Thermally reversible polymer systems by cyclopentadienylation. I. A model for termination by cyclopentadienylation of olefin polymerization

The reaction between tert ‐butylchloride ( t ‐BuCl) and dimethylcyclopentadienylaluminum (Me 2 AlCPD) was studied as a model for initiation by the tert ‐butyl cation ( t ‐Bu ⊕ ) and termination by cyclopentadienylation by the Me 2 Al(CPD)Cl ⊖ counteranion of isobutylene polymerization. All reaction products formed in this model system have been identified and quantitatively determined. A comprehensive scheme that indicates pathways to these products was developed (scheme III). It is proposed that the predominant product, tert ‐butylcyclopentadiene ( t ‐BuCPD), arises in the collapse of the t ‐Bu ⊕ ‐Me 2 Al(CPD)Cl ⊖ ion pair, mainly by CPD ⊖ transfer to the tert ‐butyl cation. The minor products are neopentane ( t ‐BuMe) and isobutylene ( i ‐C 4 H 8 ), which are probably formed, respectively, by Me ⊖ transfer to and proton loss from the t ‐butyl cation. Cyclopentadienylation selectivity increases by lowering the temperature and extrapolation of results suggests 100% cyclopentadienylation at −55°C. The t ‐BuCl/Me 2 AlCPD ratio strongly influences the overall reaction pathway. The reaction is first order in t ‐BuCl with Δ E a of ca. 7 kcal/mole (1,2‐dichloroethane or chlorobenzene solvents, +24 to −29°C). Indirect evidence indicates that the kinetic product of cyclopentadienylation is 5‐ t ‐BuCPD and that this isomer cannot be tert ‐butylated; that is, the initiation of 5‐ t ‐BuCPD polymerization by t ‐Bu ⊕ is sterically unfavorable. Detailed analysis of the chemistry and kinetics of the t ‐BuCl/Me 2 AlCPD model system holds important clues to the controlled polymerization of olefins leading to macromolecules with cyclopentadiene (CPD) termini.