Thiol-ene

Even in the absence of added transfer agents, all polymerizations may be complicated by transfer to initiator (Sections 3.2.10 and 3.3), solvent (Section 6.2.2.5), monomer (Section 6.2.6) or polymer (Section 6.2.7). The significance of these transfer reactions is dependent upon the particular propagating radicals involved, the reaction medium and the polymerization conditions. Thiol-ene polymerization consists of sequential chain transfer and reinitiation steps and ideally no monomer consumption by propagation (Section 7.5.3). 

Thiol-ene polymerization was first reported in 1938.220 In this process, a polymer chain is built up by a sequence of thiyl radical addition and chain transfer steps (Scheme 7.17). The thiol-ene process is unique amongst radical polymerizations in that, while it is a radical chain process, the rate of molecular weight increase is more typical of a step-growth polymerization. Polymers ideally consist of alternating residues derived from the diene and the dithiol. However, when dienes with high kp and relatively low A-, monomers (e.g. acrylates) are used, short sequences of units derived from the diene are sometimes formed. 

The process may be used to form linear polymers. Nuyken and Volkel224 225 described a method for tclcchclic production, based on the radical initiated reaction of difunctional transfer agents with dienes (e.g. divinyl benzene (13), dimethacrylate esters). However, currently the most common use of thiol-ene... 

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. 

UV Cross-Linking of Thiol-ene Polymers A Rheological Study... 

Commonly used monomers for UV curing include acrylates (7), styrene/unsaturated polyesters (8,9), and thiol-ene compositions (10-12). Currently, acrylate-functional systems constitute a major share of the UV curable polymers market, mainly due to their rapid curing via free radical chain polymerization. 

An FTIR spectrometer (Nicolet Magna-IR System 750) and a DTGS-KBr detector was used to obtain the IR spectra for the thiol-ene sample. Sixteen scans,... 

Thus, a semilogarithmic plot of the gel time as a function of 1/T should be linear, with the slope corresponding to the apparent activation energy. We have determined the gel times for a temperature range of 25°-50° C for a thiol-ene system consisting of stoichiometrically equivalent amounts of a trifunctional thiol, trimethylolpropane tris(2-mercaptoacetate), and a trifiinctional allyl monomer, triallyl isocyanurate. In this system, we also added 0.31% by weight of hydroquinone, to prevent premature polymerization, and 1.0% by weight of a commercial photoinitiator, Esacure TZT. 

Johnson PM, Stansbury JW, Bowman CN (2008) High-throughput kinetic analysis of acrylate and thiol-ene photopolymerization using temperature and exposure time gradients. J Polym Sci Part A Polym Chem 46 1502-1509... 

The most important attribute of the thiol-ene system is its insensitivity to oxygen thus, it is not inhibited by it. Another attractive feature is its very high cure speed. The disadvantage of the thiol-ene system is an unpleasant odor of the volatiles emitted from some polythiol compounds. ... 

Research Focus Preparation of degradable hydrogels by thiol-ene photopolymerization. Originality Ongoing 6-year investigation by this group. 

---