Macroscopic fracture toughness is dependent on chain scission at the molecular level; however, obtaining a quantitative correlation is challenging, owing to the complex interplay between network topology and tearing behavior. Quantifying the mechanism of crack formation and propagation on the single-chain level is crucial to enable molecular design of tougher materials and accurately tune macroscopic properties for specific applications. Recent studies have shown that incorporating a mixture of weak and strong mechanically scissile functional groups in networks can drastically affect macroscopic tearing energy, with weak linkers leading to weaker end-linked networks but stronger side-linked networks. While this presents potential for precise tuning of network properties, the effect of molecular-level scission mechanisms and different molecular parameters is not yet well understood on a fundamental level for such systems. In this work, we present a combination of theoretical analysis and coarse-grained fracture simulations of mixed chain-type networks to obtain a quantitative relationship between network topology, molecular scission mechanisms, and fracture toughness. Force-activated chain scission rates are governed by two crucial molecular parameters: the bond dissociation energy and the bond activation length-scale. We identify four regimes on the phase space composed of these two molecular parameters, each corresponding to a distinct tearing pathway in such mixed chain-type networks. Mapping simulated fracture properties onto this phase space reveals one distinct regime where side-linked networks exhibit toughening with incorporation of weaker linkers, whereas such an effect is not observed in end-linked networks, a behavior that is consistent with previously reported literature. This regime corresponds to networks having highly stiff linkers and strong, flexible chains. A topological analysis reveals that while the topological connectivity is similar in both network types, these differ substantially in terms of their chain length distributions. Such a topological difference, combined with the force-dependence of relative bond strengths of network chains under tension, plays a crucial role in controlling network failure, and is a major factor contributing to the contrasting toughness behavior observed in this regime. However, the range of molecular parameters exhibiting such a toughening behavior in side-linked networks is very narrow, and small variations in these shifts the tearing behavior into the other three regimes, where side-linked and end-linked networks exhibit very similar trends. This highlights the importance of choosing linker chemistries according to the desired fracture regime in order to achieve precise control over macroscopic properties.
