Conformation tacticity effects

Raman spectroscopy has been widely used to study the composition and molecular structure of polymers [100, 101, 102, 103, 104]. Assessment of conformation, tacticity, orientation, chain bonds and crystallinity bands are quite well established. However, some difficulties have been found when analysing Raman data since the band intensities depend upon several factors, such as laser power and sample and instrument alignment, which are not dependent on the sample chemical properties. Raman spectra may show a non-linear base line to fluorescence (or incandescence in near infrared excited Raman spectra). Fluorescence is a strong light emission, which interferes with or totally swaps the weak Raman signal. It is therefore necessary to remove the effects of these variables. Several methods and mathematical artefacts have been used in order to remove the effects of fluorescence on the spectra [105, 106, 107]. 

If in the study of the polymers primary structure we did not especially take into account particular characteristic parameters of the compounds because "substitution effects" are so clearly marked, it is clear that in studying the secondary structure of the polymers, we must be very careful and critical, about the preparation and characterization of the compounds as far as the isomeric conformation, tacticity, and/or crystallinity... are concerned. It is indeed expected that the effects that must be experimentally evidenced will be (very) fine, and small in amplitude. It is the reason why the compounds, whose XPS spectra are discussed below, were synthesized and characterized by specialized laboratories in the world (practical informations will be given elsewhere (40, 41). 

Bcrnoullian statistics do not provide a satisfactory description of the tacticity. 6 This finding is supported by other work.28" 38 First order Markov statistics provide an adequate fit of the data. Possible explanations include (a) penpenultimale unit effects are important and/or (b) conformational equilibrium is slow (Section 4.2.1). At this stage, the experimental data do not allow these possibilities to be distinguished. 

Experimental and theoretical results are presented for four nonlinear electrooptic and dielectric effects, as they pertain to flexible polymers. They are the Kerr effect, electric field induced light scattering, dielectric saturation and electric field induced second harmonic generation. We show the relationship between the dipole moment, polarizability, hyperpolarizability, the conformation of the polymer and these electrooptic and dielectric effects. We find that these effects are very sensitive to the details of polymer structure such as the rotational isomeric states, tacticity, and in the case of a copolymer, the comonomer composition. 

Nuclear magnetic resonance (NMR) spectroscopy is a most effective and significant method for observing the structure and dynamics of polymer chains both in solution and in the solid state [1]. Undoubtedly the widest application of NMR spectroscopy is in the field of structure determination. The identification of certain atoms or groups in a molecule as well as their position relative to each other can be obtained by one-, two-, and three-dimensional NMR. Of importance to polymerization of vinyl monomers is the orientation of each vinyl monomer unit to the growing chain tacticity. The time scale involved in NMR measurements makes it possible to study certain rate processes, including chemical reaction rates. Other applications are isomerism, internal relaxation, conformational analysis, and tautomerism. 

Polymer Conformation and Crystallinity. Beyond the stereoregularity and tacticity, the geometrical conformation of the polymer chain in the solid material could influence its electronic structure, through a modification of its valence band molecular orbitals. Indeed, a few years ago, very characteristic band structures were calculated for T, G, TG, and TGTG polyethylenes ( ). More recently. Extended Huckel crystal orbital calculations showed that for isotactic polypropylene, a zig-zag planar or a helical conformation resulted in significant changes in the theoretical valence band spectra, supporting the idea that conformation effects could be detected experimentally by the XPS method ( ). 

It is claimed that although measurements of the lower limit are reproducible determination of the upper limit is quite a problem with errors of 2-3 nm in the calculations. Apparently above this limit solvent cast films appear to be immiscible whatever the polymer tacticity. It is suggested that changes in the donor/acceptor emissions with tacticity are simply due to an effect on the chain conformation of the probability of intermolecular interactions. As a final criticism it is shown that heating a monophase blend above the lower critical solution temperature does not actually result in a significant enough change in the donor/acceptor emission ratio to be able to detect phase separation. However, it should be pointed out that the studies of... 

Discussion of the effect of the polymer backbone in Sec. 3.4 of this chapter already provided examples of highly isotactic po-ly(f )-endo,exo-5,6-di [n-[4 -(4"-methox-yphenyl)phenoxy]alkyl]carbonyl bicy-clo[2.2.1]hept-2-ene s [190] and syndiotactic poly n-[4 -(4"-methoxyphenyl)phen-oxyjalkoxy methacrylate s [42, 44] which crystallize and form more ordered meso-phases than those of the corresponding atactic polymers (Fig. 15). Although more flexible backbones are more able to achieve the conformation necessary to order, the side chains are evidently already attached to the polymer backbone of these tactic polymers with the proper configuration to order, which obviates the need to distort their conformation for such purposes. 

This polymer is included here to illustrate the effect of parent PVA tacticity on the ring conformation of the acetal. ... 

On the methodological front of these broadly based endeavors, we have exploited pericyclic processes such as the dipolar cycloadditions of nitrile oxides together with the aldol reaction and related constructions as tactical devices for the formation of new carbon-carbon bonds with high levels of stereochemical control Another important focus of these explorations has been upon the development of techniques for the manipulation and refunctionalization of hydropyrans, since this structural subunit is not only common to a variety of natural substances, but it may also be effectively exploited as a conformationally-biased template for the stereoselective construction of various skeletal arrays present in numerous natural products. In this context, we have devised a novel and highly effective strategy for the asymmetric syntheses of oxygenated natural products. The fundamental approach features the intermediacy of the hydro-3-pyranones 12, which may be accessed from the chiral furfuryl carbinols 10 via the hydroxy enediones 11 by well-established oxidative techniques (Scheme 1). A critical element of this overall planll is that the hydro-3-pyranones 12 are admirably endowed with differentiated functionality that is suitable for further elaboration by reaction with selected nucleophiles... 

To account for the effect of monomer structure on the tacticity of the polymer, Natta et al. (1963b) presented the following interpretation. Complexing of the monomer with catalyst was thought to occur with only one double bond, even for both isomeric 1,3-pentadienes which afford cis-1,4-polydiene. The idea of coordination of two double bonds was rejected for steric reasons. This cis isomer (LXXI) of 1,3-pentadiene should be too hindered in the cis conformation for double coordination. Furthermore, if double coordination of the tram isomer (LXXII) is evoked to explain the stereochemical results, then butadiene, which is even less hindered, should polymerize by 1,4-addition. 

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. 

Conformational factors. The most important conformational factor is the tacticity of vinyl-type polymers. A polymer such as poly(methyl methacrylate) can have quite different values of Tg, depending on whether it is isotactic, syndiotactic, or atactic. See Table 3 for a collection of literature data (1) on the effects of tacticity on Tg. A theoretical analysis of the effects of tacticity variations on Tg has been provided (52). 

Abe et ah have studied the conformational characteristics of a series of poly[oxy(-l-alkylethylenes)]. In the first paper a comparison of the statistical weights of various conformations in poly(propylene oxide), deduced from the solution properties of isotactic and atactic chains, were compared with conformational weights derived from a potential-energy surface, and an anomeric effect identified with a discrepancy between the two. Flexible side-chains influence the main chain in a manner which is expressible in the statistical weight matrices. The dimensions of these chains are strongly influenced by their tacticity. Polymer chains containing both sulphur and oxygen heteroatoms have received the attention of... 

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