The Grignard metathesis (GRIM) polymerization of 3-alkylthiophenes proceeds by a quasi-"living" chain growth mechanism, not by a step growth process. Kinetic studies of the Grignard metathesis polymerization of 2,5-dibromo-3-alkylthiophenes showed that the molecular weight of poly(3-alkylthiophenes) is a function of the molar ratio of the monomer to nickel initiator, and conducting polymers with predetermined molecular weights and relatively narrow molecular weight distributions (PDIs ) 1.2-1.5) can be made. Sequential monomer addition resulted in new block copolymers containing different poly(3-alkylthiophene) segments.

Low polydispersity regioregular polythiophenes with number average molecular weights ranging from 2 to 13 kDa were cast under the same conditions from solution to form a series of field effect transistors (FETs). Tapping mode AFM and grazing incidence small-angle X-ray scattering revealed that in all cases the polymers formed regular nanofibrillar morphologies with the width of nanofibrils proportional to the weight average contour length of polymer chains, indicating that conjugated backbones were oriented perpendicular to the nanofibril axes. FET charge carrier mobilities exhibited exponential dependence on nanofibril width, pointing to the decisive role of extended conjugated pathways in charge transport.

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 nano-and microscale electronic materials and devices. [4][5][6][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)Cl 2 , one would expect a fast disappearance of the monomer and increase of the polymer molecular weight toward the end of the polymerization. [8][9][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 (<1%), if any, of the dimer. 11 These results indicate the very strong preference of the Ni(0) (see Scheme 1, intermediate 3) to form a nondiffusive associated pair, resulting in near 100% formation of the trimer. All of the results indicate that the polymerization proceeds with selective oxidative addition to the growing 2-bromopolythiophene and that these regioregular polymerizations progress by a chain growth mechanism rather than a step growth.Poly(3-hexylthiophene) (HT-PHT) was prepared by the original method 2 that provides a high specificity of H-T configuration of the repeating units (>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-chlorozinc-3-hexylthiophene monomer (1) generated in situ from 2-bromo-3-hexylthiophene reacts with Ni(dppp)Cl 2 , 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 cen...

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