Polycondensation Processes

Various inorganic, organic, and organometaUic compounds are known to cataly2e this polymerization (4,8,9). Among these, BCl is a very effective catalyst, although proprietary catalysts that signiftcandy lower polymerization temperature from the usual, sealed-tube reaction at 250°C are involved in the industrial manufacture of the polymer. A polycondensation process has also been developed for the synthesis of (4) (10—12). This involves elimination of phosphoryl chloride from a monomer prepared from (NH 2 04 and PCl. ... 

Polyetherification is similar to a polycondensation process formation of high molecular weight polymer requires precise adjustment of composition to approximately 1 1 ratio of bisphenol to dihalosulfone. Trace amounts of water gready reduce the molecular weight attainable owing to side reactions that unbalance the stoichiometry (76). The reactivity of the halosulfone is in the order expected for two-step nucleophilic aromatic displacement reactions ... 

J. P. Kennedy and R. M. Thomas, Polymerisation and Polycondensation Processes, Mdvances in Chemistry Series No. 34, American Chemical Society, Washington, D.C., 1962, p. 111. 

Polymerization and polycondensation processes together with transformations of polymeric chains have been used for the preparation of PCSs. Polymerization processes include the opening of C=C or C=N-bonds and the opening of carbo- and heterocycles. 

In the AA-BB type of sulfonylation, two or more activated aromatic hydrogen atoms are commonly present in the reacting molecules. Therefore, this polycondensation process may result in different repeating units. Structural irregularities... 

Use of polycondensation processes of substituted phosphoranimines to obtain already substituted poly(organophosphazenes)... 

The fact that this polycondensation process takes place at room temperature, with careful control of both molecular weight and molecular weight distribution of the final polymers or copolymers produced are definitive advantages over the corresponding ROP reaction of (NPCl2)3 in molten state. 

M. Hofinger, W. Bose, M. Hille, and R. Bohm. Quaternary oxyalkyl-ated polycondensates, process for their manufacture and their utilization (Quatemare oxalkylierte Polykondensate, Verfahren zu deren Herstellung und deren Verwendung). Patent EP 212265,1987. 

When making theoretical considerations of polycondensation processes it is necessary to distinguish chemically identical functional groups if they differ in reactivity. Examples are primary and secondary hydroxyls in a molecule of glycerine, SA2j A2, which belong to kinetically distinct types Ax and A2. 

Monomers employed in a polycondensation process in respect to its kinetics can be subdivided into two types. To the first of them belong monomers in which the reactivity of any functional group does not depend on whether or not the remaining groups of the monomer have reacted. Most aliphatic monomers meet this condition with the accuracy needed for practical purposes. On the other hand, aromatic monomers more often have dependent functional groups and, thus, pertain to the second type. Obviously, when selecting a kinetic model for the description of polycondensation of such monomers, the necessity arises to take account of the substitution effects whereas the polycondensation of the majority of monomers of the first type can be fairly described by the ideal kinetic model. The latter, due to its simplicity and experimental verification for many systems, is currently the most commonly accepted in macromolecular chemistry of polycondensation processes. 

Alongside the radical distinction of the mechanism of this process from that of chain polymerization, linear polycondensation features a number of specific peculiarities. So, for instance, the theory of copolycondensation does not deal with the problem of the calculation of a copolymer composition which normally coincides with the initial monomer mixture composition. Conversely, unlike chain polymerization, of particular importance for the products of polycondensation processes with the participation of asymmetric monomers is structural isomerism, so that the fractions of the head-to-head and head-to-tail patterns of ar-... 

The rate constants in the reactions (29) may be conveniently envisaged as elements of symmetric matrix k. In order to calculate the statistical characteristics of a particular polycondensation process along with matrix k parameters should be specified which characterize the functionality of monomers and their stoichiometry. To this end it is necessary to indicate the matrix f whose element fia equals the number of groups A in an a-th type monomer as well as the vector v with components Vj,... va,..., v which are equal to molar fractions of monomers M1,...,Ma,...,M in the initial mixture. The general theory of polycondensation described by the ideal model was developed more than twenty years ago [2]. Below the key results of this theory are presented. 

For cases where the growth period is the same as the residence time in the reactor, as in polycondensation processes, the residence time distribution is the dominant factor influencing the molecular weight distribution. In this case one obtains a broader molecular weight distribution from a CSTR than from a batch reactor. Figure 9.12 [also taken from Denbigh (11)] indicates the type of behavior expected for systems of this type. 

The experimental results, of course, only show that steric factors and the reactivity of amino acids can have a certain influence on the sequence of the polymer in thermal reactions, but that many other parameters influence the polycondensation process. 

Synthesis of Protein-Like Copolymers Using Polycondensation Processes. 120... 

