Copolymer composition measurement methods

Figure 7.3 Calibration curves for copolymer composition measurement by the infrared method [10].

The copolymer composition measurements were done by IR ( i38sMi4si. calibration with homopolymer mixtures) and chromoto-graphic methods (43). The monomea distribution was estimated by means of the 997 cm band method, as used for 4-methylpentene-l copolymers with ethylene (Section IV.A.6), hexene-1 (Section IV.E.4b), and styrene (Section IV.E.6). The ( 997/ 91 g)cop versus C4 pi dependence, measured from the IR spectra, satisfactorily correlate with Eq. (8) for T2 1 [see Fig. 4 in (43)]. The evaluation of vinyl cyclohexane unit distribution was also studied. 

The copolymer composition equation relates the r s to either the ratio [Eq. (7.15)] or the mole fraction [Eq. (7.18)] of the monomers in the feedstock and repeat units in the copolymer. To use this equation to evaluate rj and V2, the composition of a copolymer resulting from a feedstock of known composition must be measured. The composition of the feedstock itself must be known also, but we assume this poses no problems. The copolymer specimen must be obtained by proper sampling procedures, and purified of extraneous materials. Remember that monomers, initiators, and possibly solvents are involved in these reactions also, even though we have been focusing attention on the copolymer alone. The proportions of the two kinds of repeat unit in the copolymer is then determined by either chemical or physical methods. Elemental analysis has been the chemical method most widely used, although analysis for functional groups is also employed. 

One final point should be made. The observation of significant solvent effects on kp in homopolymerization and on reactivity ratios in copolymerization (Section 8.3.1) calls into question the methods for reactivity ratio measurement which rely on evaluation of the polymer composition for various monomer feed ratios (Section 7.3.2). If solvent effects arc significant, it would seem to follow that reactivity ratios in bulk copolymerization should be a function of the feed composition.138 Moreover, since the reaction medium alters with conversion, the reactivity ratios may also vary with conversion. Thus the two most common sources of data used in reactivity ratio determination (i.e. low conversion composition measurements and composition conversion measurements) are potentially flawed. A corollary of this statement also provides one explanation for any failure of reactivity ratios to predict copolymer composition at high conversion. The effect of solvents on radical copolymerization remains an area in need of further research. 

The solvent in a bulk copolymerization comprises the monomers. The nature of the solvent will necessarily change with conversion from monomers to a mixture of monomers and polymers, and, in most cases, the ratio of monomers in the feed will also vary with conversion. For S-AN copolymerization, since the reactivity ratios are different in toluene and in acetonitrile, we should anticipate that the reactivity ratios are different in bulk copolymerizations when the monomer mix is either mostly AN or mostly S. This calls into question the usual method of measuring reactivity ratios by examining the copolymer composition for various monomer feed compositions at very low monomer conversion. We can note that reactivity ratios can be estimated for a single monomer feed composition by analyzing the monomer sequence distribution. Analysis of the dependence of reactivity ratios determined in this manner of monomer feed ratio should therefore provide evidence for solvent effects. These considerations should not be ignored in solution polymerization either. 

Many characteristic molecular vibrations occur at frequencies in the infrared portion of the electromagnetic spectrum. We routinely analyze polymers by measuring the infrared frequencies that are absorbed by these molecular vibrations. Given a suitable calibration method we can obtain both qualitative and quantitative information regarding copolymer composition from an infrared spectrum. We can often identify unknown polymers by comparing their infrared spectra with electronic libraries containing spectra of known materials. 

Reactivity ratios for 1-hexene (M ) with 5-methyl-1,4-hexadiene CM2) copolymerization at 30 c in hexane solvent using a Et2AlCl/6-TiCl3 AA catalyst system (Al/Ti atomic ratio s 1.5) were determined. The compositions of copolymers were measured by 300 MHz 1H-NMR spectroscopy. The reactivity ratios, calculated by the Tidwell-Mortimer method, were 1.1 + 0.2 for each of the two monomers. 

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... 

