Tacticity, role

The submarine or surface launched strategic weapon is turbofan-powered, with a solid propint tandem rocket booster and nuclear warhead. Conversion to tactical role involves substitution of tactical guidance, a conventional HE warhead, and a turbojet cruise engine in place of the turbofan... 

A third factor influencing the value of Tg is backbone symmetry, which affects the shape of the potential wells for bond rotations. This effect is illustrated by the pairs of polymers polypropylene (Tg=10 C) and polyisobutylene (Tg = -70 C), and poly(vinyi chloride) (Tg=87 C) and poly(vinylidene chloride) (Tg =- 19°C). The symmetrical polymers have lower glass transition temperatures than the unsymmetrical polymers despite the extra side group, although polystyrene (100 C) and poly(a-meth-ylstyrene) are illustrative exceptions. However, tacticity plays a very important role (54) in unsymmetrical polymers. Thus syndiotactic and isoitactic poly( methyl methacrylate) have Tg values of 115 and 45 C respectively. 

X. Chemistry is not only relevant on the operational level in the chemical process industry. Also on tactical and strategical level it plays an important role in all types of industries. 

Many radical polymerization systems were examined hy Challa at a/. A number of papers were devoted to polymerization of methyl methacrylate, MM, in the presence of poly(methyl methacrylate). The role of tacticity of template was stressed. Isotactic PMM forms stereocomplex with syndiotactic PMM. Polymerizations of MM in the presence of isotactic PMM were carried out and the product (template + daughter polymer) was analyzed by NMR. When MM is polymerized in the polar solvent in the presence... 

Little work was conducted on the role of the solvent. The standard conditions for most of the subsequent work involved polymerizations in dichloromethane at - 60° to — 78°. However, a few experiments using sulfur dioxide as the solvent, with phosphorus pentafluoride as the catalyst, at —78°, gave, from GlcAnBzl3, a polymer of low d.p. having only a slightly lower specific rotation.92 Whether this was the result of slightly lower stereoregularity in the polymer, or contamination with a low-d.p. fraction of different tacticity, was not established. 

The carbonyl group in 24 could be temporarily protected as the dioxolane,16 or somewhat more efficiently as the dimethyl acetal 25.17 Subsequent conventional hydroboration led stereoselectively to the P alcohol 26. Although our earliest considerations called for ultimate base-promoted elimination of the OR substituent in intermediates more advanced than 16, we later came to recognize that a methoxy derivative was not going to fulfill this role to our satisfaction. Alternatively, the tactic of forming the benzyl ether at this point ultimately proved well suited to the eventual return to an unmasked hydroxyl at this site.17,18... 

A related approach exploited a A-cyanomethyl group to serve in the dual role of a nitrogen protecting group and a latent precursor of the formaldehyde im-inium ion (e.g., 478), and this innovative modification in tactics resulted in a simplified route to the c/.v-3a-aryloctahydroindole 474 (203a,c). To this end, the amino ketone 482 was readily prepared in one step by the reaction of 1,2-bis(trimethylsilyloxy)cyclopentene with A -benzyl-A -cyanomethyl amine. When 482 was exposed to [l-(3,4-methylenedioxyphenyl)ethenyl]lithium, a mixture (1 14) of 483 and 484 was obtained. It is noteworthy that the stereochemical sense... 

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