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Living cationic polymerization of vinyl ethers by electrophile/lewis acid initiating systems. VII. Living cationic polymerization of isobutyl vinyl ether by trimethylsilyl halide/zinc halide initiating systems in the presence of p-methoxybenzaldehyde: Effects of halide anions and zinc halides
Authors:Masami Kamigaito  Mitsuo Sawamoto  Toshinobu Higashimura
Abstract:
Trimethylsilyl halides (Me3SiY), in conjunction with zinc halides (ZnX2) (Y and X:I, Br, Cl), were employed to investigate the living cationic polymerization of isobutyl vinyl ether (IBVE) in toluene at ?15°C in the presence of p-methoxybenzaldehyde; with the aldehyde and IBVE monomer, Me3SiY yields an initiating species [Me3Si? O? CHC6H4(OMe) ? CH2CH(OiBu) ? Y] that triggers the IBVE polymerization via the activation of its carbon-halogen bond (equation imageC? Y) by ZnX2 into equation imageCδ+…?Yδ?…?ZnX2. Living polymerizations occurred with the silyl iodide and bromide irrespective of the type of ZnX2, either when Y = X (Me3Sil/Znl2 and Me3SiBr/ZnBr2) or when Y ≠ X (Me3Sil/ZnBr2, Me3SiI/ZnCl2, and Me3SiBr/Znl2). With these five initiating systems, the number-average molecular weights (M?n) of the polymers increased in proportion to monomer conversion, and the molecular weight distributions (MWDs) of the polymers were narrow (M?w/M?n = 1.1?1.2). The Me3SiCl-based systems (Me3SiCl/ZnCl2 and Me3SiCl/Znl2), in (Me3SiCl/Znl2), in contrast, failed to give perfectly living polymerization; the M?n indeed increased with conversion, but the MWDs of the polymers were broader (M?w/M?n = 1.3?1.5). Thus, the living nature of the polymerizations with Me3SiY/Znx2 is primarily determined by the halogen Y in Me3SiY, which generates the terminal carbon-halogen bond (equation imageC? Y) that is activated by ZnX2 for the propagation via a species equation imageCδ+…?Yδ?…?ZnX2. For Y?, not only the iodide but the bromide anion also is suited for living cationic polymerization. The virtual absence of the effects of X in ZnX2 implies that the halogen exchange between ZnX2 and Y from Me3 SiY at the growing end (equation imageCλ+…?Yδ?…?ZnX2 ?equation imageCδ+…?Xδ?…?ZnXY) is absent or negligible.
Keywords:living polymerization  cationic polymerization  vinyl ether  trimethylsilyl halide  zinc halide  polymerization initiator
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