Novel Thermoresponsive Polymers Tunable by pH

Thermoresponsive polymers that are pH tunable were successfully synthesized by a combination of atom transfer radical polymerization (ATRP) and Cu(I) -catalyzed 1,3-dipolar cycloaddition of azide and alkynes (click chemistry). ATRP was employed to synthesize poly(2-hydroxyethyl methacrylate) (PHEMA), followed by introduction of alkyne groups using pentynoic acid, leading to PHEMA-alkyne. 2-Azidoethylamine, 2-azido-N,N-dimethylethylamine, and 2-azido-N,N-diethylethylamine were added to the PHEMA-alkyne backbone via click chemistry. Molecular weight, molecular weight distribution, and click reaction efficiency were determined by gel permeation chromatography (GPC) and H-1 NMR spectroscopy. The average molecular weight (M-n) of the resulting polymers ranged from 5.6 x 10(4) to 7.0 x 10(4) depending on the molecular architecture. The molecular weight distribution was low (M-n/M-n = 1.25-1.35). The transmission spectra of the 0.1 wt % aqueous solutions of the resulting polymers with different pH values at 650 nm were measured as a function of temperature. Results showed that the lower critical solution temperature (LCST) could be dramatically affected by solution pH. To give additional evidence for pH-responsive thermal transition, in-situ temperature-dependent H-1 NMR measurements in deuterated water (0.01 wt %) were conducted. The LCST values measured by in-situ H-1 NMR correlated well with those determined by turbidimetry.