When discussing various methods for the synthesis of protein-like HP-copolymers from the monomeric precursors (Sect. 2.1), we pointed to the possibility of implementation of both polymerization and polycondensation processes. The studies of the potentials of the latter approach in the creation of protein-like macromolecular systems have already been started. The first published results show that using true selected reactions of the polycondensation type and appropriate synthetic conditions (structure and reactivity of comonomers, solvent, temperature, reagent concentration and comonomer ratio, the order of the reagents introduction into the feed, etc.) one has a chance to produce the polymer chains with a desirable set of monomer sequences. 

The key to the successful preparation of this new composite is the identification of a surfactant, PE-b-PEG, that is capable of stabilizing the emulsion and promoting the dissolution of the PE. Then submicrometre particles of low-density PE silica and high-density PE silica are synthesized by carrying out a silica sol-gel polycondensation process within emulsion droplets of TEOS-dissolved PE, at elevated temperatures (78 and 130°C for low- and high-density PE, respectively). 

Transesterification is the main reaction of PET polycondensation in both the melt phase and the solid state. It is the dominant reaction in the second and subsequent stages of PET production, but also occurs to a significant extent during esterification. As mentioned above, polycondensation is an equilibrium reaction and the reverse reaction is glycolysis. The temperature-dependent equilibrium constant of transesterification has already been discussed in Section 2.1. The polycondensation process in the melt phase involves a gas phase and a homogeneous liquid phase, while the SSP process involves a gas phase and two solid phases. The respective phase equilibria, which have to be considered for process modelling, will be discussed below in Section 3.1. 

The transesterification and glycolysis reactions proceed via the Aac2 mechanism described above in Section 2.1. The reactions are acid catalyzed as demonstrated by Chegolya el al. [27], who added TPA to the polycondensation of PET and observed a significant increase of the apparent reaction rate. The industrial polycondensation process is accelerated by the use of metal catalysts, with these being mainly antimony compounds. 

In 1973, Bonatz et al. [101] finally showed by carefully performed experiments that the first reaction model proposed by Hoftyzer and van Krevelen [100] is correct. Thus, the polycondensation reaction (transesterihcation of bEG) takes place in the entire melt phase and the removal of EG is the rate-determining step for the overall polycondensation process. 

The chemistry of the solid-state polycondensation process is the same as that of melt-phase poly condensation. Most important are the transesterification/glycolysis and esterification/hydrolysis reactions, particularly, if the polymer has a high water concentration. Due to the low content of hydroxyl end groups, only minor amounts of DEG are formed and the thermal degradation of polymer chains is insignificant at the low temperatures of the SSP process. 

Due to different residence times needed for the esterification and the polycondensation steps, the industrial-batch polycondensation process is designed with two main reactors, i.e. one esterification reactor and one or two parallel polycondensation reactors (Figure 2.34). 

The continuous polycondensation process consists of four main process units, i.e. (1) slurry preparation vessel, (2) reaction unit, (3) vacuum system, and (4) distillation unit. The molar EG/TPA ratio is adjusted to an appropriate value between 1.05 and 1.15 in the slurry preparation vessel. In most industrial processes, the melt-phase reaction is performed in three up to six (or sometimes even more) continuous reactors in series. Commonly, one or two esterification... 

Yokoyama, H Sano, T Chijiiwa, T. and Kajiya, R., Degradation reactions in ethylene glycol terephthalate polycondensation process,. /. Jpn. Petrol. Inst., 21, 194-198 (1978). 

Hoftyzer, P. J. and van Krevelen, D. W., The rate of conversion in polycondensation processes as determined by combined mass transfer and chemical reaction, in Proceedings of the 4th European Symposium on Chemical Reactions, Chem. Eng. Sci., 139-146 (1971). 

Ravindranath, K. and Mashelkar, R. A., Modeling of polyethylene terephthalate) reactors. IX. Solid state polycondensation process,./. Appl. Polym. Sci., 39, 1325-1345 (1990). 

Devotta, I. and Mashelkar, R. A., Modelling of polyethylene terephthalate reactors - X. A comprehensive model for a solid-state polycondensation process, Chem. Eng. Sci., 48, 1859-1867 (1993). 

Figure 5.2 Effect of temperature on the molten (melt)-state polycondensation process for PET [15(b)], Reprinted from Polymer, 14, Tomita, K., Polymer 14, 50 (1973), (see references) Copyright (1973), with permission from Elsevier Science...

Figure 5.3 Effect of nitrogen gas flow rate on the solid-state polycondensation process for PET reaction conditions, 259°C for 7h initial Mn, 16500, with a particle size of 0.18-0.25 mm data obtained by gas chromatographic analysis, employing a column of dimensions 8ft x 0.7.5 in o.d. [5]. Reproduced from Hsu, L.-C., J. Macromol. Sci., Phys., B1, 801 (1967), with permission from Marcel Dekker...

Besides the pressure (vacuum) and the flow rate of the gas, temperature is the major experimental variable in SSP and is of the highest importance for the economy of the process. Temperature dependence data for the solid-state polycondensation process are shown in Figures 5.4-5.7. According to the results... 

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