| prov:value
| - Xm wherein R1 is the same as defined above, X is a halogen atom, and m is zero or a positive number of not greater than 3, compounds of formula R1 3-n Al(OR2)n wherein R1 is as defined above, R2 is a hydrocarbon group, which is identical to, or different from, R1, and n is a positive number greater than 0 but not greater than 3, and compounds of formula R1 Al(OR2)X wherein R1, R2 and X are the sam
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