Comparison of Thermoresponsive Hydrogels Synthesized by Conventional Free Radical and RAFT Polymerization

Abstract

We compared the influence of the polymerization mechanism onto the physical characteristics of thermoresponsive hydrogels. The Poly(N-isopropylacrylamide) (PNIPAAm) hydrogels were successfully synthesized using reversible addition-fragmentation chain-transfer (RAFT) and free radical polymerization (FRP). The gels were prepared while using different crosslinker feed and monomer concentration. The swelling, dye release, and hydrolytic stability of the gels were investigated in water, or in representative komostrope and chaotrope salt solutions at room temperature and at 37 °C. It was found that the swelling ratio (SR) of the RAFT gels was significantly higher than that of the FRP gels; however, an increased crosslinking density resulted in a decrease of the SR of the RAFT gels as compared to the corresponding gels that are made by FRP, which indicates the limitation of the cross-linking efficiency that is attained in RAFT polymerization. Additionally, an increased monomer concentration decreased the SR of the RAFT gels, whereas a similar SR was observed for the FRP gels. However, the SR of both RAFT and FRP gels in NaSCN and Na2SO4 solutions were similar. Finally, the rate of dye release was significantly slower from the RAFT gels than the FRP gels and the hydrolytic stability of the RAFT gels was lower than that of FRP gels in water, but maintained similar stability in Na2SO4 and NaSCN solutions.

Versatile Bilayer Hydrogel for Wound Dressing through PET-RAFT Polymerization

“…Consequently, PNIPAm-PAm block copolymers with thiocarbonylthio end groups were obtained. The key for synthesizing architecturally diverse polymers via the approach lies in the retention of the thiocarbonylthio polymer chain end throughout the polymerization. , The PET-RAFT polymerization process is gentle and without intense exothermic, so there is no need for cooling during the synthesis of PNIPAm. ,,,, On the other hand, PET-RAFT polymerization enabled temporal/spatial control over the polymerization process. , Thus, the polymerization process can be started or stopped by turning the light source on or off during hydrogel preparation. This is particularly important in dealing with the interface between hydrogel layers, where the temporal/spatial control can allow continued growth of the second layer directly upon the first layer.…”

“…This is particularly important in dealing with the interface between hydrogel layers, where the temporal/spatial control can allow continued growth of the second layer directly upon the first layer. Specifically, the growth of PAm can be started from the PNIPAm chain to form the second block, which combines the two layers in a single framework. ,,,, An apparent bilayer structure was formed in the hydrogel, as shown in Figure . Additionally, in the lateral stretching process, the mechanical adhesion between the inner layer (H m -PN n ) and the outer layer (PVA o -PAm p ) can be separated, and the interactive binding is too tight to peel off the inner layer before it is destructed (Figure S2).…”

“…On the other hand, the outer layer of hydrogel was made with high stretchability and toughness to protect both the inner layer and the wound from accidental bumping or frictions. Most importantly, we used a facile, mild, oxygen-tolerant, and biocompatible photocontrolled polymerization method to tightly connect the inner and outer layers by covalent bonding during the layer growth in a controlled way. , By incorporating photocontrol into reversible addition-fragmentation chain transfer polymerization, the photoinduced electron/energy transfer (PET)-RAFT polymerization technique can be started or stopped by turning on/off the light source during the hydrogel preparation.…”

Are RAFT and ATRP Universally Interchangeable Polymerization Methods in Network Formation?

“…The aim of this article is to compare the networks made by two reversible deactivation radical polymerization (RDRP) methods: atom transfer radical polymerization (ATRP) − and reversible addition–fragmentation chain transfer (RAFT) polymerization. , In general, RDRP networks are more regular and swell more than networks prepared by conventional free radical polymerization (FRP) processes dominated by chain breaking reactions. , This was first demonstrated by the delayed gel points and higher swelling of polystyrenes crosslinked with divinyl benzene prepared by nitroxide-mediated polymerization, as compared to those formed by FRP. ,, Moreover, it has been reported that either ATRP or RAFT gels are more uniform and homogeneous than their FRP analogs. ,,− In an FRP process, radicals are generated, propagated, and then terminated, with the entire chain growth occurring within ∼1 s. Early in the polymerization, dense nanogels are formed, which are crosslinked later during the reaction to form a heterogeneous network. In both ATRP and RAFT methods, the growing polymer chain ends are reversibly and intermittently activated to alternate between active growing radicals and dormant chain ends.…”

Phosphates-Containing Interpenetrating Polymer Networks (IPNs) Acting as Slow Release Fertilizer Hydrogels (SRFHs) Suitable for Agricultural Applications

“…Such devices guarantee more effective, economical and environmentally beneficial nutrient uptake, as they prevent fertilizers from being leached from the soil [ 4 ]. Slow release fertilizer hydrogels (SRFHs) can be prepared using many different polymerization techniques, such as solution [ 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 ] or inverse suspension polymerization [ 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 ], as well as different mechanisms—free radical polymerization [ 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 16 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ], RAFT polymerization [ 23 ], photopolymerization [ 15 , 17 , 26 ], etc. In general, there are several methods of SRFH preparation [ 3 ].…”