HPLC-Kopplung mit Gelpermeationschromatographie
Final Report Abstract
The instrument was purchased and installed to fulfil the need within the Barner-Kowollik research group to enable a precision analysis of block copolymer structures as well as complex mixtures of polymers via variable modes of chromatography. Specifically, the ability to not only conduct SEC and HPLC separately was of importance, but also the coupling of both techniques was of critical interest. In contrast to an NMR or mass spectrometer, a two dimensional chromatographic experiment cannot be used for everyday routine analysis but requires careful planning and preparation. The reason for the inherent complexity is based on the fact that for each copolymer or complex architecture system to be analysed under LACCC conditions the so-called critical eluent conditions need to be determined, i.e. the solvent composition on the HPLC dimension where the macromolecules do not elute according to their size, but only with respect to their chemical functionality. Since more than one type of polymer is present in a complex architecture design, it is necessary to determine the critical conditions for each polymer separately (which of course requires the synthesis of LACCC standards of the respective polymer). Occasionally, it is not possible to identify critical conditions for a given system. In view of the complexity of the chromatographic analysis, we have a dedicated PhD student, who works with the 2D chromatography system, supported by a highly skilled and permanently employed technical officer with long standing expertise in SEC. In the original grant application different areas of interest were identified to be studied with the 2D SEC system. The first named areas are [A] Reversible Addition Fragmentation Chain Transfer (RAFT) Polymerization for Access to Complex Architecture Polymers // Polymer Formation Processes and [B] Click Transformations of Polymers Prepared via Living/Controlled Radical Polymerization These two areas, which overlap to a certain degree, have benefited substantially from the presence of the 2D chromatography equipment. For example, an in-depth study investigated for the first time the degree of block copolymer purity and efficiency that can be achieved via the use of modular macromolecular construction approaches, while at the same time assessing the purity of individual polymer segments that can be prepared via living/controlled radical polymerization approaches. A very interesting project combines modular synthetic approaches and a – in our laboratories – newly invented controlled radical polymerization techniques (i.e. the ‘Enhanced Spin Capturing Polymerization (ESCP)’). Nitrones equipped with an alkyne functionality were subjected to ESCP or nitrone mediated radical coupling (NMRC) processes and the generated mid-chain alkyne functional polymers were employed for the modular construction of mikto-armed star polymers. The success of the modular ligation chemistry was evidenced via LACCC-SEC coupling and indicated a very clean coupling process. In addition, the switch from a RAFT system to a non-sulphur containing ROP system for the generation of narrow polydispersity block copolymers of poly(styrene) and poly(ε-caprolactone) (PCL) have been investigated for linear block copolymers and star architectures. Here, it was particularly challenging to identify chromatographic conditions that allow for an operation under the critical conditions of PCL and a new LACCC-gradient elution chromatography (GELC) method coupled to SEC was developed to solve this problem. It could be very clearly demonstrated that the target structures could indeed be produced. The four key publications in research areas [A] and [B] are of substantial length and depth. A further area of interest was defined as the investigation of acrylate containing polymer systems. Here, macromonomers were of interest as well as the structure of polymers obtained via acrylate free radical polymerization. The generation of acrylate based macromonomers is possible via a high temperature polymerization process that exploits the branching behaviour of poly(acrylate)s to place with a high fidelity vinyl end groups at the end of polymeric chains. [C] Macromonomers as Versatile Building Blocks for Variable Polymer Architectures and [E] Determination of Chain Branching Levels in Acrylate Polymers: Impact of Polymerization Conditions on the Polymer Microstructure Thus, in a further DFG funded project, macromonomers were employed as building blocks for the preparation of complex polymer designs. In one of the studies we conducted, the copolymerization behaviour of the acrylate based macromonomers with other acrylates was investigated and studied at the critical conditions of the butyl acrylate macromonomers. The microstructure of these all-acrylate systems was revealed via LACCC and SEC and in addition via ESI-MS. Furthermore, the macromonomers were transformed into initiators for ring opening polymerization and LACCC was employed to determine the effectiveness of the ROP polymerization process and to probe the efficiency of variable catalytic initiation system. To a certain amount, the 2D equipment is used by the group of Prof. Manfred Wilhelm for the elucidation of [D] Block Copolymer Structures via Anionic Polymerization. The instrument is occasionally employed to assess the purity of an anionically prepared structure to ensure its structural integrity before its use in rheological experiments. The instrument is further employed in a range of other projects e.g. to fractionate complex polymer samples via the use of the purchased fraction collector (project on the determination of Mark-Houwink-Sakurada constants for branched polymers based on branched monomers), the identification of residual initiator in free radical polymerizations (project on the RAFT based generation of synthetic rubber materials) or the identification of functional peptides (in a DFG funded project – via the Centre for Functional Nanostructures (CFN)) which are used in applications for targeted cell attachment.
Publications
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"Spin Capturing with ‘Clickable’ Nitrones: Generation of Mikto-Armed Star Polymers". Macromolecules 2010, 43, 3785–3793
Wong, E. H. H.; Stenzel, M. H.; Junkers, T.; Barner-Kowollik, C.
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"A Detailed Investigation of the Free Radical Copolymerization Behavior of n-Butyl Acrylate Macromonomers". C. Macromolecules 2011, 44, 6691–6700
Zorn, A.-M.; Junkers, T.; Barner-Kowollik, C.
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"An Efficient Avenue to Poly(styrene)-block-(ε-Caprolactone) Polymers via Switching from RAFT to Hydroxyl Functionality: Synthesis and Characterization". J. Polym. Sci. – Polym. Chem. 2011, 49, 1-10
Schmid, C.; Falkenhagen, J.; Barner-Kowollik, C.
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"Visualizing the Efficiency of Rapid Modular Block Copolymer Construction". Polym. Chem. 2011, 2, 126-136
Inglis, A. J.; Barner-Kowollik, C.
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"In-Depth LCCC-(GELC)-SEC Characterization of ABA Block Copolymers Generated by a Mechanistic Switch from RAFT to ROP". Macromolecules 2012, 45, 87-99
Schmid, C.; Weidner, S.; Falkenhagen, J.; Barner-Kowollik, C.
