Stacking of poly(3-alkylthiophene)s P3RThs and poly(4-alkylthiazole)s P4RTzs has been studied. Light scattering analysis indicates that head-to-tail (HT) type HT-P3HexTh (R = n-C6H13) gives a degree of depolarization (ρv) of 0.26 in CHCl3, which reveals that HT-P3HexTh takes a stiff structure even in the good solvent. Addition of CH3OH to CHCl3 solutions of HT-P3HexTh and head-to-head (HH) type HH-P4HepTz (R = n-C7H15) leads to π-stacking of the polymer molecules to form stable colloidal particles. The light scattering analysis of the colloidal solution of HT-P3HexTh in a 2:1 solution of CHCl3 and CH3OH reveals that HT-P3HexTh is aggregated in a parallel style. Results of filtration experiments using membranes with 0.20 and 0.02 μm pores agree with the degree of the aggregation. P3HexThs with irregular structures (P3HexTh (Fe) and P3HexTh (Ni) with HT/HH ratios of about 7/3 and 1/2, respectively) show a weaker trend to aggregate; however, P3HexTh (Fe) is considered to stack in a surface region of a stretched poly(ethylene terephthalate) PET film. A dichroism observed with the stretched PET film indicates that the π−π* absorption as well as photoluminescence of the stacked P3HexTh molecules have a transition moment along the direction of the polymer main chain. X-ray diffraction analysis of HT-P3RThs and HH-P4RTzs reveals that they take a face-to-face stacked structure with an end-to-end packing mode, except for HT-P3MeTh (R = Me). HT-P3MeTh forms a face-centered lattice with an interlayer distance of 3.51 Å. An alternative copolymer of bithiazole and 4,4‘-dibutylbithiazole is packed in an interdigitation mode. At temperatures below 0 °C, the HT-P3HexTh molecules are π-stacked in CHCl3, and the 1H NMR spectrum shows a severe magnetic effect on the thiophene ring. Solid 13C NMR data are also consistent with the π-stacking.

The crystal structure of the R-form of poly(L-lactide) was analyzed by the linked-atom leastsquares refinements for the X-ray fiber diffraction data. The space group was orthorhombic P212121, and the unit cell contained the antiparallel two chains. The chain conformation was the 2-fold s(15*2/7) helix distorted periodically from the regular s(3*10/7) helix. The ester groups were deformed from the planar trans conformation. Potential energy calculations suggested that the periodic chain distortion resulted from the impartial stress distribution due to the interchain interactions.

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Various π-conjugated copolymers constituted of π-excessive thiophene, selenophene, or furan units (Ar) and π-deficient pyridine or quinoxaline (Ar‘) units have been prepared in high yields by the following organometallic polycondensation methods:  (i) n X−Ar−Ar‘−X + n Ni(0)Lm → (-Ar−Ar‘)- n (X = halogen, Ni(0)Lm = zerovalent nickel complex), (ii) n X−Ar−X + n Me3Sn−Ar‘−SnMe3 → (-Ar−Ar‘)- n (palladium catalyzed), and (iii) a X−Ar−X + b X−Ar‘−X + (a + b)Ni(0)Lm → (-Ar) x (Ar‘)- y . Powder X-ray diffraction analysis confirms an alternative structure of a polymer prepared by the method ii. The copolymers have a molecular weight of 5.4 × 103 to 3.3 × 105 and an [η] value of 0.37 to 4.4 dL g-1. π−π* absorption bands of the copolymers generally show red shifts from those of the corresponding homopolymers, (-Ar)- n and (-Ar‘)- n , and the red shifts are accounted for by charge-transferred CT structures of the copolymers. For example, an alternative copolymer of thiophene and 2,3-diphenylquinoxaline gives rise to an absorption band at λmax = 603 nm, whereas homopolymers of thiophene and 2,3-diphenylquinoxaline exhibit absorption peaks at about 460 and 440 nm, respectively. The CT copolymers are electrochemically active in both oxidation and reduction regions, showing oxidation (or p-doping) peaks in a range of 0.39 to 1.32 V vs Ag/Ag+ and reduction (or n-doping) peaks in a range of −1.80 to −2.22 V vs Ag/Ag+, respectively. Copolymers of pyridine give unique cyclic voltammograms exhibiting p-undoping peaks at potentials much different (about 2−3 V lower) from the corresponding p-doping potentials, and this large difference between p-doping and p-undoping potentials is explained by an EC mechanism. They are converted into semiconductors by chemical and electrochemical oxidation and reduction. Copolymers of thiophene with pyridine and quinoxaline show the third-order nonlinear optical susceptibility χ(3) of about 5 × 10-11 esu at the three-photon resonant wavelength, which is 5−7 times larger than those of the corresponding homopolymers and related to the CT structure in the copolymers.