Solvent-cage efficiency

Figure 1.4 Effect of the substrate on the solvent-cage efficiency and alcohol-to-ketone ratio.

The proportion of radicals which escape the solvent cage to form initiating radicals is termed the initiator efficiency /) which is formally defined as follows... 

The high rate of decarboxylation of aliphatic acyloxy radicals is also the prime reason behind low initiator efficiencies (see 3.3.2.1.3). Decarboxylation occurs within the solvent cage and recombination gives alkane or ester byproducts. Cage return for LPO is 18-35% at 80 °C in -octane as compared to only 4% for BPO under similar conditions.144... 

When a symmetrical distribution of products is found, this is evidence for a free-radical mechanism the solvent cage is not efficient and breaks down. 

We have also investigated the kinetics of free radical initiation using azobisisobutyronitrile (AIBN) as the initiator [24]. Using high pressure ultraviolet spectroscopy, it was shown that AIBN decomposes slower in C02 than in a traditional hydrocarbon liquid solvent such as benzene, but with much greater efficiency due to the decreased solvent cage effect in the low viscosity supercritical medium. The conclusion of this work was that C02 is inert to free radicals and therefore represents an excellent solvent for conducting free radical polymerizations. 

A possible mechanism for these reactions is shown in Scheme 21 for compound 101. The absence of a solvent polarity effect on the efficiency of photoreactions of 119 and 124 might be due to a very fast rearrangement of the radical-anion 126 within solvent cages (Scheme 21, path a). In cases in which this intermediate escapes from the cage before rearrangement occurs, a significant influence of the polarity of the solvent would have been observed. This is the situation in DMA-sensitized reactions of 101,117, and 118 (Scheme 21, path b). 

Finnegan and Mattice do not go into detail as to the nature of the excited state, but they do imply that Kobsa s view40 of the mechanism might apply. This would involve going through an excited state of the carbonyl to radicals 10 and 11 in a solvent cage (just after photodissociation so that radical recoupling follows efficiently). The observed preference of 10 for the para position of 11 to the exclusion of the ortho position is apparently without precedent. 

The dimethyl ester of this acid in solution shows a quantum efficiency photochemical products. On the other hand, when the same acid is copolymerized with a glycol to form a polymeric compound with molecular weight 10,000 the quantum yield drops by about two orders of magnitude, 0.012. The reason for this behavior appears to be that when the chromophore is in the backbone of a long polymer chain the mobility of the two fragments formed in the photochemical process is severely restricted and as a result the photochemical reactions are much reduced. If radicals are formed the chances are very good that they will recombine within the solvent cage before they can escape and form further products. Presumably the Norrish type II process also is restricted by a mechanism which will be discussed below. 

The initiation step could also be positively affected by the above-mentioned transport properties, as the efficiency factor f assumes higher values with respect to conventional liquid solvents due to the diminished solvent cage effect One further advantage is constituted by the tunability of the compressibility-dependent properties such as density, dielectric constant, heat capacity, and viscosity, all of which offer additional possibilities to modify the performances of the polymerization process. This aspect could be particularly relevant in the case of copolymerization reactions, where the reactivity ratios of the two monomers, and ultimately the final composition of the copolymer, could be controlled by modifying the pressure of the reaction system. 

Braden DA, Parrack EE, Tyler DR. Solvent cage effects. I. Effect of radical mass and size on radical cage pair recombination efficiency. II. Is geminate recombination of polar radicals sensitive to solvent polarity Coord Chem Rev 2001 211 279-94. 

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