Polymerization of long chain alkyl glycidyl ethers: a platform for micellar gels with tailor-made melting points

Abstract

Controlled polymerization of long-chain alkyl glycidyl ethers (AlkGE) under anionic ring opening polymerization conditions is enabled by the addition of a crown ether (18-crown-6, K⁺). Homopolymers with molecular weights in the range of 4000 to 9000 g mol⁻¹ and side chain melting temperatures of 14 °C (C₁₂-AlkGE) and 43 °C (C₁₆-AlkGE), respectively were synthesized. Furthermore, a series of amphiphilic ABA polyether triblock copolymers with polyethylene glycol (PEG) as the hydrophilic central block (M_n = 6k, 10k, and 20k g mol⁻¹) with total molecular weights in the range of 7000 to 28 000 g mol⁻¹ and narrow dispersity (1.12 to 1.34) were prepared. Separate melting endotherms of the triblock copolymers were observed for these double-crystalline materials. Due to self-assembly of the hydrophobic alkyl chains, the addition of water or water/ethanol mixtures to the triblock copolymers leads to the formation of micellar, one-component hydrogels. By varying the repeating units of the hydrophobic PAlkGE blocks, a tunable hydrophobic effect can be achieved. Variation of the alkyl chain length enables tailoring of the melting temperatures of the PAlkGE-blocks both in bulk and hydrogels. Thermal and rheological investigation of ethanol/water gels revealed temperature-dependent mechanical behavior, indicating a transition from crystalline to flexible micellar hydrophobic domains in the range of 12 °C to 31 °C. The amphiphilic polyether triblock copolymers enable the preparation of guest-loaded thermo-responsive gels and hydrogels and bear promise for pharmaceutical applications, e.g. as drug release systems.