Copolymer analysis determination

A number of 2DLC applications have attempted to use liquid chromatography at critical conditions (LCCC) and are discussed in Chapter 17. This mode of operation is useful for copolymer analysis when one of the functional groups has no retention in a very narrow range of the solvent mixture. However, determining the critical solvent composition is problematic as it is very sensitive to small changes in composition. 

The gas chromatographic analysis of the unreacted monomers in the experiments from Table II discloses a constant C5/C8 ratio comparing the starting comonomer composition to the final composition. This means that monomer conversion is the same for 1,5-cyclooctadiene and cyclopentene in the copolymerization so that copolymer compositions are equal to the charge ratios. This result is consistent with the product analysis by 13C NMR spectroscopy where the copolymer composition is nearly identical to the starting comonomer composition. 13C NMR is used to determine the composition of the cyclopentene/1,5-cyclooctadiene copolymers as part of a detailed study of their microstructure (52). The areas of peaks at 29-30 ppm (the pp carbon from cyclopentene units) and at 27.5 ppm (the four ap carbons from the 1,5-cyclooctadiene) are used to obtain the mole fractions of the two comonomers (53, 54, 55). 13C NMR studies and copolymer composition determinations are described by Ivin (51, 56, 57) for various systems. 

The theta (0) conditions for the homopolymers and the random copolymers were determined in binary mixtures of CCl and CyHw at 25°. The cloud-point titration technique of Elias (5) as moaified by Cornet and van Ballegooijen (6) was employed. The volume fraction of non-solvent at the cloud-point was plotted against the polymer concentration on a semilogarithmic basis and extrapolation to C2 = 1 made by least squares analysis of the straight line plot. Use of concentration rather than polymer volume fraction, as is required theoretically (6, 7 ), produces little error of the extrapolated value since the polymers have densities close to unity. 

The molecular weight () of the poly(dimethyl siloxane), PDMSX, was determined by both proton NMR and non-aqueous potentiometric titration (5). Proton NMR was used routinely to determine immediately after synthesis and prior to use in any copolymerization. Experimental confirmation of percent silicon in the copolymers was determined by elemental analysis (Galbraith Laboratories). 

Copolymerization reactions Copolymerization experiments with styrene and MMA employed molar fractions of 20, 40, 60, and 80% comonomers, which were reacted in ethanol 1,2-dichIorethane 60 40 (by volume) mixtures and benzoyl peroxide as catalyst. Polymerizations were carried out at 70°C. The reactions were quenched by the addition of methanol as non-solvent, and the copolymer was isolated by centrifugation. Copolymer analysis employed UV spectroscopy for copolymers with MMA, and methoxyl content determination according to a procedure by Hodges et al. (16) in the case of styrene copolymers. Reactivity ratios were determined in accordance with the method by Kelen-Tiidos (17) and that by Yezrielev-Brokhina-Roskin (YBR) (18). Experimental details and results are presented elsewhere (15). 

Reactivity ratios between acrylated lignin model compound (Fig. 2), defined as Mi, with either MM A or S, defined as M2, were determined experimentally in accordance with standard procedures (15). These involve mixing two different vinyl monomers in various molar ratios with catalyst (i.e., benzoyl peroxide) and solvent, heating the mixture to achieve polymerization, and recovering the polymer by the addition of non-solvent, and centrifugation. The respective molar monomer fractions of the copolymer were determined by UV-spectroscopy in the cases where MMA served as M2, and by methoxyl content analysis in those cases in which S was the M2-species. The results were subjected to numerical treatments according to the established relationships of Kelen-Tiidos (17) and Yezrielev-Brokhina-Roskin (YBR) (18), and this is described elsewhere (15). 

The composition of the copolymer was determined by either NMR analysis at 90 MHz according to the equations derived by Mochel (21) or by infrared. (22) The agreement of these methods was 2% when applied to copolymer taken to 100% conversion. The reactivity ratios were calculated according to the Mayo-Lewis Plot (13,15), the Fineman-Ross Method (14), or by the Kelen-Tudos equation.(16,17,18) The statistical variations recently noted by 0 Driscoll (23), were also considered. 

C. All copolymerizations were carried out without solvent. Below 80 °C azobisisobutyronitrile was used as initiator. At 100 °C the reactions were initiated thermally. At temperatures of 50°, 60°, and 80 °C the reactions were carried out in dilatometers. At 20°C small flasks were used, and the reactions were conducted in a temperature-controlled room over a period of days. At 100 °C sealed glass tubes were preferred. The reactions were stopped at yields below 5%. The composition of the copolymers was determined by oxygen analysis in the analytical laboratories of BASF. The method for determining oxygen was developed in the Untersuchungslaboratorium of BASF (18). 

