Analytical methods, measuring copolymer composition

All IR methods for measuring copolym compositions are relative and need calibration with suitable standards. Three calibration m hods are usually used radiochemical standards, calibration with the homopolymer mixtures, and calibration with model compounds (33-36). Calibration with radiochemical standards is very precise and accurate but is limited by the availability of labelled olefins. Calibration with homopolymer mixtures is also very popular (33,37-41,43). It is a very simple method but has many drawbacks, the main one being the constitutional difference between such mixtures and real copolymers (32). Nevertheless, its use is justified if the chosen analytical bands are highly localized and if the copolymers examined have a block structure (rj rj > 1) similar to that of the polymer mixtures. The third calibration method consists in using model compounds for the determination of absorption coefficients. These model compounds are either homopolymers (42,44-47), in which case the method is in priiKiple the same as calibration with polymer mixtures, or special compoimds with structures resembling those of characteristic groups in copolymers (13,49,51). 

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. 

If precise analytical methods are available for determining both the total fraction of diene in the copolymer and the fraction of either cyclic units or pendant vinyl groups, then by making a series of such measurements for different initial monomer feed compositions, values for r, r, and a could be obtained from Equation (11). Then the remaining two parameters, r2 and could be obtained from Equation (10). 

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. 

The measurement of the dipole moments of copolymers and its analysis in terms of both sequence distribution and local chain configurations has received attention Modern computer aided analytical procedures provide in ght into the dependence of mean square dipole moment per residue on reactivity ratios, polymer composition and rotamer probabilities. One such calculation for atactic cc ly-(p-chlorostyrene-p-methylstyrene) has shovm that at constant composition, the dipole moment is quite sensitive to the sequence distribution and thus to the reactivity ratios. This dependence would be even more marked for syndiotactic chains. For cop61y(propylene-vinyl chloride) and copoly(ethylene-vinyl chloride) d le moments are again very sequence dependent, much more so than the diaracteristk ratio. It would appear that in copolymer systems dielectric measurements can be a powerful method of investigating sequence distributions. Two copolymers, p-dilcxo-styrene with styrene and with p-methylstyrene have been examined experimentally The meamrements were made on solid amorphous samples above the ass-rubber transition temperature (Tg) and they are consistent with the predictions of the rotational isomeric state model udi known reactivity ratios and rea nable replication probabilities . However, it is the view of this author that deduc-... 

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