Copolymers spectroscopic methods, composition

NMR and IR are powerful spectroscopic techniques, which provide additional information about the compositional details of a sample. However, they are often unable to differentiate between a polymer blend A + B and a copolymer consisting of A and B. For such complex polymer compositions a combination of liquid chromatography and spectroscopic methods is helpful. In his recent review Pasch [57] discusses a couple of examples. 

Quantitative analysis of copolymers is relatively simple if one of the comonomers contains a readily determinable element or functional group. However, C,H elemental analyses are only of value when the difference between the carbon or hydrogen content of the two comonomers is sufficiently large. If the composition cannot be determined by elemental analysis or chemical means, the problem can be solved usually either by spectroscopic methods, for example, by UV measurements (e.g., styrene copolymers), by IR measurements (e.g., olefin copolymers), and by NMR measurements, or by gas chromatographic methods combined with mass spectroscopy after thermal or chemical decomposition of the samples. 

When only spectroscopic methods are used, they are able to identify polymer components with respect to their chemical nature. However, in many cases, they are unable to answer the question whether two chemical structures are combined to yield a copolymer or a blend or both. For example, analyzing a rubber mixture one is able to identify styrene and butadiene as the monomer units. However, using FTIR or NMR it is impossible to decide if the sample is a mixture of polystyrene (PS) and polybutadiene (PB),or a copolymer of styrene and butadiene, or a blend of a styrene-butadiene copolymer and PB. For the latter case, even the copolymer composition cannot be determined just by running a FTIR or NMR spectrum. 

Obviously, what we would really like to do is not just have a feel for tendencies, useful as this is, but also calculate copolymer composition and sequence distributions, things that can also be measured by spectroscopic methods. We will start by using kinetics to obtain an equation for the instantaneous copolymer composition (it changes as the copolymerization proceeds). Later we will use statistical methods to describe and calculate sequence distributions. In deriving the copolymer equation, we only have to consider the propagation step and apply our old friend, the steady-state assumption, to the radical species present in the polymerization, and... 

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. 

Molecular weight distribution information obtained by size-exclusion chromatography on its own is insufficient to characterize the properties of complex polymers, such as copolymers and block and graft polymers [23,514,524]. For these polymers the chemical composition and functionality type distributions are equally important. A major obstacle to the characterization of these materials is that their molecular properties are present as joint distributions. Unlike the mass distribution the composition and functionality distributions can only be determined by separation methods that employ interactions with the stationary phase. To fully characterize a complex polymer it is not unusual to use manual or automated tandem techniques where the sample is fractionated according to its chemical or end group composition for subsequent further separation by size-exclusion chromatography to establish their mass distribution. Chromatographic methods may also be combined with spectroscopic methods to determine microstructural information. 

The composition of carbon-chain polymers with monomeric units having widely differing analytical composition, characteristic elements or groups, or radioactive labels can be readily determined. Chemical (microanalysis, functional group determination, etc.) and spectroscopic methods (infrared, ultraviolet, nuclear magnetic resonance, etc.), as well as the determination of radioactivity, yield the average composition of the polymer. The mean composition can also be determined from the refractive indices of solid samples. The composition can be calculated from the principle that the copolymer is considered to be a solution of one unipolymer (from one of the monomeric units) in the other. The composition can also be found by means of the refractive index increment dw/dc in solution, which gives the variation in refractive index with concentration. The mass fraction of the monomeric unit A can be calculated from... 

In order to calculate a copolymerization reactivity ratio, it is first necessary to determine the composition of the copolymer or of the unconverted monomer mixture (or both). Elemental analysis, spectroscopic methods (IR, UV, NMR), refractive index determination, or turbidimetric titration can be suitable for determining the copolymer composition. 

All spectroscopic methods allowing the identification of chemical structures and the quantitative determination of identified chemical functions can be used to determine the composition of a copolymer. Nuclear magnetic resonance is by far the most used method for this purpose, but infrared and Raman spectroscopy can also be used. 

Copolymer Systems.—Copolymerization of c-caprolactara and rMlodecalactam using sodium and 7 -acetyl-e-caproIactam at 160 °C was studied and copolymer composition evaluated by spectroscopic methods. Copolymerization constants were determined and used to calculate the intramolecular distribution of the respective units in the chains. This method has also been used to prepare copolyamides in a single stage process. ... 

For a detailed analysis of olefin copolymers or polyolefin blends it is important to determine the CCD in addition to the MMD. The bulk chemical composition of polyolefins can be determined quantitatively by FTIR or NMR spectroscopy. Dual information on the chemical composition as a function of molar mass can be obtained when HT-SEC is directly coupled to these spectroscopic methods. Interfacing SEC with H-NMR is a cost-intensive option for the understanding of molecular structure as a function of separation. It has, however, the advantage... 

One of the issues when monitoring an emulsion polymerization reactor is selection of the most appropriate technique [124, 126]. For instance, monomer conversion and copolymer composition can be monitored on-line by means of densim-etry, refractive index, gas chromatography, calorimetry, ultrasound, fluorescence, ultraviolet reflection, and other spectroscopic methods such as Raman, mid-range infrared, and near-infrared. 

The properties of some polymers are dependent on their microstructure for example isotactic polypropylene is crystalline whereas atactic polypropylene is amorphous. Microstructure effects are also exemplified by polybutadienes, where the mode of addition of the diene to the growing chain leads to 1,2-addition, 1,3-addition and 1,4-addition, which may be as or trans. The fraction of different addition species changes the mechanical properties of the polymer. Another example is provided by the chemical composition of a copolymer and its sequence distribution, which together determine its ultimate properties. It is thus of great importance to be able to characterize polymer micro structure. This is generally done using spectroscopic methods, specifically infrared spectroscopy and nuclear magnetic resonance spectroscopy. 

In the study of anionic copolymerization it is possible to use two types of approach. The first method is the use of the classical copolymer composition equations developed for free radical polymerization. The second is unique to anionic polymerization and depends on the fact that for living systems it is possible to prepare an active polymer of one monomer and to study its reaction with the second monomer. The initial rate of disappearance of one type of active end, or the appearance of the other type (usually determined spectroscopically) or the rate of monomer consumption gives directly the reactivity of polymer-1 with monomer-2. It is in principle possible to compare the two methods to see if additional complications occur when both monomers are present together. 

In this way, EA can be applied to determine monomer composition in copolymers and polymer blends and any other composite material. Although results from EA are comparable to those obtained from spectroscopic techniques such as IR and NMR (nuclear magnetic resonance) spectroscopies, developments in EA are needed to improve the accuracy and precision of the method. 

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