Stereosequence microstructure

Figure 1. 50.31 MHz I3C NMR spectra of PVC (a) and two partially reduced PVC s, E-V-84 (b) and E-V-21 (c). Please note the table of E-V microstructural designations in the upper right-hand corner of the Figure, where 0,1 = CH2, CHC1 carbons. Resonances correspond to underlined carbons. The assignment of different stereosequences is given in reference 2.

The physical properties (7-10) of our E-V copolymers are sensitive to their microstructures. Both solution (Kerr effect or electrical birefringence) and solid-state (crystallinity, glass-transitions, blend compatibility, etc.) properties depend on the detailed microstructures of E-V copolymers, such as comonomer and stereosequence distribution. I3C NMR analysis (2) of E-V copolymers yields microstructural information up to and including the comonomer triad level. However, properties such as crystallinity depend on E-V microstructure on a scale larger than comonomer triads. 

Conformational energies are calculated for chain segments in poly(vlnyl bromide) (PVB) homopolymer and the copolymers of vinyl bromide (VBS and ethylene (E), PEVB. Semlempirical potential functions are used to account for the nonbonded van der Waals and electrostatic Interactions. RIS models are developed for PVB and PEVB from the calculated conformational energies. Dimensions and dipole moments are calculated for PVB and PEVB using their RIS models, where the effects of stereosequence and comonomer sequence are explicitly considered. It is concluded from the calculated dimensions and dipole moments that the dipole moments are most sensitive to the microstructure of PVB homopolymers and PEVB copolymers and may provide an experimental means for their structural characterization. 

Rinaldi et al. introduced the use of correlations for the characterization of microstructures in fluoropolymers by demonstrating that highly dispersed 2D- F, C HMQC and HMBC spectra allowed the identification of different triads and pentads in a 1-chloro-l-fiuoroethylene/isobutylene copolymer, and 3D- H, C, F experiments permitted assignment of the tacticity in polymeric l-chloro-l-fluoroethylene. The latter experiment is interesting because it exploits the unique chemical shift dispersion of F nuclei for the separation of resonances originating from different stereosequences, and then uses the cross-peak multiplicities in the slices... 

Microstructures of Poly(chlorofluoroethylene)s The carbon-13 NMR spectrum of PVCF consists of a —CH2— resonance at 54.1 ppm and a —CFC1— resonance at 108.8 ppm. There is no splitting of these lines due to tacticity, nor are there any other resonances to indicate the presence of regioirregular monomer sequences. However the polymer is stereoirregular, as shown by the fluorine-19 NMR spectrum in Figure 1. There are three principal resonances spread by 3 ppm owing to triad stereosequences, with some pentad fine structure which is barely resolved. 

The possible stereosequences of the middle triads of the PCFEI pentads are listed below each of the pentads in Table 1. Triads composed only of isobutylene units do not contain stereogenic centers, and therefore do not exhibit stereosequence effects in their NMR spectra. Even a relatively simple copolymer such as PCFEI is a mixture of over 30 microstructures, thus... 

The NMR spectra presented in Fig. 20.9 [11] for the same PP samples whose H NMR spectra appear in Fig. 20.8 make the superior microstructural sensitivity of NMR plainly evident. While resonances are spread over an 30 ppm range, aU H resonances observed for PPs are within < 1 ppm of each other. In addition, the absence of homonuclear ( C- C) and the easy removal of hetero-nuclear ( C- H) scalar couplings further simplify the spectra. Both of these advantages result in the kind of microstructural sensitivity seen in the methyl carbon region of the PP spectra note that in atactic PP sample all ten possible pentad stereosequences mmmm, rrrr, mmrm, etc.) are distinctly observed. (Also see in Fig. 20.10 an expansion of the methyl region of the atactic PP spectrum observed at a higher magnetic field, which we will subsequently discuss.)... 

NMR spectrum of atactic PP were sensitive to pentad stereo-sequences. At 90.5 MHz (see Fig. 20.10), the methyl carbon resonances show sensitivity to heptad stereosequences (rmnmmmm, rrrrrr, mrmmrr, etc.) [13]. The NMR spectra of PPs are sensitive to stereosequences extending over 4 (pentads) and 6 (heptads) bonds in both directions along the PP backbone. This long-range sensitivity to microstructural detail makes NMR a valuable tool in the determination of polymer structures. 

In the case of inherently more complicated stereoblock polypropylenes containing (close-to-)atactic blocks, with all possible stereosequences present in comparable amounts, the requirements for a meaningful microstructural analysis are even more stringent. Routine NMR characterization of propylene polymers gives access to the stereosequence distribution at the pentad... 

Rricheldoif et al. [59] proposed assignments for the stereosequences of the known microstructures of PLA, which were more recently confirmed by Zell et al. [61]. Figure 8.13 presents the and C spectra of PLA synthesized using 5%... 

Figure 3.6 shows the spectrum of free-radical polymerised poly(methyl methacrylate), a polymer in which the fractions of m and r dyads are approximately equal [3]. The spectrum differs considerably from that of the isotactic polymer, and comparison of Figures 3.5 and 3.6 illustrates the power of solution NMR for the study of polymer microstructure. The results can provide information about reaction mechanism if the stereosequence peaks can be assigned. The traditional assignment methods rely on spectral comparisons with model compounds... 

By achieving agreement between the observed chemical shifts and those predicted by the y-gauche effect method, we have not only determined the microstructure (stereosequence) of this polymer, but in addition we have... 

Carbon-13 NMR generally offers the potential for greater spectroscopic resolution than HNMR and might be expected to be better suited for the analysis of PPO microstructure [43-47]. Tto expectation is realized for re-gioregular (all H-T) PPO, where CH and CH2 carbon resonances are separated by 2 ppm, and permits the unambiguous assignment [45] of PPO stereosequences. However, as we will demonstrate here, the methine and methylene carbon resonances in regioirregular (H-T, H-H, T-T) PPO do overlap [48]. 

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