High Refractive Index Polymers Based on Thiol-Ene Cross-Linking Using Polarizable Inorganic/Organic Monomers Sharad D. Bhagat, ‡ Jhunu Chatterjee, † Banghao Chen, ‡ and A. E. Stiegman* ,‡ ‡ Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States † High Performance Materials Institute, Florida State University, Tallahassee, Florida 32306, United States * S Supporting Information ABSTRACT: The self-initiation of the thiol-ene coupling reaction of tetravinyl monomers containing main group elements and trivinyl heterocycles with alkyl and aryl dithiols resulted in the formation of highly cross-linked prepolymer gels which upon final curing at 120 °C yielded hard, monolithic polymeric materials. Because of the presence of highly polarizable main group elements such as Si, Ge, Sn, and S and the relative absence of highly electronegative elements, the resulting polymers exhibited high refractive indices ranging from 1.590 to 1.703 and Abbe numbers between 24.3 and 45.0. All of the polymers were highly transparent over the UV-vis region of the spectrum. Moreover, due to the high cross-linked density achievable in specific compositions, very hard materials capable of being ground and polished could be produced. The range of compositions produced yields important structure-property relationships, indicating the effect of monomer structure on mechanical and optical properties. 1. INTRODUCTION Transparent optical-quality plastics are extremely important in many commercial optical applications, including camera and projector lenses, and in optometric applications, including prescription eyewear and contact lenses. These materials afford large advantages over metal oxide glasses in areas such as weight reduction and fracture resistance. A number of optical figures of merit must be achieved for a polymer to be useful for specific optical application. These include the refractive index (n), the Abbe number, and the percent transmission of light over the spectral range of interest. In addition, depending on the application, mechanical properties such as hardness, fracture toughness, and scratch resistance are also significant. Current optical polymer technology revolves principally around well-established polymers such as poly(methyl methacrylate)s, polycarbonates, polythioureas, polyvinylcarbazoles, and poly- isocyanates. 1 Of the various optical properties, the refractive index is one of the most significant; as it dictates the shape and size, the lens must be to be utilized in a specific application. As such, much effort has been expended in developing high refractive index polymers. The primary approach to control the refractive index in a polymer is through the incorporation of polarizable atoms and groups, either as part of the backbone or as attached pendant groups. The basis for this approach lies in the concept of molar refractivity of the constituent monomers. 2 Constitu- ents and monomers with high molar refractivity include aro- matic and other unsaturated organic groups and groups con- taining polarizable main group heteroatoms, particularly sulfur. As such, polythiourethanes, polynaphthalenes, and polvinyl- carbazoles exhibit refractive indices in the range of ∼1.6-1.70, which occupy the high end of the refractive index range for organic polymers. 1 We report here the development of hybrid organic/inorganic thermoset polymers that utilize constituent monomers containing a preponderance of highly polarizable atoms such as Si, Ge, Sn, and S. 3 These polymers exhibit refractive indices that, on average, occupy the high end of the range of polymer refractive indices from 1.590 to 1.703. They also have Abbe numbers acceptable for most applications, are highly trans- parent, and, for many examples, are extremely hard and can be fabricated and polished into optical-quality monoliths. The repeat unit of the polymer is generated through the thiol-ene coupling reaction. 4-7 As has been noted previously, polymers formed by this reaction are particularly attractive for fabricating high index optical materials due to the fact that highly polarizable sulfur atoms are an intrinsic part of the repeat unit and that polymers can, in principle, be made which contain no highly electronegative, low-polarizability atoms such as N and O. 8,9 The problem with bulk polymers made through thiol-ene coupling reactions is that monolithic materials with the requisite strength and hardness required for optical applications are difficult to achieve due to the generally poor cross-link densities realized in this type of polymerization. 8 For this reason, thiol-ene polymers have typically been made and studied as films or in hybrid compositions with other polymers to impart strength. 8,10 We have overcome these problems by increasing the monomer functionality and through the use of a slow, self-initiation polymerization process. Received: November 8, 2011 Revised: January 5, 2012 Published: January 20, 2012 Article pubs.acs.org/Macromolecules © 2012 American Chemical Society 1174 dx.doi.org/10.1021/ma202467a | Macromolecules 2012, 45, 1174-1181