Retention mechanism, isocratic and gradient-elution separation and characterization of (co)polymers in normal-phase and reversed-phase high-performance liquid chromatography

https://doi.org/10.1016/S0021-9673(99)01216-9Get rights and content

Abstract

Synthetic (co)polymers or (co)oligomers with two (or more) repeating groups show not only molar mass distribution, but also composition and sequence distribution of the individual repeat units. To characterize such two- (or more-) dimensional distribution, liquid chromatography under “critical conditions” has been suggested, where the separation according to one type of repeating units is suppressed by balancing the adsorption and the size-exclusion effects. In present work it is shown that by combination of adequately selected separation conditions in normal-phase and in reversed-phase systems, the two-dimensional distribution mode can be adjusted to result in the separation following the distribution of any of the two repeat units in ethylene oxide–propylene oxide block (co)oligomers. Based on the retention mechanism suggested, prediction and optimization of the conditions for isocratic and gradient-elution separations of (co)oligomers is possible. HPLC–MS with atmospheric-pressure chemical ionization is a valuable tool for unambiguous identification of the individual (co)oligomers and their tracking in course of method development. Gradient elution can be used for the separation and characterization of block (co)oligomers of ethylene oxide (EO) and propylene oxide (PO) according to the number of the units in one block, while the separation according to the distribution of the units in the other block is suppressed. The effects of the arrangement of the individual EO and PO blocks in the block (co)oligomers (the sequence distribution) affects significantly the retention behavior and the selection of the optimum separation conditions.

Introduction

With widespread use of synthetic polymer- or oligomer-based materials in industry, agriculture and households, the demands increase on the properties of technical products with respect to mechanical and chemical resistance, biodegradability, etc. Technical polymers and oligomers contain varying types and numbers of structural units, the distribution of which controls their quality and suitability for specific applications. For the evaluation of the quality of technical products and for further development of polymerization techniques leading to new tailor-made materials with improved properties, characterization methods are necessary to provide information on the polymer structure. In addition to classical wet analytical and spectroscopic methods, separation techniques and especially size-exclusion (SEC) and high-performance liquid chromatography (HPLC) can provide most detailed information on the composition of the polymer products.

The basic information on the character of synthetic polymers and oligomers is their molecular mass. However, technical products never are a pure single species, but are more or less complex mixtures of different compounds containing one or more structural repeat units. The composition of these mixed products can be characterized by one or more types of distribution.

(1) Oligomers or polymer products composed of a single repeat monomeric unit, U, are always mixtures containing individual species with various numbers of repeat units, n. The distribution of the species with different numbers of the repeat units represents the molecular mass distribution in the product, which can be schematically represented as: –U–, –U–U–, –U–U–U–, –U–U–U–U–, –U–U–U–U–U–,…

(2) Some polymers may contain various end groups, E1, E2,… so that the functionality type distribution can be attributed to such products, e.g.,: E1–U–U–U–U–E1, E2–U–U–U–U–E2, E1–U–U–U–U–E2

(3) (Co)polymers and (co)oligomers contain two or more different monomeric repeat units, U1, U2,… in various ratios, n1:n2…, controlling the chemical composition distribution of the product, schematically represented as: –U1–U2–U1–U1–U1–, –U1–U2–U1–U2–U1–, –U1–U2–U2–U1–U2–, –U1–U2–U2–U2–U2

(4) Finally, different monomeric units can be present in different sequences of microblocks with different numbers of the individual repeat units along the polymer chain in random or block (co)polymers. This polymer heterogeneity is characterized by the sequence distribution, such as: –U1–U2–U1–U2–U1–U2–, –U1–U1–U1–U2–U2–U2–, –U1–U1–U2–U2–U2–U1

The molecular mass distribution can be determined by size-exclusion chromatography, but various distribution types can be characterized using HPLC in normal-phase and in reversed-phase modes (interaction chromatography).

Ethylene oxide–propylene oxide (EO–PO) block copolymers are nonionic surfactants frequently used in washing machines. Because of their thickening capacity and low skin sensitivity they found also applications as the emulsifiers and solubilizers of flavors and fragrances in cosmetic products.

