Thiol-ene reactions

The second section focuses on emerging classes of photopolymerizations that are being developed as alternatives to acrylates. Three types of polymerization systems are included cationic photopolymerizations, initiator-free charge-transfer polymerizations, and a thiol-ene reaction system. The last section covers four interesting emerging applications of photopolymerization technology. 

Photosensitized thiol-ene reaction. An important class of liquid photolnltlated polymerization systems are known as the photosensitized thlol-ene reaction. The mechanism underlying this technology has been summarized In the following scheme ... 

The introduction of mono-, di-, and oligosaccharide units on dendritic surfaces is usually carried out as the final synthetic step in monodendron and dendrimer synthesis. For this purpose, well-known polymer analogous/ organic conversion steps are used amidation, esterification, reductive amina-tion, nucleophilic substitution, addition/elimination, thiol-ene reaction, 1,3-dipolar cycloaddition, and others (Fig. 5.22). 

This chapter will serve to highlight recent advances in polymer science that have been aided by the use of click chemistry. The copper(l)-catalyzed azide-alkyne cycloaddition (CuAAC) and thiol-ene reactions will be discussed first, after which the utilization of these chemical transformations in the construction and fimction-ahzation of a multitude of different polymeric materials will be outlined. Particular attention will be focused on the preparation of highly complex polymer architectures, such as dendrimers and star polymers, which exempHfy the essential role that chck chemistry has assumed in the polymer science community. 

A recent report by Hawker and coworkers described the use of both CuAAC and thiol-ene reactions for the preparation of hetero-telechehc polystyrene (PS) [49]. In these studies, styrene was polymerized from an alkene-containing ATRP... 

Figure 9 Plots of tan 5 as a function of temperature. In sample a, trifunctional thiol small molecules and trifunctional vinyl ether small molecules are mixed in the ratio 1 1, allowing for only thiol-ene reaction. In samples b-j, polymeric diacrylate is added to this mixture, increasing from 10% in sample b to 90% in sample j. Sample /r is the result of photopolymerization of the diacrylate in the absence of thiol. Radical transfer from acrylate chains to thiols results in shorter acrylate chains, and thus in flexibility and heterogeneous free volume distribution. Data obtained at a scan rate 2° C min" . ...

The S5mthesis runs via transesterification, amidation and thiol-ene reactions to get aliphatic diols containing ester, these diols are then reacted with a biobased methyl diester. Most of these polyesters display a good thermal stability with temperature at 5% weight loss in the range 330-350°C. 

Finally, a thiol-ene reaction was used with vinylphosphonate monomers using CTAs carrying hydrophobic chains, in order to improve the hydrophobic properties of metallic surfaces (Scheme 3.3). The phosphonic acid groups are used to promote adhesion of hydrophobic chains onto the... 

Scheme 3.3 Thiol-ene reaction with dimethyl vinylphosphonate.

It has been stated that the propagation processes in die thiol-ene reactions are less sensitive to oxygen than are typical free radical chain-growth polymerizations. In the presence of excess oxygen, however, peroxy radicals... 

In addition it was also reported that compounds like bicyclo[2,2,l]-heptene derivatives react up to 30 times as fast as allyl ethers. Also, it was reported early that internal olefins exhibit lower reactivity than do the terminal ones. Cyclic olefins are claimed to be less reactive than terminal ones, however, Jacobine ei al, reported that resins functionalized with norbomenes react rapidly and exothermally in the thiol-ene reaction with multifunctional thiols due to ring strain ... 

The functional group conversion is taken as the independent variable, the reaction kinetics of the ring-free thiol-ene system is expected to be identical to that of usual step-growth reactions. In the rate expressions for intermolecular reactions in thiol-enes are first-order reactions overall. The cyclization rate in thiol-ene reactions is slower compared to that in step-growth reactions. This result arises from intramolecular chain transfer reactions reducing the probability of favorable intramolecular collisions between the functional groups. Some examples of thiols and olefins are listed in Table 3.3. 

Table 3.3. Examples of Thiols and Olefins that Cnre by the Thiol-Ene Reaction...

Confirmation of delivery of the methacryloyl groups to the cell surface was performed using the thiol-ene reaction. After 3 days of HL-60 cell cultivation in the presence of 5 mM ManM, the cells were rinsed three times with the cell culture medium. RPMI-1640 medium containing 2.5 wt% thiol-terminated 4-arm poly(ethylene glycol) (PEG4IOK-SH) and... 

The thiol-ene reaction, which has been used for 110 years [1], corresponds to the simple hydrothiolation of a C=C double bond (Scheme 6.1). However, there are some peculiarities of this reaction that make it remarkably attractive today. It is a tool for researchers in all areas of organic synthesis due to recognition of its click characteristics. 

Under free-radical conditions (often induced by photochemical means), thiol-ene reactions proceed via a typical chain process with initiation, propagation and termination steps (Scheme 6.3). The mechanism involves a thiyl radical cis-trans isomerisation, with concurrent formation of a thiol-ene product. Initiation requires treatment of a thiol with a photo-initiator under irradiation, and results in formation of a thiyl radical (R-S ). Propagation involves direct addition of the thiyl radical across... 

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