prov:value
| - enum or tungsten, preferably zirconium, hafnium and titanium.R.sup.1 and R.sup.2 are identical or different and are a hydrogen atom, a C.sub.1 -C.sub.1 -, preferably C.sub.1 -C.sub.3 -alkyl group, a C.sub.1 -C.sub.10 -, preferably C.sub.1 -C.sub.3 -alkoxy group, a C.sub.6 -C.sub.10 -, preferably C.sub.6-C.sub.8 -aryl group, a C.sub.6 -C.sub.10 -, preferably C.sub.6 -C.sub.8 -aryloxy group, a C.sub
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