for about 1 to 3 hours the particular time period depending on the remaining conditions applied temperature pressure catalyst and the desired degree of polymerisation that is to say the desired viscosity theoretically the process can also be worked at pressures above 100 mbar but in that case the reaction period is lengthened considerably which is undesirable as a rule since decomposition phenomena may occur the catalysts employed are those known for this reaction according to the state of the art dialkyl zirconates are preferred the alkyl groups being able to be branched or linear and containing 1 to 24 c atoms tetraalkyl orthotitanates can also be employed alkyl group having 4 c atoms are preferred especially n butyl compounds the quantity of catalyst lies within the known range of about 0 01 to 5 by weight related to the weight of the reaction mixture preferably the quantity of catalyst is below about 1 by weight production can also be effected in such a way that all the starting products are first heated to the above mentioned temperatures of about 180 u00b0 to 300 u00b0 c preferably about 200 u00b0 to 260 u00b0 c for a period of about 2 to 5 hours with dissociation of water preferably for about 2 5 to 3 5 hours subsequently after condensation is carried out under reduced pressure within the range indicated above and at the temperatures indicated above for the above mentioned period until the.