Stimulus-Responsive Water-Soluble Polymers Based on 2-Hydroxyethyl Methacrylate

Abstract

The homopolymerization and block/statistical copolymerization of 2-hydroxyethyl methacrylate (HEMA) using atom transfer radical polymerization (ATRP) in methanol at 20 °C has been investigated. For the homopolymerizations, both high conversions and low polydispersities (M w/M n < 1.25) were obtained over a wide range of target degrees of polymerization. According to the literature, HEMA homopolymer is usually described as only water-swellable, but in this work low molecular weight HEMA oligomers (target degrees of polymerization, DPn, less than 20) exhibited water solubility over a wide temperature range (no cloud point behavior). Furthermore, for actual DPn's between 20 and 45, HEMA homopolymers exhibited inverse temperature solubility in dilute aqueous solution at pH 6.5, and their cloud points increased systematically as the DPn was reduced. Gravimetric studies indicated that “water-insoluble” HEMA homopolymers with DPn's higher than 50 were actually partially soluble: GPC studies confirmed that fractionation occurred due to preferential dissolution of the shorter chains. Furthermore, HEMA homopolymers with DPn's up to 50 are water-soluble at pH 2.2 and do not exhibit cloud points. This is attributed to protonation of the terminal morpholine groups derived from the ATRP initiator. Thus, depending on the mean DPn and the solution pH, water can be a good solvent, a marginal solvent or a nonsolvent for HEMA homopolymer. Chain extension (self-blocking) experiments conducted for the ATRP of HEMA in methanol at 20 °C using a Cu(I)Cl catalyst and bpy ligand indicated reasonable living character. Statistical copolymerizations of HEMA with other comonomers such as glycerol monomethacrylate (GMA) and 2-hydroxypropyl methacrylate (HPMA) allowed the cloud point behavior to be manipulated. Finally, a range of novel HEMA-based block copolymers were synthesized in which the HEMA block was either thermoresponsive or permanently hydrophilic, depending on its DPn and the nature of the second block. Thus, diblock copolymer micelles with either hydroxylated cores or coronas could be prepared.

Synthesis and Solubility of (Mono-) End-Functionalized Poly(2-hydroxyethyl methacrylate-g-ethylene glycol) Graft Copolymers with Varying Macromolecular Architectures

“…ature solubility (Figure 3a), similar to lower molecular weight PHEMA (cloud point temperatures ∼28-39°C in dilute aqueous solutions at pH 6.5 18 ) and PEG homopolymers. 23 For PEG, this interesting behavior has been explained in terms of entropic effects of fitting PEO molecules into the water network, 24 conformationdependent hydrophobicity, 25 and competititive etherwater and water-water H-bonding.…”

Light-Sensitive Polymeric Nanoparticles Based on Photo-Cleavable Chromophores

“…On the other hand, the water solubility of the resulting polymer is well known to crucially depend on the molecular weight of PHEMA. [270][271] As the described microgels were prepared by free radical copolymerization, no control over the molecular weight of PHEMA was achieved. As a consequence, the resulting lengths of the polymeric chains of the gel network were assumed to be in the range of restricted water solubility and thus hindering particles swelling in aqueous medium.…”

Synthesis and self-assembly of poly(ethylene oxide)-b-poly (2-hydroxyethyl methacrylate)

“…As for PEO-b-PHEMA copolymer, although PEO and PHEMA are water soluble blocks, HEMA homopolymers with DPn"s higher than 50 were actually partially soluble [20]. So PEO-b-PHEMA-4 was selected to investigate its self-assembly in water.…”

“…A recent overview on the synthetic routes toward DHBCs and some of their prominent applications was presented by Colfen [1]. Armes [20] found that copolymerizations of HEMA with other comonomers allowed the cloud point behavior to be manipulated. A range of novel HEMA-based block copolymers were synthesized in which the HEMA block was either thermo-responsive or permanently hydrophilic, depending on its degree of polymerization (DP n ) and the nature of the second block.…”