New types of phosphonous acid-terminated oligomers oligo-polystyrenylphosphonous acid (PSPA) were prepared by free radical reaction between styrene and hypophosphorous acid and characterized by IR, MS, GPC and terminal group analysis. The initiator had great influence upon the average molecular weights and structure of dominate product. The average molecular weights of PSPA I (AIBN as initiator) was obviously higher than that of PSPA II (BPO as initiator). MS data showed that there were three series of polystyrenyl compounds for PSPA I, five series for PSPA II. The main product was H-[CH(C6H5)CH2](n)-PO2H2 for PSPA I and C6H5CO2-[CH2CH(C6H5)](n)-PO2H2 or C6H5CO2[CH(C6H5)CH2](n)-PO2H2 for PSPA II. Different free radical reaction mechanisms with AIBN or BPO as initiator were proposed according to the results of MS, IR and f(n) analysis. (C) 2005 Elsevier B.V. All rights reserved.