itrile), azobis(2-methylpropionitrile), azobis(2-methylbutyronitrile), 4,4???-azobis(4-cyanobutanoic acid), dimethylazobis(2-methylpropionate), azobis[2-methyl-N-(2-hydroxyethyl)-propionamide], azobis{2-methyl-N-[2-(1-hydroxybutyl)]-propionamide} and the like; and persulfates such as potassium persulfate, ammonium persulfate, sodium persulfate and the like.