Isobutene Polymerization and Isobutene-Isoprene Copolymerization Catalyzed by Cationic Zirconocene Hydride Complexes

Abstract

The cationic zirconocene trihydrides [Cp‘4Zr2H(µ-H)2]+X-, generated from [Cp‘2ZrH2]2 with various trityl salts of weakly coordinating anions, are powerful initiators for the polymerization of isobutene (IB) and its copolymerization with isoprene (IP) (Cp‘ = C5H4SiMe3). This study is concerned with the quantification of the effects of the counteranion and of trace moisture on IB/IP copolymers and the nature of the initiating species. Polymer molecular weights increase with decreasing anion nucleophilicity in the order X = [B(C6F5)4] ˜ [H2N{B(C6F5)3}2] > [CN{B(C6F5)3}2]. Using [Cp‘4Zr2H3]+ [CN{B(C6F5)3}2]-, high copolymer molecular weights are found (Mw ˜ 5 × 105 g/mol at -35 °C). There is little reduction in either rate or molecular weight on addition of isoprene. Polymer molecular weights are substantially higher than with the Et2AlCl/tBuCl initiator system under identical conditions. Water was shown to be an important chain-transfer agent; substoichiometric quantities of water reduced activity, and copolymer molecular weights decreased linearly with increasing [H2O]. Mechanistic studies suggest that [Cp‘4Zr2H(µ-H)2]+ does not itself act as a cationic initiator but is transformed into one or more other binuclear (polynuclear?) species, accompanied by alkene insertion into the Zr-H bond and a monomer hydrogenation step. Tentative structures for these reactive intermediates are suggested

Isobutene–isoprene copolymerization initiated by [Cp∗MMe2][(n-C18H37E)B(C6F5)3] (M=Ti, Hf; E=O, S) and related compounds

“…Similar results were reported by Bochmann and co-workers [29] using bulky anions. Table 2 lists a series of complementary experiments in which the effects of increased amounts of isoprene are added to the feed.…”

“…Although several very bulky anions have been reported [15][16][17][18][19][20][21][22][23][24][25][26][27][28], only a few of these have been assessed for the carbocationic polymerizations of monomers such as isobutene [29][30][31]. For instance Bochmann and co-workers [29] used ½Cp 0 4 Zr 2 H 3 ½CNfBðC 6 F 5 Þ 3 g 2 ðCp 0 ¼ C 5 H 4 SiMe 3 Þ as an initiator system and high molecular weight IB-IP copolymers (M w $ 5 · 10 5 at À35°C) were obtained.…”

Formation and structures of cationic zirconium complexes in ternary systems (X=Cl, Me)

“…Cationic hydride species are capable of initiating isobutene polymerization [22,31,32]. When different amounts of isobutene [(a) ca.…”

Alkyl Zinc Chlorides as New Initiators for the Polymerization and Copolymerization of Isobutene

“…Classical systems that provide high molecular weight PIBs include BCl 3 ,11 chelating boranes12 and a variety of transition‐metal halides, e.g., TiCl 4 , VCl 4 and FeBr 3 13. Organometallic Lewis acids paired with weakly coordinating anions, notably AlMeCp 2 /B(C 6 F 5 ) 3 ,14 [SiMe 3 ][B(C 6 F 5 ) 4 ],15 (C 5 Me 5 )TiMe 3 /B(C 6 F 5 ) 3 ,1c, 16 and ${\rm Cp'}_{\rm 2}$ ZrX 2 /[CPh 3 ][B(C 6 F 5 ) 4 ] (X = Me, H; Cp′ = C 5 H 4 SiMe 3 )17, 18 have also proved to be highly effective initiators, as have proton initiated systems coupled with perfluoroaryl borate anions 19, 20. Some of these systems can directly provide medium molecular weight polymers by a careful selection of the initiator system and the polymerization conditions.…”