2.2. Preparation of Hydrogels

The poly(acrylic acid)-based macro-RAFT agent was prepared similarly as described elsewhere [25]. In a typical experiment (Table 1, entry E2), AAc (6.100 g, 84.50 mmol), ACPA (18.30 mg, 0.065 mmol) as an initiator, TTCA (237.0 mg, 0.650 mmol) as a RAFT agent and 1,3,5-trioxane (350.5 mg, 3.891 mmol) as an internal reference for Nuclear Magnetic Resonance (NMR) analysis were charged to a three-necked flask with a magnetic stirrer and dissolved in 1,4-dioxane (35 mL). The reaction solution was purged for 20 min with nitrogen and then degassed by a freeze-thaw cycle and sealed under nitrogen. The polymerisation was carried out at 70 ∘C in a thermostated oil bath under stirring and stopped after 80 min at 48.8% of monomer conversion (the overall monomer conversion was determined by ¹H-NMR spectroscopy in d6-DMSO by the relative integration of the internal reference peak at 5.10 ppm and the vinylic proton peaks of AAc at 5.90, 6.08, and 6.31 ppm). The polymer was recovered by precipitation in cold diethyl ether and then dried under reduced pressure overnight.

Composition, experimental conditions, and analysis of the synthesised macro-RAFT agents PAAc-TTC in 1,4-dioxane and their molecular characteristics a.

a The polymerisations were performed with a [TTCA]₀/ [ACPA]₀ ratio of 10; b Monomer conversion determined by ¹H-NMR.

To synthesise the comb-type grafted gels (GG), a mixture of PAAc-TTC, NIPAAm, MBAm, ACPA were dissolved in 3 mL ethanol and the solution was charged to a glass vial (10 mm in diameter) equipped with a magnetic stirrer. The solution was purged over a period of 20 min with nitrogen and then degassed by a freeze-thaw cycle and sealed under nitrogen. The polymerisation was conducted for at least 72 h at 70 ∘C. The obtained gels were immersed in ethanol for 5 days and in deionised water for another 7 days at room temperature with solvent being daily refreshed to remove unreacted materials and allowing equilibrium swelling. Then, gels were dried under ambient conditions for 5 days. The conversions of the monomer were calculated according to the following equation:

A comprehensive account on the samples used and their respective composition as well as the degree of monomer conversions are given in Table 2.

Composition and experimental conditions for the synthesis of PNIPAAm-g-PAAc networks by RAFT polymerisation a.

a The polymerisations were performed in ethanol (3 mL) at 70 ∘C with a ratio of [ACAP]₀/[PAAc-TTC]₀ = 1 mol % and [MBAm] = 0.05 g; b The nomenclature of the hydrogels (gel code) was given based on the chain length and the content of PAAc-TTC, respectively, while GG represented the abbreviation of graft gel. c Monomer weight conversion determined by gravimetric analysis.