Melt transesterification

Small amounts of basic catalyst ( 0.01 mol% of alkyl-ammonium, or phosphonium-salts) are mixed together with the DPC and BPA and the reaction is started at the lower temperature. Conversion is driven by the removal of phenol and the pressure is successively reduced while the temperature is increased. Simple reactors such as CSTRs, tubular reactors or falling-film evaporators can be used for the first stages. Viscosity increases dramatically from 5 mPa s up to 1000 Pa s as conversion increases and special reactors are needed for dealing with the high viscosity at high conversions [149-151]. 

The melt process depends on the reaction rates, the thermodynamic phase equilibrium, and mass transfer between the phases. Detailed mechanistic studies of the Li-catalyzed melt process have been published by Choi et al. [152], and of the solubility of DPC and phenol in polycarbonate by Webb [153]. The liquid-gas equilibrium has to take into account at least two components phenol and DPC. 

For a semi-batch operation for the first stages, optimal variations of pressure and temperature can be calculated based on the above relationships plus the assumption of phase equilibrium, or on a simple relationship for the mass transfer of each volatile component Y (Eq. (55), with the mass transfer rates per unit volume Ji of component Y , mass transfer coefficient of component i kfi, interface area per unit volume a , and equilibrium concentration [Yj at the interface). 

Mass transfer coefficients may be obtained by fitting to process data. Including DPC loss, production capacity (reaction time), and unwanted side products in a cost function, the optimization leads to a balancing of mass transfer and reaction rate. This means that an optimal process is neither entirely mass-transfer nor ki-netically controlled. To avoid side reactions that impair product quality, the lowest temperature that kinetics and mass transfer allow is chosen. The results of the semi-batch optimization can be transferred to the design of a staged, continuous process [154]. 

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Commercial aromatic polyester resins or polyarylates are a combination of bisphenol A with isophthahe acid or terephthahe acid (79). The resins are made commercially by solution polymerization or melt transesterification (47). 

Polybibenzoates are a kind of thermotropic polyesters obtained by polycondensation of 4,4 -biphenyldicar-boxylic acid (p,p -bibenzoic acid) with a diol. These polyesters contain the biphenyl group, which is one of the simplest mesogens. They are synthesized by melt transesterification of the dimethyl or diethyl ester of p,p -bibenzoic acid and the corresponding diol, using a titanium compound as catalyst, according to the following scheme ... 

These materials are segmented copolyether esters formed by the melt transesterification of dimethyl terephthalate, poly(tetramethylene ether) glycol and 1,4-butane diol. As with the thermoplastic polyurethanes, one can describe a hard segment and a soft segment, the hard segments forming crystalline areas which act as pseudocrosslinks . 

Physical properties are related to ester-segment structure and concentration in thermoplastic polyether-ester elastomers prepared hy melt transesterification of poly(tetra-methylene ether) glycol with various diols and aromatic diesters. Diols used were 1,4-benzenedimethanol, 1,4-cyclo-hexanedimethanol, and the linear, aliphatic a,m-diols from ethylene glycol to 1,10-decane-diol. Esters used were terephthalate, isophthalate, 4,4 -biphenyldicarboxylate, 2,6-naphthalenedicarboxylate, and m-terphenyl-4,4"-dicarboxyl-ate. Ester-segment structure was found to affect many copolymer properties including ease of synthesis, molecular weight obtained, crystallization rate, elastic recovery, and tensile and tear strengths. 

Polymer Preparation. The polyether-ester copolymers were prepared by titanate-ester-catalyzed, melt transesterification of a mixture of PTME glycol, the dimethyl ester of an aromatic diacid, and a diol present in 50-100% molar excess above the stoichiometric amount required (Figure 1). The reactions were carried out in the presence of no more than 1 wt %, based on final polymer, of an aromatic-amine or hindered-... 