The oxypropylene chain –[CH2–CH(CH3)–O]m–=(PO)m in the EO–PO block copolymers is hydrophobic, but the oxyethylene chain –[CH2–CH2–O]n–=(EO)n is hydrophilic. Two types of block (co)polymers with different sequence distribution can be prepared using different technologies: The (EO)n–(PO)m–(EO)n type is prepared by ethoxylation of polypropylene glycol. The reversed reaction sequence, reaction of polyethylene glycol with propylene oxide is used to prepare the second type of block (co)polymers, (PO)m–(EO)n–(PO)m. All three types of distribution – the molecular mass distribution, the chemical composition distribution and the sequence distribution – affect the properties of the technical products. With increasing average molecular mass the gelation becomes easier, whereas increasing number of PO units improves the wetting properties of the products and increasing number of EO units improves the solubility in water, but impairs the wetting properties. The (co)polymers with the (PO)m–(EO)n–(PO)m structure are characterized by reduced foam formation and gelation with respect to the (co)polymers of the (EO)n–(PO)m–(EO)n type [1].

Efficient separations of ethoxylated non-ionic surfactants can be achieved by HPLC both in normal-phase systems on columns packed with unmodified silica gel or with amino, nitrile or diol chemically bonded phases [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16] and in reversed-phase systems [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. Few reports have been published so far on the systematic investigation of their retention behavior [27], [28].

Individual ethoxylated oligomers are eluted in the order of increasing number of oligomeric oxyethylene units in normal-phase systems. In reversed-phase systems, the order of elution of the oligomers depends on the oligomeric series and on the mobile phase. For example, oligoethylene glycol phenyl ethers are eluted in the order of increasing number of oxyethylene units on an octadecylsilica column [29] and so are oligoethylene glycol octylphenyl ethers on a trimethylsilica column in mobile phases comprised of water and methanol [24], whereas oligoethylene glycol nonylphenyl ethers are almost unseparated on a C18 column in these mobile phases (Fig. 7 in Ref. [30]) and their order of elution is reversed in aqueous acetonitrile [26], [31], [32], propanol or dioxane [30], like in chromatography on a mixed-mode reversed-phase/ion-exchange column [33]. We have studied earlier simultaneous effects of the degree of polymerization, n, and of the concentration of the stronger solvent in binary mobile phases on the retention in nonionic oligomeric series, both in reversed-phase [30], [34], and in normal-phase [35] systems. Recently, we have extended this investigation to the anionic oligomeric surfactants in reversed-phase [36] and normal-phase ion-pair systems [37] and to ethoxylated alcohol surfactants with different numbers of oxyethylene units and alkyl chain lengths [38].

In the present work, we investigated possibilities of using HPLC in normal-phase and in reversed-phase modes for the determination of the molecular mass distribution, chemical composition distribution and block sequence distribution in block copolymers, with special attention to the EO–PO block copolymers.

Section snippets

Theoretical

SEC has been widely used for the characterization of the molecular mass distribution. In the separations controlled solely by size-exclusion mechanism, polymeric compounds, dependent on their size, partly penetrate into the pores of the inert column packing. As the volume of the pores accessible to the molecules decreases as the size of the molecules increases, the elution volumes, Ve, decrease with increasing molecular masses of the individual species and Ve of all sample components are in the

Materials

n-Hexane and dichloromethane were obtained from Baker (Deventer, The Netherlands), acetonitrile and 2-propanol from Labscan (Dublin, Ireland). All solvents were of HPLC grade. Water was doubly distilled in glass (with addition of potassium permanganate and sodium hydrogencarbonate). Glass cartridge columns (150×3 mm I.D.), packed with Separon SGX C18, particle size 7 μm (octadecyl silica) and Separon SGX NH2, 7 μm (aminopropyl silica), both average pore size 8 nm, were purchased from Tessek,

HPLC–APCI-MS analysis of the EO–PO block (co)oligomers

Mass spectrometric detection with APCI ionization in the positive ion mode allows sensitive detection of the samples of EO–PO (co)oligomers, which do not absorb radiation in the UV region. Further, unambiguous identification of the individual species is possible even in the chromatograms with overlapping peaks, as the ionization conditions (sample cone voltage) were adjusted to yield only [M+H]+ ions. Fig. 1 shows examples of mass spectra of overlapped peaks of the (EO)n–(PO)m–(EO)n (Slovanik,

Conclusions

Reversed-phase or normal-phase HPLC can be successfully used for the separations of EO–PO (co)oligomers. The separation selectivity for low polarity repeat PO units is better in reversed-phase systems, but the selectivity for more polar EO repeat units is superior in normal-phase systems on a bonded amine column with propanol–hexane mobile phases.

In both RP or NP separation systems, the separation of the EO–PO (co)oligomers according to one distribution mode can be suppressed and the separation

Acknowledgements

This research was supported by the Grant Agency of Czech Republic, project 203/98/0598, and by the Ministry of Education of the Czech Republic, project VS 96058.

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