Monomer sequences, assignments

The unique coupling patterns and chemical shifts observed for many sequences enabled the proton monomer sequence assignments to be made unambiguously at... 

Another serious problem in applying these methods is that unambiguous assignments of N VIR signals to monomer sequences are, as yet, only available for a few systems. Moreover, assignments are complicated by the fact that the sensitivity of chemical shifts to tacticity may be equal or greater than their... 

A general purpose program has been developed for the analysis of NMR spectra of polymers. A database contains the peak assignments, stereosequence names for homopolymers or monomer sequence names for copolymers, and intensities are analyzed automatically in terms of Bernoullian or Markov statistical propagation models. A calculated spectrum is compared with the experimental spectrum until optimized probabilities, for addition of the next polymer unit, that are associated with the statistical model are produced. 

The program will be demonstrated with poly(vinyl alcohol) for tacticity analysis and with copolymer vinylidene chloride isobutylene for monomer sequence analysis. Peak assignments in C-13 spectra were obtained independently by two-dimensional NMR techniques. In some cases, assignments have been extended to longer sequences and confirmed via simulation of the experimental data. Experimental and "best-fit" simulated spectra will be compared. 

Figure 2.8 The structure of the dimeric cytochrome bcomplex of the respiratory chain, (a) The cave for chemistry constituted by the hollow between the two monomers (the essential dimer ) in a cartoon representation. Reprinted with permission from Smith, 1998. Copyright (1998), American Association for the Advancement of Science, (b) The structure viewed perpendicular to the twofold axis and parallel to the membrane. All of the eleven subunits are completely traced and their sequences assigned. The top of the molecule extends 3.8 nm into the intermembrane space, the middle spans the membrane (4.2 nm), and the bottom extends some 7.5 nm into the matrix. Reprinted with permission from Iwata et al., 1998. Copyright (1998) American Association for the Advancement of Science.

The properties of a copolymer depend on its composition, monomer sequence and stereochemical structure. Although compositional analysis can be achieved by several methods other than NMR spectroscopy, quantitative data on monomer sequence distribution can only be obtained from NMR spectroscopy. I3C NMR chemical shifts of C=0 carbons of PMMA are sensitive to pentad to heptad stereochemical sequences. The C=0 carbon signals for the copolymers of methacrylates are also sensitive to triad comonomer sequence. Thus it should be difficult to assign both tactic and comonomer sequence signals, especially in the case of copolymers with low stereoregularity. 

The C=0 carbon spectrum of MMA and butyl methacrylate copolymer exhibits similar spectral features, that is, splittings due to both monomer sequence and stereosequence,213 214 although the signal assignments for the 25-MHz 13C NMR spectra based on only monomer sequence distribution215 had been made erroneously. 

NOESY) has been successfully used to assign the monomer sequence of this copolymer.219 For example, in the WVVI pentad shown below,... 

General Features of PVF Spectra The proton- and fluorine-decoupled 22.62 MHz carbon-13 NMR spectra of PVF prepared from PVCF (a) and commercial PVF (b) are shown in Figure 3. There are five additional peaks present in spectrum (b) from the commercial polymer which are absent in spectrum (a). These are due to aregic monomer sequences, which have been assigned according to Tonelli et al. (16). Monomer sequence triads are resolved and are denoted by the binary regiosequence pentad notation in Table 1 (l = CFH, O = CH2). 

The significance of a monomer sequence distribution is higher, the longer is the sequence length that can be attained. The complete assignment of spectrum of regio-regular... 

The NMR technique is also proving valuable, for example, in its application in assigning relative rates of attack in copolymerization. Moad et alM has shown that labelled monomer can be very useful in studying the NMR of polymers. NMR of PMMA prepared from MMA carbonyl- 3c has proved to be very convenient in the determination of the tacticity of homopolymers while NMR of copolymers prepared from labelled monomers can rapidly provide infomiation on monomer sequence distribution. 

The second method to improve the accuracy of the reactivity ratios is the use of other types of data than copolymer composition. If the monomer sequence distribution is measured as a function of comonomer feed ratio, the accuracy will usually be larger than in the case of copolymer composition versus comonomer feed ratio. One of the frequentiy used methods is the measurement of so-called triad fiactions. For example, in a copolymer of STY (S) and BA (B), the STY-centered triads are SSS, BSS + SSB, and BSB. Their fractions can usually be measured from NMR measurements. The assignments in the NMR spectra are often difficult to make, and quantitative NMR measurements can be quite tedious. In some cases even special techniques such as distortion enhancement by polarization transfer (DEPT) NMR need to be used in order to circumvent... 

Laser Raman spectroscopy has been proposed as a useful technique for probing the microstructure of copolymers. Good correlations were found between the concentrations of some isolated, dyad, triad and tetrad comonomer sequences in vinyl chloride/vinylidene chloride copolymers and certain scattering intensities [99]. The positions and intensities of particular absorption bands have also been correlated with chain microstructure in an infrared study of ethylene/vinyl chloride copolymers, previously characterised by C-NMR analysis [100]. More recently, FTIR spectra have been analysed for monad, dyad and triad monomer sequence-distribution dependencies in random styrene/acrylonitrile copolymers [101]. Changes in peak intensities from normalised spectra were correlated with microstructure probabilities assignments were given if there existed a linear relationship between peak intensity and the number fraction of a microstructure. 

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...

NMR techniques have been widely used to study copolymer composition and monomer sequence distribution. In the 1960s, Bovey used a statistical method to assign the sequence distribution of methyl methacrylate - styrene (MMA-S) and MMA-a-methylstyrene (MMA-a-MS) copolymers from H-NMR data [82]. Ito and co-workers studied the coisotacticity of MMA-S [83], MMA-a-MS [84], MMA-methyl acrylate (MMA-MA) [85] and vinylidene chloride - vinyl acetate (VC-VA) [86] copolymers by H-NMR. 

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). 

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