Configuration sequences

Look at (-)-laclic acid in Figure 9.8 for an example of how to assign configuration. Sequence rule 1 says that —OH has priority 1 and -H has priority 4, but it doesn t allow us to distinguish between -CH3 ancl -CCbH because... 

MMA polymerization is one of the most studied systems and was thought to be explicable, within experimental error, in terms of Bemoullian statistics. Moad et ai.jb have made precise measurements of the configurational sequence distribution for PMMA prepared from 13C-labeled monomer. It is clear that... 

NMR Characterization of the Compositional and Configurational Sequencing of Tri-ra-butyltin Polymers... 

Nuclear magnetic resonance has proved to be a valuable tool in determination of configurational sequences in poly(MMA) (14). In Figure 3 is shown the NMR of poly(MMA) synthesized with an anionic polymerization catalyst known to produce predominantly isotactic sequences. In these polymers, the NMR spectrum of the methylene units In the polymer backbone gives an unequivocal determination of tacticity. The methylene signal, occurring about 1.8... 

Ong SA, Toorisaka E, Hirata M et al (2008) Granular activated carbon-biofilm configured sequencing batch reactor treatment of C.I. Acid Orange 7. Dyes Pigm 76 142-146... 

Since about 1960 nuclear magnetic resonance (NMR) spectroscopy has become an important tool for the study of chain configuration, sequence distribution and microstructure of polymers. Its use started from early broad-line studies of the one-set of molecular motion in solid polymers and passed through the solution studies of proton NMR, to the application of the more difficult but more powerful carbon-13 NMR methods to both liquids and solids. 

The new configurational sequences, mmmr, mmrr and mrrm will be produced from the isotactlc polymer by Isolated Inversions of configuration. The relative intensities for the mmmr, mmrr and mrrm pentads will be 2 2 1 which allows a positive Identification of the mrrm pentad. Correspondingly, the predominantly syndio-tactlc polymer will have mm triads Inserted into rr.r se-... 

Note Configurational sequences comprising two configurational units are termed diads, those with three such configurational units triads, and so on. In order of increasing sequence lengths they are called tetrads, pentads, hexads, heptads, octads, nonads, decads, undecads, etc. 

The configurational sequence and stereosequence coincide in this particular case because there is only one site of stereoisomerism in each constitutional repeating unit (compare Definitions 2.1.3 and 2.1.4). 

Configurational sequence in which the relative or absolute configuration is defined at all sites of stereoisomerism in the main chain of a polymer molecule. 

High magnetic fields and in particular C-NMR spectroscopy allow the analysis of even longer configurational sequences (tetrads up to nonads). This proved to be important in particular for the analysis of polyolefins like polypropylene or cycloolefin copolymers (COC). These polymers are available via transition-metal mediated (Ziegler-Natta, metallocene) insertion polymerizations, and the configurational analysis provides deep insight into the respective polymerization mechanisms as well as into the structure-property relationships. 

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],... 

For simple poly(a-olefins) of practical importance, the chain stereostructure can be quantitatively determined at least at the stereotriad level [1], Longer configurational sequences can be observed by nuclear magnetic resonance, particularly using larger magnetic fields or 13C NMR spectroscopy or both. The chemical shifts due to CH3,CH2 and CH carbons are widely spaced. It is mainly the chemical shift of the pendant CH3 group carbon that is utilised for sequential analysis. 

Studied in ref. [26] is the effect of HFC reaction conditions on the configuration sequences in POCS-4. Since the mesomorphous state in PMCS-4 is formed in stereoregular trans-tactic polymers only [27, 32] and spatial configuration of initial monomers is not always fully preserved in poly-mers, the effect of HFC conditions on transformation of =Si-CI and =Si-OH centers in initial corn-pounds has been studied. The detected fact of cyclosiloxanes partial inversion at Cl atoms substitution at silicon was expected, as reported before [33, 34], More detailed description of reflex correlation was carried out in ref. [35], Symbols and mark projections of units and bonds to the pla-ne perpendicular to the cycle plane. 

The effect of chain tacticity on the phase states of POCS-6 has been studied. On the example of two POCS-6 polymers, the influence of configuration sequences on thermal transitions has been investigated [52], For this purpose, cis- and trans-isomers of 1,7-dihydro-xydecamethylcyclohexasiloxane, 1,7-dichloro-1,7-dipheny 1-3,3,5,5,9,9,11,11 -octamethyl-... 

The effect of molecular mass on occurrence of the mesomorphous phase in POCS-6 polymers is de-tected. Results displayed for POCS-6 polymers, in which configuration sequences of units alternate irregularly along the backbone, indicate that the ability to crystallization in them is disturbed. For the mesomorphous phase, strict regularity of units is not the necessary condition of its occurrence. In connection with the difference between studied POCS-6 polymers by the type of tacticity and mole-cular mass, the authors have separated effects induced by these two factors and detected more accu-rate, which type of configuration sequences creates conditions for mesophase stabilization. 

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