by substantially free of sulfonic acid groups is meant not more than 0 5 mole sulfonic acid groups based on the polymer composition also when sulfonic acid groups are replaced by carboxyl groups in the final acrylonitrile copolymer the oxidation rate during carbon fiber preparation is increased the precipitation polymerization of acrylonitrile monomer and suitable comonomers is enhanced by the presence of an initiator system comprising a peroxide and a low molecular weight volatile organic mercaptan the organic mercaptan should have low volatility the initiator system cannot contain any metal or metal ion containing compounds greater than 10 ppm the peroxide is a member selected from the group consisting of hydrogen peroxide t butyl hydroperoxide t butyl peroxide and lauroyl peroxide the mercaptan is a member selected from the group consisting of 1 thioglycerol mercapto ethanol and butyl mercaptan isomers by butyl mercaptan isomers is meant normal butyl mercaptan sec butyl mercaptan and iso butyl mercaptan by low molecular weight organic mercaptan is meant a c 1 c 6 organic mercaptan a catalytic amount of an iron compound is added to the mixture of solvent initiator and monomer systems examples of iron catalysts are ferric or ferrous nitrate ferric or ferrous chloride and ferric or ferrous ammonium sulfate the compounds can have water of hydration associated therewith an acrylonitrile.