Stereoregular polymers, symmetry

From considerations on translational symmetry in the limit of a stereoregular polymer, which are more conveniently analyzed in terms of conservation constraints on momenta at interaction vertices and within self-energy diagrams (31), each Ih line can be easily shown (see e.g. Figure 4 for a second-order process)... 

The stereocenters in all three stereoregular polymers are achirotopic. The polymers are achiral and do not possess optical activity. The diisotactic polymers contain mirror planes perpendicular to the polymer chain axis. The disyndiotactic polymer has a mirror glide plane of symmetry. The latter refers to superposition of the disyndiotactic structure with its mirror image after one performs a glide operation. A glide operation involves movement of one structure relative to the other by sliding one polymer chain axis parallel to the other chain axis. 

M-C as propagating species, 4, 1008 monomer coordination and insertion reactions, 4, 1010 monomer insertion regio- and stereochemistry, 4, 1015 overview, 4, 1005-1166 regioirregular insertions, 4, 1023 stereocontrol mechanism, 4, 1018 stereocontrol symmetry rules, 4, 1020 stereoregular polymers, 4, 1016 in Ru-Os heterodinuclear compounds, 6, 1046 in Ru-Os mixed-metal clusters, 6, 1064 semiconductor growth, conventional precursors, 12, 2 with silicon, 3, 514... 

Symmetry and Repetition Theory in Classifying Stereoregular Polymers... 

In most NMR spectra of stereoregular polymers, the key factor for the analysis is the methylene group in the polymer chain. In an isotactic molecule, the two methylene protons are not identical, and therefore, two resonances must be observed. For a syndiotactic chain, the two protons in the methylene group are identical by symmetry, so only one resonance is observed [27]. 

Ideal stereoregular polymers such as those shown in Figure 3-2 possess a translational symmetry in that the same configurations can be produced by shifting the central atoms along the chain. Molecules with a translational or rotational axis of symmetry but without mirror-image symmetry are called dissymmetric. Asymmetric molecules are molecules that do not have any axis of symmetry. Thus, it-polypropylene is dissymmetric but not asymmetric. Poly(L-alanine) or poly(D-alanine) -f-NH—CH(CH3)—CO-, is, however, an asymmetric molecule. 

Fig, 7, Conformations of the four stereoregular polymers of 1,3-butadiene with symmetry elements a) h) trayis-1 y h c) syndiotactic 1>2 d) isotactic 1,2... 

In most theoretical investigations a stereoregular polymer is described by an infinite, extended and isolated chain constructed from a periodic sequence of monomer units. In addition to translational symmetry, stereoregular polymers possess some other symmetry elements like screw axes, mirror, or glide planes. The related operations combine into groups, the line groups (I6a-c). 

Polymers such as polystyrene, poly(vinyl chloride), and poly(methyl methacrylate) show very poor crystallization tendencies. Loss of structural simplicity (compared to polyethylene) results in a marked decrease in the tendency toward crystallization. Fluorocarbon polymers such as poly(vinyl fluoride), poly(vinylidene fluoride), and polytetrafluoroethylene are exceptions. These polymers show considerable crystallinity since the small size of fluorine does not preclude packing into a crystal lattice. Crystallization is also aided by the high secondary attractive forces. High secondary attractive forces coupled with symmetry account for the presence of significant crystallinity in poly(vinylidene chloride). Symmetry alone without significant polarity, as in polyisobutylene, is insufficient for the development of crystallinity. (The effect of stereoregularity of polymer structure on crystallinity is postponed to Sec. 8-2a.)... 

Polymers, with their highly stereoregular structures, are frequently of sufficiently high symmetry for infrared spectroscopy to give only an incomplete picture of the vibrational characteristics of the compounds. In some, as many as half of the fundamental modes are infrared inactive. These non-absorbing modes can frequently be observed in the Raman effect (e.g. polyethylene where mutual exclusion" applies and at least eight modes are Raman active and infrared silent ). 

Abstract The synthesis and X-ray structure of various octahedral zirconium complexes and their catalytic properties in the polymerization of a-olefins are described. Benzamidinate, amido, allylic, and phosphinoamide moieties comprise the study ligations. For the benzamidinate complexes, a comparison study between homogeneous and heterogeneous complexes is presented. For the phosphinoamide complex, we show that the dynamic symmetry change of the complex from C2 to C2v allows the formation of elastomeric polymers. By controlling the reaction conditions of the polymerization process, highly stereoregular, elastomeric, or atactic polypropylenes can be produced. The formation of the elastomeric polymers was found to be the result of the epimerization of the last inserted monomer to the polymer chain. 

As an example for a one-dimensional property transfer, the preparation of optically active co-polymers using chiral template molecules is described. In this case, the optical activity in the polymer arises from the chirality of the configurational arrangement of the main chain (main chain chirality). Symmetry considerations show that there are several possible structures both for stereoregular homo- and for co-polymers in which this type of chirality can be expected [4]. 

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