Chain Growth Mechanism for Regioregular Nickel-Initiated Cross-Coupling Polymerizations

Poly(3-alkylthiophenes) are conducting polymers that have good solubility, environmental stability, and processability.1 The synthesis of regioregular polythiophenes has produced defect-free, structurally homogeneous, head-to-tail coupled poly(alkylthiophenes) (HT-PATs) that have greatly improved electronic and photonic properties over regiorandom analogues.2,3 Regioregular polythiophenes have led to a multitude of important and novel nanoand microscale electronic materials and devices.4-7 Very recently, we have developed an endgroup functionalization methodology of HT-PATs that allows for the synthesis of a plethora of well-defined block copolymers that form nanowires with high electrical conductivity.7 Here, we have discovered that the nickel-initiated regioregular polymerization of alkylthiophenes proceeds by a chain growth mechanism and does not occur by the accepted step growth mechanism.8 We also observed that the degree of polymerization of the synthesized poly(alkylthiophenes) increases with the conversion and can be predicted by the molar ratio of monomer to nickel initiator. On the basis of our experimental results, we predict that nickel-initiated crosscoupling polymerization is essentially a living system, giving PATs with low polydispersities (PDIs). In a cross-coupling step polymerization catalyzed by Ni(dppp)Cl2, one would expect a fast disappearance of the monomer and increase of the polymer molecular weight toward the end of the polymerization.8-10 On the basis of the experimental results, we observed that relatively high molecular weight polymer forms almost immediately. As a model reaction, we have also found that 2 equiv of a variety of aryl dibromides and 1 equiv of an aryl organometallic (either magnesium or zinc) always gives a near quantitative yield of the trimeric aryl and minor amounts ( 98% H-T coupling).12 The mechanism of the cross-coupling chaingrowth polymerization is outlined in Scheme 1. The first step in the mechanism, where the 2-bromo-5-chlorozinc3-hexylthiophene monomer (1) generated in situ from 2-bromo-3-hexylthiophene reacts with Ni(dppp)Cl2, yielding the organonickel compound (2), is as it has been described by others.9,13 We differ in our mechanism in that reductive elimination of 2 immediately forms an associated pair [3‚4] of the tail-to-tail aryl halide dimer (4) and nickel (0) (3). The dimer 4 undergoes fast oxidative addition to the nickel center generating 5, in view of the fact that the formation of the complex 3‚4 eliminates potential separation of 4 from 3. Subsequently, growth of the polymer chain occurs by insertion of one monomer at a time as shown in the reaction cycle (5 f 6 f [3‚7] f 5), where the Ni(dppp) moiety is always incorporated into polymer chain as an end group. Addition of various Grignard reagents (R′MgX) at the end of polymerization results in end-capping of HT* Corresponding author: e-mail rm5g@andrew.cmu.edu. Figure 1. Conversion (filled symbols) and logarithm of monomer concentration (open symbols) vs time plots for 2-bromo-3-hexylthiophene polymerization at different concentration of Ni(dppp)Cl2 initiator (23-25 °C); [M]0 ) 0.075 mol/ L: (9, 0) [M]0:[Ni(dppp)Cl2] ) 136:1; (b, O) [M]0:[Ni(dppp)Cl2] ) 57:1; (2, 4) [M]0:[Ni(dppp)Cl2] ) 49:1.