Thiol-Enes: Fast Curing Systems with Exceptional Properties. Performance Materials, USA. Chemistry and Biochemistry, USA

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1 Thiol-Enes: Fast Curing Systems with Exceptional Properties Charles Hoyle, 1,2 Tolecial Clark, 2 Tai Yeon Lee, 1 Todd Roper, 1 Bo Pan, 1 Huanyu Wei, 1 Hui Zhou, 1 and Joe Lichtenhan 3 1 The University of Southern Mississippi, School of Polymers and High Performance Materials, USA 2 The University of Southern Mississippi, Department of Chemistry and Biochemistry, USA 3 Hybrid Plastics, USA Introduction The use of light to cure systems for applications ranging from protective coatings on floor tiles and optical fibers to coatings on compact discs has grown steadily since the first commercial uses were reported over fifty years ago. 1 Virtually all free-radical photocurable systems today are based upon the following components: functionalized oligomers, monofunctional and multifunctional monomers to control viscosity and aid in development of the crosslinked networks, additives such as antioxidants and UV screeners which help protect the final cured films, and photointiators which absorb light and produce initiating radical species. The use of ultra-high light intensity sources that provide light at wavelengths corresponding to the absorption spectra of the photoinitiators ensure that the coatings are produced rapidly in the presence of air. While there have been many advances in the use of UV light to cure coatings rapidly in many environments, there are still many challenges that remain and opportunities for the future abound. Herein we highlight recent work from our lab on thiol-ene chemistry. Since thiol-ene curing is an extremely versatile chemistry that allows for free-radical curing in air, many potential applications are expected to emerge for its use. Thiol-ene polymerization is unique in that it is a step growth type polymerization that proceeds by a free-radical chain mechanism. In the mid 1970s, Morgan, Ketley and coworkers 2-4 showed that exposure of mixtures of multifunctional thiols and multifunctional enes resulted in crosslinked networks characterized by high functional group conversion. Applications included rapid printing processes and conformal coatings for electronic packaging. Two articles have reviewed all aspects of thiol-ene photopolymerization. 5,6 Thiol-Ene Photocurable Systems Below in Figure 1 is a typical example of a multifunctional ene and a multifunctional thiol that have been employed in many past investigations. Standard cleavage photoinitiators that produce radicals can be used to photocure thiol-ene mixtures. Alternatively, hydrogen abstraction type initiators can be used, or it is even possible to directly excite the thiol groups and induce an efficient cleavage of the bond to give thiyl and hydrogen radicals that initiate the radical chain process: this results in addition of thiols across ene double bonds. After the initial interest in the 1970s that resulted from the work of Morgan and Ketley, 2-4 the use of thiol-enes as matrix resins for commercially viable photocurable systems decreased and were replaced by acrylates. Recently, due to their ability to eliminate oxygen inhibition, rapid cure rates, and the ability to create networks with a variety of mechanical and physical properties, their use has experienced a revival. 5,6 Triallyl Ether H SH SH SH Trimethylolpropane tris(mercaptopropionate) Figure 1. Typical multifunctional thiol and ene

