Monomer sequences, assignments resonances

Carbon-13 spectroscopy has been used very effectively by Corno and coworkers [115-117] to characterize the distributions of monomer sequences in copolymers derived from episulfides using anionic catalysts. Although chiral monomers were not employed in these studies, it is worth noting that tacticity effects had a relatively small effect on the resonance patterns observed, but that the chemical shifts of in-chain carbon atoms in different sequences were s ibstantially different. On the basis of assignments and empirical shift parameters developed by Corno, et al., the spectra of stereoregular ethylene sulfide-propylene sulfide copolymers and propylene sulfide-isobutylene sulfide copolymers should be readily analyzed. Studies on copolymers derived from racemic monomers indicate them to have random structures a similar result can be e3q>ected for copolymers derived from optically active monomers. 

Figure 5.1 shows typical NMR spectra (methylene and methine regions) of the copolymers (THF-soluble fraction) prepared by [Me2Si(C5Mc4)(N Bu)] TiCL, (l,2,3-Me3C5H2)TiCl2(0-2,6- Pr2C6H3) catalysts in the presence of a MAO cocatalyst [13b]. Table 5.1 also summarizes the assignments of resonances for poly(ethylene-co-styrene) in the NMR spectrum based on the distortionless enhancement by polarization transfer (DEPT) spectrum and data reported previously [12-18], and monomer sequences in the copolymer are shown in Scheme 5.1. As described below, the naicrostructures for the resultant poly(ethylene-co-styrene)s depend on the catalysts used. As shown in Figure 5.2, the glass transition temperature (Tg) as measured by DSC increased with an increase in the styrene content (-8.1 to 58.3 °C). This is because, as...

Figure 7.9 and Figure 7.10 show the expanded C-NMR spectra of the carboxyl carbon and the Cj carbon of phenyl in the copolymer, respectively. The split resonance peaks for one kind of carbon make it possible to investigate the monomer sequence distribution. The triads were assigned as shown in Figures 7.9 and 7.10. 

Kapur and Brar [192] report the C-NMR spectra of a series of acrylonitrile-methyl methacrylate copolymers. The primary structure, including monomer composition, monomer sequence distribution and triad tacticity of A/M copolymers were determined on the basis of C[ H]-NMR analysis and compared with the calculated fractions. The resonance of carbonyl and nitrile carbons were assigned to different cotactic triads by considering the possible electronic interactions between the central monomer unit and its immediate neighbours. 

The T resonances in the 73-74 ppm region have multiple-bond correlations in the HSQC-TOCSY spectmm to proton resonances of S and/or S , methylenes. Therefore, they are attributed to Xm groups such as those found in stmcture 20. Analysis of these data provided resonance assignments for all the S and T type carbons for the stmctures in Scheme 1. Detailed analysis of expansions of the peak-containing regions of the 2D-NMR data provided complete resonance assignments for all monomer sequences up to the tetrad level in poly(EV). 

A question posed by Wong and Poehlein [53] regarding the monomer system of styrene-acrylic acid is whether sequence distribution information can be determined in NMR spectra. In this work, they examined and C-NMR spectra of S-AA copolymers to determine the resonances sensitive to the copolymer microstructure. The compositions of the copolymers were measured by NMR and the reactivity ratios calculated. The triad sequences were assigned by experiments and the Alfrey-Mayo (AM) statistics-kinetics model. 

Wong and Poehlein [53] concluded that the sequence distribution of styrene-acrylic copolymers can be measured by C-NMR of the carboxyl carbon and the Cj carbon of phenyl in S-AA copolymers. Low-conversion copolymer composition data obtained by NMR at different initial monomer ratios were used with the Kelen-Todos plot method to determine the reactivity ratios of ra = 0.13 and rs = 0.38. The resonance peaks split by triads were assigned and confirmed by comparing experimental triad values with those calculated from the Alfrey-Mayo statistics-kinetics model. 

A study of the NMR spectra of (acrylonitrile-methyl methacrylate) copolymers (PAM), with very low percentages in M monomer, allows a quantitative determination of AMA and MMA or AMM triad sequences from the methoxy resonances. The resolution of the complex pattern of the a-methyl resonances of isolated M units has been attempted. If our assignments are correct, the analysis of the cotactic pentad sequences (with a central M unit) has revealed that the configurations of the copolymer chain do not follow the Bernouillian or the first order Markoffian statistics. 

Brown and Cudby [6] and Randall [7] have analyzed the C-spectra of a series of propylene-butene-1 copolymers prepared using an isospecific catalyst system. The enchainment of the monomer units was essentially isotactic and head-tail. Resonances observed for the homopolymers, assignments made previously by Fish and Dannenberg [78], the Grant-Paul relationships and variations in resonance intensity with copolymer composition were used to make assignments for the resonances of methine, methylene and methyl carbon atoms. Triad and some tetrad sequence distribution measured from the spectra were consistent with Bernoullian distributions over the entire range of copolymer compositions examined. 

In order to be useful for structure analysis, the observed resonances must be assigned to chemical structures. Eor copolymer analysis, the enormity of the problem of assigning the observed resonances can be recognized by the following simple considerations. For the 21 most common vinyl and vinylidene monomers, there are a total of 210 possible binary copolymers and 1330 ternary copolymers [29]. These copolymer combinations can also have an alternating, random, or block sequence structure so that the total number of possible copolymers is quite large. Experimentally, only about 30 of these copolymers have been studied. This extremely complex situation suggests that computer simulation of the spectra of the copolymers is helpful in the interpretation of the NMR spectra [30]. 

Additionally, three resonances are observed for the a-methyl proton resonances which exhibit substantially different relative intensities between the two polymers. These three resonances are assigned to the stereoregular triad sequences. Those monomer units that are flanked on both sides by units of the same configuration are termed isotactic triads (i). Those monomer units that have units of opposite configuration on both sides are syndiotactic triads (s). Those monomer units that have a unit of the same configuration on one side and a unit of opposite configuration on the other side are heterotactic triads (h). These three triad a-methyl resonances have the same chemical shifts in the spectra of the different stereoregular polymers but have very different intensities in each spectrum. The relative intensities of these a-methyl proton triad resonances provide a measure of the triad probabilities. 

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