Triad configurational sensitivity

There are only three unique triad combinations, mm, mr and rr thus a methyl configurational sensitivity to just nearest neighbor configurations would produce only three resonance in the methyl region of the C-13 spectrum. From an earlier spectrum of the amorphous polymer, we noted at least ten methyl resonances. We must therefore consider the situation where the next-nearest as well as nearest neighbor configurations are affecting the chemical shift, that is. 

The ester methyl resonance could in principle give the same information but in most solvents, as in chloroform, it is not configurationally sensitive). In Fig. 2, we see three a-methyl proton resonances having the same chemical shifts but very different intensities in each spectrum. From their measured areas, the triad probabilities are readily determined. 

The configurational sensitivity falls within a range from triad to pentad for most vinyl polymers. In non-crystalline polypropylenes, three distinct regions corresponding to methylene (46 ppm), methine (28 ppm) and methyl (20 ppm) carbons are observed in the C-NMR spectrum. (The chemical shifts are reported with respect to an internal tetramethylsilane (TMS) standard.) The spectrum of a 1,2,4-trichlorobenzene solution at 125 "C of a typical amorphous PP is shown in... 

The benzenoid C-l resonance of styrene units in acrylonitrile-styrene copolymers is particularly sensitive to the sequence of the chain relative configurations of triad sequences can be determined by quantitative evaluation of carbon-13 signals [524], Micro-structures of other vinyl polymers such as polystyrene [525], polypropylene oxide [526], and polyalkyl acrylates [527] have also been investigated by 13C NMR. 

For a given sample, the tacticity was found to be identical in the two solvents. For PMMA, the chemical shifts of the a-methyl protons are most sensitive to poljnner configuration. Therefore, triad tacticity information was obtained by integration of the a-methyl resonances. All polymer configurations are expressed in terms of triad tacticities. 

By NMR analysis of (acrylonitrile-methyl-methacrylate) copolymer (PAM), the methoxy resonance of the M units (M methyl-methacrylate) adjacent to A (A acrylonitrile) is shifted to lower magnetic fields [1]. This phenomenon is due to the electronegativity of the substituant (CN) of the A units [1]. The resonances of a-methyl protons of the polymethyl methacrylate (PM), which are quite sensitive to the configurational effects of isotactic (i), heterotactic (h), and syndiotactic (s) triad sequences, could have a very complicated pattern in NMR spectra of PAM. 

For a complete description of the monomer sequence distribution and relative stereochemical configuration of monomer sequences, at least in terms of N- and M-centred triads, it is necessary to take into consideration as many as 10 different triads with a central N unit, which may be magnetically distinguishable as is shown in the scheme of Figure 7.22. Since both monomeric repeating units have a quaternary pseudoasymmetric carbon, from a stereochemical point of view 10 triads with a central M unit sensitive to tacticity must also be considered. The statistical analysis of the monomer sequence distribution and of the stereochemical configuration of the copolymer sequences was carried under the following assumptions with respect to the chemical composition of copolymer sequences, it was assumed that... 

Gerken and Ritchey [190] reported the C-NMR of series of acrylonitrile-methyl methacrylate (A/M) copolymers and determined the copolymer microstructure. They reported the configuration probabilities of A/M copolymers and Bernoullian statistics were found to describe the monomer configuration, a result in conflict with the conclusions of Pham [191]. Gerken and Ritchey [190] assigned the resonance lines of carbonyl and nitrile carbons to different triad cotactic sequences on the assumption that M and A monomer units have the same sensitivity to the tacticity of an adjacent A monomer unit. Also they did not calculate cotactic triad fractions from C-NMR spectra and did not compare experimental cotactic triad fractions with those expected from configurational probabilities have been made. 

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