Winstein spectrum

The experimental observations may be explained by the mechanism shown in Scheme 5 together with the following speculations. The initial event of star synthesis is the formation of 2 from 1 (under the influence of BCI3 or TiC [7]) which in the presence of excess BCI3 gives 3. The existence of an equilibrium between dormant and active species such as 2 and 3, respectively, in living IB polymerization has been discussed in detail [7]. The ionicity of the active form 3 and its position in the Winstein spectrum [7] depend on experimental conditions (i.e., nature and concentration of the coinitiator, additives (e.g., organic bases), temperature, solvent polarity, etc.). 

Propagation. Regardless of its degree of polymerization, the carbanion and its metal counterion Me+ can be expected to exist in a spectrum of different forms, ranging from a covalently bonded species at one extreme to separate ions at the other, with a contact ion pair and a solvent-separated ion pair as intermediates (Winstein spectrum) [77,78] ... 

All species in the Winstein spectrum 11.53 might contribute to propagation. Leaving all possibilities open, the propagation rate can be written... 

The model is based on the proposition that all carbocationic polymerizations of olefins, alkyl vinyl ethers, etc., involve a spectrum of species with differing ionicitieS connected by equilibria formally expressed by the Winstein ionicity spectrum 1] the Winstein spectrum starts with a covalent species and progresses through increasingly polarized and ionized species, to fully ionized solvated ( free ) ion pairs ... 

For the sake of simplicity the large number of possible entities in the Winstein spectrum can be reduced to three representative species [1, 24] ... 

Thus the model consists of a series of intimately interconnected Winstein spectra. The individual Winstein spectra show only four representative species of the many possible. The total number of Winstein equilibria in a polymerization of this sort will depend on the number average degree of polymerization (DPn). For example, the trimerization of a monomer M induced either by HX or RX in the absence of chain transfer, will include four interconnected Winstein spectra one Winstein spectrum for ion generation, followed by one for cationa-tion of the first monomer unit, and two additional Winstein spectra for the two... 

Initiation is visualized to occur in two consecutive steps ion-generation and cationation [1]. Initiation in impurity-induced polymerization consists of ion-generation from HX shown in the first Winstein spectrum on the right followed by protonation of M by any one of these species. Similarly, for purposely-induced initiation by RX, the first line on the left depicts ion-generation, which is followed by cationation of M. The chemistries of cationation and propagation are necessarily very similar [1]. 

In general, these anions are associated with a counterion, typically an alkali metal cation. The exact nature of the anion can be quite varied depending on the structure of the anion, counterion, solvent, and temperature [3-5]. The range of possible propagating species in anionic polymerization is depicted in terms of a Winstein spectrum of structures as shown in Equation 7.2 for a carbanionic chain end (R ) [3, 6]. In addition to the aggregated (associated) (I) and unaggregated (unassociated) (2) species, it is necessary to consider the intervention of free ions (5), contact... 

Polar Solvents A change in the reaction medium from hydrocarbon to polar solvents causes changes in the nature of the alkali metal carbanions, which can be interpreted in terms of the Winstein spectrum of ionic species as shown in Scheme 7.11 [3, 6]. Thus, in addition to the aggregated (1) and unaggregated (2) species that can exist in hydrocarbon solution, in polar solvents it is necessary to consider the intervention of free ions (5) and the contact (3) and solvent-separated (4) ion-paired carbanion species as propagating species as shown in Scheme 7.11. 

In general, as the polarity (dielectric constant) and solvating ability of the medium increase, a transition to more ionic species (a shift in the Winstein spectrum from left to right) occurs. In weakly polar solvents such as diox-ane (e = 2.21), the kinetics of styrene propagation exhibits pseudo-tirst-order behavior as illustrated in Equation 7.17,... 

Third, the kinetics of initiation with Grignard reagents and alkyllithiums is further complicated by association of ion pairs. The association, as described by the Winstein spectrum (Scheme 2.3), is influenced by steric hindrance as well as temperature, solvent, and concentration. Normally, the position in this series (Scheme 2.3) varies from aggregated species in hydrocarbon solvents to contact ion pairs and solvent separated ion pairs in the presence of polar solvents. Decreasing the concentration and increasing... 

Thus, because this reaction would be even more exothermic, it would be predicted based on the Hammond Postulate [107] that there would be less charge transfer onto the benzylic carbon in the transition state. The fact that p values in THF are much larger than in hydrocarbon solution can also be explained by the effect of solvent on the counterion solvation of the counterion would decrease the interaction of the counterion with the carbanion which would result in more delocalization of charge. Or in terms of the Winstein spectrum of ionic species [3, 108], solvation of the counterion and an increase in the dielectric constant of the medium would shift the structure towards a more ionic species. 

The idea of the stretched bonds and the whole Winstein spectrum popular at one point of time is finally put to rest. Extensive discussion on the allegedly living GVP is given in a paper by Matyjaszewski and Sigwalt. These authors analyzed the GGVP in terms of reversible deactivation in the propagation step either by the formation of covalent species (Scheme 36) or... 

Considering the potential complexity of active propagating species based on the Winstein spectrum of carbanionic stmaures (see eqn [2]), it would be anticipated that the following general rate equation (eqn [17]) would be applicable for anionic polymerization of all styrene and diene monomers that exhibit first-order kinetic dependencies on monomer concentration. 

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