Molten -state polycondensation

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. 

For the solubility of TPA in prepolymer, no data are available and the polymer-solvent interaction parameter X of the Flory-Huggins relationship is not accurately known. No experimental data are available for the vapour pressures of dimer or trimer. The published values for the diffusion coefficient of EG in solid and molten PET vary by orders of magnitude. For the diffusion of water, acetaldehyde and DEG in polymer, no reliable data are available. It is not even agreed upon if the mutual diffusion coefficients depend on the polymer molecular weight or on the melt viscosity, and if they are linear or exponential functions of temperature. Molecular modelling, accompanied by the rapid growth of computer performance, will hopefully help to solve this problem in the near future. The mass-transfer mechanisms for by-products in solid PET are not established, and the dependency of the solid-state polycondensation rate on crystallinity is still a matter of assumptions. 

Figure 5.1 Reaction mechanism for the synthesis of polyesters (e.g. poly (ethylene terephthalate)) via molten (melt)-state polycondensation... 

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...

The occurrence of fluorescence is often related to inappropriate processing conditions in molten-state and solid-state polycondensation (SSP) (presence of oxygen, high temperature, long retention time, etc.), as well as the later drying of chips where prolonged residence times can occur. 

High-molecular-weight polyesters cannot be made by polymerization in the molten state alone - instead, post-polymerization (or polycondensation) is performed in the solid state as chips (usually under vacuum or inert gas) at temperatures somewhat less than the melting point. The solid-state polycondensation of polyesters is covered in detail in Chapters 4 and 5. 

Contrary to the high-pressure polycondensation, when the polycondensation of the salt monomers was conducted in a molten state under atmospheric or reduced pressure for the preparation of the polyimides having Tm below 300°C, this often led to the formation of crosslinked aliphatic polyimides that were insoluble even in concentrated sulfuric acid. Therefore, the high-pressure polycondensation process provides a simple and effective method for the synthesis of the linear polyimides with well-defined structures that caused high crystallinity, compared with the other synthetic methods. 

A side reaction often occurring in polycondensation reactions that take place in the molten state is the formation of cyclic oligomers. As already mentioned (Section 5.2), this is an important group of reactions in the preparation of poly(ethylene terephthalate). The reactions that lead to cyclooligomerization are, of course, not degradative the cyclic material is in equilibrium with linear polymer molecules. 

Polycondensations can be carried out in an aqueous or a solvent medium, or they can be performed while the reactions are in a liquid or in a molten state. In industry, reactions of polyfunctional monomers leading ultimately to the tridimenaonal-network molecules of thermosetting resins are usually interrupted at a stage where the polymers still are soluble and fusible. They then are shipped to the fabricators, who convert them by heat curing processes into the final thermosetting product. 

Broza et al. ° prepared new poly(butylene terephthalate) (PBT)-nanotube composites by introducing the oxidised SWNTs into the reaction mixture during two-stage polycondensation of buthylene terephthalate in a molten state. 

Unlike polycondensation polymers, polymers of addition polymerization such as polyethylene and polypropylene when depolymerized in inert atmosphere (39) or in supercritical water (37) do not convert to just the monomer, but a homologous series of oligomers (alkanes and alkenes). Compared to pyrolysis in argon, for polyethylene, the portion of the lighter products increases in supercritical water depolymerizations conducted at 693 K and water densities of 0.13 and 0.42 g/cm. The 1-alkene to n-alkane ratio also increases in supercritical water and with density. These are shown in Figure 11. These results are attributed to the fact that in argon pyrolysis, the reaction proceed in the molten state of the polymer, whereas in supercritical water, some of degradation products... 

PolyCethylene terephthalate) is produced by polycondensation of bis(hydroxy-ethyl)terephthalate (BHET) or its oligomers. BHET may be synthesized both by the reaction of dimethyl terephthalte (DMT) and ethylene glycol (EG) and by the direct esterification of EG with terephthalic acid. Although direct esterification has recently gained importance, the DMT method remains the main process for obtaining BHET. This latter process provides the formation of DMT solution in EG, the transesterification of DMT with EG and distillation of methanol with formation of BHET, and finally, BHET polycondensation. The DMT EG molar ratio is 1 2 with an excess of EG (0.2 to 0.5 mol). Preheating of EG to 120 to 160°C and introduction of DMT in the molten state shortens the DMT dissolving time. 

Poly (olefin-ester) multiblock copolymers belong to a quite new and fast developing class of polyester elastomers [58-60]. The copolymer with PBT rigid segments and flexible polyisobutylene (PIB) segments was prepared by polycondensation in the molten state with DMT, BD, and functionalized PIB, in the presence of titanium tetrabutoxide as catalyst. a,o -Dianhydride [58] and dihydroxyl-terminated PIB of molecular weights 1000, 2200, 4800, and 10000 g/mole were used (Schemes 17 and 18). A multiblock copolymer with PBT and... 

As mentioned in Section 3, typical aramid-6-polyether elastomers are synthesized by the polycondensation reaction of polyether diol with the aramid compound I in the presence of transesterification catalysts. Under these conditions, the synthesis of aramid-6-polyester elastomers gave only low molecular weight elastomers with a broad segment length distribution due to transesterification reactions of the polyester segments. This result inidicated that in the obtaining of aramid-6-polyester elastomers, transesterification catalysts should be avoided. Later, a method for the obtaing of this type of elastomers was developed, which consisted in the copolymerization of an activated acyl lactam-terminated aramid compound II with polyester diols in the molten state, in the absence of transesterification catalysts [40,42]. Compound II was obtained by the reaction of N-(p-aminobenzoyl) caprolactam with terephthaloyl chloride, as shown in Scheme 8 [61]. 

Polycondensations by means od DBOP or TPP also proved useful for the preparation of hb polymides which served as precursors of polyimides [47-49] ((a) Formula 11.6) or polybnezoxazole)s (from (b) or (c) in Formula 11.6 [50-54]. For the best polyimide a DB of 0.48 and a Mw of 188 kDa were determined (by SEC-MALLS). Polycondensations of di or multifunctional aromatic amines and carboxylic acids in the melt are usually not successful, because the high temperatures (>300 °C) are needed to keep the reaction products in the molten state to cause partial decarboxylation. However, in the case of monomer (c). Formula 11.5, polycondensation at 235 °C proved successful, and a Mw of 74 kDa with a PD of 2.6 was achieved [54]. 

The understanding of the SSP process is based on the mechanism of polyester synthesis. Polycondensation in the molten (melt) state (MPPC) is a chemical equilibrium reaction governed by classical kinetic and thermodynamic parameters. Rapid removal of volatile side products as well as the influence of temperature, time and catalysts are of essential importance. In the later stages of polycondensation, the increase in the degree of polymerization (DP) is restricted by the diffusion of volatile reaction products. Additionally, competing reactions such as inter- and intramolecular esterification and transesterification put a limit to the DP (Figure 5.1). 

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