Nuclear magnetic resonance spectroscopy monomer sequence distribution

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. 

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 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. 

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). 

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