Photo-induced ligation of acrylonitrile-butadiene rubber: Selective tetrazole-ene coupling of chain-end-functionalized copolymers of 1,3-butadiene

A highly selective photo-induced nitrile imine mediated tetrazole-ene coupling (NITEC) of chain-end-functionalized nitrile-butadiene rubber (NBR) is reported, providing nitrile rubbers with molar masses of up to 48 000 g·mol-1. NBR was obtained via the reversible addition-fragmentation chain transfer (RAFT) mediated copolymerization of acrylonitrile and 1,3-butadiene employing a novel photoreactive tetrazole-functionalized trithiocarbonate. The herein reported tetrazole-functionalized trithiocarbonate represents - to the best of our knowledge - the first ever reported photoreactive RAFT agent capable of undergoing light-induced ligations with enes. Molar masses of the tetrazole-functionalized NBRs were in the range of 1000 to 38 000 g·mol-1 with dispersities between 1.1 to 1.6. By an appropriate choice of the tetrazole substituents, a reaction of the in situ formed enophile with the double bonds or the nitrile moieties of the incorporated monomer units within the polymer backbone - present in high excess relative to the dipolarophile linker molecule - was not observed. Underpinned by DFT calculations, the selectivity was identified to originate from a reduced LUMO energy level of the maleimide linker compared to the nonactivated backbone olefins when employing a nitrile-imine of moderate reactivity. © 2013 American Chemical Society