Carbon-13 nuclear magnetic resonance was used to determine the molecular structure of four copolymers of vinyl chloride and vinylidene chloride. The spectra were used to determine both monomer composition and sequence distribution. Good agreement was found between the chlorine analysis determined from wet analysis and the chlorine analysis determined by the C nmr method. The number average sequence length for vinylidene chloride measured from the spectra fit first order Markovian statistics rather than Bernoullian. The chemical shifts in these copolymers as well as their changes in areas as a function of monomer composition enable these copolymers to serve as model... 

The problem of the computation accuracy of these kinetic parameters is dependent first of all on the validity of the copolymer composition determination. As a criterion here one may use the closeness to each other of the values of this composition obtained via the different experimental methods. It is possible to judge about the degree of such a closeness using Tables 6.1 and 6.2 where the data on both chemical analysis and spectroscopy are presented. One can see that, as for the considered cases, the different experimental methods provide quite close values of the copolymer compositions within the accuracy in the range of 5%. Authentic evidence concerning the feasibility to reach such a degree of accuracy is furnished by the data on copolymer composition obtained via independent methods in the different systems, for instance, under the copolymerization of p-chlorstyrene with methyl acrylate [32], of 4-methylstyrene with methyl methacrylate or acrylonitrile [213], and also of styrene with acrylic or methacrylic acids [214],... 

Structure and composition of synthesized cyclolinear carbosiloxane copolymers were determined by functional and ultimate analysis, IR and NMR spectral data. Some parameters of copolymers are shown in Tables 3 and 4. 

Copolymer Analysis. Even though the overall copolymer composition can be determined by residual monomer analysis, it still is necessary to have reliable quantitative techniques for copolymer composition measurements on the actual copolymer, mainly because concentration detectors for SEC or HPLC are sensitive to composition and because the conversion histories are not always available. Some of the techniques used to determine copolymer composition are melt viscometry (46), chemical analysis, elemental analysis, infrared spectroscopy (IR), Nuclear Magnetic Resonance (NMR), ultra-violet spectroscopy (UV), etc. Melt viscometry, chemical and elemental analysis are general techniques that can be applied to almost any polymer. The spectroscopic techniques can be applied depending on the ability of the functional groups present to absorb at specific wavelengths. 

Spectrometric Analysis. Spectroscopy has been extensively used for polymer and copolymer analysis. (59-69). The kind of information available from different spectroscopic techniques as well as the instrumentation required depends on the region of the electromagnetic spectrum in which absorption is taking place. Recent investigations (63) on the use of spectrophotometers for copolymer analysis have shown that the response from spectrophotometers is sometimes sensitive to the microstructure of the polymer molecules and that calibration of spectrophotometers with absolute measurements on the microstructure (i.e. NMR) may be necessary in order to obtain reliable quantitative information on concentration and copolymer composition determinations. 

Later work by Frechet et al. attempted to prepare ABA diblock copolymers using the same methodology [255]. Two polyether dendrons were connected to a dihydroxy functional TEMPO moiety, which was then used to initiate, as well as control, the polymerization of St and formed dumbbell-shaped block copolymers (Scheme 29). The ABA block copolymers were formed however, there was some initial contamination from mono- and bis-dendritic species, which were removed by column chromatography. In addition, XH NMR analysis determined that the purified ABA triblock copolymers were also contaminated with AB diblock copolymers. This was attributed to the persistent radical effect, as well as inherent problems with the mobility of the bulky dendron counter radical, both of which will result in the AB diblock copolymers. Thermal polymerization may also contribute to diblock formation, as the radicals generated will be trapped by the counter radical dendron, but would not contain the initiating dendron moiety. Pure ABA triblock copolymer was obtained after two purification cycles using column chromatography [255]. 

Recently, the Research Group on NMR, SPSJ, assessed reliability of copolymer analysis by NMR using three samples of radically prepared copolymers of MMA and acrylonitrile with different compositions. 1H and 13C NMR spectra of the copolymers were collected from 46 NMR spectrometers (90 500 MHz) and the composition and sequence distribution were determined.232 Table 14 summarizes the monomer reactivity ratios determined by 13C NMR analysis. The large difference between rxx and r2X indicates the presence of a penultimate effect in this radical copolymerization, as previously reported.233 The values of riy, especially rxx, depended on the comonomer feed ratio, suggesting higher order of neighbouring unit effect on the reactivity of chain-end radicals. 

Analytical Procedures. The purity of all copolymers, i.e. absence of monomers, was checked by thin layer chromatography (TLC). Composition of the DHA -co-4VP copolymers was determined from elemental analysis data obtained by Micro-Analysis, Inc., Wilmington, Del. Compositions of the DHA-co-NVP copolymers were determined by non-aqueous titration, using 0.1N perchloric acid and gentian violet indicator in glacial acetic acid (11). Isocyanate was determined as reported previously (12). 

