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, k gel, was determined in aqueous solutions with varying both prepolymer volume fraction, ϕ, and molecular weight, M w, of the prepolymers. It was clearly found that the value of k gel is independent of both ϕ and M w, and is comparable to that of the corresponding linear-PEG system. The k gel 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.