Bimetallic Catalysis for Styrene Homopolymerization and Ethylene−Styrene Copolymerization. Exceptional Comonomer Selectivity and Insertion Regiochemistry

Abstract

This communication reports the styrene homopolymerization behavior and ethylene-styrene copolymerization behavior of the covalently linked bimetallic constrained geometry catalyst (mu-CH2CH2-3,3'){(eta5-indenyl)[1-Me2Si(tBuN)](TiMe2)}2 (Ti2), which is the first single-site catalyst that effects not only styrene homopolymerization with high activity, but also efficient ethylene-styrene copolymerization over a broad styrene composition range (0-76% at 20 degrees C, 1.0 atm ethylene pressure). In styrene homopolymerization, a 50x increase in polymerization activity is achieved with Ti2 vs the mononuclear analogue, Ti1, using an identical trityl borate cocatalyst and polymerization conditions. In ethylene + styrene copolymerization, Ti2 enchains approximately 20% more styrene than Ti1 under identical reaction conditions. 13C NMR spectroscopy indicates that greater than two consecutive styrene units are enchained in the copolymer backbone produced by Ti2 + Ph3C+B(C6F5)4-. End group analysis of the styrene homopolymer produced by Ti2 + Ph3C+B(C6F5)4- suggests that 1,2-regiochemistry is installed in approximately 50% of the initiation steps. This unusual microstructure is believed to be related to the bimetallic catalyst structure.

Dinuclear dialkoxo-bridged cyclopentadienylsiloxo titanium complexes

“…Neither of the chloro derivatives 1a and 1b reacted with B(C 6 F 5 ) 3 . However, the ethyleneglycolate derivative 1a reacted with Al(C 6 F 5 ) 3 with substitution of the dialkoxo ligand by two pentafluorophenyl groups to give [{TiCl(C 6 F 5 )} 2 (m-{(h 5 -C 5 Me 4 SiMeO) 2 (m-O)})] (5) as the unique titanium product (Scheme 2).…”

“…However, 9B was unstable at ambient temperature. The same reaction employing Al(C 6 F 5 ) 3 as Lewis acid afforded the anal-…”

“…This process required de-coordination of the Al(C 6 F 5 ) 3 moiety from the oxygen atom in compound 6, recovering the initial reaction products as a consequence of the weak O-Al interaction. Formation of compound 7 as the major reaction product can also be justified by the different steric and electronic characteristic of the phenylenedialkoxo bridge, with regard to the ethylenedialkoxo bridge, which would make the approach of Al(C 6 F 5 ) 3 to the bridging oxygen atoms difficult, rather favouring exchange of the chloro ligand with the C 6 F 5 group. However, it is important to note that formation of 7 should be independent of the formation of adduct 6, de-coordination of Al(C 6 F 5 ) 3 from the oxygen atom of 6 is required to obtain 7.…”

“…A solution of 3a (0.100 g, 0.14 mmol) and B(C 6 F 5 ) 3 (0.071 g, 0.14 mmol) in CH 2 Cl 2 (2 mL) were stirred for 5 min. The volatiles were removed under vacuum leaving an oil that was washed with hexane (2 ¥ 2 mL) to give 8B as an orange solid (0.146 g, 85%).…”

“…36, 37 The ease of formation of bridges caused complex A to be inactive in ethylene polymerization, although the methyl derivative [(TiMe 2 ) 2 (m-{(h 5 -C 5 Me 4 SiMeO) 2 (m-O)})] was active in MMA polymerization. 36 Abstraction of two alkyl groups was only possible in the reaction of complex [(TiBz 2 ) 2 (m-{(h 5 -C 5 Me 4 SiMeO) 2 (m-O)})] with excess B(C 6 F 5 ) 3 for several days, although the transformation was not complete.…”