Polymerization of Cysteine Functionalized Thiophenes

Different synthetic pathways leading to polythiophenes (PTs) containing units derived from methyl N-(tert-butoxycarbonyl)-S-3-thienyl- L-cysteinate (1) and methyl N-(tert-butoxycarbonyl)-S-(2-thien-3-ylethyl)-L-cysteinate (2) were investigated. The oxidative coupling with FeCl3 applied to N-deprotected monomer 1 generates a chemically fleeting PT, whereas when applied to N-deprotected monomer 2 generates a mixture of oligomers. Two co-polymers bearing cysteine moieties, poly{[methyl N-(tert-butoxycarbonyl)-S-3-thienyl-L-cysteinate]-cothiophene} (co-PT1) and poly{[methyl N-(tert-butoxycarbonyl)-S-(2-thien-3-ylethyl)-L-cysteinate]-co-thiophene} (co-PT2), were eventually synthesized through Stille coupling of 2,5-bis(trimethylstannyl)thiophene and 2,5-dibromo derivative of compound 1 and through the post-functionalization with protected cysteine of a tosylate co-polymer, poly{[2-(3-thienyl)ethyl 4-methylbenzenesulfonate]-co-thiophene} (co-PTTs). UV–vis, CD, NMR and GPC analyses evidenced that these polymers are able to form chiral self-assembling structures, due to the formation of a hydrogen bond network and to p-stacks, not only in the solid state but also in solution.