By Stanislaw Penczek, Przemyslaw Kubisa, Krzysztof Matyjaszewski
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Extra resources for Cationic Ring-Opening Polymerization of Heterocyclic Monomers
1,3-Dioxolane was used as the model monomer and there are two groups of opinions about the structure of the growing species in polymerization. One group of authors maintains that poly-1,3-dioxolane grows exclusively as a macro-ring 149-151) with insertion of the monomer molecules in the propagation step and with release of the monomer molecule from the growing macro-ring in the depropagation step. J I V. , ) CH: - O - C H 2 - - C H 2 0 - ' C H 2 - O ~ - j ,, / (65) (Insertion by a concerted four-center mechanism) Another group 51, 152-156) sees propagation of 1,3-dioxolane in more conventional terms assuming that not cyclic but linear macromolecules grow, having active centers alkoxycarbenium or oxonium at their ends: + +/ .
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.
Chemical reactions preceding the actual initiation step may markedly depend on the structure o f the monomer involved and o f the F-C initiator. : 2 AIBr3 ~-~AtBr~, AIBr4 (43) The preformed cation can then be added to the monomer molecule. In the polymerization o f heterocycles such a possibility can be omitted because Lewis acids like BF3 readily form complexes with all nucleophilic reagents such as ethers, ketones, sulfides, or amines. g. the BFa/Et20 complex is equal to K = 140 mole -1 • I at 25 °C103).