Mayr, Herbert; Schneider, Reinhard; Schade, Christian (1988): Kinetic and Thermodynamic Studies of Carbenium Ion Additions Towards Alkenes. In: Makromolekulare Chemie / Macromolecular Symposia, Vol. 13/14: S. 43-59




The ionisation (Ar2CHCl + BC13 v=± Ar2CH+BCi;) and dissociation (Ar2CH+BCi; ^=*Ar2CH+ + BCl^) equilibria of diarylmethyl chlorides i n boron trichloride/ dichloromethane solution have been studied by conductimetry, photometry and *H NMR spectroscopy. Small differences i n the UV-vis spectra of diarylcarbenium tetrachloroborates, which have been observed i n solutions of low and high tetrachloroborate concentration, can be attributed to the formation of 1 :1 ion-pairs i n the more concentrated solutions. Low temperature calorimetry was used to determine the heats of addition of diarylcarbenium tetrachloroborates to 2~methyl-1 -pentene (Ar2CH+BCi; + H2C=CRRT ArjCH-CHj-CRR'Cl + BC13), and it is estimated that the standard free enthalpy of t h i s reaction is greater than 0 for systems with pKR+ > -2.6. Kinetic studies have shown that paired and unpaired diarylcarbenium tetrachloroborates exhibit identical reactivity towards alkenes. A rationalisation for the different situation in carbocationic and carbanionic polymerisation i s presented. The rate constants for the initiation of isobutene, styrene and isoprene polymerisation by diarylcarbenium ions have been determined, and it is shown, how propagation rate constants can be estimated from linear free energy relationships.