Subject tacticity

The chlorination of polypropylene has been the subject of several fundamental studies and a variety of products is obtainable according to the tacticity of the original polymer and to the extent of chlorination. 

The polymerization of MMA has been shown to be subject to enantiomorphic site control when the Ci-symmetric a .va-lanthanocene complexes (196) and (197) are employed as initiators.463 When the (T)-neomenthyl catalyst (196) is used, highly isotactic PMMA is produced (94% mm at — 35 °C), whereas the (-)menthyl derived (197) affords syndiorich PMMA (73% rr at 25 °C). NMR statistical analysis suggests that conjugate addition of monomer competes with enolate isomerization processes, and the relative rate of the two pathways determines the tacticity. 

The measurement of polymer configuration was difficult and sometimes speculative until the early 1960 s when it was shown that proton NMR could be used, in several instances, to define clearly polymer stereochemical configuration. Bovey was able to identify the configurational structure of poly(methylmethacrylate) in terms of the configurational triads, mm, mr and rr, in a classic example (3). In the case of polypropylene, configurational information appeared available but was not unambiguously accessible because severe overlap complicated the identification of resonances from the mm, mr and rr triads (4). Several papers appeared on the subject of polypropylene tacticity but none totally resolved the problem (5). 

Thirdly, if new molecules could be synthesized which were to maximize properties and energy, such molecules could be useful even if costly. Much effort has been expended toward synthesizing new compounds for use as liquid propellants. Since that area of the propellant industry is the subject of two other chapters in this volume, I will avoid any detailed discussion of this area. However, in many tactical weapons systems, propellants can be used whose costs would normally be considered prohibitive for a propulsion application. This results from the fact that the rest of the missile propulsion, guidance, and warhead systems are so expensive and the amount of propellant so small that propellants costs become unimportant. The larger the missile becomes, and the more propellant it contains, the greater becomes the consideration of propellant cost. However, the fact that propellant cost may not be crucial has justified much research to prepare new propellants. In fact, several new propellant or propellant ingredients have been synthesized and will find their way into propulsion systems. 

The tacticity of anionically prepared polystyrenes has been the subject of extensive study by a number of groups of workers, mostly by means of 13C-NMR spectroscopy. From a study of the aromatic Cl resonances, Matsuzaki and coworkers found 234) that there is a tendency towards syndiotacticity when using -butyl-Iithium in toluene as initiator. From the sensitivity of the CMR spectrum to the nature of the solvent employed it was concluded that the polymerization did not conform to Bernoullian statistics. Randall examined the methylene resonances in the CMR spectrum and concluded that butyllithium initiated polystyrene is essentially atactic 235) and that propagation is Bernouillian. Uryu et al.236) examined polystyrene... 

The energetic nitramines, RDX and HMX, provide excellent impulse and nonsmoky exhaust, but their use gives propellants that may detonate when subjected to shock or impact. The possibility of unwanted violent burning or detonation of propellants during transport or storage has resulted in recent emphasis on the development of insensitive (reduced hazard) rocket propellants for tactical applications. Special attention are required in using these energetic materials.47... 

The effect of salts on radical polymerizations is the subject of growing attention. It appears that the addition of salts to the active centre may affect not only the rate, but also the method of monomer addition, i.e. the regularity or tacticity of the product. The indicated possibilities have not been technically exploited so far. 

Inlhe course of this chapter high explosives only are referred to, a dassification which excludes powder. Methods of handling are not Kxven, as practical training is essentiat in this subject The chapter oonones itseli to suggesting some of the tactical uses of H.E. both in defence and in attack. 

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