Chemical-composition distributions

Beyers et aV° in the Polymer Research Division of BASF-AG used in-line transflectance NIR to monitor methyl methacrylate (MMA) and iV,7V-dimethylacrylamide (DMAAm) monomers in a copolymerization reaction. The work in this paper is of interest as it illustrates an example of calibration development done off-line with a very limited number of prepared calibration samples. The value of the measurement is to control the end properties of the products resulting from the copolymerization reaction. The end properties are related to many parameters including the intramolecular chemical composition distribution (CCD). The... 

These materials, however, as a rule exhibit rather broad chemical composition distribution. Block copolymers may contain important amounts of parent homopolymer(s) [232,244,269], In any case, it is to be kept in mind that practically all calibration materials contain the end groups that differ in the chemical composition, size, and in the enthalpic interactivity from the mers forming the main chain. In some cases, also the entire physical architecture of the apparently identical calibration materials and analyzed polymers may differ substantially. The typical example is the difference in stereoregularity of poly(methyl and ethyl methacrylate)s while the size of the isotactic macromolecules in solution is similar to their syndiotactic pendants of the same molar mass, their enthalpic interactivity and retention in LC CC may differ remarkably [258,259]. 

Fast high-pressure fractionation of tails in the molar-mass-distribution and/or the chemical-composition-distribution of a (co)polymer [47] during the polymerisation process will indicate eventual drifts in (co)polymer composition. Several important polymer properties depend strongly on such tails in the (co)polymer distribution. On--line fractionation data are strongly needed. 

Today, the majority of all polymeric materials is produced using the free-radical polymerization technique [11-17]. Unfortunately, however, in conventional free-radical copolymerization, control of the incorporation of monomer species into a copolymer chain is practically impossible. Furthermore, in this process, the propagating macroradicals usually attach monomeric units in a random way, governed by the relative reactivities of polymerizing comonomers. This lack of control confines the versatility of the free-radical process, because the microscopic polymer properties, such as chemical composition distribution and tacticity are key parameters that determine the macroscopic behavior of the resultant product. 

The composition distribution of the graft copolymers obtained by the macromonomer method has been shown theoretically to be statistically broader than in the corresponding conventional linear copolymer, due to the high MW of the macromonomer branches [45, 46]. This has been experimentally confirmed by Teramachi et al. with PSt macromonomers, 23 or 24, copolymerized with MMA [47-49]. The chemical composition distribution was found to broad-... 

On-line SEC-NMR has successfully applied to the determination of chemical composition distribution in ethylene-propylene-diene terpolymers (EPDM) by the use of H-NMR at 750 MHz [24], EPDM containing 2-ethylidene-5-norbornene (ENB), as a diene... 

Strictly speaking, Balke s system combined SEC in the first dimension with a mixed mode separation in the second dimension. Since SEC separates with respect to hydrodynamic volume and not molar mass, the copolymers under investigation could not be quantified with respect to molar mass distribution (see discussion in Sect. 3). The effect of the SEC separation was simply to obtain fractions with narrower molar mass distribution as compared to the total sample. Considering this fact, it is clear that for chemically heterogeneous copolymers no quantitative data can be obtained from the first dimension. Only the second dimension, separating with respect to chemical composition, can provide quantitative information on the chemical composition distribution. Accordingly, a coupled information on both MMD and CCD was not available for this system. 

The mapping of ethoxylated fatty alcohols and ethylene oxide-propylene oxide block copolymers by 2D chromatography was discussed by Trathnigg et al. [95]. They combined LAC and SEC and were able to determine the chemical composition distribution and the molar mass distribution of the polyethers. 

To overcome the difficulties of ESI-MS, Simonsick and Prokai added sodium cations to the mobile phase to facilitate ionization [165,166]. To simplify the resulting ESI spectra, the number of components entering the ion source was reduced. Combining SEC with electrospray detection, the elution curves of polyethylene oxides) were calibrated. The chemical composition distribution of acrylic macromonomers was profiled across the molar mass distribution. The analysis of poly(ethylene oxides) by SEC-ESI-MS with respect to chemical composition and oligomer distribution was discussed by Simonsick [167]. In a similar approach aliphatic polyesters [168], phenolic resins [169], methyl methacrylate macromonomers [169] and polysulfides have been analyzed [170]. The detectable mass range for different species, however, was well below 5000 g/mol, indicating that the technique is not really suited for polymer analysis. 

As has been demonstrated, the combination of selective separation techniques with powerful spectroscopic detectors enables complex polymers to be analyzed with respect to all possible types of molecular heterogeneity. Chemical composition distribution can be monitored across the molar mass distribution. Steric and functional peculiarities can be detected over the entire molar mass range. 

Using SEC-ESIMS we studied the products of a macromonomer synthesis in which MMA (2) and BA (3) MS were loaded in an 80 20 (wt/wt) weight ratio. The details of the synthesis are reported in Experimental Details. The SEC-ESIMS data will allow us to profile the chemical composition distribution across the MWD. From these data we should be able to measure the relative efficiency of our chain-transfer agent for methacrylates versus acrylates. 

We wanted to monitor the chemical composition distribution across the MWD. This can be accomplished by summing all the selected-ion plots for the MMA BAq, MMA BAi, MMA BA2, and MMA BAs, respectively. We used a 2-Da window to obtain the selected-ion plots. For example, the selected-ion plot for the MMA2BA trimer (351 Da eq... 

SEC-ESIMS is a valuable tool for polymer characterization. Compounds are separated based on their hydrodynamic size in solution, but upon detection, an absolute molecular weight is also furnished. Only 1% of the SEC effluent is required for ESIMS analysis, thereby accommodating the popular SEC detectors. SEC-ESIMS provides an attractive solution to the calibration of SEC without the use of external calibrants. Chemical composition distribution information on copolymers is easily afforded provided the individual monomers differ in molecular weight. The successively acquired mass spectra contain narrow fractions of the overall distribution that simplifies the analysis of complex formulations. Unfortunately, we have not been able to provide structured details on materials beyond 5000 Da due to the low resolution of the quadrupole mass spectrometer. Nevertheless, SEC-ESIMS is an exciting hyphenated techniques for polymer characterization. 

Fig. 1 Molar mass distribution with overlaid chemical composition distribution of a styrene-MMA block copolymer with poor block formation.

The use of different modes of liquid chromatography facilitates the separation of complex samples, selectively, with respect to different properties like hydrodynamic volume, molar mass, chemical composition, or functionality. Using these techniques in combination, multidimensional information on different aspects of molecular heterogeneity can be obtained. If, for example, two different chromatographic techniques are combined in a cross-fractionation mode, information on chemical composition distribution and molar mass distribution can be obtained. Reviews on different techniques and applications involving the combination of GPC and various LC methods can be found in the literature [6-8]. 

Synthetic copolymers have both molecular-weight and chemical composition distributions and copolymer molecules of the same molecular size, which are eluted at the same retention volume in SEC, may have different molecular weights in addition to different compositions. This is because separation in SEC is achieved according to the sizes of molecules in solution, not according to their molecular weights or chemical compositions. 

---