Thiol−Yne Photopolymerizations: Novel Mechanism, Kinetics, and Step-Growth Formation of Highly Cross-Linked Networks

Abstract

Radical-mediated thiol−yne step-growth photopolymerizations are utilized to form highly cross-linked polymer networks. This reaction mechanism is shown to be analogous to the thiol−ene photopolymerization; however, each alkyne functional group is capable of consecutive reaction with two thiol functional groups. The thiol−yne reaction involves the sequential propagation of a thiyl radical with either an alkyne or a vinyl functional group followed by chain transfer of the radical to another thiol. The rate of thiyl radical addition to the alkyne was determined to be approximately one-third of that to the vinyl. Chain-growth polymerization of alkyne and vinyl functionalities was only observed for reactions in which the alkyne was originally in excess. Analysis of initial polymerization rates demonstrated a near first-order dependence on thiol concentration, indicating that chain transfer is the rate-determining step. Further analysis revealed that the polymerization rate scaled with the initiation rate to an exponent of 0.65, deviating from classical square root dependence predicted for termination occurring exclusively by bimolecular reactions. A tetrafunctional thiol was photopolymerized with a difunctional alkyne, forming an inherently higher cross-link density than an analogous thiol−ene resin, displaying a higher glass transition temperature (48.9 vs −22.3 °C) and rubbery modulus (80 vs 13 MPa). Additionally, the versatile nature of this chemistry facilitates postpolymerization modification of residual reactive groups to produce materials with unique physical and chemical properties.

Porous polymer particles—A comprehensive guide to synthesis, characterization, functionalization and applications

“…From the several proposed click type reactions in the literature [326][327], two of them received much attention within the polymer society: Cu(I) catalyzed azide-alkyne cycloaddition (CuAAC) [328][329][330][331][332] and the addition of a thiyl radical to olefins (thiol-ene and thiol-yne) [230,[333][334] (Scheme 2).…”

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

“…[17][18][19][20][21] It was found earlier that the addition of the first thiol to the alkyne is the rate-limiting step, which is followed by the fast second thiol addition to the intermediate thiol-alkene. 15,16 The conditions and corresponding degree of functionalization by thiol-yne reactions are summarized in Table 4. 1) PU numbers correspond to numbers in Table 3.…”

“…Therefore, highly efficient reactive systems that do not contain any metal catalyst are often desired. In this respect, light-mediated thiol-ene 13,14 and thiol-yne [15][16][17][18][19][20][21] radical reactions have become widely used as they effectively combine some classical benefits of coupling reactions with the advantages of a photoinitiated process resulting in a powerful method for chemical synthesis and tailorable material fabrication.…”

Sequential thiol‐ene/thiol‐ene and thiol‐ene/thiol‐yne reactions as a route to well‐defined mono and bis end‐functionalized poly(N‐isopropylacrylamide)

“…The vinylthioether product is able to undergo a second, faster, regiospecific (2), formally thiol-ene, reaction with the newly generated thiyl radical yielding the intermediate bisthioether radical that undergoes chain transfer with another thiol molecule quantitatively yielding the target bisthioether with exclusive 1,2-orientation. 30 However, one important difference between the radical thiol-ene and radical thiol-yne reactions, at least in the case of the double addition products, is that the thiol-yne reaction of a terminal acetylene results in the generation of a stereocenter and thus the final 1,2-addition product is a mixture of (R) and (S) enantiomers, and as such the radical thiol-yne reaction is not stereoselective.…”

“…However, researchers have recently begun to evaluate this potentially highly versatile reaction. Fairbanks et al 30 described radical-mediated thiol-yne stepgrowth polymerization as a route to highly crosslinked networks and conducted a detailed kinetic analysis of the process simultaneously noting the similarities with the radical thiol-ene process. Chan et al 31 reported the synthesis and physical properties of high refractive index polysulfide networks formed via the photopolymerization of a range of alkyldithiols with a series of alkyldiynes, and most recently, Chan, Hoyle, and Lowe 6 described the first example of a sequential, quantitative nucleophilic thiol-ene/radical thiol-yne process for the synthesis of a range of branched structures including examples with potential biomedical significance.…”