Polymer modification chain and termini

Poly(3-alkylthiophene)s are chemically robust, withstanding strong reductants including boranes [67] and LiAlH4 [72]. The electron-rich backbone is, however, readily functionalized by oxidative methods. Li and co-workers exploited this to replace the 4-proton with Cl, Br or NO2 functionality [73-75]. Reaction at the a-methylene was noted in some instances. Subsequent Pd-catalyzed cross-coupling of the perbrominated polymer could effect 99% derivatization. Oxidation renders the backbone susceptible to nucleophilic attack. Li et al. found that pyridine derivatives efficiently reacted at the 4-position of the radical cation, functionalizing up to 60 % of the putative polaron pentads. Use of l-methyl-4-(4 -pyridyl)pyridinium salts yielded viologen substituents [76]. 

Terminal ring functionalization has been explored as a route to block copolymer synthesis. The Stille coupling approach affords Br and RsSn termini on an isolable product. Iraqi and Barker isolated a low-A/ fraction (DP 14) then homo-cross-coupled to increase Af (DP ra 28), thus demonstrating telechelic utility [43]. The nature of the nickel-initiated cross-coupling polymerization (GRIM method) allows chain extension from the quasi-living end until the reaction is terminated. Controlled blocks of HT-PHT with HT-PDDT were prepared by changing the monomer [63]. 

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