High molecular weight acrylonitrile–butadiene architectures via a combination of RAFT polymerization and orthogonal copper mediated azide–alkyne cycloaddition

Abstract

α-Functional nitrile butadiene rubber (NBR) building blocks were employed in the copper mediated 1,3-dipolar Huisgen coupling upon addition of 1,4-bis(azidomethyl)benzene (4). Polymer–polymer coupling afforded linear polymers with molecular weights ranging from 2500 g mol−1 to 97 000 g mol−1 and polydispersities from 1.1 to 1.6. The α-functional NBR building blocks were obtained via the reversible addition–fragmentation chain transfer (RAFT) copolymerization of acrylonitrile (AN) and 1,3-butadiene (BD) at 100 °C, utilizing the high temperature azo initiator 1,1′-azobis(cyclohexane-1-carbonitrile) and chlorobenzene or acetone as solvents. A novel alkyne-functional trithiocarbonate 2 was synthesized in 64% yield via the N,N′-dicyclohexylcarbodiimide mediated coupling of 2-((dodecylsulfanyl)carbono-thioyl)sulfanyl propanoic acid (DoPAT, 1) and propargyl alcohol. 2 was shown to be an efficient controlling agent for the controlled/living radical copolymerization of acrylonitrile and 1,3-butadiene. The use of copper mediated azide–alkyne cycloaddition was extended towards the side-chain modification of acrylonitrile–butadiene rubbers as well as applied in the synthesis of branched and cross-linked NBR structures. For this purpose an acrylonitrile-1,3-butadiene–propargyl methacrylate (PMA) terpolymer of 3900 g mol−1 with a PDI of 1.3 was synthesized by a DoPAT-mediated RAFT polymerization. Herein, monomers were employed in the ratio of 56 : 35 : 9 (BD : AN : PMA). The ability of the terpolymer to undergo side-chain modification was demonstrated upon addition of 1-undecane azide. Cross-links were established via addition of 1,4-bis(azidomethyl)benzene. The current study provides the first successful approach to employ an orthogonal conjugation technique on this technically important class of synthetic rubbers.