Polymerization of tetrahydrofuran

Claim 1

In a process for the preparation of polybutylene glycol carboxylic acid diesters of the formula R--CO--O--CH.sub.2 CH.sub.2 CH.sub.2 CH.sub.2 --O).sub.n CO--R.sup.1 where R and R 1 are identical or different and each is alkyl, a carboxyl-containing aliphatic radical which may or may not be ethylenically unsaturated, or a carboxyl-containing aromatic radical and n is an integer from 2 to 200, by polymerizing tetrahydrofuran, wherein the tetrahydrofuran, after removal of the catalyst used in the preparation of the tetrahydrofuran, is treated, before polymerization, with a strong mineral acid, an organic sulfonic acid, silica gel and/or a bleaching earth, the treating agent is removed from the tetrahydrofuran, and the tetrahydrofuran is then polymerized in the presence of one or more carboxylic acid anhydrides and a polymerization catalyst, the improvement which comprises: using the polymerization catalyst a bleaching earth containing less than 3% by weight of water, said catalyst being arranged in a fixed bed, and passing a mixture of pretreated tetrahydrofuran and carboxylic anhydride through said fixed bed.