Power of Ultra Performance Liquid Chromatography/Electrospray Ionization-MS Reconstructed Ion Chromatograms in the Characterization of Small Differences in Polymer Microstructure

Abstract

From simple homopolymers to functionalized, 3-dimensional structured copolymers, the complexity of polymeric materials has become more and more sophisticated. With new applications, for instance, in the semiconductor or pharmaceutical industry, the requirements for the characterization have risen with the complexity of the used polymers. For each additional distribution, an additional dimension in analysis is needed. Small, often isomeric heterogeneities in topology or microstructure can usually not be simply separated chromatographically or distinguished by any common detector but affect the properties of materials significantly. For a drug delivery system, for example, the degree of branching and branching distribution is crucial for the formation of micelles. Instead of a complicated, time-consuming, and/or expensive 2D-chromatography or ion mobility spectrometry (IMS) method, that also has its limitations, in this work, a simple approach using size exclusion chromatography (SEC) coupled with electrospray ionization (ESI) mass spectrometry is proposed. The online coupling allows the analysis of reconstructed ion chromatograms (RICs) of each degree of polymerization. While a complete separation often cannot be achieved, the derived retention times and peak widths lead to information on the existence and dispersity of heterogeneities. Although some microstructural heterogeneities like short chain branching can for large polymers be characterized with methods such as light scattering, for oligomers where the heterogeneities just start to form and their influence is at the maximum, they are inaccessible with these methods. It is also shown that with a proper calibration even quantitative information can be obtained. This method is suitable to detect small differences in, e.g., branching, 3D-structure, monomer sequence, or tacticity and could potentially be used in routine analysis to quickly determine deviations.

Detection challenges in quantitative polymer analysis by liquid chromatography

“…Epping et al. recently explored the use of LC‐ESI‐MS in the microstructure determination of copolymers; by combining retention time, chromatographic peak width, and MS information, it was possible to elucidate in depth microstructure and topology information on various glycol and ethoxylate oligomers [90]. Furthermore, by using separation at LCCC conditions coupled to ESI‐MS insight into the MWD, end‐group distribution, CCD, as well as the sequence of ethylene oxide‐propylene oxide copolymers could be obtained [91].…”

Co-Polymer Sequence Determination Over the Molar Mass Distribution by Size-Exclusion Chromatography Combined with Pyrolysis - Gas Chromatography

“…The sequence in notoriously hard to determine, as it requires quantitative measurements of the numbers of subunits in the chain (i.e. diads or triads), which can only be achieved by a limited number of methods [ 13 , 16-18 ] Methods to study the sequence of co-polymers include infrared spectroscopy [ 17 , 19 ], mass spectrometry (MS) [20][21][22] , pyrolysis -gas chromatography (Py-GC) [23] and nuclear-magneticresonance spectroscopy (NMR) [ 12 , 13 , 16 , 24-26 ]. Despite the availability of various methods, co-polymer sequence determination is rarely straightforward.…”

Resolving Isomers of Star-Branched Poly(Ethylene Glycols) by IMS-MS Using Multiply Charged Ions

“…Steric hindrance of neighboring arms, however, may cause asymmetric arm growth, resulting in architectural dispersity. , Detailed characterization of the three-dimensional (3-D) architecture of star-branched polymers is difficult or impossible using well-established polymer methods. Light scattering, size exclusion chromatography, and viscometry, or a combination, are employed in determining average properties of polymer architectures such as polymer size and shape. − Methods such as (ultra)­high resolution mass spectrometry (MS) coupled with MS/MS have demonstrated structural characterization in complex polymeric samples. , These methods are limited to specific polymer compositions such as the presence of a fragile linker group for differentiation of block copolymer arm length …”