With BPO as an initiator in solution; the comb-copolymer of PSt macromer and vinyl acetate was synthesized. The composition and branching degree of comb-copolymer were deter mined by IR spectroscopy. It is suggested that a reasonably accurate estimate of components and branching degree would be given by the equations respectively:A700/A1740=0.4194Cst/CVAC+ 0.1469 and A700/A1740 = 0.12n + 0.0155. When Mn of Macromer is not the standard 20000

the branching degree of graft copolymer is n'=n (20000/Mn)

where Mn is the numeral average molecular weight of macromer usedin the process of copolymerization

n is branching degree per 1000 VAC units.From the experimental evidence available

it was found that branching degree of graft co-polymer depends on Mn of macromer. When the monomer feed ration and Mn of macromer changed slightly

the branching degree did not change basically;when Mn of macromer was re-duced

the branching degree of graft copolymer was increased. The styrene macomer feed ra-tio changed from 5 to 8% then the styrene contents of copolymer increased to ≥55%. It was shown that macromer is more active than vinyl acetate in the process of copolymerization.