Solvent polarity effects, initiator

An investigation into the initiation mechanism of copolymerization of ethyl vinyl ether and acrylonitrile by /-butoxyl radicals lias shown that the reaction between the two monomers competes successfully with radical trapping by the nitroxide radical trap (5).37 The /-butoxyl radicals react 3-6 times faster with ethyl vinyl ether than acrylonitrile the authors proposed that this is due to selective interaction of one monomer with the radical species rather than a solvent polarity effect. 

In order to establish the mechanism of the reaction, the solvent effect on the reaction of pyridinyl radical with dibromomethane was investigated. As the results listed in Table 1 show, there is no solvent effect on the rate of the reaction. How could one reconcile the formation of a salt with the lack of solvent polarity effect on the rate Since the initial state (Py + RX) is not very polar (pyridinyl radical with a 1-ethyl group is soluble in n-hex ine, BrCH2Br has a dipole moment of ca. 1 Debye), the lack of... 

Mechanism Initial state Transition state Relative charge in transition state Rate effect of increase in solvent polarity... 

Saturated hydrocarbons show a slight stabilizing effect on the initial state but a very destabilizing effect on the transition state, consistent with arguments based on solvent polarity. 

Research described in this section concerns effects of solvent polarity, temperature, monomer and initiator concentration on the polymerization of a-methylstyrene with Si-H containing initiator/Me3Al system for the synthesis of HSi-PaMeSt and of desirable molecular weight. 

Table 10. The effect of solvent polarity on the polymerization of isobutylene by the HSi(CH3)2CH2CH29 CH2Q/Me3Al initiating system...

Solvatochromic pareuaeters, so called because they were Initially derived from solvent effects on UV/visible spectra, have been applied subsequently with success to a wide variety of solvent-dependent phenomena and have demonstrated good predictive ability. The B jo) scale of solvent polarity is based on the position of the intermolecular charge transfer absorption band of Reichardt s betaine dye [506]. Et(io> values are available for over 200 common solvents and have been used by Dorsey and co-%rarkers to study solvent interactions in reversed-phase liquid chromatography (section 4.5.4) [305,306]. For hydrogen-bonding solvents the... 

Aryl and, more so, chlorine substituents on silicon enhance thermal stability of silacyclobutanes. The rate of the first-order thermal decomposition of silacyclobutanes varies inversely with the dielectric constant of the solvent used. Radical initiators have no effect on the thermal decomposition and a polar mechanism was suggested. Thermal polymerization of cyclo-[Ph2SiCH212 has been reported to occur at 180-200°C. The product was a crystalline white powder which was insoluble in benzene and other common organic solvents [19]. 

As pericyclic reactions are largely unaffected by polar reagents, solvent changes, radical initiators, etc., the only means of influencing them is thermally or photochemically. It is a significant feature of pericyclic reactions that these two influences often effect markedly different results, either in terms of whether a reaction can be induced to proceed readily (or at all), or in terms of the stereochemical course that it then follows. Thus the Diels-Alder reaction (cf. above), an example of a cycloaddition process, can normally be induced thermally but not photochemically, while the cycloaddition of two molecules of alkene, e.g. (4) to form a cyclobutane (5),... 

Effect of Solvent Polarity. Quasiliving polymerization of IBVE was also attempted in a nonpolar solvent, n-heptane, ap -50 and -70°C. Figure 5 gives results obtained at -70°C. The Mn versus Wj ve plots obtained at this temperature are strongly curved. Importantly, the N values were less than unity at the beginning of the reactions and they increased beyond unity with increasing WibVE Evidently initiation is slow in nonpolar media due to incomplete ionization of the initiator (i.e., N >1). 

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

Another, more indirect but perhaps more efficient method, would be to determine K for a number of typical systems, perhaps by use of model compounds, and then to select for kinetic experiments initiator systems for which K is so great that [Pn+] is effectively equal to c0, so that then the simple Equation (1) with [Pn+] = c0 is applicable. The trouble is that this method will probably only work for fairly polar solvents, because it is to be expected that Kp will be smaller, the less polar the solvent. This effect is probably one of the factors responsible for the improbably low kp value obtained by Higashimura for styrene in benzene solution [7]. In any case, for solvents of low polarity the participation of paired cations must be taken into account, which makes the relevant equations rather more complicated, but does not alter the relevance and importance of equilibrium (i). 

Several factors have been invoked to explain the aqueous rate acceleration aggregation of the reactants leading to micellar catalysis, effects connected with the internal pressure of the solvent, polarity of the solvent, H-bonding interactions with the solvent, and hydrophobic interactions (A y < 0). The initial literature was rather controversial, and there was a strong need for a systematic study using physical-organic techniques. 

---