2 A brief description of the general mechanism for the free-radical thiol-ene polymerization process is given in Scheme I. The thiol-ene polymerization revolves around a two step propagation sequence: the first step is the addition of a thiyl radical into the ene to generate a carbon centered radical, and the second step is a subsequent hydrogen abstraction by the carbon centered radical produced in the first step. Initiation RSH + PI (if used) RS + ther Products Propagation 1 RS + H 2 C=CHR RSH 2 C-CHR Propagation 2 RSH 2 C-CHR + RSH RSH 2 C-CH 2 R + RS Termination RS + RS RSSR RS RS + RSH 2 C-CHR RSH 2 C-CHR RSH 2 C-CHR RSH 2 C-CHR + RSH 2 C-CHR RSH 2 C-CHR Scheme I. General thiol-ene photopolymerization mechanism. A crosslinked polymer is only produced when monomers with combined average functionality of greater than two. Since the polymerization proceeds by a free-radical process, the rate of polymerization is rapid. In the thiol-ene free-radical polymerization, the gel point (the point at which an infinite network is produced) is predicted to occur 4 according to the standard gel equation. The gel formation for thiol-enes is quite different from polymerization of multifunctional acrylate monomers where microgelation, which results in a heterogeneous polymerization medium and reaction diffusion controlled termination kinetics, occurs in the early stages of polymerization. Basically, the thiol-ene step growth polymerization proceeds to relatively high conversion in a medium that has quite a low viscosity before gelation. According to Jacobine, 5 this type of step growth free-radical polymerization process allows the shrinkage which accompanies free-radical polymerization to take place in a homogeneous medium of low viscosity. Hence, the shrinkage in a thiol-ene polymerization does not result in delamination of films to the same extent as occurs with traditional acrylate type free-radical polymerization where shrinkage occurs during/after the crosslinked network has begun to form extensively. Furthermore, literature reports 4 indicate that there is little oxygen inhibition of the freeradical polymerization of thiol-enes since the peroxy radicals that are formed by reaction of oxygen with the carbon centered radicals readily abstract a hydrogen from a thiol to continue the free-radical chain process: oxygen inhibition readily occurs with acrylate based radical polymerization processes since there is only one propagation step (reaction of a carbon centered radical chain end with the double bond of a monomer) and no facile hydrogen abstraction process as occurs in propagation step 2 in Scheme I. An example of the lack of oxygen inhibition on thiol-ene photocuring is seen in Figure 2 where oxygen has little effect on the polymerization exotherm of the trifunctional thiol and trifunctional allyl ether whose structures are given in Figure 1 initiated with a cleavage type photoinitiator (DMPA -- dimethoxyphenyl acetophenone). Trifunctional acrylates show an equivalent exotherm to that obtained for the trithiol/triallyl ether mixture in nitrogen, but essentially no exotherm in air due to oxygen inhibition using an equivalent light intensity. Heat flow, mw Nitrogen Air Time, min

3 Figure 2. Photo-DSC of trithiol/triallyl ether mixture in air and nitrogen. Relative rates for the addition of thiols to a several enes including simple alkenes (such as 1-heptene, 2-pentene, and 1,6-hexadiene), allyl ethers, vinyl acetate, alkyl vinyl ethers, conjugated dienes, styrene, and even acrylates have been published as described in the two recent review articles. 5,6 In addition, work in other labs as well as ours indicates that virtually any monomer with an ene functionality will participate in a radical polymerization process with thiols including propenyl ethers, unsaturated esters, and styryloxys. This, of course, makes thiol-ene chemistry very versatile since coatings resins that contain one or more of the ene monomer types can be readily formulated due to the wide variety of monomers available which impart different physical properties to the final cured films. If the thiol is combined with a multifunctional acrylate monomer such as 1,6-hexanediol diacrylate (HDDA), it has been reported that 4 a free-radical polymerization occurs that involves the two step thiolene reaction resulting in addition of the thiol group across the acrylate double bond as well as homopolymerization of the acylate groups. Previously Gush and Ketley 4 showed that mixtures of acrylates and thiols resulted in quite rapid polymerization rates in air when conventional photoinitiators were used. It was proposed that peroxy radicals formed by reaction of propagating carbon centered radical chain ends with oxygen could abstract a hydgrogen from thiol groups thereby generating a thiyl radical capable of continuing a free-radical chain propagation process. Finally, we mention that thiol-ene based systems which have no added photoinitiator can be used in a variety of unique applications. We have been able to photocure thiol-ene samples that are several centimeters thick in relatively short time periods in the absence of an added photoinitiator. The thick cured plastics are optically clear and can potentially exhibit a variety of physical and mechanical properties depending upon the chemical structure of the multifunctional ene used. Thin films can also be readily cured in air. Thiol-Ene Based Photocurable PSS Inorganic/rganic Composites The incorporation of nanoparticles into UV cured coatings for improvement in physical, mechanical and chemical properties represents a significant opportunity in developing advanced coatings and films. While nanoparticles can be dispersed directly in photocurable resins using appropriate dispersing agents, it would be advantageous to functionalize nanoparticles with reactive groups that can be used to attached the particle to the crosslinked network as it forms. ne particularly important class of nanoparticle are based on polyhedral oligomeric silsesquioxane (PSS) cages. By incorporating PSS nanoparticles directly into coatings, it should be possible to improve film properties such as flame retardancy, thermal stability, hardness and hydrophobicity. Figure 3 shows a simplified pictorial of functionalized PSS particles that we have synthesized and incorporated into photocurable crosslinked networks. In the present case we limit our discussion to a modified PSS that has 6 vinyl groups as shown where R = CBu. High solubility and compatibility of the PSS in coatings resins is aided by incorporation of the ester group. VinylPSS Cage mixture 10 + : HS 1 4 R R 1 RT NH2 6 R = CH, CBu, ctyl R 1 = Alkyl S R 4

