Bound-Ion-Radical Mechanism

According to this mechanism, the catalyst adsorbs monomer on its surface [Fig. 9.7(a)] and initiation occurs when an adsorbed monomer is polarized by some 

In bound-ion-radical mechanism for ethylene polymerization, initiation can occur either through a chemisorbed ethylene molecule or a chemisorbed hydrogen atom. With chemisorbed ethylene as initiator, polymerization occurs simultaneously at two sites and the growing polymer, which is attached at each end (Fig. 

Problem 9.11 Metal-oxide catalyzed polymerization of ethylene was carried out in benzene solution in a stirred autoclave with a suspension of hydrogen-reduced molybdena-alumina catalyst (Friedlander and Oita, 1957). The pressure was maintained nearly constant by repressuring the autoclave with ethylene as it was consumed in the polymerization process. Temperatures of 200-275°C were studied. The ethylene concentration in solution was controlled by adjusting the pressure (in the range 625 to 1000 psi) at any particular temperature. The ethylene uptake rate (rate of pressure drop, dPIdt) was mea- 

Derive suitable expressions to explain the aforesaid experimental results, considering that the polymerization takes place in an adsorbed layer of ethylene with initiation by adsorbed hydrogen and transfer and termination processes as illustrated in Fig. 9.8. 

Since chemisorbed ethylene disappears by initiation reaction with adsorbed hydrogen, pro-pagation reaction with surface-bound polymer chain, and polymer chain transfer with monomer (Fig. 9.8), one may write (Friedlander and Oita, 1957)  

According to this mechanism, the catalyst adsorbs monomer on its surface pig. 9.7(a)] and initiation occurs when an adsorbed monomer is polarized by some speeies on the eatalyst surfaee converting it to an ion (or a radieal or an ion-radieal pair) bound to the surfaee pig. 9.7(b)]. Fhop-agation takes place along the surfaee pig. 9.7(e)] and the polymer ehain is eventually terminated and desorbed from the surface, being replaeed by fresh monomer. The ehain termination may be caused by transfer with monomer or spontaneous transfer or detaehment from the surface. The molecular regularity of the polymer formed depends on the surfaee layer line-up of the adsorbed monomer molecules. 

a plot of (dP/dt)/Wc ,t vs. [C2H4]i at higher temperatures should t straight lines passing through the origin, as observed experimentally. 

If ethylene initiation is considered, instead of hydrogen initiation, the growing polymer is attached at both the ends and is converted to a single attachment by transfer or termination at one of the ends. The equations describing polymerization take exactly the same form as in the case of initiation by hydrogen but are complicated by extra terms dealing with interconversion of the polymer growing from one or both ends. 

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Two general mechanisms have been proposed to explain the formation of polymers with precipitated catalysts (a) the bound-ion-radical mechanism and (b) the bound-ion-coordinate mechanism. The bound-ion-radical mechanism involves chain growth in a chemisorbed layer of monomer molecules initiated by radicals or ion-radicals bound to the surface of the catalyst, while the coordinate mechanism involves chain growth from a complex ionic center in the catalyst. 

Figure 9.9 Bound-ion-radical mechanism for polymerization on a catalyst surface showing (a) adsorbed monomer (b) initiation (c), (d),etc., propagation. (After Ref. 27.)...

Problem 9.13 The coordinate and bound-ion-radical mechanisms although apparently quite dissimilar, have many features in common. If, in the bound-radical hypothesis, the surface involved decreases to the limiting case of three points of contact, the two mechanisms would appear to be quite similar [28]. Explain this similarity by applying the idea of growth on the surface, used in the bound-radical hypothesis, to the coordinate mechanism, considering a surface with only three points of contact. 

Some ion-radical mechanisms are implicated in the DNA damage others can be described as repairing the damage. In normal DNA, bases belonging to the two opposite strands are bound by relatively weak hydrogen bonds. In case of complementary base pairs, several hydrogen bonds are formed they are depicted in Scheme 3.71. 

The process resembles free radical polymerization, but propagation is not by free radicals but by bound ion-radicals which are fixed to the catalyst surface. A feature of this proposal is that the metal-carbon bond which is the active polymerization site moves through the adsorbed monomer layer. (See Reference 9 for a fuller account of the mechanism of polymerization with metal oxides.)... 

The impact of ozone on MTBE has been studied intensively in literature. For the reaction of ozone with MTBE two major reaction mechanisms have to be considered firstly the direct reaction of ozone with MTBE and secondly the elimination via so-called AOP (advanced oxidation processes), where the reaction is induced by OH radicals as oxidants. The mechanism is strongly dependent on the pH. At low pH values, direct reaction with ozone prevails, especially if functional groups with high electron density are present (e.g., olefinic double bounds). As the milieu is getting more alkaline, radical mechanisms gain importance since ozone is decomposing into OH radicals in the presence of hydroxyl ions. Erom pH 11 only radical reactions are taking place [64]. 

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