Macroscopic fracture toughness is dependent on chain scission at the molecular level; however, a quantitative correlation is challenging owing to the complex interplay between network topology and tearing behavior. 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 the change in network toughness dependent upon the chemistry used for network preparation (i.e. side-linked vs. end-linked chemistry). However, the effect of the molecular network design parameters is not yet well understood on a fundamental level. This work presents 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 in networks with a mix of weak and strong bonds. There exist four distinct fracture regimes in the design space composed of bond dissociation energy and bond activation length for both bond types, out of which one regime corresponds to stiff linkers and strong flexible chains where preferential toughening of side-linked over end-linked networks may be observed. This is attributed to the force-dependence of relative bond strengths of network strands under tension, which, in combination with substantial differences in topological distribution of linkers in the two networks, plays a crucial role in controlling fracture. Such a chemically-driven toughening mechanism is physically similar to entanglement-driven toughening of side-linked networks observed in previous experimental studies. This work highlights the importance of appropriately selecting linker chemistries according to the desired fracture regime in order to achieve precise control over macroscopic properties.