poly are prepared by an improved process which makes it possible to directly esterify terephthalic acid and 1 , 4 - butanediol without excessive loss of the diol reactant to side reactions . in the process , the conditions are controlled to promote rapid removal of the water as formed . it is also advantageous to minimize the contact time between 1 , 4 - butanediol and terephthalic acid . illustratively , 1 , 4 - butanediol will be heated to 175 °- 275 ° c ., terephthalic acid is gradually added , and byproduct water is removed as formed . in another embodiment , a mixture of terephthalic acid and an excess of 1 , 4 - butanediol is heated and byproduct water is removed by entrainment in the excess 1 , 4 - butanediol distilled from the reaction zone .