Hunter-Yohe mechanism

Tater, Gantmakher and Medvedev [43] revived the Hunter-Yohe mechanism with the addition that they supposed it to operate only in polar solvents, the idea being that the zwitterion would thus be stabilised by solvation. The experiments of Colclough and Dainton [36] with the system styrene-stannic chloride-nitrobenzene, and those of Tongworth, Plesch and Rutherford [44] with isobutene-titanium tetrachloride-various alkyl chloride solvents, then showed that in these systems a co-catalyst is required and that therefore the HY-MG mechanism cannot be operative. It has in fact now been abandoned by its originators, who have conceded that in the styrene-SnCl4-EtCl system a co-catalyst is required [45]. 

This is, of course, the Hunter-Yohe mechanism, involving a zwitterion, but with this monomer it does not seem implausible in particular, the objection that it involves a large charge separation, which has some force in the case of a sterically hindered hydrocarbon such as isobutene, is probably not valid here, especially in view of the flexibility which... 

Direct initiation (i.e. without the intervention of a co-initiator), either according to the Hunter-Yohe mechanism (3) ... 

After the discovery of cocatalysis, Gantmakher and Medvedev revived this mechanism claiming that it could be operative in polar sdvents. This was however disputed notably by Colclou and Dainton and by Plesch s school Later studies have led to the conclusion that the unmodified Hunter-Yohe mechanism can be reasonably well applied to direct initiation schemes involving titanium tetrachloride ... 

AH the evidence gathered in this section must now be rationalised into a reasonable mechanism. Titanium tetrachloride is monomeric in methylene chloride even at low temperature. Sli t interactions exist between TiCl4 moIecules but these do not amount to any strong association. Given the spectroscopic observations concerning the initial stoicheiometry of initiation and the structure of the products of this reaction, we are led to formulate a revival of the old Hunter-Yohe mechanism Thus, the first step in the direct initiation processes involving TiCl4 and olefins takes the form ... 

It will be shown in the following discussion that (i) and (ii) are extremely unlikely, but that (iii) and (iv) are at least possible under some conditions. Hunter-Yohe and Medvedev-Gantmakher mechanism (HY-MG). The first theory of cationic polymerization [41] was based on the supposition that metal halide and monomer form a complex which is so polar that it can be considered as a zwitterion, and that polymerization proceeds from the positive end of this complex... 

Before the publication of Colclough and Dainton s work, Gantmakher and Medvedev [11] had revived Hunter and Yohe s theory of direct initiation, but restricted it to solvents of moderately high dielectric constant. They maintained that in such solvents neither a protonic acid nor an alkyl halide co-catalyst is required. The experiments of Colclough and Dainton make this appear highly unlikely, although they do not disprove it completely. It is important to realise that several types of initiation could co-exist in the same system even if in certain systems co-catalysis by alkyl halides were proved, this does not exclude the existence of a concurrent direct initiation by the Hunter-Yohe, Gantmakher-Medvedev mechanism. 

Having ruled out other reasonable possibilities, we are left with two mechanisms vdiich can account for direct initiation, viz., the Hunter-Yohe zwitterion reactiwi and the selfionisation of the Lewis acid followed by the addition of the cation onto the monomer double bond. Before discussing the relevance of each one of them, some remarks must be made on the nature of the interactions between Lewis acids and olefins and on the possible conjugation of Lewis acids with anions. 

In 1930, Hunter and Yohe proposed the following mechanism for the cationic polymerisation. They assumed the formation of a complex between the Lewis acid and the monomer. 

Whilst this view was widely accepted as explaining many of the observations, a few workers retained for some time a modified version of Hunter and Yohe s direct initiation mechanism to explain a limited range of phenomena. The point at issue is whether the polymerisation of an olefinic hydrocarbon can be initiated by a metal halide alone, without the participation of a co-catalyst, which might be water, a hydroxylic impurity, or a... 

These considerations imply a mechanism which is similar to that proposed by Hunter and Yohe for the polymerization of olefins in the presence of aluminum chloride (see p. 68). 

Hunter and Yohe laid out the first mechanism of carbocationic polymerization in 1933 (13). According to their proposal, the metal halide-monomer interaction leads to the formation of a zwitterion (the German word zwitter stands for between ) ... 

Lewis Acids. Certain Lewis acids or their mixtures are capable of initiating carbocationic pol5nnerization in the absence of a cation or proton sonrce. This type of initiation is called direct initiation. Hunter and Yohe (13) proposed the formation of zwitterions. The more accepted mechanism is auto-ionization of a Lewis acid dimer, followed by its addition to the monomer (eq. 4) ... 

---