Controlled Chain-Growth Kumada Catalyst Transfer Polycondensation of a Conjugated Alternating Copolymer

The Ni-catalyzed Kumada catalyst transfer polycondensation of a novel biaryl monomer, 2-(4-bromo-2,5-bis(2-ethylhexyloxy)phenyl)-5-chloromagnesiothiophene (2), afforded the respective pi-conjugated alternating copolymer poly(thiophene-alt-p-phenylene) (PTPP). Under optimized conditions, the polystyrene-equivalent number-average molecular weight (M-n) of the copolymers prepared using this method were varied between 6.4 and 39 kDa by adjusting the initial monomer-to-catalyst ratios and, in all cases, the resulting materials exhibited narrow polydispersity indices (PDIs <= 1.33). Moreover, the M-n of the copolymers produced were found to increase linearly with monomer conversion. The ability of PTPP to be utilized as a macroinitiator for further copolymerization was confirmed through a series of chain extension experiments as well as block copolymerizations involving 2-bromo-5-chloromagnesio-3-hexylthiophene. MALDI-MS analysis showed that the major population of the PTPP prepared using the aforementioned method contained H/Br end groups, as would be expected for efficient catalyst transfer during the polymerization with minimal occurrence of chain termination. Collectively, these results were consistent with a controlled polymerization reaction and constituted the first such example in which a conjugated polymer with an alternating repeat unit was produced via a chain-growth process.