Regioregularity

The asymmetry of 3-substituted thiophenes results in three possible couplings when two monomers are linked between the 2- and the 5-positions. These couplings are  

These three diads can be combined into four distinct triads, as shown in Fig. 3.19. 

The triads are distinguishable by nuclear magnetic resonance (NMR) spectroscopy, and the degree of regioregularity can be estimated by integration. 

A regiorandom copolymer of 3-methylthiophene and 3-butylthiophene possessed a conductivity of 50 S/cm, while a more regioregular copolymer with a 2 1 ratio of HT to HH couplings had a higher conductivity of 140 S/cm. Films of regioregular poly[3-(4-octylphenyl)thiophene] (POPT) with greater than 94% HT content possessed conductivities of 4 S/cm compared to 0.4 S/cm for regioirregular POPT. 

The difference between absorption and emission maxima, the Stokes shift, also increases with HH dyad content, which was attributed to greater relief from conformational strain in the first excited state. 

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Optically active polymers are potentially very useful in areas such as asymmetric catalysis, nonlinear optics, polarized photo and electroluminescence, and enantioselective separation and sensing.26 Transition metal coupling polymerization has also been applied to the synthesis of these polymers.27 For example, from the Ni(II)-catalyzed polymerization, a regioregular head-to-tail polymer 32 was obtained (Scheme 9.17).28 This polymer is optically active because of the optically active chiral side chains. 

The microstructure and architecture of polymers can also gready influence die properties of die polymers. For example, poly(3-substituted thiophene)s could have three microstructure joints s-trans (head to tail), s-trans (head to head), and s-cis (head to tail) (Fig. 9.3). The regioregular head-to-tail poly(3-substituted thiophene)s exhibit higher electrical conductivity values and higher... 

The monomer 19 can also be polymerized using analogous methods of initiation to those employed in organic polymer science. Radical initiators afford regioirregular polymers, whereas anionic initiators add selectively to the phosphorus atom of the P=C bond and thus yield a regioregular polymer [85]. The product of the initial addition of MeLi across the P=C bond, Mes(Me)P-CPh2Li, was identified spectroscopically. The polymers obtained from anionic initiation are spectroscopically identical to those obtained from the thermolysis. Reasonable molecular weights (ca. 5000-10,000 g mol 0 are obtained when methyllithium is used as an initiator. 

Liu, J., Sheina, E., Kowalewski, T. and McCullough, R. D. (2002) Tuning the electrical conductivity and self-assembly of regioregular polythiophene by block... 

Several isospecific Ci-symmetry catalysts have also been described including (12-15). When activated with [Ph3C]+ [B(C6F5)4]-, (12) affords highly regioregular i-PP (mmmm = 95%) with the stereochemical defects predominantly being isolated rr triads, consistent with a self-correcting enantiomorphic site-control pathway. 2,73 The isospecificity was therefore explained by a mechanism... 

Bao, Z. Dodabalapur, A. Lovinger, A. 1996. Soluble and processable regioregular poly (3-hexylthiophene) for thin film field-effect transistor applications with high mobility. Appl. Phys. Lett. 69 4108 1110. 

Kobashi, M. Takeuchi, H. 1998. Inhomogeneity of spin-coated and cast non-regioregular poly(3-hexylthiophene) films. Structures and electrical and photophysical properties. Macromolecules 31 7273-7278. 

Bao, Z. Lovinger, A. 1999. Soluble regioregular polythiophene derivatives as semiconducting materials for thin film field-effect transistors. Chem. Mater. 11 2607-2612. 

Ong, B. Wu, Y. Liu, P. 2005. Design of high-performance regioregular polythiophenes for organic thin-film transistors. Proc. IEEE 93 1412-1419. 

Molecular mechanics analyses of the kind described in previous sections are able to rationalize not only the stereoselectivity (and stereospecificity) of regioregular primary insertion steps but also the stereoselectivities relative to occasional secondary monomer insertions as well as relative to primary insertions following these secondary insertions.37d... 

Pomerantz et al. [493] prepared regioregular polymers HHTT-400 and HHTT-401 (Scheme 2.66) and compared them with previously synthesized irregular polymers 400 and 401 (Scheme 2.63) [485]. Regioregular polymers showed blue shifts in absorption for solution and films (23-30 nm and 57-79 nm, respectively Table 2.4), interpreted in terms of shorter conjugation length. Blue shifts in PL and EL were much less-pronounced (10-15 nm) and the PLED showed 2-4 times lower 4>el (Table 2.4) [493]. 

FIGURE 2.26 PL and absorption spectra of thin films of regioregular polythiophenes HT-396 (a) and HHTT-396 (b), spin coated on a fused silica substrate. (From Barta, P., Cacialli, F., Friend, R.H., and Zagorska, M., J. Appl. Phys., 84, 6279, 1998. With permission.)... 

McCullough Method of Preparation of Regioregular HT Poly(3-Alkylthiophenes) (Chart 2.162)... 

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