Polycondensation irreversible

Let us address first the processes of irreversible polycondensation of an arbitrary mixture of monomers. The functional groups A1,...,Ai,...,Am act here as kineti-cally independent elements, and the scheme of the elementary reactions of condensation between them... 

As distinct from the ideal model of irreversible polycondensation the exact... 

Fig. 1. The dependence of weight fraction of gel on conversion of functional groups A under irreversible polycondensation of monomer SA3 described by the simplified FSSE model with kinetic parameters K k k0 and K2=k2/k0. The curves are depicted proceeding from the results of calculations at values of these parameters equal to kx=1, k2=0.1 (a), and kx=1, 2=10 (b)...

The results reported above have been extended to the general case of irreversible polycondensation of an arbitrary mixture of monomers (characterized by arbitrary matrix of functionalities f and the composition vector v) under the conditions of the applicability of the FSSE model [26]. 

Furthermore, polycondensation of LA with and widiout catalyst under closed and open conditions at different temperatures has been studied [28]. The experimental results are used to estimate the polymerization rate constant and the equilibrium constant. Accoimting for the actual rate of water removal, the model encompasses all possible situations and, in particular, the two limiting cases of complete water removal (ideal irreversible polycondensation) and no water removal (ideal reversible polycondensation). 

For irreversible polycondensations, recent studies by Kricheldorf [24, 25] using MALDI mass spectroscopy have in several circumstances detected quite an appreciable concentration of ring molecules. Unfortunately, dependence of ionization and thus of instrumental response factor of polymer molecules on the nature of the end groups [26] prevents a quantitative exploration of those findings. But it can be concluded that extension of kinetic modeling in order to take into account the presence of rings is more important than was previously acknowledged. 

Methods for predicting molecular weight distributions at chemical equUibrium and for irreversible polycondensations are presented with some detail below see Sections 3.4.3 and 3.4.4. 

Devolatilization in irreversible polycondensations is carried out in the later stages of the process and is similar to other polymerizations. For reversible polycondensations, however, it differs from the devolatilization of other polymers in a number of ways. 

The kinetics has been studied by Enikolopyan et al. [242] and Gao [243], among others. Branching formation occurs to a low extent and can usually be neglected the reaction can be described as a linear irreversible polycondensation AXA + BYC, with A = -OH, B = -Cl, and C = epoxide (ECH and oligomers have different reactivities). 

This approach can only deal with ring-forming reactions either for a limited number of the smallest rings, or alternatively, for linear polycondensations. The important practical case of the irreversible polycondensation of AXA + BYB + BYC (C being an inert group) leads to the rate laws in Eqs. (131) for the molecules with the six possible combinations of end groups P, . .. P and rings C , where... 

If any transient (dynamic) heterogeneity is present in a gelling solution, it will be arrested in the gel network when the timescale of the sol-gel transition is short enough to freeze the snap-shot structure of the heterogeneity. In the present sol-gel system, both the structural evolution due to phase separation and the structure freezing by sol-gel transition take place as a result of irreversible polycondensation reaction. The frozen stmcture depends, therefore, on the onset of phase separation relative to the freezing point by sol-gel transition. The earlier the phase separation is initiated relative to the sol-gel transition, the coarser the resultant structure becomes, and vice versa. 

Example 8.4 For a second-order, irreversible polycondensation reaction with rate proportional to the concentrations of reactive A and B groups, obtain expressions for conversion and number-average chain length as a function of time for a stoichiometrically equivalent batch. 

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