Unsaturated polyester resins usually have to be diluted with a cross-linking agent and a reactive diluent into a viscous liquid with appropriate viscosity, and then made into various forms of materials by co-curing to achieve their excellent processability. Therefore, the compatibility with the crosslinking agent is one of the most important criteria for whether the unsaturated polyester can be used as a commercial resin. Compatibility with cross-linking agents is generally similar to solvents with similar solubility parameters and similar polarities. In addition to the commonly used principles of solvents such as electron donation and acceptance, it must be fully noted that the unsaturated polyester must not be crystallized. Because the molecular chains of unsaturated polyesters are generally highly polar and can easily cause crystallization, more attention is required. The first thing to note is to avoid connecting more than two CH2- groups, such as 1,3-groups, such as 1,3 propanediol, 1,4-butanediol, 1,6 hexanediol, etc. It is almost difficult to obtain a resin that does not contain crystals and has good miscibility with the crosslinking agent. In addition, if the molecular chain contains para-benzene chain links and any other symmetrical chain links are also easy to crystallize and must be avoided. In contrast, the ortho-benzene substituent is an advantageous group to prevent crystallization. In addition, the introduction of polar groups such as hydroxyl groups, isooxy acid ester groups (urethane groups) in the molecular chain will increase the crystallinity and reduce the compatibility with non-polar cross-linking agents such as styrene. It is unfavorable to form a clear, transparent and compatible unsaturated polyester resin.