Solution Properties and pH Responsiveness

The most utilized PAI congeners, namely B-PEI and L-PEI, have quite different solubility behaviour. B-PEI is soluble in water, independent of solution pH, and various organic solvents, while L-PEI in its free base form is insoluble in water and most organic solvents at room temperature, except lower alcohols, due to the formation of insoluble L-PEI crystals. Aqueous solutions of the L-PEI freebase also display temperature-dependent solubility behaviour as it becomes soluble in water above 64 °C. FTIR spectroscopy has confirmed that this phase transition is due to a melting transition from a crystalline zig-zag state to the hydrated random coil state.When cooled from the heated soluble state, the polymer forms a crystalline fibre-based hydrogel, which can be chemically crosslinked with glutaric anhydride.  

Detailed studies on the pH-dependent solubility of L-PEI have shown that ca. 10% of protonated amines is required to disrupt the polymer crystal structure and to dissolve the polymer in water. This solubility behaviour also explains the incomplete alkaline hydrolysis of PAOx when converting it to L-PEI. Hence L-PEI is usually employed as a salt, invariably the hydrochloride salt, and the properties of other salts are largely unknown. L-PEI displays an overall pTsTg of 7.2-7.9, whereas B-PEI of 25 kDa displays a bulk pkTa of 8.4 with the individual pTsTg of the 1°, 2° and 3° amines being ca. 9, ca. 8 and 6-7, respectively. Hence at physiological pH of 7.4, approximately 55% of the amines in L-PEI will be protonated in comparison to 90% of the amines of B-PEI. 

For evaluation and understanding of the properties of PEI in biological applications, it is crucial to know the molar mass and dispersity of the polymers. However, unlike for most other polymers, SEC of PEI is not trivial. Even though B-PEI is soluble in many common organic solvents, these polymers cannot be analysed by SEC using common solvents as eluent due to strong interactions between the amines and the column, while L-PEI is 

These alternative methods are rather time-consuming and provide, however, no information on the distribution of molecular weight. Therefore, SEC analysis is the preferred method, even though HFIP has to he utilized as eluent. 

The most interesting physical property of PEI is the cationic charge. This allows the complexation with anionic materials, but also creates a challenge in analysis. The p Tg of the amines varies depending on their nature and environment, and can he used to tune the properties. Unfortunately, veiy little is known about the solubility and p/Tg of other PAIs. 

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