Of degree, conversion

The general reactivity of higher a-olefins is similar to that observed for the lower olefins. However, heavier a-olefins have low solubihty in polar solvents such as water consequentiy, in reaction systems requiting the addition of polar reagents, apparent reactivity and degree of conversion maybe adversely affected. Reactions of a-olefins typically involve the carbon—carbon double bond and can be grouped into two classes (/) electrophilic or free-radical additions and (2) substitution reactions. 

Quantum, by contrast, converted an ethylene—carbon monoxide polymer into a polyester-containing terpolymer by treatment with acidic hydrogen peroxide, the Baeyer-Villiger reaction (eq. 11). Depending on the degree of conversion to polyester, the polymer is totally or partially degraded by a biological mechanism. 

The amount of catalytic coke that is formed depends on the type of catalyst used ia the FCCU, the coking tendency of the feed, the degree of conversion of the feed, and the length of time the catalyst is exposed to the feed (eq. 2) (11). 

Food Applications. A number of features make en2ymes ideal catalysts for the food industry. They are all natural, efficient, and specific work under mild conditions have a high degree of purity and are available as standardi2ed preparations. Because en2ymatic reactions can be conducted at moderate temperatures and pH values, simple equipment can be used, and only few by-products are formed. Furthermore, en2ymatic reactions are easily controUed and can be stopped when the desired degree of conversion is reached. 

Theoretical plots of ee (substrate) and eep (product) as a function of c are shown in Figure 2a and b. It can be seen that the ee increases with the extent of conversion. Consequently the enantiomeric purity of the substrate can be increased by sacrificing the yield and carrying out the reaction to higher degrees of conversion. Conversely, if high purity product is required the conversion should be terminated at early stages. 

In contrast to the hydrolysis of prochiral esters performed in aqueous solutions, the enzymatic acylation of prochiral diols is usually carried out in an inert organic solvent such as hexane, ether, toluene, or ethyl acetate. In order to increase the reaction rate and the degree of conversion, activated esters such as vinyl carboxylates are often used as acylating agents. The vinyl alcohol formed as a result of transesterification tautomerizes to acetaldehyde, making the reaction practically irreversible. The presence of a bulky substituent in the 2-position helps the enzyme to discriminate between enantiotopic faces as a result the enzymatic acylation of prochiral 2-benzoxy-l,3-propanediol (34) proceeds with excellent selectivity (ee > 96%) (49). In the case of the 2-methyl substituted diol (33) the selectivity is only moderate (50). 

Kinetic Resolutions. From a practical standpoint the principal difference between formation of a chiral molecule by kinetic resolution of a racemate and formation by asymmetric synthesis is that in the former case the maximum theoretical yield of the chiral product is 50% based on a racemic starting material. In the latter case a maximum yield of 100% is possible. If the reactivity of two enantiomers is substantially different the reaction virtually stops at 50% conversion, and enantiomericaHy pure substrate and product may be obtained ia close to 50% yield. Convenientiy, the enantiomeric purity of the substrate and the product depends strongly on the degree of conversion so that even ia those instances where reactivity of enantiomers is not substantially different, a high purity material may be obtained by sacrificing the overall yield. 

In using Eq. (14-66), therefore, it should be understood that the numerical values of will be a complex function of the pressure, the temperature, the type and size of tower packing employed, the hq-uid and gas mass flow rates, and the system composition (for example, the degree of conversion of the liquid-phase reactant). 

An alternative approach is to copolymerise only up to a limited degree of conversion, say 40%. In such cases although there will be some variation in composition it will be far less than would occur if the reaction is taken to completion. 

The rate equation involves a mathematical expression describing the rate of progress of the reaction. To predict the size of the reactor required in achieving a given degree of conversion of reactants and a fixed output of the product, the following information is required ... 

Influence of back mixing (macromixing) on the degree of conversion and in continuous chemical reaction operation. 

Fig. 7.9. Measurements of the degree of conversion of ct p silicon nitride at a fixed time and various temperatures are thought to show the strong influence of shock modification on the high temperature dissolution [84B01].

Chemical Factors. These involve mainly the kinetics of the reaction. The design must provide sufficient residence time for the desired reaction to proceed to the required degree of conversion. 

