Ethylene polymerization on a SiH4-modified Phillips catalyst: detection of in situ produced alpha-olefins by operando FT-IR spectroscopy

Ethylene polymerization on a model Cr(II)/SiO2 Phillips catalyst modified with gas phase SiH4 leads to a waxy product containing a bimodal MW distribution of alpha-olefins (M-w < 3000 g mol(-1)) and a highly branched polyethylene, LLDPE (M-w approximate to 10(5) g mol(-1), T-m = 123 degrees C), contrary to the unmodified catalyst which gives a linear and more dense PE, HDPE (M-w = 86 000 g mol(-1) (PDI = 7), Tm = 134 degrees C). Pressure and temperature resolved FT-IR spectroscopy under operando conditions (T = 130-230 K) allows us to detect alpha-olefins, and in particular 1-hexene and 1-butene (characteristic IR absorption bands at 3581-3574, 1638 and 1598 cm(-1)) as intermediate species before their incorporation in the polymer chains. The polymerization rate is estimated, using time resolved FT-IR spectroscopy, to be 7 times higher on the SiH4-modified Phillips catalyst with respect to the unmodified one.