UV‐vis spectroscopic study revealed that vinylcyclohexane interaction with zirconocene catalyst is enhanced on light exposure. If oxygen is present it interacts with the catalyst, hindering vinylcyclohexane interaction and consequently lowering the catalyst activity. The catalyst activity can be recovered with light exposure if low O2 concentrations are used since in light the zirconocene‐vinylcyclohexane interaction is favoured over the zirconocene‐O2 interaction. Moreover, 1H NMR studies of the reaction products showed that the structures of the vinylcyclohexane‐based species produced with light irradiation were identical to the species produced in dark. This indicates that light exposure of the catalytic system does not change the reaction mechanism.Absorption spectrum of [(nBuCp)2ZrCl2]/MAO/VCH. (a) reaction in dark, (b) reaction with irradiation, (c) reaction with O2 treatment in dark and (d) reaction with O2 treatment with irradiation.magnified imageAbsorption spectrum of [(nBuCp)2ZrCl2]/MAO/VCH. (a) reaction in dark, (b) reaction with irradiation, (c) reaction with O2 treatment in dark and (d) reaction with O2 treatment with irradiation.
Summary: Propene polymerisation was conducted with three ansa‐zirconocene complexed: Me2Si(2‐Me‐4‐PhInd)2ZrCl2 (A), Me2Si(2‐Me‐4‐PhInd)2ZrClNMe2 (B) and Me2Si(2‐Me‐4‐PhInd)2ZrClNEt2 (C). Methylalumoxane (MAO) or [HNPhMe2][B(C6F5)4] was used as cocatalyst. The influence of cocatalysts and triisobutylaluminum (TIBA) on polymerisation activity, molecular weight and polymer microstructure was studied. Furthermore, the alkylation and activation of the complexes were examined using UV/VIS spectroscopy. Complex A was the most active in polymerisation, but polymers produced by B and C had in general higher molecular weights. Replacing MAO partially by TIBA increased the molecular weight and prevented catalyst deactivation. According to the UV/VIS measurements, all complexes have low reactivity towards TMA and TIBA. Similar cationic species were formed of all three studied zirconocenes in the reaction with MAO. Additional TIBA did not affect the type of active species formed in the reaction of zirconocene and MAO, but increased the activity of B and C.UV/VIS spectrum of complex A with MAO and with a MAO/TIBA mixture.imageUV/VIS spectrum of complex A with MAO and with a MAO/TIBA mixture.
The activation of 1-siloxy-substituted zirconocene with MAO occurs slowly, in 4−6 h. The
slow activation process involves three distinct ligand to metal charge transfer (LMCT) band shifts in the
UV/vis spectrum before the maximum LMCT absorption energy and intensity of the third appearing
band is reached. In addition, at very high MAO concentrations, a fourth band at 460 nm begins to form.
On the contrary, the corresponding 2-siloxy-substituted as well as the unsubstituted zirconocene react
with MAO in less than a minute, immediately producing the final species, as confirmed by only one
observed LMCT band shift. The 1-silyl-substituted zirconocene reacts with MAO, producing the final
active species in 1 h. UV/vis spectroscopic observations were interpreted with the aid of molecular modeling
studies, demonstrating that the interactions of the oxygen donor atom of the siloxy substituent with
electron deficient Zr center and coordinatively unsaturated Al in MAO play a prominent role in the
activation process.
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