4 Figure 3. Functional PSS. We have mixed the above six functional ene with the trithiol and triallyl ether in Figure 1 with increasing concentration from 0 to 100 mol % of the six functional PSS ene: only results for 0, 20, and 100 mol% of the six functional PSS ene are given in Table I. The results in Table 1 indicate that a combination of the two enes (six functional PSS ene and three functional alllyl ether) gives the maximum Shore hardness value. Apparently, when the PSS concentration is too large, agglomeration occurs and the hardness is reduced. Interestingly, the low-temperature glass transition of the thiol-triallyl matrix is not greatly influenced by the PSS. Percent PSS Hardness (GPa) T g ( o C) 0 % % % (weak) Table I. Shore hardness and glass transitions of thiol-triallyl ether-pss ene films. ne of the salient features of including PSS into any film is its ability to limit flame spread. As can be seen in Table II, as the mol percent of PSS in the matrix is increased, the rate of flame spread for samples that have been ignited is severely reduced. The best compromise in properties occurs for the sample with 20 mol% of the six functional PSS ene where high hardness and improved flame spread are both manifested. Percent PSS Time (s) Rate (in.2/min.) 0 % % % Table II. Flame spread rates of thiol-triallyl ether-pss ene films. Additional physical property assessment, including Hysitron measurements, have been made for a wide variety of PSS systems indicating that incorporation of mol % functionalized PSS into thiol-ene photocurable coatings results in optimum film performance.

5 Conclusions We have shown that the physical and mechanical properties of photocurable thiol-enes can be readily modified by incorporatiing functionalized PSS nanoparticles into the films. Careful control of the composition is required to maximize film properties such as hardness, while other properties such as flame spread increase proportionally with PSS content. ther benefits of incorporating PSS nanoparticles into photocured thiol-ene coatings are under investigation. Acknowledgements The authors acknowledge Fusion UV Curing Systems for funding this work. References 1. Coffman, D. D., U. S. Patent 2,508,005, Morgan, C. R.; Magnotta, F.; Ketley, A. D. J. Polym. Sci.: Polym. Chem. Ed., 1977, 15, Morgan, C. R.; Ketley, A. D. J. Polym. Sci.: Polym. Lett. Ed., 1978, 16, Gush, D. P.; Ketley, A. D. Mod. Paint. Coat., 1978, 68, Jacobine, A. T. Thiol-Ene Photopolymers in Radiation Curing in Polymer Science and Technology: olymerication Mechanisms, Vol. III, eds, Fouassier, J. P.; Rabek, J. F., 1993, Elsevier Science Publishers Ltd., New York, pp Hoyle, C. E.; Lee, T. Y.; Roper, T. M. J. Polym. Sci.: Polym. Chem. Ed., 2004, 42, 5301.