopropyl xylan was also observed during the reaction, as the reaction mixture became heterogeneous within a few minutes after addition of the initiator. The heterogeneous conditions might have favored sidereactions, due to reduced accessibility of double bonds, lowering the coupling efficiency (Derboven et al, 2013). It was found that the addition of hydrochloric acid was requisite to drive the conversion of double bonds to higher values, by keeping thioacetic acid protonated and capable of participating in the reaction.…”

Functionalization of polyurethanes by incorporation of alkyne side-groups to oligodiols and subsequent thiol–yne post-modification

“…Hence, a sufficient amount of photoinitiator is necessary to achieve a high functionalization degree, as also noted earlier. 3,24,25 Besides, as side reactions such as disulfide bond formation or thiyl radical combination are inevitable in thiol-yne reactions, 2,3 the use of an excess amount of thiol was employed to obtain full conversion. Good agreement between conversion of alkyne groups and the amount of attached thiol was observed, which indicates that two thiol molecules reacted with one alkyne group.…”

Visible light‐induced thiol‐ene reaction: A new strategy to prepare Α,ω‐dithiol and Α,ω‐divinyl telechelic polythiolether oligomers

“…It is worth mentioning that UV-triggered thiol-ene reactions can result in the formation of undesirable side products. 44 Thus, there has been growing interest in the development of thiol-ene reactions using visible light as the energy source in recent years. Meanwhile, over the past several years, interest in new synthetic methods for preparing well-defined polymers with controlled chain-end functional groups has been rejuvenated.…”