Molar-mass analysis

To first fractionate the polymer on the basis of composition (TREE) followed by molar mass analysis (GPC)... 

To overcome the problems related to SEC of complex polymers, multidetector systems have been developed over the years. One approach is the combination of SEC with multiple concentration detectors. If the response factors of the detectors for the components of the polymer are sufficiently different, the chemical composition of each slice of the elution curve can be determined from the detector signals. Typically, a combination of UV and RI detection is used another possibility is the use of a diode array detector. In the case of non-UV-absorbing polymers, a combination of RI and density detection yields information on chemical composition. Similar information can be obtained by coupling SEC with spectroscopic detectors like FTIR, NMR, or mass spectrometry (MS). This approach is addressed in Section 2.03.5. Such detector combinations, however, are normally not able to differentiate between copolymers and polymer blends. In this case, it might be more suitable to carry out a separation according to chemical composition in a first step followed by a molar mass analysis (see for more details. Section 2.03.6). 

The most useful approach for the molar mass analysis of complex polymers is the coupling of molar mass-sensitive detectors to the SEC. Since the response of such detectors depends on both concentration and molar mass, they have to be combined with a concentration-sensitive detector... 

Figure 12 Schematic representation of a SEC-MALLS setup (a) and molar mass analysis of a polyamide 12 (b). Reprinted from Laun, S. Pasch, H. Longieras, N. Degoulet, C. Po/ymer2008,49,4502, with permission from Elsevier.

The linear column (PSS SDV 5 /mm linear) has a wider molar mass fractionation range while keeping the analysis time roughly the same. Therefore the slope of the calibration curve is much steeper and the resolution will be poorer in this case. The second column with a single pore size (PSS SDV 5 /mm 1000 A) separates only below 50,000 Da, but does this very efficiently in the same time. 

One of the most important properties of a chromatographic column is the separation efficiency. A measure of this parameter could be the difference of the retention volume for two different compounds. The result of a GPC analysis is usually, however, only one large peak, and a separation into consecutive molar mass species is not possible. Additionally there is no standard for higher molar masses consisting only of a species that is truly monodisperse. Therefore, the application of the equation to the chromatographic resolution of low... 

Huber, A. (1992). In Analysis of Polymers/Molar-Mass and Molar-Mass Distribution of Polymers, Polyelectrolytes and Latices (W.-M. Kulicke, ed.), Hiithig Wepf Verlag, 61, 248. 

A large excess of 1,2-ethanediol (threefold) is reacted with adipic acid until carboxyl groups have reacted. The excess of 1,2-ethanediol is distilled off under vacuum in the presence of Ti(OBu)4 as hydroxy-ester interchange catalyst. The reaction can be followed by SEC analysis of samples withdrawn from reaction medium and stopped when the desired molar mass has been reached (Scheme 2.48). 

J.9 You are asked to identify compound X, which was extracted from a plant seized by customs inspectors. You run a number of tests and collect the following data. Compound X is a white, crystalline solid. An aqueous solution of X turns litmus red and conducts electricity poorly, even when X is present at appreciable concentrations. When you add sodium hydroxide to the solution a reaction takes place. A solution of the products of the reaction conducts electricity well. An elemental analysis of X shows that the mass percentage composition of the compound is 26.68% C and 2.239% H, with the remainder being oxygen. A mass spectrum of X yields a molar mass of 90.0 g-moF. (a) Write the empirical formula of X. (b) Write... 

Suppose that 10.0 g of an organic compound used as a component of mothballs is dissolved in 80.0 g of benzene. The freezing point of the solution is 1.20°C. (a) What is an approximate molar mass of the organic compound (b) An elemental analysis of that substance indicated that the empirical formula is C3H2C1. What is its molecular formula (c) Using the atomic molar masses from the periodic table, calculate a more accurate molar mass of the compound. 

An elemental analysis of epinephrine resulted in the following composition 59.0% carbon, 26.2% oxygen, 7.15% hydrogen, and 7.65% nitrogen by mass. When 0.64 g of epinephrine was dissolved in 36.0 g of benzene, the freezing point decreased by 0.50°C. (a) Determine the empirical formula of epinephrine, (b) What is the molar mass of epinephrine ... 

If it is possible to analyse end groups of a particular specimen of polymer, it may be possible to use the data to determine number average relative molar mass. If the molecules are branched the degree of branching can be measured from a combination of end group analysis and relative molar mass determination (determined by an alternative method). 

One major drawback of end group analysis is that it rapidly becomes inaccurate as relative molar mass increases. This arises because the percentage of the end groups becomes smaller and smaller, and hence more and more uncertainty attaches to the numerical values of end group content that may be obtained. To illustrate this point, let us consider a polyester with acid end groups being determined by titration. Results for such titrations are shown in Table 6.5. 

