| prov:value
| - M1 is titanium, zirconium, or hafnium; (X1) and (X2) are independently a substituted cyclopentadienyl, a substituted indenyl, or a substituted fluorenyl; one substituent on (X1) and (X2) is a bridging group having the formula ER1R2, wherein E is a carbon atom, a silicon atom, a germanium atom, or a tin atom, and E is bonded to both (X1) and (X2), and wherein R1 and R2 are independently an alkyl gr
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