Elastomeric polyether-ester block copolymers were prepared by melt transesterification of poly(tetramethylene ether) glycol of molecular weight approximately 1000 with a variety of diols and esters. The ease of synthesis and the properties of these thermoplastic copolymers have been related to the chemical structure and concentration of the ester hard segments. 

LC polymers of the ester type have been the most widely synthesized because they can be prepared by low temperature solution polymerization methods and melt transesterification reactions from available or readily attainable monomers. High temperature ester interchange polymerization, however, is somewhat limited in usefulness because it often leads to polymer structures of poorly defined sequence... 

PECs were prepared by monobutyltin oxide-catalyzed, melt transesterification of mixtures of HBA, DHBP, and DPC. 

A segmented polyether-polyester (H49) was synthesized via melt transesterification from 1-4 dimethyl terephthalate, 1-4 butanediol and poly(tetramethylene ether)glycol ... 

Researchers at GE and Bayer independently developed commercially feasible synthetic processes for BPA-PC in the 1950s and began commercial production in the early 1960s. Bayer was awarded the U.S. patent for PC produced via the interfacial polymerization process and GE the U.S. patent for the melt transesterification process. However, until recently, the major part of BPA-PC was produced via the interfacial process. Further information on the history of PC development can be found in previously published reviews. 

Hytrel is a random segmented polyester made (60. 61) by the equilibrium melt transesterification of dimethyl terephthalate (DMT), 1,4-butanediol, and polytetramethyleneglycol (PTMEG) (Reaction 16) ... 

Polycarbonates of numerous bisphenols have been extensively studied. However, most commercial polycarbonates are derived from bisphenol A. At first, both direct-reaction and melt-transesterification processes were employed (Figure 4). In direct-reaction processes, phosgene reacts directly with bisphenol A to produce a polymer in a solution. In transesterification, phosgene is first reacted with phenol to produce diphenyl carbonate, which in turn reacts with bisphenol A to regenerate phenol for recycle and molten, solvent-free polymer. Transesterification is reported to be the least expensive route. It was phased out, however, because of its unsuitability to produce a wide range of products. 

Several polymers were synthesized to initiate the development of this concept (30). In one case (single amino acid), poly(palmitoyl-hydroxypro-line [(Pal-Hpr)-ester] was obtained by melt transesterification of Af-Pal-Hpr-Me in the presence of aluminum isopropoxide as catalyst. In a second case (dipeptide), the tyrosine (Tyr) dipeptide carbobenzyoxy-Tyr-Tyr-Hex was cyanylated at the tyrosine side chain hydroxyl groups, to yield car-bobenzoxy-Tyr-Tyr-Hex-dicyanate. By solution polymerization of equimolar quantities of carbobenzoxy-Tyr-Tyr-Hex and carbobenzoxy-Tyr-Tyr-Hex-dicyanate in tetrahydrofuran, poly(carbobenzoxy-Tyr-Tyr-Hex-iminocarbonate) (polyCTTH) was obtained with Mn = 11,500 and Mw = 19,500. 

Copolyesters of poly(ethylene terephthalate) and 4-acetoxybenzoic acid (PET / oxybenzoate) were synthesized by Jackson et al. through high temperature melt transesterification. However intricate details pertaining to the polyesterification kinetics have remained unexamined. In this system, insertion of 4-oxybenzoate moieties, with stiff rod like conformations, into flexible PET chains fosters the development of thermotropic character within a definite range of copolyester composition. 

A 250 ml glass reactor as shown in Figure 1 was used for the melt transesterification kinetic investigations. 

Maleic anhydride, 130, 206, 211, 213-215 Mannich base salt, 246, 248 Medium temperature shift, 61 Melt transesterification reaction, 233 Membrane air separation ... 

In the melt transesterification process, bisphenol A and diphenyl carbonate are heated to high temperatures... 

Semiflexible MCLCPs were prepared following two basic conceptual procedures. The first one (melt transesterification) involves the chemical modification of pre-... 

