Polyesters with pendant hydroxyl side groups such as (R,S)and (S)-poly(tetramethy1ene ma1ate)s and (R,R)-poly(tetramethy1ene tartrate)s of moderately high molecular weights were prepared by polycondensation, at 105 "C in 1-methyl-2-pyrrolidone for 16 to 80 h, from potassium malate or tartrate and 1,4-dibromobutane. It was shown by a detailed analysis of the 'H and 13C NMR spectra of the prepared polyesters that no branching occurs in the course of the reaction. The nature of the end-groups was also clearly established by NMR spectroscopy. The presence of large cycles in the final products cannot be completely excluded.0 1993, Huthig & Wepf Verlag, Base1 CCC 0025-1 16X/93/$05.00
Variable range hopping (VRH) conduction in the insulating three-dimensional n-CdSe samples has been studied over the entire temperature range from 0.03 to 1 K. In the absence of a magnetic field, the low temperature conductivity of the three samples (A, B and C) obeys the Mott VRH conduction with an appropriate temperature dependence in the prefactor ( D 0 exp OE .T 0 =T / p with p 0:25/. This behavior can be explained by a VRH model where the transport occurs by hopping between localized states in the vicinity of the Fermi level, E F , without creation of the Coulomb gap (CG). On the contrary, no Efros-Shklovskii VRH is observed, suggesting that the density is constant in the vicinity of the E F .
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