Oligomers titrations

Determined by end-group titration b Average number of siloxane (Si—O) repeat units in the oligomer ... 

Capture ELISA on selected oligomeric fractions of formalin-treated RNase A (see curves 3-7 in Fig. 15.9) also reveal that the plateau values increase with an increase in the number of cross-linked molecules in the fractions. This is due to an increasing proportion of bound epitopes per binding site or, in other words, epitope density on the surface. Thus, the nearly identical plateau values for the titration of native RNase A and formalin-treated unfractionated RNase A (curves 1 and 2 in Fig. 15.9) are fortuitous, being caused by the particular composition of oligomers present in the formalin-treated RNase A preparations that was analyzed. 

Synthesis and characterization of well-defined, a,w-terminated difunctional siloxane oligomers are discussed. Detailed procedures on the preparation of primary amine- and hydroxy-terminated oligomers are given. Control of the average molecular weight (Mn) and also the possible variations in the backbone structure and composition are explained. The effect of these variations on the physical, thermal and chemical properties of the resulting materials are discussed. Characterization of these oligomers by FT-IR, NMR and UV spectroscopy, potentiometric titration and DSC are summarized. 

In the oligomer map two almost typical titration curves (B, Q are obtained. Their positions on the log (lLlo/[Ni]o) scale indicate that the corresponding intermediates occur in high steady-state concentrations. 

Analysis of Bisphenol-A Carbonate Oligomers for Chloroformate Chlorine. Chloroformate chlorine end groups were determined by a modified Volhard procedure in which the bischloroformate first reacts with aqueous pyridine to convert chloroformate to chloride ion. The chloride ion is extracted into water and titrated by the Volhard procedure. 

As to Eq. (7), it is to be remembered that AG, in a general case is a function of p. Therefore, the experimental dependencies of p on concentration, chain length of oligomer and temperature may be employed to find thermodynamic parameters only for a fixed value of p, e.g., for p = 0.5 using Eqs. (8 a- b). These equations have been taken by various authors to calculate the enthalpy and entropy of complex formation between simple synthetic oligomers and polymers 28). In a number of cases the correspondence between the values of complex formation enthalpy thus obtained and determined, either by calorimetry or by potentiometric titration 26), has been found satisfactory although it is obvious that in a general case these values do not necessarily coincide. 

When the end-capping is carried out in two separate steps the titration of the acidic groups give an accurate view of the oligomer molecular weights. This method also allows control of the formation of the imide by following the disappearance of the amic acid. Various titration techniques are available but the most common is to use a tetralkylammonium hydroxide in methanol, the equivalent point being determined by potentiometry [6,16]. 

Above pH 8 the titration curve shows uptake of a second equivalent of base corresponding to production of (en)Pd(OH)2. This second section of the titration curve is also flattened, owing to break up of oligomers by uptake of a second hydroxide at high pH. Though little reaction occurs directly by this route, we define the second acidity constant in the usual way. 

The oligomer molecular weights were characterized by both UV-visible spectra (20, 21) and/or potentiometric titrations (22, 23). Details of the measurements are provided in these papers. The block copolymers also were characterized by intrinsic viscosity and in some cases by membrane osmometry and gel permeation chromatography. Additional characterization studies are continuing and will be reported later. A typical synthesis of a 5000-5000 polysulfone-S-polycarbonate-A copolymer via interfaciar polymerization is described below. 

Bis-S-polysulfone oligomer (5.0 g, Mn 5300 by titration) and Bis-A-polycarbonate oligomer (5.0 g, Mn 5000 by UV) were dissolved in 300 mL of tetrachloroethane in a hood. The solution was somewhat hazy even at these concentrations. (By contrast, Bis-A-polysulfone oligomers and Bis-A-polycarbonate oligomer yield clear solutions at comparable... 

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