Figure 2. Chemical modification of PET via a melt transesterification reaction with p-acetoxybenzoic acid, to obtain a liquid-crystalline copolyester.

Figure 3. Synthesis of an LC polyester through a melt transesterification reaction between the mesogenic 4,4 -diacetoxybiphenyl and a diol.

Lei, G.D. and Choi, K.Y. (1992) Kinetics of melt transesterification of dimethyl terephthalate with bis(2-hydroxyethyl) terephthalate in the synthesis of... 

Choi, K.Y. (1987) A modeling of semibatch reactors for melt transesterification of dimethyl terephthalate with ethylene glycol. Polym. Eng. Sci., 27, 1703-1712. 

TEEEs are typically produced by condensation polymerization of an aromatic dicarboxylic acid or ester with a low MW aliphatic diol and a polyalkylene ether glycol. Reaction of the first two components leads to the hard segment, and the soft segment is the product of the diacid or diester with a long-chain glycol. This can be described as a melt transesterification of an aromatic dicarboxylic acid, or preferably its dimethyl ester, with a low MW poly(alklylene glycol ether) plus a short-chain diol. ... 

The effects of 17 different catalysts were studied in the preparation of this same polymer. A higher temperature melt-transesterification was... 

In 1975, Kellyprepared and characterized a number of polyesters based on 2,5-disubstituted furans in various states of reduction. In this study, 2,5-disubstituted-furan, -dihydrofuran, and -tetrahydrofuran monomers were polymerized using solution, melt-transesterification, ring-opening, and interfacial techniques. These monomers included diacids, diols, diacid chlorides, diesters, dicarboxylic acid anhydrides, as well as monomers based on 5-hydroxymethyl-2-furoic and tetrahydrofuroic acids and esters, and bycyclic lactones containing the tetrahydrofuran ring. A thorough review of previous work done in the area of poljnner synthesis, based on 2,5-disubstituted furan derivatives is reported. It is reported that when... 

Based on this work on aromatic polyesters, Schnell et al. [30] and Fox [31] independently prepared hnear, bigh-melting, bigh-molecular-weight aromatic polycarbonates in 1953—1954 that were derived from 4,4 -dihydroxy-diphenylalkane monomers. These aromatic polycarbonates could be prepared either (1) by a two-phase interfacial method (a modified Schotten-Baumann reaction) or (2) by a melt transesterification (monomers-as-solvent) process using diphenyl carbonate [32]. Versus earher aUphatic polycarbonates, the aromatic polycarbonates were unique in tbat they could be made into water-clear (colorless) transparent structures tbat possessed excellent long-term mechanical properties. 

It is not surprising, given the concurrent research and commercialization efforts that were focused on bringing BPA-based polycarbonates to the market in the late 1950s, that a patent interference arose. Bayer was eventually awarded the first U.S. patents on BPA-based polycarbonates [35] and for the interfacial method of production. GE was issued a patent covering the melt transesterification production process [36]. 

Although there is an extensive volume of work on the polymerization of polycarbonate and polycarbonate copolymers, today there are only two basic commercial condensation processes used to produce the majority of commercial product interfacial and melt transesterification. For a more in-depth review of these two processes as well as others that are not commercially significant, the following resource is recommended [49]. 

From a manufacturing standpoint, the interfacial process is capital-intensive to purify the resin solution, isolate and dry the resin, and recycle solvents and brine. With melt transesterification, because it is a solventless process, the only recycle streams that must be dealt with are those related to the recovery of phenol for reuse in the production of DPC. Hence, there is no need to invest in solvent recovery infrastructure with the melt process, and polymer purification units and dryers can likewise be avoided. However, these investments are somewhat diminished by the investment required for the preparation and purification of DPC. 

From a product standpoint, the major benefit of the melt transesterification approach is that it produces resin with a most probable (thermodynamic) molecular weight distribution. Hence, under normal processing conditions, the anhydrous resin exhibits no tendency to change molecular weight or the melt flow index (MFI) (via redistribution). 

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