Irradiation tacticity

Transformations in the Solid State. From a practical standpoint, the most important soHd-state transformation of PB involves the irreversible conversion of its metastable form II developed during melt crystallization into the stable form I. This transformation is affected by the polymer molecular weight and tacticity as well as by temperature, pressure, mechanical stress, and the presence of impurities and additives (38,39). At room temperature, half-times of the transformation range between 4 and 45 h with an average half-time of 22—25 h (39). The process can be significantly accelerated by annealing articles made of PB at temperatures below 90°C, by ultrasonic or y-ray irradiation, and by utilizing various additives. Conversion of... 

There are a few exceptions to this general rule. One of the few examples of an effect on polymer stereochemistry was provided by Dais et al.m who found that polymerization of 31 above the cmc initiated by y-irradiation at 25 °C yields polymer composed entirely of syndiolaclic dyads P(m) =0. When the double bond was distant from the polar head group in 32, the tacticity observed was similar to that observed in solution polymerization / ( )-0,18. Polymerization of 31 at higher temperatures (50 °C) initiated by AIBN also showed no sign of tacticity control. The stcrcospccific polymerization of 31 was attributed to organization of the methacrylate moiety on the surface of the micelle. 

Early in the program, critical components (e.g.,the turbomolecular pump) and circuit boards were tested for their ability to survive neutron and gamma irradiation rates and doses similar to those that would be received from exposure to the detonation of a tactical nuclear device. All components were powered up at the start of the gamma irradiation tests but not during the neutron irradiation tests. Circuit boards were protected by circumvention circuits that powered down critical circuits in 10 to lOOps upon detecting radiation. All components survived the nuclear radiation tests. This unusual performance was noted with positive commendations by the staff at the White Sands Missile Range, where the tests were performed. Tests of the fully integrated CBMS II system, installed in a reconnaissance vehicle, will be conducted in the future. 

Remarkable differences in the G values of F12 were also recorded in atactic and syndiotactic polystyrene. The tacticity itself was also affected by the irradiation. Fligh-temperature irradiation with y-rays caused the reduction of isotacticity of isotactic PMMA... 

Poly(PO) formed with 17 as initiator under irradiation showed virtually the same NMR pattern as that for the polymer formed in the dark at 70 C, where the resonance due to the methyl group was very simple, indicating that the polymer consists of regular head-to-tail linkages. The diad and triad tactic-ities of the polymer, as determined by NMR [82], indicated the atactic struc-... 

A further chiral auxiliary-based tactic exploited tricarbonyl( 76-arene)chromium complexes of aromatic imines 71, which reacted under ultrasound (US) irradiation with a-bromoesters in a predictable stereochemical course to give comparable amounts of /S-aminoesters and / -lactams, as outlined in equation 44127. Chromium decomplexation is eventually achieved by photochemical oxidation under air. 

Polypropylene (PP).—The effects of stereoregularity in u.v.-irradiated PP upon the behaviour of the free radicals so produced have been investigated.192 The radicals formed at low temperatures were largely —CH2CH(CH2)CHa— and methyl, and were produced in large yield in samples of poor tacticity. On warming, acyl radicals were observed, especially in atactic and stereoblock PP. 

The most pronoimced decrease in the melting temperature is observed with isotactic polypropylene, indicating that the crystalline perfection of polypropylene is more highly affected by ionizing irradiation than that of polyethylene. Obviously, the process includes not only the main chain scissions but also the change in tacticity of polypropylene macromolecules. 

The homonuclear Overhauser enhancements explained in section 4.2.1 may also in principle be exploited for spatial and conformational information. However, this is only possible when the proton shifts are unusually sensitive to tacticity, because otherwise they cannot be resolved. They can be investigated by either 1-D or 2-D methods. The 1-D method involves selective irradiations, but is relatively quick and easy to quantify. The 2-D method, NOESY [38], is more elegant. In both cases one must take precautions to avoid spin-diffusion if quantitative information is required. Spin-diffusion is discussed in chapters 6 and 7 in the context of solid-state NMR, where its effects are more serious. What happens, in brief, is that the Wq term in (equation 4.6) can become very large for a proton pair, so that spin energy flows freely between most of the protons in the sample. Thus all selectivity is lost. In practice, spin diffusion is not too serious in a typical semi-mobile polymer solution, so that qualitative distance information may easily be obtained. The method is particularly promising for random copolymers, where more resonances are available for selective irradiation, because of the many variations of local sequence, as well as of tacticity. 

David and co-workers (32) confirmed the occurrence of direct main-chain scission and reported that racemization also occurred during radiolysis of syndio-tactic PMMA. In the presence of a radical scavenger, no racemization was found to take place, suggesting that radical processes are involved in the racemization process (33). These results therefore indicate that irradiation must induce some temporary main-chain scission, which is followed hy recombination of the scission radicals within the cage leading to a concomitant change in the stereochemistry of the polymer. The extent of the racemization observed for a y-ray dose of 8 MGy was reported to be approximately 24%. 

Instead of a reducing agent for decomposing peroxides or azo compounds, UV irradiation can be applied for splitting initiator. Millan et al. [319,321,366] describe the use of an ultraviolet beam to activate initiator decomposition at low temperatures. In a series of experimental procedures it was established that at 0°C a molecular weight of Mn = 320000 can be obtained. Results similar to those discussed for borane-initiated polymerization in reference to tacticity, thermal stability, and branching were obtained. This technique has very little practical value because special equipment is required. 

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