Atactic oriented

Excimer formation has been studied in polystyrene and poly(a-alkylstyrenes)189 (PS), poly(vinylcarbazole),139>140 poly-(2-vinylnaphthalene), and poly-(4-vinyl-biphenyl).141 For polystyrene films, David et a/.189 showed that the fluorescence yield increased with increasing crystallinity, at both ambient temperature and 77 K. The contribution of excimer fluorescence yield increased in the sequence atactic (0.7) < atactic oriented (0.60) < isotactic amorphous (0.28) < isotactic crystallized (0.01), with normal yields relative to excimer given in parentheses. Similar results were obtained for poly(vinylcarbazole), PVCZ, although the contribution of excimer fluorescence at 77 was independent of crystallinity. The results can be interpreted in terms of electronic energy migration to low-energy defect sites from which excimer emission can occur. In PVCZ copolymers with fumaronitrile (10), diethyl fumarate (11), and diethyl maleate (12) (Scheme 6),... 

Atactic (Section 31.2) A chain-growth polymer in which the substituents are randomly oriented along the backbone. 

The above comments were directed against atactic PVN, in which the nitrate groups are randomly oriented about the backbone of the... 

There are three principal stereochemical types of poly(l-alkene)s, illustrated in Scheme 8.38 for polypropylene. In isotactic polypropylene 80 (i-PP) all methyl substituents have the same relative orientation (m). The scheme shows the stereochemistry with the usual Fischer projection underneath. In syndiotactic PP (81, s-PP) every second CHMe unit has the opposite stereochemistry to the first, while in atactic PP (82, a-PP) the orientation of the methyl substituents is random. In some polymers there is partial order, i. e. only every second monomer orientation is random (83, hemi-isotactic PP). 

Atomistic simulation of an atactic polypropylene/graphite interface has shown that the local structure of the polymer in the vicinity of the surface is different in many ways from that of the corresponding bulk. Near the solid surface the density profile of the polymer displays a local maximum, the backbone bonds of the polymer chains develop considerable parallel orientation to the surface [52]. This parallel orientation due to adsorption can be one of the reasons for the transcrystallinity observed in the case of many anisotropic filler particles. 

Atactic PMMA is a brittle amorphous polymer with low elongation at break. Without molecular orientation, PVDF has also a low elongation under tensile stress as shown in Figine 7.9. However, the elongation of PVDF/PMMA blends... 

Lefebvre, D., Jasse, B., Monnerie, L. Fourier Transform Infrared Study of Unaxially Oriented Poly(2,6-dimethyl 1,4-phenylene oxide)-Atactic Polystyrene Blends, submitted to Polymer... 

Methyl groups alternate from one side to the other in syndiotactic polypropylene. (c) The spatial orientation of the methyl groups is random in atactic polypropylene. 

The infrared spectrum of a sample of atactic polystyrene is shown in Fig. 15 [Liang and Krimm (114)]. The band positions are given in Table 16, on the left-hand side of the "Infrared column. Raman data for atactic polystyrene [Signer and Weiler (198) Palm (163)] are also included in the table. Polarized spectra of oriented specimens of isotactic polystyrene have recently been published [Tadokoro, Nishiyama, Noza-kura, and Murahashi (227) Tadokoro, Nozakura, Kitazawa, Yasuhara, and Murahashi (222) Takeda, Iimura, Ya-mada, and Imamura (222a) Morero, Man-tica, Ciampelli, and Sianesi (140a)]. The spectrum of the isotactic polymer differs slightly from that of the atactic polymer, and these differences (above 500 cm-1) are shown... 

The bilateral coverage of each coordination site in such a catalytic active site by a CH and a CH3 group (both in a /1-position to the bridgehead C atom) appears to render them indifferent to the enantiofacial orientation of an incoming a-olefin molecule, and thus atactic poly(a-olefin) is formed [30]. 

A final example of a stereoselective heterogeneous catalytic system is the work of Laycock, Collacott, Skelton and Tchir.17 Layered double hydroxide (LDH) synthetic hydrotalcite materials were used to stereospecifically polymerize propylene oxide [PO] to crystalline isotactic and liquid atactic poly(propyleneoxide) [PPO]. These authors suggest that the LDH surface acts as other inorganic or organometallic coordination initiators or catalysts by providing specific surface orientations for propylene oxide monomer. X-ray powder diffraction showed some loss of crystallinity after calcination and X-ray photoelectron spectroscopy showed an enhancement of Mg/Al content due to restructuring of the Mg and A1 surface atoms. The surface was also rich in Cl ... 

Atactic polymer—A polymer in which the groups bonded to the main chain are arranged in random spatial orientations. A polymer with an absence of tacticity. 

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