Kinetic Aspect on Gelation Mechanism of Tetra-PEG Hydrogel

We carried out a kinetic study on the gelation reaction of AB-type cross-end coupling of two tetra-arm poly­(ethylene glycol) (Tetra-PEG) prepolymers having amine (Tetra-PEG-NH2) and activated ester (Tetra-PEG-NHS) terminal groups by ATR-IR and UV spectroscopies. The reaction rate constant for the gelation of Tetra-PEG, kgel, was determined in aqueous solutions with varying both prepolymer volume fraction, ϕ, and molecular weight, Mw, of the prepolymers. It was clearly found that the value of kgel is independent of both ϕ and Mw, and is comparable to that of the corresponding linear-PEG system. The kgel value is obtained to be around 70 dm3 mol–1 s–1, which is much smaller than the reaction rates of typical diffusion-controlled reaction (e.g., 108–109 dm3 mol–1 s–1) and of cross-linking photopolymerization (104–105 dm3 mol–1 s–1). From these results, we concluded that the gelation reaction of Tetra-PEG gel is not diffusion-limited but reaction-limited process, i.e., the diffusion motion is much faster than the reaction rate. It is thus expected that Tetra-PEG prepolymer chains can diffuse in the solution during gelation process, leading to homogeneity and high-strength of Tetra-PEG gel. These discussions imply that, in order to achieve high-efficient and homogeneous gel, it is necessary to choose reaction groups so as to undergo reaction-limited reaction.