Novel reactive phenylethynyl-endcapped diluents were synthesized and physically mixed with fluorinated cooligoimides (molecular weights of 5000 g/mol) derived from 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 2,2'-bis(trifluoromethyl)-4,4'-diamino biphenyl (TFDB) and para-phenylenediamine (p-PDA) at 25/75 M ratio, and 4-phenylethyethynylphthalic anhydride (PEPA) to formulate blends served as matrix resins for developing high-performance composites with excellent processabilities. The uncured blend shows very low minimum melt viscosity ( < 1 Pa s) and extended process window achieved using molten solvation of reactive diluent to prevent intermolecular interactions and recrystallization at higher temperature. This blend was converted to interpenetrating polyimide network through thermal-curing of phenylethynyl end-groups in each constituent. The morphology, relaxation transition and mechanical property were investigated as functions of mass fraction and chemical configuration of reactive diluent. The covalent incorporation of crosslink structures restricts segmental motion while backbone linkage induces strength and toughness retention. This cooperative effect endows homogeneous networks with high glass transition temperatures (435 degrees C) and flexural strength (130 MPa) simultaneously.