The polymerization initiator may include persulfates such as potassium persulfate and ammonium persulfate, azo compounds such as 2,2???-azobis(isobutylonitrile) and 4,4???-azobis(4-cyanovaleric acid), organic peroxides such as benzoyl peroxide and methyl ethyl ketone peroxide, and redox type initiators composed of combination of i) any of organohydroperoxides such as diisopropylbenzene hydroperoxi