The free radical polymerization of styrene was carried out in the presence of a new found chain transfer monomer, p-vinyl benzene sulfonyl chloride (VBSC), which possesses both a chain transfer group and a polymerizable double bond. Branched polystyrene was formed during the polymerization, as indicated by multi-peaks gel permeation chromatography (GPC) curves of the products, the increase in the number-average molecular weight and molecular weight distribution along with monomer conversions. The structure of the obtained polystyrene was analyzed by $^1H$ nuclear magnetic resonance (NMR) spectroscopy. The results showed that with increasing VBSC in the feed, the degree of branching and VBSC unit in the copolymer increased and a shortest polystyrene arm arose from the highest VBSC content in the feed, suggesting that the composition and structure of the branched polystyrenes could be tuned by the amount of VBSC in the feed. By tracing the structure change in the copolymer at various stages of polymerization, the main polymerization process can be regarded as the copolymerization of VBSC with styrene first and then chain transfer to polymeric radical to form branched polystyrene. This strategy is facile and less expensive than the other method.