Polycondensations in the Melt

Syntheses of aliphatic polyesters and polyamides by 02 + 2 polycondensations in the melt were first studied by Carothers in the years 1930-1937 (see Chap. 3). Since that time Nylon-6,6 type aliphatic polyamides were and are prepared from equimolar monomer mixtures usually supplied in the form of diamine-dicarboxylic acid salts. In the case of aliphatic polyesters two different procedures are applicable, depending on the volatility of the reaction partners. Combinations of low boiling esters such as dimethyl succinate and low boiling dieols (e.g., ethanediol) are best polymerized in equimolar feed ratios Equimolar feed ratios are again advisable for combinations of long a,m-alkanediols and dicarboxylic acids or their 

The predominance of diol terminated linear chains reduces the influence of end-biting , but the formation of cyclics by back-biting cannot be suppressed. The role of ring-chain equilibria and the extraction and characterization of cyclic oligoesters is discussed in Chap. 5. However, it was also demonstrated by means of MALDI-TOF mass spectrometry that high molar mass PBT contains cyclic polymers with masses at least up to 12 kDa and not only cyclic oligomers [8]. 

---

Preparation of a Liquid Crystalline (LC), Aromatic Main-Chain Polyester by Polycondensation in the Melt... 

A highly activated AB2-monomer is 3,5-bis(trimethylsiloxy) benzoyl chloride (3-3) that leads to a high DB of 60% in a bulk polycondensation, as the once-reacted monomer is activated for the second condensation step the DB was increased to 64% by slow monomer addition to trimethylolpropane (2-(hydroxymethyl)-2-ethylpropane-f,3-diol) (TMP) as a core molecule. As mentioned above, 2,2-bis(hydroxymethyl) propanoic add (bisMPA) (3-6) is a major monomer for aliphatic hb polyesters (Boltom) but shows cydization and other side reactions (e.g., etherification) during polycondensation. A comparable easy-to-handle monomer is bis(4-hydroxyphenyl) pentanoic acid that can be easily polycondensed in the melt or solution, leading to polyesters with a DB of 50% and with only a low tendency for side reartions. Smet et al. presented highly branched copolyesters by a combination of AB2-polycondensa-tion and the ROP of -caprolactone. ... 

The final stage of a polycondensation in suspension represents a polycondensation in the solid state. Polycondensation in the solid state is particularly suitable for producing polyamides. Here, too, a continuous precondensation is first performed to molecular weights between 1000 and 4000. The products are then spray-dryed and are polycondensed further at temperatures of 200-220 "C under nitrogen. This further polycondensation occurs relatively quickly. In order to obtain molecular weights between 1000 and 15,000 with polymers from hexamethylene diamine and adipic acid, the time required at 216°C is 16 h. If the molecular weight of the precondensate is increased to 4000, however, 2 h is sufficient. Since the temperatures are lower than for polycondensation in the melt, better end products are also obtained (less discoloration, etc.). 

This morphology is preserved in the block copolymers even though they were prepared by polycondensation in the melt. The analysis provides also interesting information on the flexible block. 

Another high melting polyamide fiber, Nylon-4,6 was commercialized after 1990 by DSM in Europe under the trademark Stanyl . The attractive properties of this polymer were known since the days of Carothers [6-9], but due to the high Tm (around 300 °C) it proved to be impossible to prepare a high molar mass polymer without discoloration by polycondensation in the melt. In the 1970s Caymans et al. [10]. elaborated a successful, two-step procedure. At first a prepolymer is produced in water under pressure at 200-215 °C. The second step... 

The synthesis of high-molar-mass PLA and PGA by two-step polycondensations of lactic and glycolic acids, respectively, has recently been reported.374,375 It involves the formation of a low-molar-mass oligomer followed by a polycondensation step either in the solid state374 or in the melt under vacuum.375 The procedures are detailed in Section 2.4.1.5.2. 

Novel poly(aryloxydiphenylsilane) is prepared from Bisphenol AF (2,2-bis(4-hydroxyphenyl)-l, 1,1,3,3,3-hexafluoropropane) (1) and dianilinodiphenylsi-lane (30) by melt polycondensation at elevated temperatures under reduced pressure of 1 to 2 Torr (Scheme 18).27 The molecular weight of the poly-(aryloxydiphenylsilane) derived from anilinosilane and bisphenols have been reported to be highly dependent on the reaction temperature in the melt polycondensation.28... 

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. 

Currently, the accepted interpretation of experimental evidence is that the polycondensation of PET in industrial reactors is dominantly controlled by diffusion of EG in the melt phase [1, 6, 8, 102-110]. 

The second approach employs a detailed reaction model as well as the diffusion of EG in solid PET [98, 121-123], Commonly, a Fick diffusion concept is used, equivalent to the description of diffusion in the melt-phase polycondensation. Constant diffusion coefficients lying in the order of Deg, pet (220 °C) = 2-4 x 10 10 m2/s are used, as well as temperature-dependent diffusion coefficients, with an activation energy for the diffusion of approximately 124kJ/mol. 

