Poly(N-isopropylacrylamide) hydrogels fabricated via click chemistry: well-defined α,ω-bis propargyl linear poly(N-isopropylacrylamide)s as crosslinkers

Abstract

This work provides a facile method to regulate swelling properties and/or to impart special functions for click poly(N-isopropylacrylamide) (PNIPA) hydrogels, by adjusting the chain length of crosslinkers or by introducing other functional groups.

Preparation of Well-Defined Propargyl-Terminated Tetra-Arm Poly(N-isopropylacrylamide)s and Their Click Hydrogels Crosslinked with β-cyclodextrin

“…As expected, hydrogels made from higher molecular-weight PNIPAs have greater ESR values in swollen state. As have been elaborated in our previous paper [ 25 ], the higher molecular weight of PNIPA implies longer distance between crosslinks, which will provide larger free volume to accommodate more water molecules. On the other hand, the higher the PNIPA molecular weight is, the less is the hydrophobic parts content; meanwhile the dosage of less hydrophilic azido-β-CD crosslinker also decreases in gel fabrication.…”

“…However, this ideal hypothesis is of probable existence due to the low concentration of raw materials (20 wt %); and we have tried to lower the concentration to 15 wt %, finding that the obtained gel almost cannot self-stand. Herein, the gelation was accomplished in half an hour using 20 wt % polymer solution in DMF, and the final gel fraction of each sample was over 90 %, under the catalysis of a common CuSO 4 /ascorbic acid system, the same as our previously published works [ 24 , 25 ]. In a study about click PNIPA hydrogels [ 21 ], however, towards PNIPA solutions in DMF with as high as 80 wt % concentration, the authors finally selected CuI/ N , N -diisopropylethylamine (DIPEA) pair through examining many catalyst systems including CuSO 4 and Cu(I) salts in combination with various reducing agents and ligands because only CuI/DIPEA system behaved highly efficient.…”

“…Correspondingly, the appearances of gels also altered from transparent expansive states to white huddled ones gradually at elevated temperatures; that is to say, the inherent homogeneous state of swollen gels became heterogeneous ascribed to the aggregation of PNIPA chains in network as the temperature rose. Volume phase transition temperature (VPTT), a parameter to characterize temperature sensitive hydrogels, can be regarded as the temperature at which the swelling ratio decreased to a half of its value at initial temperature [ 25 ]. Therefore, the curves of ESR vs. temperature in Figure 5 a were normalized based on the data at 10 °C, as displayed in Figure 5 b, where water retention was defined as the ratio between ESR at a certain temperature and that at 10 °C.…”

“…The molecular weights ( M n ) of RAFT T-PNIPA samples were obtained by the methods as described in our previous paper [ 25 ]. Equation (1) was used to calculate M n (theory) based on monomer conversion that was estimated according to 1 H NMR spectra of unseparated polymerizing resultant including polymer and residual monomers.…”

“…We have ever prepared a well-defined linear PNIPA carrying pendant azido groups using RAFT polymerization and the following modification, which was click-crosslinked with multi-alkynyl terminated small molecules to get well-defined PNIPA hydrogels [ 24 ]. Furthermore, the same linear azido-PNIPA was crosslinked with several α,ω-bis propargyl PNIPAs with different molecular weights that were synthesized using a bis propargyl-RAFT agent, tailoring a series of click hydrogels with tunable properties [ 25 ].…”