Isotactic chain segment

The second generalisation is that the relaxation times of comparable main-chain atoms will increase, at all temperatures, as more conformations become energetically available to the chain. This may lead to a modest dependence of Ti upon local tacticity. In vinyl polymers, there are indeed several reports of somewhat longer values for isotactic chain segments relative to syndio-tactic segments. These are discussed later in more detail with respect to RIS theory (section 4.5). 

It was shown in section 4.3 that often lengthens significantly for all carbons in isotactic chain segments in dissolved polymers [37]. This may be explained by the RIS analysis in the previous section, given the reasonable assumption that the transitions between conformations is rapid. Furthermore, the analysis shows that the relaxation parameters are insensitive to the rate of the local motions, within this limit. For example, an i-PMMA dyad must have two equally weighted conformations, loosely tg and gt, on symmetry grounds. The calculation described above also finds that tt has a very similar weighting. In contrast, an s-PMMA dyad has only one main conformation, tt, with a smaller contribution from gg. Thus more extensive local motion is to be anticipated in short i-PMMA sequences than in short s-PMMA sequences, in the same overall chain. Similar observations and calculations have been made with poly(acrylonitrile). 

The case of isotactic polypropylene (iPP) presents some differences with respect to those just discussed. While both sPP and PET adopt in their mesophases disordered, extended, essentially non-helical conformations, iPP is characterized by a unique, relatively well ordered, stable chain structure with three-fold helical symmetry [18,19,36]. More accurately we can state that an iPP chain segment can exist in the mesophase either as a left handed or as the enantiomeric right-handed three-fold helix. The two are isoener-getic and will be able to interconvert only through a rather complex, cooperative process. From a morphological point of view Geil has reported that thin films of mesomorphic iPP quenched from the melt to 0 °C consist of... 

This model was introduced by Frisch, Schuerch, and Szwarc (38) who discussed the chirality of vinyl polymers assuming that effects due to the chain length and the nature of the terminal groups were negligible. Arcus (58) spoke of nonterminal chain segments. The infinite chain model was explicitely used by Natta, Danusso, Corradini, Farina, and others (30-32). Natta, Pino, and Mazzanti cited this model in 1957 (35). It represents a simplification of the one with different terminal groups used by Natta in his first paper on isotactic polymers. 

In an earlier discussion (254) polymers in which the chirality depends only on the presence of chiral side groups were said to be nonintrinsically chiral, in contrast with intrinsically chiral polymers where the chirality is independent of the internal structure of the substituent. Substituted carbon atoms in the polymers described in the next paragraphs are often indicated as true or classic asymmetric carbon atoms. In this way one can distinguish between carbon atoms whose four substituents are constitutionally different in the proximity of the atom under consideration, from the tertiary atoms of vinyl isotactic polymers. For these, only the different length of the two chain segments and/or the stmcture of tire end grmips make all the ligands different from each other. 

The sp3 stereocenter (i.e., C ) in XII is chirotopic, like the case of poly(propylene oxide), since the first couple of atoms of the two chain segments are considerably different. The isotactic structures are optically active while the syndiotactic structures are not optically active. 

These definitions are clarified by considering a portion of a polymer chain such as XVII. Chain segment XVII has a total of 9 repeating units but only 8 dyads and 7 triads. There are 6 meso dyads and 2 racemic dyads (m) —, (r) —, There are 4 isotactic, 2 heterotactic, and 1 syndiotactic triads mm) = (mr) — j, (rr) — A. 

Spin-lattice relaxation times of carbon-13 in different polypropylene stereosequences differ slightly while nuclear Overhauser enhancements are almost identical (1.8-2.0) [533] isotactic sequences display larger Tx values than the syndiotactic stereoisomers. Other vinyl polymers behave correspondingly [534]. Carbon-13 spin-lattice relaxation times further indicate that dynamic properties in solution depend on configurational sequences longer than pentads. The ratio 7J(CH) 7J(CH2) varies between 1.6 to 1.9 thus, relaxation can be influenced by anisotropic motions of chain segments or by unusual distributions of correlation times [181],... 

Figure 8 Segments of isotactic (a), syndiotactic (b), atactic (c), and hemiisotactic polypropylene (d) chains. Segments of erythro-6 soXaci c (e), f/ reo-diisotactic (f), and disyndiotactic (g) poly-diolefin chains. The modified Fischer projection is shown. For parts, (a)-(c) a zigzag representation is also reported.

The difference in f3C-Tj between polymers with different tacticities but the same chemical structure results not only from differences in the chain segmental motion between stereoregular polymers or sequences but also from differences in preferred conformations between the stereoregular polymers or sequences which lead to different average distances for the interaction of a carbon with a proton of a neighbouring monomeric unit.298 The solvent dependence of l3C-T for stereoregular PMMAs has been explained by the solvent-dependent conformation of the polymer chain.299-301 In the case of H-Tj the mechanism of relaxation is rather complicated and the observed T cannot be directly related to the segmental mobility of the polymer chains. However, the H-T] values of poly (alkyl methacrylate )s were found to be parallel with the values, i.e. the Tx of the protons in the isotactic poly-... 

The extent of oxidative bond scissions in the insoluble R fractions (in analogy to polyethylene data (12)) may be concentrated in the noncrystalline isotactic PP chain segments (total initial crystallinity is about 50%, but increases presumably owing to secondary crystallization, particularly during thermooxidative degradation at 150°C). 

Fig. 33. C—H stretching vibrations in polystyrene (symmetric and asymmetric modes are not resolved). 70% isotactic film (normal line) and atactic film (bold line) show that the first one is much richer in the aromatic component than the second one. Isotactic film exposes preferentially the phenyl groups at the surface whereas the atactic one exposes phenyl groups and chain segments in comparable amounts. Spectra were normalized to the aromatic component.

Draw six-unit chain segments of isotactic and syndiotactic polystyrene. 

For the growth of isotactic polypropylene chains and higher polyolefin chains at the chiral coordination sites of ansa-metallocene catalysts, the following explanation is now firmly established [11,12] Formation of the new C-C bond requires that the a-olefin substituent and the C(a)-C(P) bond of the metal-bound polymeryl chain are oriented anti to each other along the incipient C-C bond, while the C(a)-C(P) chain segment must reside in an open quadrant of the chiral metallocene coordination site. The latter is thus considered to control the enantiofacial orientation of the a-olefin in the insertion transition state TS (Fig. 4) by way of the C(a)-C(P) chain-segment lever . 

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