Linear Poly(ethylene imine)s by Acidic Hydrolysis of Poly(2-oxazoline)s: Kinetic Screening, Thermal Properties, and Temperature-Induced Solubility Transitions

The kinetics for the acidic hydrolysis of two water-soluble poly(2-oxazoline)s with methyl and ethyl substituents were investigated. It could be observed that poly(2-ethyl-2-oxazoline) (PEtOx) and poly(2-methyl-2-oxazoline) (PMeOx) are hydrolyzed following a linear relation with time. Various polymer lengths and concentrations were investigated, revealing that both parameters had no influence on the hydrolysis kinetics. Comparison between PEtOx and PMeOx revealed that the smaller PMeOx side group could be removed faster. Furthermore, a series of linear poly(ethylene imine) (PEI) were synthesized with varying M/I ratio from 5 to 200 to elucidate structure−property relations. Thermal measurements indicated that the Tg of PMeOx was increasing up to a M/I ratio of 100. PEI, on the other hand, is crystalline and exhibits a melting temperature which shows a similar increase with M/I ratio as observed for the Tg of PMeOx. Solubility measurements in water and water/ethanol mixtures indicated a solubility transition to a soluble state at elevated temperatures. The solubility transition was observed to fit with the melting temperature, indicating that the PEI crystals need to melt in solution before solubilizing. Moreover, when adding ethanol to the water solution, the formed crystals are less stable due to the improved solubility of PEI in ethanol, causing a decrease in cloud point temperature. Upon cooling, PEI crystallizes at much lower temperatures compared to dissolution, causing a large hysteresis presumably due to extensive intermolecular hydrogen bonding, which also plays a role in the formation of the hydrate crystals.