Initiator efficiency solvent effects

The low conversion initiator efficiency of di-r-butyl pcroxyoxalatc (0.93-0.97)1-1 is substantially higher than for other peroxyeslers [/-butyl peroxypivalale, 0.63 /-butyl peroxyacetate, 0.53 (60 °C, isopropylbenzene)195]. The dependence of cage recombination on the nature of the reaction medium has been the subject of a number of studies. 12I,1<>0 20CI The yield of DTBP (the main cage product) depends not only on viscosity but also on the precise nature of the solvent. The effect of solvent is to reduce the yield in the order aliphatic>aromatie>protic. It has been proposed199 that this is a consequence of the solvent dependence of p-scission of the f-butoxy radical which increases in the same series (Section 3.4.2.1.1). 

Et2 All coinitiators and MeCl, MeBr and Mel solvents at various temperatures. The H20 /EtAiei2/n-pentane system was also briefly investigated. A large number of comparative molecular weight and conversion data were gathered. The effect of MeX on the polymerization was investigated in detail. In this section, conversion and initiator efficiency data will be discussed. 

The second important solvent effect on Lewis acid-Lewis base equilibria concerns the interactions with the Lewis base. Since water is also a good electron-pair acceptor129, Lewis-type interactions are competitive. This often seriously hampers the efficiency of Lewis acid catalysis in water. Thirdly, the intermolecular association of a solvent affects the Lewis acid-base equilibrium242. Upon complexation, one or more solvent molecules that were initially coordinated to the Lewis acid or the Lewis base are liberated into the bulk liquid phase, which is an entropically favourable process. This effect is more pronounced in aprotic than in protic solvents which usually have higher cohesive energy densities. The unfavourable entropy changes in protic solvents are somewhat counterbalanced by the formation of new hydrogen bonds in the bulk liquid. 

Typical equations for the dissociation of AIBN and BPO are shown below. It should be pointed out that because of recombination, which is solvent-dependent, and other side reactions of the created free radical (R ), the initiator efficiency is seldom 100%. Hence, an efficiency factor (/) is employed to show the fraction of effective free radicals produced. 

Another example of a carbonyl compound whose excited singlet reacts before crossing over to a triplet is provided by coumarin. Direct irradiation in ethanol gives very low yields of a cw-fused dimer.432 However, inclusion of benzophenone results in only trans-fused dimer, even if all the light is initially absorbed by the coumarin.433 Apparently, excited singlet coumarin transfers its singlet excitation to benzophenone, which then crosses over to its triplet and transfers its triplet excitation to coumarin. Triplet coumarin then dimerizes fairly efficiently ( = 0.25) to a tranj-fused system. Morrison has demonstrated sizeable solvent effects on this photodimerization.434 Yields of the cis-dimer are increased by polar solvents and probably arise from excited singlet excimers.119... 

A number of other polar monomers have been polymerized with butyllithium, nominally in hydrocarbon or aromatic solvents. In almost all cases the monomer concentration was so high that the effective dielectric constant was much greater than in a pure hydrocarbon. All show rather complex behaviour. The degree of polymerization of the polymer formed is always much higher than the initial monomer-catalyst ratio so that a simple scheme involving only initiation and propagation reactions is not applicable. Only precipitable polymer was isolated, so it is not sure if the low initiator efficiencies are due to low polymer formation or to side reactions of butyllithium with the monomer. In addition most systems studied stop before complete conversion of the monomer. Evidently the small fraction of active polymer chains formed... 

However, the introduction of the solvent into the polymerization medium poses new problems. The solvents must be pure, without inhibiting and transfer agents. Every solvent takes part in the polymerization process its effect is almost never limited to the mere physical dilution of the monomer. It solvates the active centres it participates in processes connected with energy and impulse transfer often it serves as a transfer agent (so that the degrees of polymerization of solution-polymerized products are usually lower compared with bulk-polymerized polymers) it may form complexes with some component of the system it modifies initiation efficiency by the cage effect etc. 

Other advantages of C02-based polymerizations are that there is no chain transfer to the solvent, and that the production of unstable end groups can be dramatically reduced. Guan et al [4] studied the decomposition of 2,2 -azobis-(isobutyronitrile) (AIBN) in SCCO2. It was found that initiator efficiencies greater than 80% were possible due to the low viscosity of CO2 and negligible solvent cage effects. Additionally, analysis of the decomposition products showed that there was no chain transfer to CO2. 

An increase in the bulkiness of the alkyl group of the Grignard reagent has the same effect in polar solvent the initiator efficiency and polymer yield increases, and the molecular... 

Burnett et al.63,64 observed an anomalous rate increase in the polymerization of methyl methacrylate in halobenzene. Although the experimental data did not indicate any solvent effect on the rate of decomposition of azobisisobutyronitrile, the efficiency of initiation varied with solvents. Since an enhanced rate of incorporation of initiator fragments and incorporation of solvent fragments into the polymer were not observed, a mechanism describing the increase in the initiator efficiency through the participation of an initiator-solvent-monomer complex was postulated [Eq. (2.4)]. Henrici-Olive et al.65) reported, however, that the rate of the azobisisobutyronitrile decomposition at 50 °C, measured spectroscopically, is higher in halo-benzene than in benzene. Burnett et al.66) found a similar enhanced rate effect of halobenzenes with other initiators, supporting his mechanism. 

We have also studied the kinetics of free radical initiation in CO2 using azobis(isobutyronitrile) (AIBN) as an initiator [35]. These experiments were accomplished using high pressure UV spectroscopy, and illustrated that AIBN decomposes more slowly in CO2 than in traditional hydrocarbon solvents, yet the initiator efficiency is much greater in CO2 due to the reduced solvent cage effect in the low viscosity supercritical medium. The main conclusion drawn from this work was that CO2 can therefore be employed effectively as a solvent for free radical polymerizations and remains an inert solvent even in the presence of highly electrophilic hydrocarbon radicals. 

The factor f, called initiator efficiency, takes into account that not all the primary radicals R effectively initiate polymer chains some can be lost due to the so-called cage effect. This implies secondary reactions of the radicals within a cage of solvent surrounding the initiator [5] (the effect can be more pronounced at high conversions/viscosities due to diffusion limitations). The values of / usually lie in the range 0.3-0.8. 

It is one of the causes of inefficiency among initiators. The average time for recombination of free radicals inside a cage and the time for their diffusion out of the cage is about 10 s. In addition, the efficiency of the initiator is affected by the monomer and the solvent. Viscosity of the medium is inversely proportional to the initiator efficiency, because the more viscous the solution, the greater the cage effect. " ... 

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