Sorting successive

During the early years of this century, organic chemists were enjoying success in determining the structures of ordinary-sized organic molecules, and this probably contributed to their reluctance to look beyond structures of convenient size. Physical chemists were interested in intermolecular forces at this period, and the idea that polymers were the result of some sort of association between low molecular weight constituent molecules prevailed fora long while. 

All polymer molecules have unique features of one sort or another at the level of individual repeat units. Occasional head-to-head or tail-to-tail orientations, random branching, and the distinctiveness of chain ends are all examples of such details. In this chapter we shall focus attention on two other situations which introduce variation in structure into polymers at the level of the repeat unit the presence of two different monomers or the regulation of configuration of successive repeat units. In the former case copolymers are produced, and in the latter polymers with differences in tacticity. Although the products are quite different materials, their microstructure can be discussed in very similar terms. Hence it is convenient to discuss the two topics in the same chapter. 

Radiometric ore sorting has been used successfully for some uranium ores because uranium minerals emit gamma rays which may be detected by a scintillation counter (2). In this appHcation, the distribution of uranium is such that a large fraction of the ore containing less than some specified cut-off grade can be discarded with tittle loss of uranium values. Radioactivity can also be induced in certain minerals, eg, boron and beryllium ores, by bombarding with neutrons or gamma rays. 

T abular. A typical distributioa as measured by modem iastmmeatatioa can iaclude size information on tens of thousands and even millions of iadividual particles. These data can be Hsted ia a computer and then sorted iato a series of successive size iatervals, keeping track of the measured quantity, such as number, surface area, or mass, within each group. For narrow size distributions it may be sufficient to group the data ia linear iatervals, such as 0—1, 1—2, 2—3 p.m, etc, and then Hst the iatervals as a perceat value of the whole. 

There are less exotic ways of increasing the strength of cement and concrete. One is to impregnate it with a polymer, which fills the pores and increases the fracture toughness a little. Another is by fibre reinforcement (Chapter 25). Steel-reinforced concrete is a sort of fibre-reinforced composite the reinforcement carries tensile loads and, if prestressed, keeps the concrete in compression. Cement can be reinforced with fine steel wire, or with glass fibres. But these refinements, though simple, greatly increase the cost and mean that they are only viable in special applications. Plain Portland cement is probably the world s cheapest and most successful material. 

But, to repeat, the extent of the diffusion of Mendeleev s scheme is not the issue here—no one disputes that this was widely (though not universally) accepted by 1890 in the scientifically advanced countries. The question at issue is the relative strength of the roles played in this diffusion by evidence of various sorts and in particular by the successful predictions of new elements. 

We attempted to prepare Pu(V) stabilized as K2PUF7, Rb2PuF7, and CsPuFg, but were only successful in repeating the preparation of CsPuFt ( 1). Since there is a paucity of information concerning syntheses of this sort, we cite possible grounds for our unsuccessful as well as our successful efforts. 

It is by means of successive reactions of this sort that simple alkanes like methane and ethane give rert-butyl cations in super acid solutions (p. 219). °... 

In highly exothermic reactions such as this, that proceed over deep wells on the potential energy surface, sorting pathways by product state distributions is unlikely to be successful because there are too many opportunities for intramolecular vibrational redistribution to reshuffle energy among the fragments. A similar conclusion is likely as the total number of atoms increases. Therefore, isotopic substitution is a well-suited method for exploration of different pathways in such systems. 

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