Poly(diallyldimethylammonium chloride) was the first quaternary ammonium polymer approved for potable water clarification by the United States Public Health Service, and has historically been the most widely produced cationic polyelectrolyte. There have been several studies on the kinetics (26-37) and uses of diallyldimethylammonium chloride (DADMAC) (38-45) however, there have been no investigations in inverse microsuspension, the most common industrial method of polymerization. Furthermore, there is considerable disagreement between published reactivity ratios, probably because no satisfactory analytical methods have been described in the literature for residual monomer concentration or copolymer composition. For other commercially important quaternary ammonium polymers, such as dimethylaminoethyl methacrylate and dimethylaminoethyl acrylate, few kinetic data are available (46-51) only Tanaka (37) measured the reactivity ratios. 

Analytical Methods. Historically, the copolymer composition of cationic acrylic polymers has been measured by conductiometric (28), silver nitrate (29), or colloid titration (52, 53). Chromatographic methods have been reported for acrylamide monomer (54-56) however, no such methods have been employed for quaternary ammonium monomers. In this chapter, a new HPLC method (Nalco) is described for the simultaneous determination of both comonomers. Colloid titration is described in the next paragraph and was used only for comparison purposes. 

The error-in-variables method was used to estimate the reactivity ratios. This method was developed by Reilly et al. (57, 58), and it was first applied for the determination of reactivity ratios by O Driscoll, Reilly, and co-workers (59, 60). In this work, a modified version by MacGregor and Sutton (61) adapted by Gloor (62) for a continuous stirred tank reactor was used. The error-in-variables method shows two important advantages compared to the other common methods for the determination of copolymer reactivity ratios, which are statistically incorrect, as for example, Fineman-Ross (63) or Kelen-Tiidos (64). First, it accounts for the errors in both dependent and independent variables the other estimation methods assume the measured values of monomer concentration and copolymer composition have no variance. Second, it computes the joint confidence region for the reactivity ratios, the area of which is proportional to the total estimation error. 

This procedure was designed to provide a measure of the absorptivity of the hydrophobic monomer through the use of a model. The use of a monomeric model compound to represent equivalent chromophores attached to polymer molecules was recently reported for polystyrene 15, 16). UV methods were also used in the determination of copolymer composition and reactivity ratios for isocyanate-containing acrylate polymers (IT). 

The second method to improve the accuracy of the reactivity ratios is the use of other types of data than copolymer composition. If the monomer sequence distribution is measured as a function of comonomer feed ratio, the accuracy will usually be larger than in the case of copolymer composition versus comonomer feed ratio. One of the frequentiy used methods is the measurement of so-called triad fiactions. For example, in a copolymer of STY (S) and BA (B), the STY-centered triads are SSS, BSS + SSB, and BSB. Their fractions can usually be measured from NMR measurements. The assignments in the NMR spectra are often difficult to make, and quantitative NMR measurements can be quite tedious. In some cases even special techniques such as distortion enhancement by polarization transfer (DEPT) NMR need to be used in order to circumvent... 

Although the application of model discrimination methods should improve our ability to discriminate, there are still many questions to be answered. How much will the application of model discrimination methods improve copolymerization modeling Which model discrimination method is the most reliable and efficient Which of the available copolymerization measurements (copolymer composition, triad fraction or rate data) will provide the most information ... 

The application of a low-angle laser light-scattering (LALLS) detector for the measurement of absolute molecular mass of molecules eluted from SEC columns is very attractive, because the construction of a calibration curve in advance is not required. However, this method is not generally applicable to copolymers, because the intensity of light scattering is a function not only of the molecular mass of the copolymer but also of the specific refractive index in the solvent used as the mobile phase. The value of the specific refractive index for the copolymer is a function of composition. In general, the correct treatment of LS measurement for copolymers requires measurements in at least three solvents of different specific refractive index increments. 

If the measurements are carried out with the same solvent-precipitant-temperature system on copolymers of different composition, then the values of (03) for the contents of the various monomeric units of the copolymers lie on a straight line (Figure 6-17). It has been shown experimentally that the method yields data concerning the average composition of the copolymer, provided that copolymer compositions vary only slightly... 

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