Efficiencies of various metallocene/reducing agent catalyst systems in the hydrogenation of polystyrene-b-polybutadiene-b-polystyrene block copolymers

Abstract

Various metallocenes, including bis( 5 -cyclopentadienyl)cobalt, bis( 5 -cyclopentadienyl)nickel, and bis( 5 -cyclopentadienyl)titanium dichloride, combined with various reducing agents, including n-butyllithium, phenyllithium, and triethylaluminum, have been evaluated for their catalytic efficiencies in the hydrogenation of polystyrene-b-polybutadiene-b-polystyrene (SBS) block copolymer. The efficiencies were determined by monitoring the extent of saturation of double bonds on the polybutadiene segment of the copolymer using FTIR and 1 H-NMR spectroscopy. The cobaltocene/nbutyllithium catalyst system was found the most active. The effects of H 2 pressure and the ratio of n-butyllithium to cobaltocene ratio on the hydrogenation efficiency were also investigated.

Synthesis and characterization of poly (styrene‐b‐[(butadiene)1−x‐(ethylene‐co‐butylene)x] ‐b‐styrene) star‐like molecular polymers produced by partial hydrogenation of SBS

“…In regards to the co‐catalyst (Li), the results demonstrate that the Li/Ti molar ratio played an important role in determining the catalytic capacity of this system. For a given catalyst/polymer ratio, an appropriate amount of co‐catalyst was required to obtain the optimal composition suitable for polymer hydrogenation, as previously reported in the literature . In regards to the effect of hydrogen, as indicated earlier, polymer hydrogenation with an organometallic catalyst involves the adsorption of hydrogen by the organometallic group, forming an intermediate that is essential for the hydrogenation process to occur .…”

“…Similarly, different titanocenes have been used for hydrogenating diene‐containing polymers: bis‐(cyclopentadienyl)titanium(III) , monocyclopentadienyl titanium , Ti( η 5 ‐C 5 H 5 ) 2 R 2 , where R is any tri‐substituted phenyl group , Ti( η 5 ‐C 5 H 5 ) 2 (PhOR′) 2 , where R′ is an alkyl group , Ti( η 5 ‐C 5 H 5 )Cl 2 (OC 6 H 4 Y‐ p ), where Y is Cl, H or OCH 3 . Catalysts made up of titanocenes and methyl benzoate or nanometric NaH have been used for polymer hydrogenation , and mixtures of substituted and unsubstituted bis(cyclopentadienyl) nickel or cobalt and organolithium reagents have been used for the hydrogenation of conjugated dienes . Relative smaller amounts of organotitanium compounds alone or in combination with reducing agents such as n ‐butyllithium (0.25–1.50 mmol Ti/100 g of polymer) allow effective hydrogenation of dienes containing polymers.…”

“…An important aspect of the polymer hydrogenation process is the interaction between the organometallic compound and hydrogen. Studies on heterogeneous and homogeneous catalytic processes have suggested that the organometallic compound adsorbs hydrogen, thus producing the true catalytic species for polymer hydrogenation . However, the actual active site for polymer hydrogenation with homogeneous catalytic systems has not been precisely identified.…”

“…However, the actual active site for polymer hydrogenation with homogeneous catalytic systems has not been precisely identified. Nevertheless, in homogeneous systems, it has been demonstrated that the saturation rate of the 1,2‐vinyl units of polybutadiene is higher than that for the 1,4‐ cis or 1,4‐ trans units .…”

Homogeneous Catalytic Hydrogenation of Polymers

“…These catalysts consist mainly of halides or aryls of cyclopentadienyl Group III or IV metals (e.g., RMX 1 X 2 X 3 , where R is an unsubstituted or substituted cycopentadienyl group; M is metal; X 1 , X 2 and X 3 may be either the same or different selected from halogen atoms, aryl groups, aryloxy groups or carbonyl groups, etc., and one of them may be an unsubstituted or substituted cyclopentadienyl group) which are often treated with organolithium reducing agents, and have been applied for homogeneous polymer hydrogenation 80]. These catalysts consist mainly of halides or aryls of cyclopentadienyl Group III or IV metals (e.g., RMX 1 X 2 X 3 , where R is an unsubstituted or substituted cycopentadienyl group; M is metal; X 1 , X 2 and X 3 may be either the same or different selected from halogen atoms, aryl groups, aryloxy groups or carbonyl groups, etc., and one of them may be an unsubstituted or substituted cyclopentadienyl group) which are often treated with organolithium reducing agents, and have been applied for homogeneous polymer hydrogenation 80].…”