Living Coupling Reaction in Living Cationic Polymerization. 1. Coupling Reaction of Living Polyisobutylene

The living coupling reaction of living polyisobutylene (PIB), prepared by the 2,4,4-trimethyl-2-chloropentane/TiCl4/hexane:methyl chloride (60:40, v:v)/−80 °C system, has been studied using 1,3-bis(1-phenylethenyl)benzene (MDDPE), 2,2-bis[4-(1-phenylethenyl)phenyl]propane (BDPEP), and 2,2-bis[4-(1-tolylethenyl)phenyl]propane (BDTEP) as coupling agents. The reaction of living PIB with MDDPE yielded the monoadduct, possibly due to delocalization of positive charge over the meta-substituted benzene ring upon monoaddition and thereby decreased reactivity of the second double bond. Using BDPEP and BDTEP which have two diphenylethylene (DPE) moieties separated by an electron-donating spacer group, rapid and quantitative coupling was achieved independently of the chain length of the original PIB. The coupled product exhibited doubled molecular weight and narrowed molecular weight distribution. Direct evidence of the quantitative coupling reaction was also obtained by comparison of the 1H NMR spectra of the samples before and after the coupling reaction. Kinetic studies by 1H NMR spectroscopy indicated the coupling reaction of living PIB by BDPEP is a consecutive reaction where the second addition is faster than the first one. By kinetic treatment of the experimental results, it was found that the second addition is about 5 times faster than the first one. As a result, high coupling efficiency was also observed when excess BDPEP was used.