The mechanical properties of ionomers can be appreciably altered by the manner in which the ionomer is prepared and treated prior to testing. Some of the factors that are influential are the degree of conversion (neutralization) from the acid form to the salt form, the nature of the thermal treatment or aging, the type of counterion that is introduced, the solvent that is used for preparation of thin films, and the presence and nature of any plasticizers or additives that may be present. In the scope of this chapter, it is not possible to provide a complete description of the influence of each of these variables on the wide variety of ionomers that are now commercially available or produced in the laboratory. Instead, one or more examples of the changes in properties that may be induced by each of the processing variables is presented and discussed. 

In most ionomers, it is customary to fully convert to the metal salt form but, in some instances, particularly for ionomers based on a partially crystalline homopolymer, a partial degree of conversion may provide the best mechanical properties. For example, as shown in Fig. 4, a significant increase in modulus occurs with increasing percent conversion for both Na and Ca salts of a poly(-ethylene-co-methacrylic acid) ionomer and in both cases, at a partial conversion of 30-50%, a maximum value, some 5-6 times higher than that of the acid copolymer, is obtained and this is followed by a subsequent decrease in the property [12]. The tensile strength of these ionomers also increases significantly with increasing conversion but values tend to level off at about 60% conversion. 

The best combination of properties of polyethylene-based ionomers, such as stiffness, strength, transparency, and toughness, are realized at partial degrees of conversion of about 40-50% [13]. The initial increase in properties is a result of the presence of ionic interactions, which strengthen and stiffen the polymer. There is, however, some loss of crystallinity as a result of the presence of the ionic groups. When the loss of crystallin-... 

For partially crystalline ionomers, such as those based on copolymers of ethylene and methacrylic acid, even time or aging at room temperature can have an effect on mechanical properties. For example, upon aging at 23°C, the modulus of the acid form of the copolymer increased 28%, while in the ionomer form, the increase ranged up to 130%, with the specific gain in modulus depending on the degree of conversion and on the counterion that was present [17]. 

Aside from ion content, a wide range of properties is available in ionomers by control of various processing variables, such as degree of conversion (neutralization), type of counterion, plasticizer content and thermal treatment. Various examples illustrating possible effects of these variables on mechanical relaxation behavior and on such mechanical properties as stiffness, strength, and time- or energy-to-fracture have been given. 

Conversion (or degree of conversion) refers to the fraction of the feed or f raction of some reactant in the feed which has been converted into products. 

If the PBR is less than unity, the oxide will be non-protective and oxidation will follow a linear rate law, governed by surface reaction kinetics. However, if the PBR is greater than unity, then a protective oxide scale may form and oxidation will follow a reaction rate law governed by the speed of transport of metal or environmental species through the scale. Then the degree of conversion of metal to oxide will be dependent upon the time for which the reaction is allowed to proceed. For a diffusion-controlled process, integration of Pick s First Law of Diffusion with respect to time yields the classic Tammann relationship commonly referred to as the Parabolic Rate Law ... 

A Comparison of the Degrees of Conversion to Cyclohexanol (xi) in the Hydrogenation of Phenol and of Cyclohexanone, respectively ... 

For the case of MMA polymerization with a source of f-butoxy radicals (DBPOX) as initiator and toluene as solvent, most initiation may be by way of solvent-derived radicals"1"" (Scheme 3.9). Thus, a high proportion of chains (>70% for 10% w/v monomers at 60 °C22) will be initiated by benzyl rather than 1-butoxy radicals. Other entities with abstractable hydrogens may also be incorporated as polymer end groups. The significance of these processes increases with the degree of conversion and with the (solvent or impurity) monomer ratio. 

The relative importance of these mechanisms, and the value of the overall kt, depends on the molecular weight and dispersity of the propagating species, the medium and the degree of conversion. The value of k, is not a constant ... 

The extent of branching, of whatever type, is dependent on the polymerization conditions and, in particular, on the solvent and temperature employed and the degree of conversion. Nozakura et at.1 1 found that, during bulk polymerization of VAc, the extent of transfer to polymer increased and the selectivity (for abstraction of a backbone vs an acetoxy hydrogen) decreases with increasing temperature. 

All relaxation curves exhibited more than one phase at various degrees of conversion and at different temperatures. This clearly rules out the all-or-none mechanism (AON) although the AON model is able to fit easily to the measured equilibrium transition curve. However, a mechanism has been proposed which allows the existence of side... 

---