| http://www.w3.org/ns/prov#value | - The polyethers preferably containing two hydroxyl groups suitable for use in accordance with the invention are also known per se, and may be obtained by polymerization of tetrahydrofuran or epoxides such as ethylene oxide, propylene oxide, butylene oxide, styrene oxide or epichlorohydrin, on their own, for example in the presence of BF3, or by the addition of these epoxides, optionally in admixtur
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