By Stanislaw Penczek, Przemyslaw Kubisa, Krzysztof Matyjaszewski
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Extra resources for Cationic Ring-Opening Polymerization of Heterocyclic Monomers
C÷. . . . . H (64) H A number of arguments support this mechanism for the polymerization of cyclic ethers: 1. Carbenium ions that should operate in the SN l-like mechanism are strong hydride abstractors; thus, the primary carbenium ions easily rearrange through the intramolecular hydride shift to the more stabilized secondary or tertiary carbenium ions (reactions observed in e. g. cationic vinyl polymerization) ~42). Neither hydride abstraction including intermolecular transfer nor isomerization (the latter argument has already been used by Medvedev a.
63) The stereochemicalcourse of this reaction requires a linear transition state; this nucleophilic attack, as shown below, leads to the inversionof configuration at the carbon atom in the a-position of the oxonium ion: /CH2~. •.. --CH2--O. . . . . C÷. . . . . H (64) H A number of arguments support this mechanism for the polymerization of cyclic ethers: 1. Carbenium ions that should operate in the SN l-like mechanism are strong hydride abstractors; thus, the primary carbenium ions easily rearrange through the intramolecular hydride shift to the more stabilized secondary or tertiary carbenium ions (reactions observed in e.
The ability o f Lewis acids to initiate p o l y m e r i z a t i o n w i t h o u t any cocatalyst added (as in Eq. (48)) depends o n b o t h the ring strain and the nucleophilicity o f m o n o m e r as well as o n the Lewis acid strength. F o r example, while BF 3 cannot initiate t h e p o l y m e r i z a t i o n o f T H F (ring strain ~ 5 . 0 k c a l - m o l e - 1), it does initiate the p o l y m e r i z a t i o n o f t h e t h r e e - m e m b e r e d oxirane (ring strain ~ 20 k c a l . m o l e - 1 ) giving a c o m p l e x w i t h BF 3, which is o n l y stable b e l o w - 8 0 °C.