From Table 6.5 quantitative determination of end groups can be seen to be increasingly uncertain particularly above a value of of 10 000. Nonetheless, end group analysis may be useful in certain circumstances, particularly for lower molar mass polymers and oligomers, where it may be a fairly straightforward approach to obtaining useful data. 

Begin by calculating the mass of one mole of caffeine (because the percentage analysis is reported to only three significant figures, we need the molar mass to no more than two decimals) ... 

If the molar mass of the compound is not known, the best we can do is to find the simplest formula that agrees with the elemental analysis. This simplest formula, or empirical formula, contains the smallest set of whole-number subscripts that match the elemental analysis. The empirical formula of caffeine is C4 H5 N2 O. 

Analysis of ibuprofen, the active ingredient in several over-the-counter pain relievers, shows that it contains 75.7% carbon, 8.8% hydrogen, and 15.5% oxygen. The mass spectrum of ibuprofen shows that its molar mass is less than 210 g/mol. Determine the chemical formula of this compound. 

C03-0083. Nicotine is an addictive compound found in tobacco leaves. Elemental analysis of nicotine gives these data C 74.0%, H 8.65%, N 17.35%. What is the empirical formula of nicotine The molar mass of nicotine is 162 g/mol. What is the molecular formula of nicotine ... 

C03-0089. Combustion analysis of 0.60 g of an unknown organic compound that contained only C, H, and O gave 1.466 g of carbon dioxide and 0.60 g of water in a combustion analysis. Mass spectral analysis showed that the compound had a molar mass around 220 g/mol. Determine the empirical formula and molecular formula. 

Table 6.10 reports the main areas of application of the various ionisation methods and the principal ions detected. A breakdown of MS techniques applied to various types of analytes is as follows thermally stable, low-MW Cl, El thermally instable, low-MW APCI (FLA, LC-MS), ESI and high-MW DCI, FD, FAB, LD, ESI (FLA, LC-MS, CZE-MS). Soft ionisation techniques such as FL, FAB and LD are useful for the detection of non-volatile, sometimes oligomeric, polymer additives. Recent developments in ionisation techniques have allowed the analysis of polar, ionic, and high-MW compounds, previously not amenable to mass-spectrometric analysis. Figure 6.4 shows the applicability of various atmospheric pressure ionisation techniques in terms of molar mass and polarity. 

The retention behaviour of oligomers in RPLC/-NPLC-APCI-MS (35 to 1500 Da) has been described [637]. In isocratic chromatography only a limited number n of oligomers can be separated, as the retention usually increases excessively at high n, so that gradient elution is necessary for successful separation of samples with broader molar mass distribution. RPLC-APCI-UV/QITMS was used for the analysis of accelerators (CBS, MBT, MBTS, BT) extracted... 

SEC-GC-FID, according to Figure 7.40, has been used to carry out the simultaneous determination of the polymer average molecular masses and molar mass distribution and the concentration of additives [984]. The effluent was split and adsorbed on PTV packing material before GC analysis. The choice of PTV... 

The behavior of the different amines depends on at least four factors basicity, nucleophilicity, steric hindrance and solvation. In the literature (16), 126 aliphatic and aromatic amines have been classified by a statistical analysis of the data for the following parameters molar mass (mm), refractive index (nD), density (d), boiling point (bp), molar volume, and pKa. On such a premise, a Cartesian co-ordinate graph places the amines in four quadrants (16). In our preliminary tests, amines representative of each quadrant have been investigated, and chosen by consideration of their toxicity, commercial availability and price (Table 1). 

Bravo, 1984). Hybrids of these systems, where chromatography and electrophoresis are used in each spatial dimension, were reported nearly 40 years ago (Efron, 1959). Belenkii and coworkers reported on the analysis of block copolymers by TLC (Gankina et al., 1991 Litvinova et al., 1991). Two-block copolymers of styrene and f-butyl methacrylate were separated first with regard to chemical composition by TLC at critical conditions, followed by a SEC-type separation to determine the molar masses of the components. 

For analyzing structure-property relationships, a variety of PEO-g-PVA copolymers were prepared, differing in the VAc-to-PEO ratio and the molar mass of PEO. The analysis of the copolymers by IR and 1H- and 13C-NMR showed the presence of both PEO and PVA. A small C=0 absorption was still present and was explained by a nonquantitative saponification. SEC showed polydispersities (Mw/M ) of around 5, with a small tailing to the low molar mass side. The latter was probably caused by the relatively low molar mass PVA homopolymer formed by the chain transfer reaction of VAc, both to the PEO and its acetate functionality. 

Statistical and block copolymers based on ethylene oxide (EO) and propylene oxide (PO) are important precursors of polyurethanes. Their detailed chemical structure, that is, the chemical composition, block length, and molar mass of the individual blocks may be decisive for the properties of the final product. For triblock copolymers HO (EO) (PO)m(EO) OH, the detailed analysis relates to the determination of the total molar mass and the degrees of polymerization of the inner PPO block (m) and the outer PEO blocks (n). 

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