Determination of Monomer Ratios in Copolymers

Quantitative determination of the weight percentage of the various bound monomer units of copolymers and terpolymers is a very important aspect of polymer analysis. 

Such data can have very important implications for various polymer characteristics oxidative stability, thermal stability, mechanical properties and flexibility, and elasticity and tensile properties. 

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Determination of monomer ratios in copolymers, e.g., ethylene oxide-vinyl chloride (Figure 3.4), tetrafluoroethylene-hexafluoropropylene. 

Smith [841] has discussed applications of pyrolysis techniques for polymeric systems with emphasis on the qualitative identification of components in a copolymer or polymer blend, identification of low-level polymer contaminants, characterisation of copolymer sequencing, differentiation between copolymers and physical blends of homopolymers, determination of monomer ratios in copolymers, and the study of polymer kinetics and degradation mechanisms. Pyrolysis destroys the stereostructure of the polymers. Gaseous components generated from pyrolysis of a wide variety of polymers have been analysed both off-line and on-line by IR spectroscopy to determine (quantitatively) the major components of the parent resin, e.g. rubbers... 

Van Doremaele and co-workers [22] applied H-NMR spectroscopy to the determination of monomer ratios in styrene methyl acrylate copolymers 400-MHz H-NMR spectra were obtained in CDCI3 solutions at 25 °C. Expansions of the methoxy region display additional fine splitting due to combined configurational (i.e., tacticity) and compositional sequence effects. Mean copolymer composition (mole fraction styrene, Fj) can be readily obtained by using absorbances, which represent the total peak areas of the aromatic and methoxy proton resonances, respectively. The initial feed (q = [S]/[M]), the average copolymer composition, and the conversion are summarised in Table 4.6, [S] = concentrated styrene in feed, [M] = concentration of methyl acrylate in feed. 

Shouting and Poehlein [29] showed that H-NMR is a valuable tool for the quantitative determination of monomer ratios in styrene-acrylic acid copolymers. In this work, results from H-NMR and C-NMR were compared. The composition results obtained by the two methods are compared in Table 4.7. 

Wang and Poehlein [88] showed that H-NMR is a valuable tool for the quantitative determination of monomer ratios in styrene - acrylic acid copolymers. 

Van Doremaele and co-workers [89] applied H-NMR spectroscopy to the determination of monomer ratios in styrene methyl acrylate copolymers. Figure 4.10 depicts, as a typical example, the 400 MHz H-NMR spectrum of a low-conversion solution styrene - methyl acrylate copolymer dissolved in CDCI3 at 25 °C, whereas in... 

The arrangement of monomer units in copolymer chains is determined by the monomer reactivity ratios which can be influenced by the reaction medium and various additives. The average sequence distribution to the triad level can often be measured by NMR (Section 7.3.3.2) and in special cases by other techniques.100 101 Longer sequences are usually difficult to determine experimentally, however, by assuming a model (terminal, penultimate, etc.) they can be predicted.7 102 Where sequence distributions can be accurately determined Lhey provide, in principle, a powerful method for determining monomer reactivity ratios. 

This chapter is concerned with the precise sequence in which monomer units exist in a homopolymer. This information is quite different from that covered in methods for the determination of the ratio in which monomer units occur. Sequencing in copolymers is more complicated and is dealt with separately in Chapter 7. 

Pyrolysis-gas chromatography (Py-GC) can be used to obtain fine detail regarding minor, but often very important detail, concerning polymer structure such as monomer sequences and, also, branching, crosslinking copolymer structure and the nature of the end-groups, and can also be used to elucidate polymer structure between adjacent monomer units in copolymers and to determine monomer ratios in copolymers. 

This is known as the copolymer equation, and is a mathematical expression from which it is possible to determine the ratio of monomer units in the increment of copolymer formed from a given ratio of monomer molecules. 

The use of nmr, —carbon-13 nmr in particular—, has given a much deeper insight into the structure of SO2 copolymers than was possible by the older, traditional method of analytically determining monomer ratios in the polymer as a function of the monomer feed. In fact, it can be safely said that the use of nmr has conpletely revolutionized the study of copolymers. (The impact of nmr on copolymer studies is studiously ignored in all polymer textbooks, which tend to reflect the status of the field twenty years ago.)... 

The nature of the polymer latex is determined by the monomer ratio in the copolymer and this property of the latex affects strength values in manner similar to that obtained with the polymer-cement ratio. The effects of polymer-cement ratio on strength are presented in Table 6.14 [87]. 

The HBA/HNA system provides a more suitable system for study, since it is prepared by melt polymerization of the two monomers and is far more stable at elevated temperatures compared to the PHBA/PET. The HBA/HNA copolymers are soluble in pentafluorophenol permitting use of NMR techniques to characterize diad sequences. In Fig. 13b,c the 13CNMR spectrum of the carboxyl carbon region of the HBA/HNA copolyesters of the 73/27 and 48/52 systems is shown [34]. Also shown in Fig. 13a,d are the spectra of 13C enriched HBA and HNA containing copolymers permitting unique identification of the diad sequences. As a result of this technique it was possible to determine the reactivity ratios of the two monomers by analyzing the 50/50 copolymer after polymerization to a molar mass value of 2000 [35]. Examination of the copolymer by 13C NMR showed the same ratio of monomers as in the starting... 

A vinyl monomer M, (1 mole) is copolymerized with another monomer M2 (2 moles). The reactivity ratios are r, = 0.6 and r2 = 1.66. Determine the mole fraction of monomer M in the initially formed copolymer. What is the mole fraction of monomer M, in the polymer being formed when two moles of the monomers are converted to a copolymer Draw the graph of the cumulative and the instantaneous copolymer compositions vs. the conversion. 

Another important recent contribution is the provision of a good measurement of the precision of estimated reactivity ratios. The calculation of independent standard deviations for each reactivity ratio obtained by linear least squares fitting to linear forms of the differential copolymer equations is invalid, because the two reactivity ratios are not statistically independent. Information about the precision of reactivity ratios that are determined jointly is properly conveyed by specification of joint confidence limits within which the true values can be assumed to coexist. This is represented as a closed curve in a plot of r and r2- Standard statistical techniques for such computations are impossible or too cumbersome for application to binary copolymerization data in the usual absence of estimates of reliability of the values of monomer feed and copolymer composition data. Both the nonlinear least squares and the EVM calculations provide computer-assisted estimates of such joint confidence loops [15]. 

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