A sulfur-sulfur cross-linked polymer synthesized from a polymerizable dithiocarbamate as a source of dormant radicals.

The dithiocarbamates (DTCs) have a special place within polymer chemistry due to their versatile role as thermal and photochemical initiators.[1] In particular the role of dithiocarbamate esters as “iniferters” in “living” radical polymerizations has practical significance.[2] The living nature of DTC ester-initiated photopolymerization derives from the extreme difference in reactivity of the radicals created by homolytic cleavage of the DTC ester bond. This gives rise to a reactive carbon-centred radical and a much less reactive, or dormant, dithiocarbamate radical, in which the unpaired electron is delocalized over the DTC structure. The chemistry of the latter is dominated by recombination to reform covalent bonds. It is the ability of dormant radicals to recapture the carbon-centred radical after addition of monomer to form a new DTC ester that imparts living character to DTC ester-initiated photopolymerization. Compounds of this type have been used in the synthesis of telechelic polymers,[3] block[4,5] or graft copolymers[5,6] and hyperbranched polymers.[7] In the latter context, Otsu et al.[8] synthesized the first example of a DTC ester of a polymerizable derivative; the 4-vinylbenzyl ester of N,N-diethyldithiocarbamic acid, creating “inimer” a compound that combines iniferter and monomer functionalities. Inimer could be polymerized photochemically to form hyperbranched polymers or by conventional thermal initiation to form linear polymers with pendant DTC ester groups. These polymers are macro-iniferters, which allow the photochemical grafting of a range of other monomers. As well as the inimer of Otsu,[8] similar compounds, based on methacrylates, have been reported.[9] In polymers of these compounds, irradiation will generate active macroradicals and dormant dithiocarbamyl radicals as low molar mass fragments. The macroradicals will react rapidly with monomer, oxygen or with dithiocarbamyl radical to reform DTC esters. The situation will be different however if the dithiocarbamyl radical is part of the polymeric system and the active radical is the low molar mass fragment. A likely scenario under these circumstances is that during prolonged irradiation, the more readily diffusible carbon-centred radicals will dimerise, forming stable low-molar mass by-products, leading to an excess of dithiocarbamyl radicals attached to the polymer. This will result in the creation of a relatively stable population of polymeric dormant radicals. The physics and chemistry of immobilized dithiocarbamyl dormant radicals is expected to be an exciting field of investigation, since materials containing stable radicals may possess useful photo-responsive magnetic, mechanical and electrical properties. Polymers with other stable radical functionalities, such as nitroxy radicals, have been reported as novel battery materials[10] and alkoxamines in the side chain[11] or main chain[12] and have been used as thermally-activated systems for the reorganization of polymeric materials, via radical crossover reactions. Similarly, reversible decross-linking to radical fragments has been shown to occur in response to light or pressure, in the case of triphenylimidazole dimers, present as crosslinks on a methacrylate backbone.[13] In order to develop new materials to explore the properties of polymeric dithiocarbamyl radicals, we designed a monomer, superficially similar to inimer, [8] but reversing the orientation of the molecule, as shown in Scheme 1, such that the dithiocarbamyl residue is the polymerizable part of the molecule.