A xanthate/BPO initiator system was used to initiate the reversible addition-fragmentation-chain transfer (RAFT) solution polymerization of vinyl acetate to prepare polyvinyl acetate (PVAc). Alcoholysis of PVAc to produce polyvinyl alcohol（PVA：S-RAFT）.Gel Permeation Chromatography (GPC) was hired to analyze the molecular weight and distribution of the samples. 1H-NMR was used to study the tacticity of PVA .The aggregation state of PVA was investigated by FTIR. By comparing the tacticity and aggregation state of 6 different PVA raw materials and 3 commercial polarizing base films, it is found that the tacticity of S-RAFT and the optical base film is similar and the content of syndiotacticity (rr) and crystallinity are higher than that of PVA raw material. After comparing the properties of polarizing films made of S-RAFT, PVA raw materials and commercial base film, it is found that S-RAFT has good film forming properties, high degree of polarization, and high tensile modulus. It is expected to be a raw material for the preparation of high-end polarizers. In addition, by comparing with the structure of PVA with traditional free radical polymerization alcoholysis, the process of preparing high syndiotactic PVA by RAFT using xanthate as chain transfer agent is deduced in detail. A novel "cis Fragmentation" mechanism of xanthate is proposed.