Five families of new controlled epoxy thermosets (CENs) using three monophenol chain terminators were prepared to study systematic changes in the structure and amount of the monophenol and the initial molecular weight between crosslinks (M-c,M-i) on the properties of epoxy thermosets. Glass transition temperature (T-g) decreases with monophenol mole fraction (chi) in proportion to both the concentration and flexibility of the chain terminator. Distinct serial relations for T-g depression were observed for the three M-c,M-i families. Dynamic mechanical analysis (DAM) shows significant perturbations of the relaxation behavior with added terminator as evidenced by decrease in peak tan delta and in post T-g damping. The rubbery coefficients of thermal expansion (CTE) increases with monophenol concentration only at chi > 0.05 and shows distinct curvature versus temperature, but is largely invariant with monophenol flexibility. The thermal stability of terminated CENs decreases only slightly with chi and little difference was found with monophenol structure. Most surprisingly, fracture toughness decreases markedly and discontinuously with chi depending on M-c,M-i. The values of the critical monophenol concentration at which fracture toughness markedly decreases (chi(c)) are inversely proportional to Mc,i but are independent of monophenol flexibility. No correlation of chi(c) with any of the calculated network structure parameters was apparent. (C) 2008 Wiley Periodicals, Inc.