Abstract:The cationic polymerization of isobutene by the "inifer"-technique was studied. Preliminary experiments are described which suggest that useful polymer can be prepared only when BCI, is rapidly added to the premixed monomer/inifer mixture and that such polymerizations do not go to completion. Some questions regarding the mechanism are discussed and remain open.0025-1 16X/83/$03 .OO
“…It is responsible for the strong rate increase observed in polymerization of isobutyl vinyl ether initi-ated by an alkyl iodide in the presence of tetrabutylammonium perchlorate. 61 The small effect of the tetrabutylammonium iodide results from an increase of the ionic strength.…”
The observation of various features of living polymerizations (‘theoretical’ molecular weights, narrow molecular weight distribution) in a large number of carbocationic polyerizations has led to the conclusion that termination and transfer were suppressed in these ‘living’ systems, while they played a predominant role in the ‘classic’ carbocationic polymerizations. This suppression has been attributed to the presence of new types of active sites, such as ‘stretched’ covalent bonds or ‘stabilized’ carbocations.
In the present article, a comparison of the data observed with the ‘classic’ and the ‘living’ systems shows that the active sites are similar, with only a change in the relative proportion of carbocationic species, which reduces the rate and favors complete initiation. This means that these ‘living’ systems are generally only apparently living, since termination and transfer reactions also occur and are hidden because the molecular weights of the ‘living’ polymers are typically low. Transfer and termination may, however, be considerably reduced or even suppressed by working at very low temperatures, as is also the case for ‘classic’ systems.
“…It is responsible for the strong rate increase observed in polymerization of isobutyl vinyl ether initi-ated by an alkyl iodide in the presence of tetrabutylammonium perchlorate. 61 The small effect of the tetrabutylammonium iodide results from an increase of the ionic strength.…”
The observation of various features of living polymerizations (‘theoretical’ molecular weights, narrow molecular weight distribution) in a large number of carbocationic polyerizations has led to the conclusion that termination and transfer were suppressed in these ‘living’ systems, while they played a predominant role in the ‘classic’ carbocationic polymerizations. This suppression has been attributed to the presence of new types of active sites, such as ‘stretched’ covalent bonds or ‘stabilized’ carbocations.
In the present article, a comparison of the data observed with the ‘classic’ and the ‘living’ systems shows that the active sites are similar, with only a change in the relative proportion of carbocationic species, which reduces the rate and favors complete initiation. This means that these ‘living’ systems are generally only apparently living, since termination and transfer reactions also occur and are hidden because the molecular weights of the ‘living’ polymers are typically low. Transfer and termination may, however, be considerably reduced or even suppressed by working at very low temperatures, as is also the case for ‘classic’ systems.
“…Among aralkyl precursors, cumyl derivatives activated by BCl 3 or TiCl 4 have been extensively used. , For instance, the cationic polymerization of isobutene initiated by the cumyl chloride/BCl 3 system was described as one of the first “living” carbocationic systems, and in this polymerization, the cumyl chloride was considered as an initiator as well as a transfer agent (a so-called inifer , ). The first controlled carbocationic polymerization of α-methylstyrene was achieved using the same initiating system .…”
Study of the ionization of cumyl chloride by boron trichloride was
carried out using stopped-flow UV−visible and 1H NMR spectroscopy in methylene
chloride. Cumyl chloride is very weakly ionized
to form the cumyl cation (λmax = 330 nm) which is
stable at low temperature (<−60 °C) with
tetrachloroborate anion since no elimination could be observed. At
that temperature, owing to the presence
of traces of α-methylstyrene in the cumyl chloride solutions, the
ionization step was immediately followed
by the addition of this monomer, resulting in the formation of the
dimeric (or oligomeric) cation (λmax =
348 nm), which slowly cyclized into the corresponding indanic
derivatives. At −65 °C, the equilibrium
constant of ionization of cumyl chloride is much smaller than that of
the chlorinated dimeric (or oligomeric)
species. At room temperature, dehydrochlorination of the cumyl
cation leading to α-methylstyrene was
evidenced and the indanic dimer was the only final
product.
“…The analogous process using biscumyl chloride, 2, and triscumyl chloride, 3, led to the formation of bi-and trifunctional polymers. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] While these polymerizations were not living under the original reaction conditions, slight modifications led to living systems. 5,[20][21][22][23][24][25][26][27][28][29][30] In contrast to these findings we had observed the exclusive formation of the 1:1 product P1, 31 when a mixture of isobutylene and cumyl chloride, 1, was treated with the weaker Lewis acid ZnCl 2 ‚OEt 2 32 in dichloromethane.…”
Section: Introductionmentioning
confidence: 99%
“…The formation of telechelic polymers by cumyl chloride 1 (1-chloro-1-methyl-ethyl)benzene/BCl 3 -initiated polymerizations of isobutylene has first been reported by Kennedy − and Nuyken (Inifer mechanism). The analogous process using biscumyl chloride, 2 , and triscumyl chloride, 3 , led to the formation of bi- and trifunctional polymers. − While these polymerizations were not living under the original reaction conditions, slight modifications led to living systems. ,− …”
The living oligomerization of isobutylene initiated by cumyl chloride 1 or by the telechelics P1 or P2 in presence of BCl3 and benzyltriethylammonium tetrachloroborate has been investigated. The tertiary chlorides P2-P7 were separated by HPLC and analyzed as spectroscopically pure compounds. A kinetic model has been derived from which the gross propagation rate constants for the early propagation steps can be calculated. It has been found that the low reactivity of the adduct of cumyl chloride to one isobutylene unit (P1) is responsible for the broad MWD in the cumyl chloride/BCl3/BCl4 --initiated oligomerization of isobutylene. In contrast, a narrow MWD could be achieved by using the 2:1 product P2 in combination with BCl3/BCl4 -as initiator.
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