Nuclear magnetic resonance spectroscopy Ethylene - propylene copolymers

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. 

The study of ethylene and propylene copolymerisation, on vanadium and titanium catalysts of various compositions [70], led to the conclusion that studied catalytic systems contain two or three types of AC. This conclusion has been made as a result of the analysis of the MWD curves, carbon nuclear magnetic resonance spectroscopy analysis, and copolymers composition fractionation data. The analysis of a large number of copolymer fractions, produced by their dissolution in several solvents at various temperatures, has indicated the existence of several types of AC different both in stereospecificity and in reactivity. According to the authors of [70], a combination of copolymer fractionation results with gel chromatography data indicates the presence of two or three types of AC. 

Johnson-Plaumann and co-workers [1] used infrared and nuclear magnetic resonance spectroscopy (NMR) to quantitate the composition of various ethylene - propylene copolymers. Absolute concentrations were measured by IR spectroscopy. An IR calibration curve was obtained by plotting the absorbance ratio 6.82 pm/7.26 pm versus concentration of monomer units. 

The difficulty results, in part, from the fact that only a small fraction of the chemical bonds, generally less than one in a thousand, are involved in me-chanochemical processes. The concentration of connecting units is therefore at the detection limit and below for traditional analytical methods such as conventional nuclear magnetic resonance and infrared spectroscopy. The sensitivity can, of course, be enhanced by techniques such as cumulative, multiple scans, Fourier transform analysis, and difference techniques for detection to one part in ten thousand and better. It may yet be difficult to determine whether polymers are linked by chemical bonds or whether they are simply intimate mixtures. For this distinction, other tests can be of value. For example, the difference between blocks and blends for ethylene-propylene polymer systems has been distinguished by thermal analysis [5]. In many cases, simple extraction tests can distinguish between copolymers and blends. For example, for rubber milled into polystyrene, the fraction of extractable rubber is a measure of mechanochemistry. Conversely, only the rubber in this system is readily cross-linked by benzoyl peroxide after which free polystyrene may be conveniently extracted [6]. In another case, homopolymers of styrene and methyl methacrylate can be separated cleanly from each other and from their copolymers by fractional precipitation [7]. The success of such processes, of course, depends on both the compositions and molecular weights involved. 

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