## 1 Introduction  
In response to the urgent need to develop new high-performance and halogen-free fire resistant materials with low environmental impact [1], different alternative flame retardant (FR) systems have been introduced. In many applications, traditional inorganic, non-halogenated flame retardants such as ammonium polyphosphate (APP) and aluminum or magnesium hydroxides (ATH, MDH) are widely used, but there is still a need to find more effective solutions. A vast majority of the proposed solutions are based on different reactive or additive organophosphorus or nitrogen containing additives that can act in gas phase and/or in condensed phase [2; 3; 4; 5]. In addition, various families of radical generators have been prepared and their viability in a series of polymer applications has been recently demonstrated [6; 7; 8; 9; 10; 11; 12; 13]. Most plastic materials require specific FRs for optimal fire safety; furthermore, FR materials must often fulfill very application-specific fire safety demands in order to pass various fire safety standards established for plastic components. Coating, construction, textiles, electric/electronic (E&E) and transportation industries have additionally various requirements on the additives used in their polymer materials; temperature stability, being solid or liquid, viscosity, polarity and miscibility are only a few properties which need to be matched between a suitable FR, the polymer, the production process and the product. Hence, the optimal flame retardant solution and the obtained level of flame retardancy depend on the polymer type and application. In this respect, flame retardants that can be compounded at temperatures \(\geq\)230 \({}^{\circ}\)C, have high hydrolytic stability, and do not require high loadings (below 10% by weight) in order to achieve compliance with UL94 V-0 would be highly desired. In addition, they should have a reasonable price compared to the halogen-containing flame retardant systems. Along with phosphorus, nitrogen and silicon, sulfur is one of the known flame retardant elements, but compared to the others, sulfur containing compounds have been much less extensively studied as flame retardants. However, disulfide based radical generators alone or in combination with phosphorus compounds have shown good flame retardant effects both in polypropylene (PP) and polystyrene (PS) [12; 14]. Replacing P=O bonds with P=S in 9,10-dihydro-oxa-10-phosphaphenanthrene-10-oxide (DOPO) enhanced gas phase FR activity in epoxy matrix [15]. An approach to combine P, N and S to an effective char forming FR for thermoplastic polyurethane has also been reported recently [16]. We have discovered that sulfenamides, a specific group of compounds containing a bond of divalent sulfur and trivalent nitrogen, which by homolytical cleavage generate aminyl and thiyl radicals, provide excellent flame retarding properties to polymer substrates [17]. Sulfenamides (SN) contain two reactive centers, and can be attacked both by nucleophiles on S and by electrophiles on N. Formation of sulfenamides was reported already in 1911 by [PERSON] [18]. Since then, sulfenamides and related structures have been extensively studied as intermediates in organic and peptide synthesis and used in various applications including pharmaceuticals, fungicides and rubber vulcanization [19; 20; 21]. One of the very attractive features of sulfenamide compounds is their versatility; by changing the \(R\) groups attached to S and N, very different kinds of chemical and physical properties can be obtained. Hydrophobicity, thermal properties and stability in different chemical environments can be tailored to fit the application-specific requirements. Both small molecules and polymers can be easily synthesized. The synthesis of first polysulfenamides using secondary diamines and dithiosuccinimid sulfenyl transfer agent was described in 2011 [22; 23; 24]. These polymers are based on aliphatic and cycloaliphatic constituents, and they readily degrade in water.