In this study a controlled polymerization of styrene (S) and methyl methacrylate (MMA) with tetraphenylbisphosphine (TPhBP) as the initiator in bulk was carried out, using UV light as the energy source. The yield of polymerization was up to 15% for PS and up to 62% for PMMA. TPhBP is decomposed to form 9PPh 2 radicals, or is oxidized to form bisphosphine oxide. Only non-oxidized TPhBP initiates the photo-polymerization. For S and MMA an insignificant portion of photo-and thermal-polymerization was detected. In the presence of oxygen the phosphine end-groups of the polymer are oxidized to 1PO(Ph) 2 groups. The molecular weights of PMMA were determined from 1 H NMR spectra and the molecular weights of PMMA and PS were determined by gel permeation chromatography (GPC) measurements. With increasing concentration of the initiator the molecular weight decreases and at a high concentration of the initiator the molecular weight decreases with conversion. The polydispersity decreases with increasing concentration of TPhBP. Termination occurs by combination of primary radicals 9PPh 2 , by combination of macroradicals, and by chain transfer to the initiator. Because of different types of termination the ratio between the tail 1CH 2 P(Ph) 2 and the head 1C(CH 3 ,COOCH 3 )P(Ph) 2 groups is about 4 to 1, and decreases with increasing concentration of the initiator.