Steric interactions conjugated polymers

The pattern presented by heats of polymerization of various monomers appears to be very well explained by two dominant factors elimination of resonance energy of conjugation, and steric interactions between substituents. The importance of the value of the heat of polymerization in determining the temperature above which reversal of the conversion of monomer to polymer occurs has been stressed by Dainton and Ivin. ... 

Pyrrole offers two positions that can be substituted to obtain soluble polymers, the N- and the 3-/or 4-positions on the aromatic ring. In general, N-substitution lowers the conductivity caused by the loss of conjugation due to steric interactions, which leads to a torsion of adjacent pyrrole units the advantage of the N-substitution is the improved environmental stability and the more straightforward structural characterization [155]. For the preparation of N-substituted and ring-substituted PPys oxidative chemical and electrochemical... 

Roncali and coworkers [81-83] reported the related polymers 55a and 55b as shown in Chart 12.6. These polymers were prepared by oxidative polymerization of the bithienyl precursors, which were obtained in a manner similar to the thieno[3,4-fo]pyrazine terthienyls shown in Scheme 12.10. While polymer 55b exhibits properties consistent with analogs 54a-b (Amax= 1070 nm Eg = 0.83 eV) [83], the Eg of 55a is reported to be 0.36 eV [81,82]. This latter value is believed to be one of the lowest Eg values reported for a conjugated polymer. The significant difference between the 55a and 55b was attributed to weakening of the interchain interactions as a result of the sterics introduced by the EDOT side chain [83]. 

Another strategy proposed by Sannicolo et al. consists of the use of cyclopentabithiophene precursors with either a 16-crown-5-ether ring coplanar to the bithiophene moiety (74) or a 15-crown-5-ether perpendicular to it (75) [180]. This approach considerably reduces the steric interactions between the complexing site and the conjugated backbone. Consequently, the precursors were readily electropolymerized into extensively conjugated polymers as confirmed by optical data. The analysis of electrochemical properties in the presence of alkali cations showed that while poly(75) is completely insensitive to change of the cationic species, the redox potential of poly(74) undergoes a 350 mV positive shift in the presence of Na" " [ 180]. 

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