Sp2-hybridized carbenium ion

The degenerative nature of propagation results in reformation of the same active species, but with monomer consumption and chain growth. Although the monomer s thermodynamic polymerizability is independent of the mechanism, the mechanism and structure of the active species determines the rate of monomer conversion. The structure of the active species involved in carbocationic polymerizations was discussed in Section II detailed information on the reactivities of model species was presented in Chapter 2, with the conclusion that covalent precursors do not react directly with alkenes, but must first ionize to sp2-hybridized carbenium ions. Only the resulting carbenium ions can add to double bonds. 

Propagation is the most important elementary reaction in which a macro-molecular chain is formed. Control in new carbocationic polymerizations, in which well-defined polymers are prepared, might be explained by new mechanisms of propagation and new types of active centers involved. However, as discussed briefly in Section IV.B.4, we believe that only two types of species with different degrees of ionization are involved sp3-hybridized dormant species and sp2-hybridized carbenium ions [Eq. (43)] ... 

Large a-deuterium KIEs were found for carbenium ion reactions and small KIEs for Sn2 reactions, and since the Crl 11(D) stretching vibration becomes stronger as the sp3 hybridized substrate is converted into the sp2 hybridized carbenium ion in an SN1 reaction, an inverse KIE, not the large normal KIEs observed for SN1 reactions, should be observed. Therefore, it was suggested that the magnitude of the KIE was... 

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