Vitrimers are polymer networks that undergo molecular network rearrangement through associative dynamic bond exchange. Despite their ability to undergo structural reconfiguration, these networks remain insoluble in good solvents due to preserved overall network connectivity. In this work, we examine how varying the cross-link density (Nxl) influences the structure and properties of vitrimer networks. Polybutadiene (PB) vitrimers incorporating dioxaborolane cross-links were synthesized via a photo-initiated thiol-ene click reaction, targeting Nxl values ranging from 2 to 15 cross-links per chain. The number of thiol-ene linkages per PB chain was determined by disassembling the networks with excess diols and analyzing the resulting fragments via nuclear magnetic resonance spectroscopy. Higher values of Nxl were associated with higher glass transition temperatures and plateau moduli, as measured by differential scanning calorimetry and variable temperature small-amplitude oscillatory shear, respectively. Analysis using the phantom network model provided insights into network defects. Additionally, vapor swelling experiments further indicated that PB vitrimers had fewer defects than permanent networks. Comparisons of vitrimers to PB networks with permanent cross-links emphasize the critical role of dynamic associative cross-links in governing material properties.
