Radioactively tagged initiators

Kinetic study of this reaction usually requires sampling the polymerizing mixture and analyzing for the concentrations of the various reaction species at different polymerization times. Vofsi and Tobolsky in 1965 reported the use of radioactively tagged initiator (10), while Saegusa amd coworkers in 1968 developed a "phenoxy end-capping" method in which the oxonium ion is trapped with sodium phenoxide and the derived phenyl ether at the polymer chain end quantitatively determined by UV spectrophotometry (11). 

Many methods have been proposed for determining the efficiency,/ of the initiator. The most direct method depends on a quantitative assay of the polymer for initiator fragments, and its comparison with the amount of initiator decomposed. This is not difficult in those cases where the initiator leaves a reactive endgroup on the polymer or is radioactively tagged. 

Such a mechanism of carbon monoxide interaction with active centers is compatible with the data on the slow copolymerization of CO with ethylene found for the ethylene polymerization by some one-component catalysts This copolymerization may proceed also in the case of two-component catalysts resulting in an increase of the number of radioactive tags in the polymer with time (see Fig. 1). Arguments have been given that the rapid increase of polymer radioactivity in the initial period (5-10 min) is due to the insertion of the first CO molecule into the active metal-carbon bond. 

The attachment of a radioactive tag to the polymer is a much more versatile method however, inherent changes of the macromolecule may result. The significance of these alterations should be assessed from the viewpoint of the purpose of the experiment. A compromise between this and the former method involves the use of a radioactive polymerization initiator which is incorporated into the polymer chain. However, the specific activity which can be achieved is very low. 

A disk of radioactively tagged benzoic acid 1 cm in diameter is spinning at 20 rpm in 94 cm of initially pure water. We find that the solution contains benzoic acid at 7.3 10 g/cm after 10 hr 4 min and 3.43 10 g/cm after a long time (i.e., at saturation). (a) What is the mass transfer coefficient Answer 8 10 cm/sec. (b) How long will it take to reach 14% saturation (c) How closely does this mass transfer coefficient agree with that expected from the theory in Example 3.4-3 ... 

In the carbon-14 expts, HMX/RDX product was isolated qualitatively, separated Into its components, and each component assayed for carbon-14 beta radioactivity using a liquid scintillation counting technique (Ref 11). DPT-l4C was isolated as an intermediate product from the reaction mixt and similarly radioassayed. For the nitrogen-15 tagged AN expts, HMX and RDX were assayed mass spectrometrically for i5N/i4N ratios from which atom %1SN contents were calcd. In die course of these expts, each tagged species was added initially and also at subsequent stages of the reaction process. The important observations and results are summarized as ... 

Autoradiographs of GBH surfaces sometimes show a "strike where the aqueous solution of heparin was delivered by a pipette, and ran down the film, as seen in Figure 1, which shows the two sides of an adsorption sack with graphite-benzalkonium surface. The match of the adsorption pattern on the two surfaces indicates that it was a matter of initial contact with solution, not a property of the surfaces. This pattern was produced on an initially dry GB surface. The solution was 90% cold heparin, 10% tagged heparin, at 67 fig. tagged heparin/ml. The average heparin adsorption, calculated as total (10X radioactive heparin adsorption) from depletion of the solution, was 2.7 fig./cm.2 Sack was rotated 20 hours, with a bubble, at 37 °C. 

Direct detection of fast positrons is possible with scintillator detectors as the processes involved are the same as for gamma rays without the initial Compton or photoelectric event. In positron experiments the detection of fast positrons by thin plastic scintillators has been put to best use in time-of-flight and lifetime studies [21, 22], Here almost 100% of the positrons from a radioactive source or MeV beam which pass through a thin scintillator disc deposit enough energy (> 20keV) to create photons which produce a useable time-zero tagging pulse in a PM. (See secs. 3.5). 

The metabolism of radioactive oleate and linoleate was examined in tissue-slices from cotyledons with active TAG biosynthesis (14-16 DAF). Typical results from time-course studies are presented in Fig. 2. With oleate it is evident that PC is the most heavily labelled lipid over the initial time period and that at later times the radioactivity rapidly accumulated in TAG. Interestingly, over 90 of the radioactivity in the PC was located at the position sn-2 (Table 1). The rapid turnover of acyl... 

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