Pentad configurations

Homopolymers.—Vinyl Monomers and Derivatives. Robinson et al. have described an exhaustive study of the measurement of poly(vinyl chloride) tacticity by H n.m.r. It was found that different solutions of the same polymer were less reproducible than repeat runs on the same solution i.e. sampling errors were greater than instrumental factors. Overall, one must use several samples for maximum precision, and combine results from both CH and CHj signals. The sensitivity of C resonance frequencies to small structural changes is illustrated by reports of the resolution of configurational pentads in polymers of... 

When the chain migrates from one site to the other without insertion we obtain two stereocentres of the same absolute configuration and our pentad analysis will show one meso (m) in the series of (r) relationships. 

An inspection of the number of unique configurations taken five at a time (pentads) shows that there are ten unique pentad configurations mmmm, mmmr, rmmr, mmrm, mmrr, rmr, rmrr, mrrm, mrrr and... 

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-... 

This amorphous polypropylene, therefore, has a tendency toward short blocks of meso and racemic sequences. This structural conclusion is not readily apparent from a simple inspection of the pentad distribution although the dominant pentads are mmmm, mmmr, rrrr, rrrm and mmrr as indicated by the above formula for the average configurational structure. 

Zambelli, Lxrcatelli, Bajo, and Bovey (143) proposed assignment of the pentads by comparison with the spectra of two diastereomeric mixtures of the 3,5,7,9,11,13,15-heptamethylheptadecane enriched in C on the 9-methyl. In the synthesis, which started from optically active precursors of known configuration, a selection of the stereoisomers occurred, which permitted the assignments to be made by means of intensity criteria. These authors reached the same conclusions as Stehling and Knox. A parallel analysis was effected with the methylene subspectrum by using 2,4,6,8,10,12-hexamethyltridecane enriched in C at position seven (144, 145). 

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. 

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],... 

Diads (triads, pentads) View of two (three, five) combined monomer units at any position in a polymer chain and investigating (e.g., by 13C NMR spectroscopy) the configuration of the tertiary carbon atoms. The term meso is used if they have the same configuration the term racemo is used if they have the opposite configuration. 

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