Polymers with Upper Critical Solution Temperature in Aqueous Solution
In this work polymers with upper critical solution temperature (UCST) in water have been synthesized and characterized. The task was to answer the question why polymers with UCST in water were largely underrepresented in basic and applied research and to develop a universal approach for the synthesi...
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|In this work polymers with upper critical solution temperature (UCST) in water have been synthesized and characterized. The task was to answer the question why polymers with UCST in water were largely underrepresented in basic and applied research and to develop a universal approach for the synthesis of new polymers with UCST.
Drawing on the example of poly(N-acryloylglycinamide) (poly(NAGA)), a polymer that is well known since 1964, it was shown that traces of ionic groups drastically decrease the UCST. In the past ionic groups have been introduced unintentionally by either acrylate impurities in the monomer, hydrolysis of the polymer side chains and/or usage of ionic initiators or chain transfer agents giving a feasible explanation why the UCST of poly(NAGA) remained unreported until 2010 as a part of this work. The synthesis of NAGA was modified in order to yield acrylate free monomer and the crystal structure was determined. A detailed analysis of the UCST suppressing effects of ionic groups followed. Only 1 ionic group in 1000 repeating units of the polymer is sufficient to completely suppress the UCST behavior. Therefore, conventional analytic methods like NMR, IR, elemental analysis, thin layer chromatography, etc. were not sensitive enough. Special methods like capillary electrophoresis and flame atomic absorption spectroscopy had to be employed to prove the hypothesis. The UCST of poly(NAGA) relies on thermally reversible hydrogen bonding. The sensitivity to ionic groups in the polymer may be explained by the very low heat of transition which could be determined by using ultrasensitive differential scanning calorimetry. Furthermore, in a prelimenary dynamic and static light scattering study it could be shown that when polymer aggregates collapsed upon cooling below the phase separation temperature their density increased by more than two orders of magnitude. In order to study the effect of polymer endgroups and the molecular weight distribution on the phase transition controlled radical polymerization via the radical addition fragmentation transfer process (RAFT) was performed. As expected, polymers with an ionic endgroup showed no UCST in pure water but in electrolyte solution. The addition of salts influenced the cloud points according to the Hofmeister series of ions.
The phase transition temperature of poly(NAGA) was shifted by copolymerization with hydrophobic comonomers like butyl acrylate and styrene. The choice of monomer turned out to be crucial for the sharpness and reversibility of the phase transition. Fundamental knowledge obtained from poly(NAGA) and copolymers was subsequently transferred to simple commercially available monomers. A set of rules were established how to obtain polymers with freely tunable UCST in water as well as electrolyte solution. This hypothesis was proven by the synthesis of poly(methacrylamide) and poly(acrylamide-co-acrylonitrile), actually very old polymer systems, that showed a UCST. Finally, knowledge gained in this and previous works was condensed into a review article allowing the conclusion of further structure-property-relationships.