Radically post-crosslinked polymers are made by a process comprising the steps of: (1) forming a polyurethane (a*) by reacting a compound (a) with an aliphatic and/or an aromatic isocyanate and a compound (c) wherein compound (a) is the product of the reaction of an epoxidized fatty acid ester and/or an epoxidized triglyceride with acrylic acid and/or methacrylic acid and wherein compound (c) is a polyol ester having two or more C═C double bonds per molecule; (2) forming a crosslinked polyurethane by reacting the polyurethane (a*) with a radical initiator (b). The polymers are useful as components in composites comprised of natural and/or synthetic fibers.