To increase the PET molecular weight beyond 20 000 g/mol (IV = 0.64 dL/g) for bottle applications, with minimum generation of acetaldehyde and yellowing, a further polycondensation is performed in the solid state at low reaction temperatures of between 220 and 235 °C. The chemistry of the solid-state polycondensation (SSP) process is the same as that for melt-phase polycondensation. Mass-transport limitation and a very low transesterification rate cause the necessary residence time to increase from 60-180 minutes in the melt phase to... 

Polycondensation of highly viscous polyesters in the melt phase is limited. The removal of the volatile by-products becomes more difficult due to diffusion inhibited by the increased viscosity of higher-IV polyesters. In addition, undesirable side reactions due to thermal degradation impede the growth of the molecular chains. As a consequence, the reaction rate decreases and decomposition reactions dominate, thus resulting in a decrease in the melt viscosity [2], As it is able to address these limitations, SSP has become the method of choice and is therefore so popular. 

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). 

For the polymerization, either in the melt or solid phase, the reaction is driven to the polymer by removing ethylene glycol. The polymerization reaction is typically catalyzed by solutions consisting of antimony trioxide or germanium oxide. Both polycondensation catalysts also catalyze the reverse reaction, which is driven by an excess of ethylene glycol at melt conditions, generally above 255 °C. The polymerization reaction follows second-order kinetics with an activation energy of 22 000 cal/mol [6],... 

Polycondensation of diols with dicarboxylic acids is often performed in the melt. However, it does not always lead to high-molecular-weight polyesters. Sometimes, the starting materials or the resulting polyester are thermally unstable at the high condensation temperatures. If the reactants and the polyester are well soluble, one can carry out the polycondensation in solution (see Example 4-2). The elimination of water from diols and dicarboxylic acids frequently occurs rather slowly. In such cases suitable functional derivatives of the diols and dicarboxylic acids (esters or anhydrides) can be used instead of the direct condensation, as described in Sect. 4.1.1.3. 

In principle, the attainment of chemical equilibrium can be accelerated by catalysts however, in contrast to polyester formation, catalysts are not absolutely essential in the above-mentioned polycondensations. The first two types of reactions are generally carried out in the melt solution polycondensations at higher temperature, e.g., in xylenol or 4-fert-butylphenol are of significance only in a few cases on account of the poor solubility of polyamides. On the other hand, polycondensation of diamines with dicarboxylic acid chlorides can be carried out either in solution at low temperature or as interfacial condensation (see Sect. 4.1.2.3). 

Step-growth polymerization, 22, 24-25, 23, 84-86, 86,90-92,114-115, 261 compared with chain-growth polymerization, 88-89, 88-89 interfacial polymerization, 91-92 laboratory activities on synthesis of nylon, 228-230 synthesis of polyesters in the melt, 231-233 synthesis of polyurethane foam, 234-237 molar mass and, 86, 86 polycondensation of poly ethylene terephthalate), 90-91 polymers produced by, 86 types of monomers for, 90 Stereochemistry, 28, 37-39,41-42, 70 tacticity, 103-105 Stereoisomers, 41 Stereoregularity, 70 Stiffness, 142, 261 Strain, 142-143, 261 Strength... 

Just as the products of polycondensation are greatly varied, so are the reaction conditions used in their production. Some are produced in the melt (many polyamides and polyesters), some initially in the melt but with extensive polymerization continuing in the solid state (polyurethane foams and elastomers), in solution (some polyurethane fibres) or in non-homogeneous liquid systems (some polycarbonates, very high melting polyamides). 

Polymerization in the melt is widely used commercially for the production of polyesters, polyamides, polycarbonates and other products. The reactions are controlled by the chemical kinetics, rather than by diffusion. Molecular weights and molecular weight distributions follow closely the statistical calculations indicated in the preceding section, at least for the three types of polymers mentioned above. There has been much speculation as to the effect of increasing viscosity on the rates of the reactions, without completely satisfactory explanations or experimental demonstrations yet available. Flory [7] showed that the rate of reaction between certain dicarboxylic acids and glycols was independent of viscosity for those materials, in the range studied. The viscosity range had a maximum of 0.3 poise, however, far below the hundreds of thousands of poises encountered in some polycondensations. 

Solution polycondensation, i.e., where the monomers and polymer are soluble in the solvent(s), is also kinetically controlled. It is closely related mechanistically to reactions in the melt, but the choice of the solvent may influence both the rate and equilibrium, and hence the molecular weight. This process is utilized commercially in the production of a variety of organic coatings and certain solution-spun fibres, e.g., certain spandex types and also very high-melting aromatic polymers. 

---