Kinetic Chain Lengths in Highly Cross-Linked Networks Formed by the Photoinitiated Polymerization of Divinyl Monomers: A Gel Permeation Chromatography Investigation

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Abstract
Highly cross-linked networks formed by the photoinitiated polymerization of multifunctional monomers are finding application in the field of biomaterials because of their chemical versatility, reaction control, and ability to polymerize under physiological conditions. Typically, degradation is introduced into these networks via the cross-links and leads to the release of nondegradable but water-soluble kinetic chains formed during the chain polymerization process. In this study, gel permeation chromatography (GPC) was used to characterize kinetic chain length distributions in highly cross-linked systems that are being developed for orthopedic applications. By polymerizing divinyl monomers to various conversions and subsequently degrading them, we investigated the aspects of network structural evolution related to kinetic chain formation. In general, the average kinetic chain length increased with conversion until the onset of autodeceleration, when the kinetic chains decreased in length as the propagation reaction became diffusion-controlled. The distribution of kinetic chains also changed when different initiation conditions (i.e., initiator concentration and incident light intensity) were used, and a decrease in the kinetic chain lengths was observed at higher initiation rates. Finally, kinetic chain lengths were examined as a function of depth in thick samples polymerized with different light intensities and with a photobleaching initiator. Light attenuation through the sample led to different initiation rates as a function of depth and, consequently, spatial heterogeneity in the network structure as measured by the distributions of kinetic chains.