With this much interest in polyethylene, many attempts have been made to use low frequency NMR in either the time domain or frequency domain to monitor and control the production more rapidly. Auburn International (now part of Oxford Instruments) developed a widely adopted system based on the time domain spectrometers [21]. In this case, sample preparation is no longer an issue since the system accepts either powder or pellets and no solvent is used. The Auburn systems determine crystalline and amorphous ratios, viscosity, melt index and molecular weight For other types of polymers, the list of advertised measurements include tacticity, rubber content, copolymer analysis, and various rheological properties. These values are determined by correlating several routine but laborious methods with the decay of the NMR signal under various pulse sequences. The man-hours... 

The synthesised copolymers are light-yellow liquid or solid systems, depending on the length of dimethylsiloxane chain, dissolve well in ordinary organic solvents, with tl p of 0.05-0.07. The structure and composition of synthesised copolymers was determined by means of elementary analysis, by IR and NMR spectra data. Some physical-chemical properties of comb-type methylsiloxane copolymers with carbocyclosiloxane fragments in the side chain are presented in Table 6.12. 

A problem frequently encoxmtered in copolymer analysis is that the MS peaks can be assigned to two or more isobaric structures. In this case, the peak intensity experimentally observed comes from several contributions. An automated procedure to find composition and sequence of the copolymers analyzed has been developed to cope with this problem of determining the sequence of copolymers when a mass spectroscopic peak has a multiple structural assignment. 

NMR has received much attention as a method for copolymer analysis. Both copolymer composition and sequence can be determined. and C-NMR are both used, but often C-NMR is preferred, since the signals possess better resolution. 

Off-line SEC-NMR is certainly more labour-intensive and time-consuming than on-line SEC-NMR. Nevertheless, this difference can be minimized by reducing the number of fractions. Clearly, the reduction cannot go beyond a certain limit, otherwise it will cause a loss of accuracy in the measurement of copolymer properties. The hypothesis that it is possible to reduce the time for copolymer analysis without the cited loss of accuracy was put forward by Murphy et al. (24) in another context (2D-chromatography). It would be interesting to check whether the hypothesis is valid also in SEC-NMR and to develop a methodology which allows the determination of the optimal conditions, viz., those where the cited loss of accuracy is still negligible. 

Nevertheless, the utility of SEC in copolymer analysis is not impaired, if one applies SEC keeping the limitations described above in mind. The rapidity, simplicity, and capability of obtaining much data on copolymers by SEC overcome those drawbacks. In this chapter, the methods of determining molecular mass averages and MMD, as well as composition and chemical heterogeneity, are described. Other separation modes of HPLC used with the size-exclusion mode, such as the adsorption mode, are also explained. 

The composition of the isolated copolymer was determined from (a) the nitrogen elemental analysis, and (b) the sulfur elemental analysis. Both figures were corrected for the small amount (4-12%) of water associated with the polymer. 

Eventual homopolymers are eliminated and block copolymers are fractionated as described elsewhere . Molecular weight of polyvinyl blocks and copolymers are measured by osmometry, viscosimetry and gel permeation chromatography and the composition of the copolymers is determined by analysis of the nitrogen content of the copolymer, U.V. spectroscopy and n.m.r. spectroscopy as already described , ... 

The hydrolysis-gas chromatography technique applied to copolymer analysis has already been described in the literature and is accurate to within about 2%. However, it contains a systematic error in that it is difficult to ensure complete conversion of the monomer units to their respective methyl esters and some dehydration to the corresponding or,)3-unsaturated ester (e.g. hydroxybutyrate to crotonate) usually occurs. The liquid chromatography technique suffers from similar problems but the hydrolysis to free acid using perchloric acid tends to be a cleaner reaction than methanolysis and it is our experience that comonomer ratios can be determined with an accuracy better than 1%. 

Zimm-Schulz distribution (88,89). Using an analysis method similar to that used previously on the poly(Q -MeSty)-6Zoc -poly(4-vinyl pyridine) system, the MMD of both parts of the copolymer were determined. The data analysis method was claimed to verify the random coupling hypotheses. The hypothesis (90) that the polydispersity of individual blocks is higher than the polydispersity of the whole polymer was confirmed (85). That is, block copolymers with narrow MMD have broad complex chemical composition distribution. 

Using MAS C NMR, the resonances at 127 and 136 ppm were attributed to the cis and trans double bonds and that at 178 ppm was for the carbonyl groups. The composition of the copolymers was determined by elemental analysis, radioassay of C content in the labeled copolymers and NMR. The copolymers were found to have a composition of 95, 90, and 87 mol% acetylene. The molecular weight of the insoluble copolymers was determined by radioassay of the T content after adjustment for the kinetic isotope effect (KEF). At a reaction temperature of —78 C, copolymers in the form of thin film were obtained with A/n = approximately 4000, whereas at a reaction temperature of 25 C, only powder-like products with A/n = approximately 1000 were obtained. The homopolyacetylene was found to have A/ = 11 000 using... 

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