Patent ID: 8512466

Claim:
A phosphorus-containing oligomer composition represented by structural formula (1) below (in the formula, R 1 represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, an alkoxy group having 1 to 4 carbon atoms, or a phenyl group; n is the number of repeating units and an integer of 1 or more; X is a structural unit represented by structural formula (x1) below; Y is a hydrogen atom, a hydroxyl group, or a structural unit represented by the structural formula (x1); and, in the structural formula (x1), R 2 , R 3 , R 4 , and R 5 each independently represent a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, a phenyl group, or an aralkyl group), wherein the phosphorus-containing oligomer composition is a mixture of a phosphorus-containing compound whose n is 1 in the structural formula (1) and phosphorus-containing oligomers whose n is 2 or more in the structural formula (1), and the content of the phosphorus-containing oligomers whose n is 2 or more in the structural formula (1) is in the range of 5% to 90% in terms of peak area in GPC measurement.