continuous multistep , economical , energy - saving process for making polybutylene terephthalate in which the butanediol - 1 , 4 used to transesterify dimethyl terephthalate is supplied from the condensed vapors volatilized during the precondensation and polycondensation steps of the process . these vapors , which contain low - soluble esters of terephthalic acid as well as butanediol , are condensed by contact with fresh butanediol , and volatilized butanediol containing the esters is recycled to the initial transesterification stage for reaction with dimethyl terephthalate . no additional butanediol is required for the reaction .