Isobutene polymerization initiated by [CP*TiMe₂]⁺ in the presence of a series of novel, weakly coordinating counteranions

Abstract

The highly electrophilic borane B(C 6 F 5 ) 3 reacts with n-octadecanol (n-C 18 H 37 OH) and n-octadecanethiol (n-C 18 H 37 SH) to form the 1:1 adducts (n-C 18 H 37 EH)B(C 6 F 5 ) 3 (E ϭ O or S). The latter are acidic and react with Cp*TiMe 3 in methylene chloride and toluene to give methane and the complexes [Cp*TiMe 2 ][(n-C 18 H 37 E)B(C 6 F 5 ) 3 ], which are very good initiators for the carbocationic polymerization of isobutene (IB) from Ϫ40 to Ϫ20°C. High conversions to high molecular weight polyisobutene (PIB) in methylene chloride and moderate conversions to high molecular weight PIB in toluene are observed and are consistent with the anions [(n-C 18 H 37 E)B(C 6 F 5 ) 3 ] Ϫ being very weakly coordinating. Although polymerization in methylene chloride is too rapid for the temperature to be controlled, polymerization in toluene is slower, and the temperatures can be controlled so that Arrhenius-type plots of the logarithm of the number-average molecular weight versus T Ϫ1 ϭ 1/T may be obtained. Activation energies for the degree of polymerization in these polymerization reactions and similar polymerizations carried out with n-C 18 H 37 EH:borane ratios of 1:2 and with the activators [Ph 3 C][B(C 6 F 5 ) 4 ] and Al(C 6 F 5 ) 3 range from Ϫ11 to Ϫ27 kJ mol Ϫ1 , values comparable to those for most conventional IB polymerization initiators. However, the values of the weight-average and number-average molecular weights are unusually high for the temperatures used, and this is consistent with current theories of the role of weakly coordinating anions.

High‐Molecular‐Weight Isobutene–Isoprene Copolymers Containing Unusually High Isoprene Content

“…[11] In a typical experiment, the glass reactor was immersed in liquid nitrogen (À196 8C) and methyl chloride (10 mL) was added and frozen. A solution of a freshly prepared solution of n-C 17 H 35 CO 2 H (17 mg, 0.06 mmol) and B(C 6 F 5 ) 3 (64 mg, 0.12 mmol) in 2 mL of methylene chloride was then added and quickly frozen.…”

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

“…Comparative results are shown in Table 1, experiments 1, 2, 9 and 10. As can be seen, conversions are higher in the more polar solvent, as has been noted previously for this initiator system [7,32], but IB homopolymerizations in toluene give materials with very high weight average molecular weight ($5 · 10 5 ). Conversions and molecular weights did not change with time, indicating that the polymerizations were normally completed within one hour.…”

“…However cationic polymerizations exhibit high propagation rates even at low temperatures, and the corresponding heat released can be difficult to control unless the viscosity of the reaction mixtures is kept low and the solution is extremely well mixed. To minimize the effects of heating, all of the polymerization experiments reported here were carried out both in a reactor immersed in a cold bath and under reduced pressure such that the accompanying refluxing of the liquid mixtures served both to maximize mixing and to remove much of the heat of reaction [32]. Solvents and IB were dried with care to minimize the moisture content to lowest feasible levels.…”

“…Polymerizations were carried out using a 50 mL glass reactor equipped with a cold finger condenser (dry ice/ isopropanol) and an attached 12 mL graduated flask as described previously [32]. In a typical experiment, 12 mL of IB were condensed into the graduated flask which contained $100 mg of isoprene.…”

“…We [32,33], which behave as effective carbocationic initiators of IB homopolymerization at temperatures in the range À40 to À10°C. High conversions to high molecular weight polyisobutene (PIB) in methylene chloride and moderate conversions to high molecular weight PIB in toluene were observed, consistent with the anions [(n-C 18 H 37 E)B(C 6 F 5 ) 3 ] À being very weakly coordinating.…”