Nuclear Magnetic Resonance monomer sequences

High-resolution nuclear magnetic resonance spectroscopy, especially 13C NMR, is a powerful tool for analysis of copolymer microstructure [Bailey and Henrichs, 1978 Bovey, 1972 Cheng, 1995, 1997a Randall, 1977, 1989 Randall and Ruff, 1988], The predicted sequence length distributions have been verihed in a number of comonomer systems. Copolymer microstructure also gives an alternate method for evaluation of monomer reactivity ratios [Randall, 1977]. The method follows that described in Sec. 8-16 for stereochemical microstructure. For example, for the terminal model, the mathematical equations from Sec. 8-16a-2 apply except that Pmm, Pmr, Pm and Prr are replaced by p, pi2, p2j, and p22. 

C nuclear magnetic resonance spectroscopy can be employed to study changes in copolymer sequence distribution brought about by differences in monomer feed profiles. Sequence distributions characteristic of conventional, staged, and power-feed copolymers are easily distinguishable in a model system of the type described here. 

Carbon-13 nuclear magnetic resonance was used to determine the molecular structure of four copolymers of vinyl chloride and vinylidene chloride. The spectra were used to determine both monomer composition and sequence distribution. Good agreement was found between the chlorine analysis determined from wet analysis and the chlorine analysis determined by the C nmr method. The number average sequence length for vinylidene chloride measured from the spectra fit first order Markovian statistics rather than Bernoullian. The chemical shifts in these copolymers as well as their changes in areas as a function of monomer composition enable these copolymers to serve as model... 

If you have been working your way through this epic in a more or less linear fashion, then you might have started to ask yourself some fundamental questions such as, How do you know if a vinyl polymer is isotactic, or atactic, or whatever How do you know the composition and sequence distribution of monomers in a copolymer How do you know the molecular weight distribution of a sample This last question will have to wait until we discuss solution properties, but now is a good point to discuss the determination of chain microstructure by spectroscopic methods. The techniques we will discuss, infrared and nuclear magnetic resonance spectroscopy, can do a lot more than probe microstructure, but that would be another book and here we will focus on the basics. 

Carman,C.J., Wilkes,C.E. Monomer sequence distribution in ethylene-propylene elastomers. I. Measurement by carbon-13 nuclear magnetic resonance spectroscopy. Rubber Chem. Technol. 44,781-804 (1971). 

Nuclear magnetic resonance (NMR) is a physical process in which nuclei in a magnetic field absorb and reemit electromagnetic radiation. Analysis of NMR spectra allows the determination of polymer composition, and the distribution of monomer units can be deduced from the diad and triad sequences by NMR spectral analysis. For characterization of polymer, the extracted polymer wiU be dissolved in CDCI3 followed by NMR analysis. The NMR spectrum for PHB shows three characteristic signals. A doublet at 1.53 ppm represented the methyl group (CH3) coupled to one proton while a doublet of... 

Carbon-13 nuclear magnetic resonance (NMR) spectroscopy represents the only direct method to analyze the polymer regio- and stereostructure (isotacticity level). The structural information derived from this technique is not limited to diads (sequence of two monomer imits) but generally includes triads or pentads and, in some cases, nonad or undecad levels have been reached, as well. Nevertheless, it is important to keep in mind that these values are only mean values referring to the whole polymer sample and, in some cases, they are not sufficient to discriminate samples with a different distribution of defects (see later). 

Nuclear magnetic resonance has become a very powerful, versatile tool for studying the structures of polymers and copolymers. The value of this technique is that it enables one to measure the relative concentrations of nuclei in particular environments in polymer chains. The technique is very discriminating and can be used, for example, to determine the relative concentrations of groups in triadic (e.g., BBB vs. ABA) or pentadic (e.g., BBBBB vs. Ab6bB) environments in A —B copolymers. In addition, nuclear magnetic resonance can measure the relative amounts of various monomer junctions (diads, e. g., B — B or A — B) or of particular monomer sequence junctions (tetrads, e.g., BB —AB, or hexads, e.g., BBB —AAA), provided the junctions contain nuclei that can be studied. 

Nuclear magnetic resonance spectroscopy. Ethylene-propylene copolymers can contain up to four types of sequence distribution of monomeric units. These are propylene to propylene (head-to-tail and head-to-head), ethylene-propylene and ethylene to ethylene. These four types of sequences and the average sequence lengths of both monomer units, i.e., the value of n, below can be measured by the Tanaka and Hatada [15] method. 

---