Chemical composition and molecular weight distribution

SAN copolymers are complex mixtures possessing heterogeneity in both chemical composition and molecular weight distribution. Consequently, analytical characterization of these materials is complex, but absolutely critical for... 

Mori S. (1988), Determination of chemical composition and molecular weight distributions of high-conversion styrene-methyl methacrylate copolymers by liquid adsorption and size exclusion chromatography. Anal. Chem., 60,1125-1128. 

Considerable effort is being exerted currently to determine the compositional and molecular weight distributions of copolymers. The basic idea behind these techniques is fractionation of the copolymer by composition and subsequent measurement of molecular weight distributions of chemically uniform fractions by SEC. The problems with SEC characterizations that have been mentioned are circumvented if the samples are indeed compositionally uniform. 

FIGURE 13.15 Low composition drift of Q9 in a batch reaction and semibatch experiments in which F. was predictively arranged to either decrease or increase. In the latter case, a blend of Am/Q9 copolyelectrolyte and hompolymeric Q9 was produced. Reprinted (adapted) with permission from Kreft T, Reed WF. Predictive control of average composition and molecular weight distributions in semibatch free radical copolymerization reactions. Macromolecules 20()9 42 5558-5565. 2009 American Chemical Society. 

Synthetic copolymers are always poly disperse, i.e., they consist of a large number of chemically similar species with different molar masses and different chemical compositions. Owing to this polydispersity, characterization of copolymers does usually not provide the number of individual molecules or their mole fraction, mass fraction, etc. but requires the use of continuous distribution functions or their averages. Continuous thermodynamics, developed by Ratzsch and Kehlen [1], can be directly applied to the calculation of thermodynamic properties, including phase equilibria, because this theoretical framework is based completely on continuous distribution functions, which include all the information about these functions and allow an exact mathematical treatment of all related thermodynamic properties. Continuous thermodynamics have been used for calculation of phase equilibria of systems containing two-dimensional distributed copolymers [1-8]. The purpose of this contribution is the application of continuous thermodynamics to copolymer fractionation according to the chemical composition and molecular weight. 

X HERMAL FIELD-FLOW FRACTIONATION (ThFFF) separates polymers according to their molecular weight and chemical composition. The molecular weight dependence is well understood and is routinely used to characterize molecular weight distributions (1-4). However, the dependence of retention on composition is tied to differences in the thermal diffusion of polymers, which is poorly understood. As a result, the compositional selectivity of ThFFF has not realized its full potential. How-... 

Pectin, a structural, cell-wall polysaccharide of all higher plants, like most other polysaccharides, is both polymolecular and polydisperse, i.e., it is heterogeneous with respect to both chemical structure and molecular weight (jl). From molecule to molecule, in any sample of pectin, both the number and percentage of individual monomeric unit types will vary, and the average composition and distribution of molecular weights can vary with the source, the conditions used for isolation, and any subsequent treatments. Because both parameters determine physical properties, various functional types of pectin can be produced by controlling the source, isolation procedure, and subsequent treatment(s)... 

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. 

Mechanical properties, as in the case of other properties, can be organized in terms of their relationship to polymer structure, depending on the functional groups present in the polymer backbone and the side groups from the main chain. Other factors that affect the mechanical properties of a polymer, besides its chemical composition, are molecular weight and molecular weight distribution. 

Polymer supercritical-fluid systems show complex phase behavior (for a general overview see also [80]). Polymer solubihty in these systems depends on (apart from temperature, pressure, and concentration) the chemical nature, molecular weight, and molecular-weight distribution of the polymer and on the comonomer composition in the case of copolymers. 

There are many reports in the literature in which chemical and molecular weight distributions have been estimated concurrently by employing multiple detectors to sense the SEC eluant. An example is the use of an ultraviolet detector, which is sensitive to the residues of monomers like styrene but does not see acrylate or methacrylate monomer residues. For reasons already discussed, the success of such analyses depends on whether the SEC separation has actually been on the basis of molecular weight and whether the relative concentrations of the eluting species were measured independently of possible variations in their compositions. This probably varies from case to case. 

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