In this work, we investigated the mechanism involved in the photoinitiated cross-linking of the polyester poly(propylene fumarate) (PPF) using the initiator bis(2,4,6-trimethylbenzoyl) phenylphosphine oxide (BAPO). It was hypothesized that BAPO has the ability to cross-link PPF into solid polymer networks, without the use of a cross-linking monomer, because two pairs of radicals, both involving a fast adding phosphinoyl radical, were formed upon UV irradiation of BAPO. Spectroscopic investigation first confirmed the addition of BAPO derived radicals to the PPF olefin. Investigations of fumarate conversion and bulk network properties were then undertaken, using the BAPO initiator and a monoacylphosphine oxide (MAPO) initiator which contains a single photolabile bond. Results show that a single BAPO phosphinoyl radical was primarily responsible for the formation of a highly cross-linked PPF network and the additional radical pair which may be formed does not dramatically alter fumarate conversion or bulk network properties. From these results, the network structure of BAPO initiated, photo-cross-linked PPF may be deduced. Finally, this study demonstrates a method for inferring cross-linked network structures by contrasting properties of bulk materials formed from similar cross-linking initiators.