Two castor oil-based monomers, (1) norbornenyl-functionalized castor oil (NCO), which has similar to 0.8 norbornene rings per fatty acid chain and (2) norbornenyl-functionalized castor oil alcohol (NCA), which has similar to 1.8 norbornene rings per fatty acid chain, have been prepared. Ring-opening metathesis polymerization (ROMP) of different ratios of NCO/NCA using the 2nd generation Grubbs catalyst results in rubbery to rigid biorenewable-based plastics with crosslink densities ranging from 318 to 6028 mol/m(3). Increased crosslink densities result in shorter gelation times, better incorporation of the monomers into the polymer network, and much less soluble materials in the bulk materials. The increased crosslink densities obtained by adding NCA enhance the thermal properties, including the glass transition temperature (T-g) and room temperature storage modulus, which increase from 17.1 degrees C to 65.4 degrees C and from 2.4 MPa to 831.9 MPa, respectively. The TGA results, where T-10 increased from 285 degrees C to 385 degrees C, illustrate that improved thermal stabilities can be obtained for thermosets with higher crosslink densities. Young's modulus (11-407 MPa), tensile strength (1.6-18 MPa) and toughness (0.14-1.6 MPa) are also improved dramatically with higher crosslink densities. Published by Elsevier Ltd.