Bimetallic Effects in Homopolymerization of Styrene and Copolymerization of Ethylene and Styrenic Comonomers:  Scope, Kinetics, and Mechanism

Abstract

This contribution describes the homopolymerization of styrene and the copolymerization of ethylene and styrenic comonomers mediated by the single-site bimetallic "constrained geometry catalysts" (CGCs), (mu-CH2CH2-3,3'){(eta(5)-indenyl)[1-Me2Si(tBuN)](TiMe2)}2 [EBICGC(TiMe2)2; Ti2], (mu-CH2CH2-3,3'){(eta(5)-indenyl)[1-Me2Si(tBuN)](ZrMe2)}2 [EBICGC(ZrMe2)2; Zr2], (mu-CH2-3,3'){(eta(5)-indenyl)[1-Me2Si(tBuN)](TiMe2)}2 [MBICGC(TiMe2)2; C1-Ti2], and (mu-CH2-3,3'){(eta(5)-indenyl)[1-Me2Si(tBuN)](ZrMe2)}2 [MBICGC(ZrMe2)2; C1-Zr2], in combination with the borate activator/cocatalyst Ph3C+ B(C6F5)4- (B1). Under identical styrene homopolymerization conditions, C1-Ti2 + B1 and Ti2 + B1 exhibit approximately 65 and approximately 35 times greater polymerization activities, respectively, than does monometallic [1-Me2Si(3-ethylindenyl)(tBuN)]TiMe2 (Ti1) + B1. C1-Zr2 + B1 and Zr2 + B1 exhibit approximately 8 and approximately 4 times greater polymerization activities, respectively, than does the monometallic control [1-Me2Si(3-ethylindenyl)(tBuN)]ZrMe2 (Zr1) + B1. NMR analyses show that the bimetallic catalysts suppress the regiochemical insertion selectivity exhibited by the monometallic analogues. In ethylene copolymerization, Ti2 + B1 enchains 15.4% more styrene (B), 28.9% more 4-methylstyrene (C), 45.4% more 4-fluorostyrene (D), 41.2% more 4-chlorostyrene (E), and 31.0% more 4-bromostyrene (F) than does Ti1 + B1. This observed bimetallic chemoselectivity effect follows the same general trend as the pi-electron density on the styrenic ipso carbon (D > E > F > C > B). Kinetic studies reveal that both Ti2 + B1 and Ti1 + B1-mediated ethylene-styrene copolymerizations follow second-order Markovian statistics and tend to be alternating. Moreover, calculated reactivity ratios indicate that Ti2 + B1 favors styrene insertion more than does Ti1 + B1. All the organozirconium complexes (C1-Zr2, Zr2, and Zr1) are found to be incompetent for ethylene-styrene copolymerization, yielding only mixtures of polyethylene and polystyrene. Model compound (mu-CH2CH2-3,3'){(eta(5)-indenyl)[1-Me2Si(tBuN)][Ti(CH2Ph)2]}2 {EBICGC[Ti(CH2Ph)2]2; Ti2(CH2Ph)4} was designed, synthesized, and structurally characterized. In situ activation studies with cocatalyst B(C6F5)3 suggest an eta(1)-coordination mode for the benzyl groups, thus supporting the proposed polymerization mechanism. For ethylene-styrene copolymerization, polar solvents are found to increase copolymerization activities and coproduce atactic polystyrene impurities in addition to ethylene-co-styrene, without diminishing the comonomer incorporation selectivity. Both homopolymerization and copolymerization results argue that substantial cooperative effects between catalytic sites are operative.

Synthesis of ethylene–styrene copolymer containing syndiotactic polystyrene sequence by trivalent titanium catalyst

“…7,8 Recently, several researchers have reported the development of more efficient catalysts for the copolymerization of ethylene and styrene: a half-titanocene featuring an aryloxy-type ligand, a titanium complex having a pendant-type cyclopentadienyl ligand and a rare-earth metal-based catalyst. [9][10][11][12][13][14][15][16][17] However, there have been only limited reports on the synthesis of copolymers containing long syndiotactic styrene-styrene sequences. Hou and co-workers 18 have reported that rare-earth metal-based catalyst systems promote not only the syndiotactic homopolymerization of styrene, but also copolymerization with ethylene, in which the Cp¢Sc(CH 2 SiTMS) 2 /Ph 3 CÁB(C 6 F 5 ) 4 [Cp¢¼C 5 Me 4 (CH 2 TMS)] catalyst system produces a multi-block copolymer consisting of a crystalline SPS part and an ethylene-rich part.…”

Group 4 metal complexes for homogeneous olefin polymerisation: a short tutorial review

“…This suggests an unusual insertion regiochemistry, which the authors suggest is a result of a favourable interaction with the proximate metal centre as is depicted in Fig. 14 [175,176].…”

Olefin Polymerization with Half-Metallocene Catalysts

“…In contrast, the bimetallic system (Ti 2 2B 1 or Ti 2 B 2 in Scheme 2.13) exhibited significantly higher catalytic activities for styrene polymerization than the mononuclear system (2Ti 1 B 1 or 2Ti 1 B 2 ), although the resultant polymer possessed atactic stereoregularity (Table 2.12) (Reviewing article for nuclearity and cooperativity effects in binuclear catalysts and cocatalysts for olefin polymerization [124,208,209]. Scheme 2.13 Catalyst/cocatalyst nuclearity matrix for constrained geometry catalysts [124] A drastic improvement in the styrene incorporation in the copolymerization could be achieved by using a dinuclear catalyst system (Ti 2 2B 1 ), synthesis of a copolymer with a high styrene content (76 mol%) has thus been achieved in this catalysis (Table 2. 12).…”