Polymer HPLC oligomers

In this chapter, high-performance liquid chromatography of oligomers and (high) polymers (polymer HPLC) will be briefly presented. As mentioned in Section 16.1, there exist several monographs, chapter in books, and review papers on this subject, for example [1-33], Most of them contain numerous examples of the HPLC separation and molecular characterization of particular macromolecular substances. Therefore, this chapter discusses almost exclusively the general principles of polymer HPLC and only few selected examples of practical applications will be mentioned for illustration. 

For polymer HPLC, important parameter is viscosity of polymer solution. It increases with the molar mass of macromolecules and also with the improving thermodynamic quahty of solvent that is with the size of macromolecules in solution. High viscosity of polymer solutions prevents execution of liquid chromatography measurements at elevated concentration, usually above 1% or 10 mg.mL for polymers with intermediate molar mass. Oligomers can be treated at even tentime higher concentration. On the other hand, extreme solution viscosity of ultra-... 

There are a number of detection options, some used primarily for GPC and others that have use for GPC as well as other modes of HPLC. The differential refractometer, viscometer, and light-scattering detectors are associated mostly with GPC, while absorbance detectors such as the UV/ visible or photodiode array (PDA) are widely used in all HPLC modes, including GPC. The UV/visible and PDA are especially useful for characterizing polymers and oligomers with chromophoric groups and for HPLC analyses of additives. Mass spectrometry is also used for some analyses. This is described in Sec. ILF. 

Fig. 3. Graph based on an hplc chromatogram of a commercial PTMEG of molecular weight = 1000. The bars represent the weight percentage of the individual oligomer fractions. The degree of polymerization is the number of repeating monomer units per polymer chain.

Applications The determination and quantification of oligomers from PET has been carried out using various techniques Soxhlet extraction followed by gravimetric analysis with identification by HPLC-DAD, selective precipitation of the polymer (from a trifluoroacetic acid solution), and chloroform extraction under pressure in a sealed Parr bomb [112]. Heating of a 1 g sample in 20 mL chloroform at 100 °C for 2h allows a precision of 5 %. 

Various techniques have been used for the determination of oligomers, including GC [135], HPLC [136-138], TLC for polystyrene and poly a-methyl-styrene [139] and SEC for polyesters [140,141]. GC and PyGC-MS can also profitably be used for the analysis of the compositions of volatile products formed using different flame retardants (FRs). Takeda [142] reported that volumes and compositions of the volatile products and morphology of the char were affected by FRs, polymers (PC, PPE, PBT) and their reactions from 300... 

In SEC analysis of additive extracts from polymers, the effect of the extraction solvent on the mobile phase is less critical than in HPLC analysis. The extraction solvents typically employed generally do not interfere with the SEC mobile phases. Moreover, the same solvents are often used both as extraction solvent and as mobile phase. Therefore, there is no need to evaporate the extract to dryness prior to analysis and then to redissolve it in a suitable solvent. Typical extraction procedures often produce extracts that generally contain a small amount of wax. Frequently, removal of such oligomers from an extract is necessary, e.g. by means of precipitation, centrifuging, precolumn filtration or protection (use of a reversed-phase guard column). In SEC separations the presence of polyolefin wax does not usually disturb provided that the MW of the wax is higher than that of the analysed compounds. 

Chromatographic analysis of these products demonstrates that these diphosphonates do not form a single 99mTc complex. For example, as many as 8-10 different "mTc species are resolvable on HPLC analysis [68-78]. Results from these and other studies indicate that 99mTc complexes with diphosphonate ligands form a mixture of oligomers and polymers that may include Sn [78], The only Tc-MDP structure, reported by Libson et al. [79], shows Tc-MDP to be a 1 1 polymer in which each Tc atom is coordinated to two diphosphonate ligands and each diphosphonate is coordinated to two Tc centers. 

The key to a controlled molecular weight build-up, which leads to the control of product properties such as glass transition temperature and melt viscosity, is the use of a molar excess of diisopropanolamine as a chain stopper. Thus, as a first step in the synthesis process, the cyclic anhydride is dosed slowly to an excess of amine to accommodate the exothermic reaction and prevent unwanted side reactions such as double acylation of diisopropanolamine. HPLC analysis has shown that the reaction mixture after the exothermic reaction is quite complex. Although the main component is the expected acid-diol, unreacted amine and amine salts are still present and small oligomers already formed. In the absence of any catalyst, a further increase of reaction temperature to 140-180°C leads to a rapid polycondensation. The expected amount of water is distilled (under vacuum, if required) from the hot polymer melt in approximately 2-6 h depending on the anhydride used. At the end of the synthesis the concentration of carboxylic acid groups value reaches the desired low level. 

High-performance liquid chromatography (HPLC) techniques are widely used for separation of phenolic compounds. Both reverse- and normal-phase HPLC methods have been used to separate and quantify PAs but have enjoyed only limited success. In reverse-phase HPLC, PAs smaller than trimers are well separated, while higher oligomers and polymers are co-eluted as a broad unresolved peak [8,13,37]. For our reverse-phase analyses, HPLC separation was achieved using a reverse phase. Cl8, 5 (Jtm 4.6 X 250 mm column (J. T. Baker, http //www.mallbaker.com/). Samples were eluted with a water/acetonitrile gradient, 95 5 to 30 70 in 65 min, at a flow rate of 0.8 mL/min. The water was adjusted with acetic acid to a final concentration of 0.1%. All mass spectra were acquired using a Bruker Esquire LC-MS equipped with an electrospray ionization source in the positive mode. 

In this case, enthalpic interactions within the HPLC system exceed the exclusion effects (Eigure 16.3b). The retention volumes of polymer species as a rule exponentially increase with their molar masses. The limitations of the resulting procedures were elucidated in Section 16.3 the retention of (high)polymers is usually so large that these do not elute from the column (Section 16.6). Therefore, the majority of enthalpy controlled HPLC procedures is applicable only to oligomers—up to... 

B. Trathnigg, M. Kollroser, D. Berek, S.H. Nguyen, D. Hunkeler, Quantitation in the analysis of oligomers by HPLC with ELSD, in Chromatography of Polymers Hyphenated and Multidimensional Techniques, T. Provder, ed., ACS Symp. Sen, Washington, DC, 731, 178, 1999. 

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