p acetoxystyrene unit and 33 by mol of so 2 unit further this bipolymer was subjected to gpc analysis to give a weight average molecular weight of 390 000 as calculated in terms of polystyrene and a polydispersibility of 1 8 dioxane 60 ml was placed in a 200 ml conical flask followed by adding thereto p acetoxystyrene sulfone copolymer 2 g obtained in example 1 agitating the mixture for a long time to prepare a uniform solution adding 1n hydrochloric acid aqueous solution 4 ml to the solution agitating the mixture at room temperature for 24 hours for hydrolysis adding to the resulting solution dichloromethane 40 ml transferring the solvent layer into a separating funnel washing the layer with a large quantity of water to make the layer neutral and introducing the neutralized solvent layer into a large quantity of hexane to precipitate the polymer the resulting polymer was subjected to ir spectra analysis as a result most of acetoxy group was extinct and the polymer was a p hydroxystyrene sulfone bipolymer in addition even according to the process shown in example 1 wherein p hydroxystyrene and so 2 were subjected to polymerization reaction in the presence of a polymerization initiator it was also possible to obtain the p hydroxystyrene sulfone bipolymer but since p hydroxystyrene homopolymer is coexistent with the above bipolymer it is necessary to separate the homopolymer into a 100 ml pressure glass tube were introduced newly distilled p chlorostyrene 38 ml and azobisisobutyronitrile 40 mg followed by removing oxygen inside the tube in the same manner as in example 1 adding.