Temperature induced solubility transitions of various poly(2-oxazoline)s in ethanol-water solvent mixtures

The solution behavior of a series of poly(2-oxazoline)s with different side chains, namely methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, pentyl, hexyl, heptyl, octyl, nonyl, phenyl and benzyl, are reported in ethanol-water solvent mixtures based on turbidimetry investigations. The LCST transitions of poly(2-oxazoline) s with propyl side chains and the UCST transitions of the poly(2-oxazoline) s with more hydrophobic side chains are discussed in relation to the ethanol-water solvent composition and structure. The poly(2-alkyl-2-oxazoline) s with side chains longer than propyl only dissolved during the first heating run, which is discussed and correlated to the melting transition of the polymers

Thermoresponsive poly(2-oxazoline) block copolymers exhibiting two cloud points: complex multistep assembly behavior

Aqueous solutions of poly(2-oxazoline) block copolymers consisting of a 2-ethyl-2-oxazoline block and a block consisting of a random copolymer of 2-ethyl-2-oxazoline and 2-n-propyl-2-oxazoline (PEtOx-block-P(EtOx-stat-PropOx)) have been studied by dynamic light scattering (DLS), static light scattering (SLS), and turbidimetry. Even at temperatures significantly below the lower critical solution temperature (LCST), polymer unimers are found to coexist with a few large aggregates with an open structure. When heated, the systems exhibit an intricate transmittance behavior whereby the samples becomes visually clear again after an initial cloud point and then exhibit a second cloud point at even higher temperatures. The DLS data indicate that the aggregates formed around the first cloud point restructure and fragment into smaller micelle-like structures ascribed to further dehydration of the more hydrophobic PPropOx containing block, causing the samples to become optically clear again. The observed fragmentation is confirmed by the SLS experiments. At even higher temperatures, both blocks become hydrophobic, causing the formation of large, compact aggregates, resulting in a second cloud point

Efficient cationic ring-opening polymerization of diverse cyclic imino ethers: unexpected copolymerization behavior

The recently developed fast microwave-assisted cationic ring-opening polymerization procedure for 2-oxazolines seems to be ideally suited for slower polymerizing cyclic imino ether monomers. In this study we report the effect of the cyclic imino ether structure on the polymerization rate under exactly the same microwave-assisted conditions revealing that indeed less reactive cyclic imino ethers, including 2-oxazines as well as 4- and 5-substituted 2-oxazolines, can be polymerized to at least 50% conversion for the slowest monomer, namely 5-methyl-2-butyl-2-oxazoline, within 10 h. In addition, the copolymerization behavior of 4-ethyl-2-butyl-2-oxazoline with 2-methyl-2-oxazoline and 2-phenyl-2-oxazoline unexpectedly revealed faster incorporation of the less reactive 4-ethy1-2-buty1-2-oxazoline monomer compared to 2-phenyl-2-oxazoline due to the increased bulk of the latter monomer amplifying the sterical hindrance for polymerization onto the 4-ethyl-2-butyl-2-oxazolinium propagating species

Star-shaped poly(2-oxazoline)s by dendrimer endcapping

The synthesis of star-shaped poly(2-ethyl-2-oxazoline) is reported by direct end-capping of the living polymer chains with dendritic multiamines. The end-capping kinetics after addition of a first generation polypropylenimine dendrimer are discussed based on monitoring by size exclusion chromatography, revealing less efficient end-capping with larger poly(2-ethyl-2-oxazoline) chains and increasing dendrimer generation. In addition, it is demonstrated that the solution viscosity and cloud point temperature of the star-shaped polymers are much less affected by chain length compared with their linear analogues

Thermal, mechanical, and surface properties of poly(2-N-alkyl-2-oxazoline)s

Thermal, mechanical, and surface properties of a library of poly(2-oxazoline)s are investigated. These polymers are suitable to study structure/property relationships as their cationic ROP and the relative facile monomer synthesis allow for control over the molecular structure. The number of carbon atoms in the linear side-chain is systematically varied from methyl to nonyl. Relations between chemical structures, thermal transitions, surface energies, and elastic moduli are discussed. It is shown that the mechanical and thermal properties of the polymers depend on the presence of a crystalline phase in the material. The amorphous polymers reveal a decrease in the reduced moduli along with a decrease in their respective glass transition temperature with increasing length of the side-chain

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 110 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 T-g 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 it similar increase with M/I ratio as observed for the T-g 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

Water uptake of poly(2-N-alkyl-2-oxazoline)s : influence of crystallinity and hydrogen-bonding on the mechanical properties

Poly(2-oxazoline)s are suitable materials to study structure-property relationships as their preparation by a living cationic ring-opening polymerization procedure and the relatively facile monomer synthesis allow accurate control over the molecular structure. In this contribution, the number of carbon atoms in the linear side-chain is systematically varied from a short methyl-to a long nonyl-group. As some of the materials are known to be hygroscopic, the effect of water uptake on the mechanical properties is investigated in detail. The combination of water uptake measurements, FT-IR spectroscopy and indentation revealed that only the samples with very short side-chains show significant hygroscopicity, while samples with longer side-chains exhibit crystalline behavior. Furthermore, depending on the polymer structure, it could be differentiated between side-chain and main-chain crystallinity

Thermoresponsive poly(2-oxazine)s

The monomers 2-methyl-2-oxazine (MeOZI), 2-ethyl-2-oxazine (EtOZI), and 2-n-propyl-2-oxazine (nPropOZI) were synthesized and polymerized via the living cationic ring-opening polymerization (CROP) under microwave-assisted conditions. pEtOZI and pnPropOZI were found to be thermoresponsive, exhibiting LCST behavior in water and their cloud point temperatures (TCP) are lower than for poly(2-oxazoline)s with similar side chains. However, comparison of poly(2-oxazine) and poly(2-oxazoline)s isomers reveals that poly(2-oxazine)s are more water soluble, indicating that the side chain has a stronger impact on polymer solubility than the main chain. In conclusion, variations of both the side chains and the main chains of the poly(cyclic imino ether)s resulted in a series of distinct homopolymers with tunable TCP