Palladium‐Catalyzed Chain‐Growth Polycondensation of AB‐type Monomers: High Catalyst Turnover and Polymerization Rates

Chain-growth catalyst-transfer polycondensations of AB-type monomers is a new and rapidly developing tool for the preparation of well-defined p-conjugated (semiconducting) polymers for various optoelectronic applications. Herein, we report the Pd/PtBu3-catalyzed Negishi chain-growth polycon- densation of AB-type monomers, which proceeds with unpre- cedented TONs of above 100 000 and TOFs of up to 280 s 1 .I n contrast, related AA/BB-type step-growth polycondensation proceeds with two orders of magnitude lower TONs and TOFs. A similar trend was observed in Suzuki-type polycondensa- tion. The key impact of the intramolecular (vs. intermolecular) catalyst-transfer process on both polymerization kinetics and catalyst lifetime has been revealed. p-C onjugated (semiconducting) polymers have become an important class of materials for applications in polymer solar cells, field-effect transistors, and light emitting diodes. (1) p-Conjugated polymers are generally produced by step-growth polymerizations, most often by Pd-catalyzed Stille (2) and Suzuki (3) polycondensations with the so-called AA/BB approach. (4) p-Conjugated polymers synthesized in such a way frequently suffer from a low degree of control over molecular weight (MW) which is undesirable for optoelec- tronic applications. Another drawback of step-growth Stille and Suzuki polycondensations is that they are relatively slow processes because of the moderate nucleophilicity of tin- and boron-organics. With these methods, synthesis of high MW polymers (which are especially attractive for applications) (5) usually requires long reaction times, high temperatures, and high loadings of expensive Pd catalysts. (2) The formation of toxic byproducts is another drawback inherent to Stille polycondensation. On the other hand, Negishi type polycon- densation, which utilizes non-toxic and strongly nucleophilic zinc-organic-based monomers, is a promising technique for industrial-scale production. (6) Impressive progress was ach- ieved in the last decade in the development of new, more efficient Pd catalysts for cross-couplings of small molecules. (6) Particularly, it was demonstrated that the use of bulky and electron-rich ligands enables, in many cases, cross-couplings to proceed under mild conditions, with low catalyst loadings and involving otherwise inactive electrophiles. (7) Turnover numbers (TONs) (8) above 100 000 where demonstrated in Suzuki, Negishi, and Heck cross-couplings, making them attractive for commercial applications. (9) Surprisingly, mech- anistically related polycondensations underwent little improvement since their discovery. (2, 3) Chain-growth catalyst-transfer polycondensations of AB- type monomers is a new and rapidly developing alternative for the preparation of well-defined conjugated homo-, gradient-, and block-copolymers. (10) Unlike the situation with step-growth polycondensations, promising results were achieved in chain-growth polycondensations initiated by new- generation catalysts. (10-12) Particularly, it was shown that Pd/