Abstract
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.
Highlights
The cationic ring opening polymerization (CROP) of 2-oxazolines, which can be used to prepare well defined polymers with narrow molar mass distributions, was discovered back in 1966 [1,2,3,4]
The majority of such temperature induced solubility transitions are based on the lower critical solution temperature (LCST) behavior of polymers in aqueous solution, i.e., the polymer is dissolved at low temperature by favorable hydration while increasing the temperature leads to an entropy driven dehydration resulting in a collapse of the hydrophobic polymer chains [18,21,23]
The solubility behavior of this series of poly(2-oxazoline)s was investigated at a polymer concentration of 5 mg/mL in ethanol-water solvent mixtures ranging from pure water to pure ethanol
Summary
The cationic ring opening polymerization (CROP) of 2-oxazolines, which can be used to prepare well defined polymers with narrow molar mass distributions, was discovered back in 1966 [1,2,3,4]. Little is known about the solution behavior of polymers in water-ethanol mixtures, despite such mixtures being known to exhibit interesting abnormal mixing properties due to the presence of hydration shells around the ethanol molecules [40,41,42] This peculiar mixing behavior has been demonstrated to result in solubility maxima for several drug molecules in aqueous ethanol [43,44] as well as for poly(methyl methacrylate) [45,46]. Schematic representation of the structures of the investigated 2-oxazolines and the corresponding poly(2-substituted-2-oxazoline)s prepared by cationic ring-opening polymerization (CROP)
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