Relative Reactivity of C4 Olefins toward the Polyisobutylene Cation.

Abstract

The capping reaction of living polyisobutylene cation (PIB+) with 1-butene, cis-2-butene, trans-2-butene, or 1,3-butadiene in hexanes (Hex)/methyl chloride (MeCl) 60/40 (v/v) solvent mixtures at −80 °C was studied. The reaction products were characterized by elemental analysis, gel permeation chromatography, and NMR spectroscopy. Monoaddition (capping) was observed with 1-butene and cis-2-butene but not with trans-2-butene, which did not react with PIB+. Monoaddition of 1,3-butadiene followed by instantaneous halide transfer from the counteranion and selective formation of the 1,4-adduct were observed in Hex/MeCl 60/40 (v/v) solvent mixtures at −80 °C at [1,3-butadiene] ≤ 0.05 mol L-1. The polymerization of isobutylene (IB) was studied in the presence of 1-butene, cis-2-butene, trans-2-butene, or 1,3-butadiene as capping agent in Hex/MeCl 80/20 to 40/60 (v/v) in the temperature range of −40 to −80 °C. From the limiting conversion and limiting number-average degrees of polymerization the reactivity ratio, kp±/kc± (where kp± is the absolute rate constant of propagation for IB and kc± is the cross-propagation (capping) rate constant from PIB+ cation to the C4 olefins), was calculated. Since the propagation of IB (addition of IB to PIB+) does not have an enthalpic barrier, the activation energy for cross-propagation was calculated to be 5.1−6.0 kJ mol-1. By extrapolating the plot of kp±/kc± vs hexanes (vol %), the reactivity ratios were calculated for 100% hexanes at −80 °C. From the cross-propagation activation energy and the calculated reactivity ratios at −80 °C in 100% hexanes, the reactivity ratios for 0 °C have also been calculated. According to these values at 0 °C, isobutylene is ∼17 times more reactive than 1,3-butadiene, ∼543 times more reactive than 1-butene, and ∼294 times more reactive than cis-2-butene.