Different Reactant Systems

For many step polymerizations there are different combinations of reactants that can be employed to produce the same type of polymer (Table 1-1). Thus the polymerization of a hydroxy acid yields a polymer very similar to (but not the same as) that obtained by reacting a diol and diacid  

On the other hand, it is apparent that there are different reactant systems that can yield the exact same polymer. Thus the use of the diacid chloride or anhydride instead of the diacid in Eq. 2-120 would give exactly the same polymer product. The organic chemical aspects of the synthesis of various different polymers by different step polymerization processes have been discussed [Elias, 1984 Lenz, 1967 Morgan, 1965]. Whether one particular reaction or another is employed to produce a specific polymer depends on several factors. These include the availability, ease of purification, and properties (both chemical and physical) of the different reactants and whether one or another reaction is more devoid of destructive side reactions. 

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In order to properly control the polymer molecular weight, one must precisely adjust the stoichiometric imbalance of the bifunctional monomers or of the monofunctional monomer. If the nonstoichiometry is too large, the polymer molecular weight will be too low. It is therefore important to understand the quantitative effect of the stoichiometric imbalance of reactants on the molecular weight. This is also necessary in order to know the quantitative effect of any reactive impurities that may be present in the reaction mixture either initially or that are formed by undesirable side reactions. Impurities with A or B functional groups may drastically lower the polymer molecular weight unless one can quantitatively take their presence into account. Consider now the various different reactant systems which are employed in step polymerizations ... 

The different reactant systems employed in step polymerizations can be classified into... 

Scheme VI describes a reaction system in which two different reactants yield a common product.

These two complementary mles are intuitively obvious, e.g. can be simply derived by considering the lateral attractive and repulsive interactions of coadsorbed reactants and promoters as already shown in section 4.5.9.2. They can explain all the observed promotionally induced kinetics for more than sixty different catalytic systems (Table 6.1). As an example these two rules can explain all the observed changes in kinetics orders with [Pg.299]

It is theorized that between the complex network structure of the unaccelerated system and the simpler network structure of the accelerated system, structures made up of the two models represent natural-rubber vnlcani7ares made at various times and temperatures of cures, with different reactant concentrations, and showing the effects of other variants. 

In this case study, nine elementary steps were invoked to construct reaction schemes that describe the reactions of three different reactants, with six of the elementary steps common to the three reactions. The success of these analyses suggests that the parameters for these elementary steps may be applied to systems with similar chemistry (i.e., reduction of oxygenates) at similar coverage regimes. 

Through investigation of complete theories with various other one-step Arrhenius rate functions (for example, two-reactant systems with reaction orders different from unity), it may be shown that equation (20) always follows from equation (18) to the present order of approximation. 

Analogous behavior is attributable to Lewis numbers differing from unity in adiabatic systems [108], [111]. In terms of the Lewis number Lej of the reactant in a one-reactant system, the relevant parameter that corresponds to H is... 

The formulation of Section 9.5.1 has served to remove the chemistry from the field equations, replacing it by suitable jump conditions across the reaction sheet. The expansion for small S/l, subsequently serves to separate the problem further into near-field and far-field problems. The domains of the near-field problems extend over a characteristic distance of order S on each side of the reaction sheet. The domains of the far-field problems extend upstream and downstream from those of the near-field problems over characteristic distances of orders from to /. Thus the near-field problems pertain to the entire wrinkled flame, and the far-field problems pertain to the regions of hydrodynamic adjustment on each side of the flame in essentially constant-density turbulent flow. Either matched asymptotic expansions or multiple-scale techniques are employed to connect the near-field and far-field problems. The near-field analysis has been completed for a one-reactant system with allowance made for a constant Lewis number differing from unity (by an amount of order l/P) for ideal gases with constant specific heats and constant thermal conductivities and coefficients of viscosity [122], [124], [125] the results have been extended to ideal gases with constant specific heats and constant Lewis and Prandtl numbers but thermal conductivities that vary with temperature [126]. The far-field analysis has been... 

There are many important functions of state. Those of importance in chemistry determine the conditions of equilibrium in chemical systems and hence the equilibrium distribution of different reactants and products among the various phases. 

The next step is measurement, or theoretical calculation when possible, of the average rates of absorption per unit interfacial area of the chemical system in the laboratory model where A l and ka are adjusted to be the same as in the packed column. These measurements are carried out for different liquid and gas compositions representative of different levels in the column and are reported as plots of versus p for different reactant concentration contours. Knowledge of these absorption rates is essential for predictive calculation of the column length h, as the consecutive values of tp from the stirred cell must be used to integrate Eq. (131) between the inlet and outlet conditions ... 

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